Fluids doi: 10.3390/fluids8100258

Authors: Phil Ligrani Valerie Hietsch Mengying Su

In past investigations of elastic instabilities and elastic turbulence, almost no attention has been devoted to the effects and influences of inertial phenomena. Within the present investigation, Nusselt number data are provided to illustrate the relative influences of inertia and polymeric viscoelastic phenomena within a rotating Couette flow (RCF) environment. Data are provided from experimental measurements of local surface heat transfer characteristics for different flow passage heights, one radial position, and different values of disk rotational speed for polyacrylamide polymer concentrations &rho; of 0 ppm, 100 ppm, 150 ppm, and 300 ppm. With this approach, data for a wide range of shear rate&nbsp;&gamma;&#729;&nbsp;values, Weissenberg numbers, and first normal stress difference values are provided. Nusselt number data are provided as dependent upon a newly developed&nbsp;P&prime;&nbsp;parameter, equal to&nbsp;ReEI/Re0.22, which collapse into a single distribution over the range of&nbsp;P&prime;&nbsp;values considered which range from 0 to about 182. Such characteristics indicate that the&nbsp;P&prime;&nbsp;parameter provides an appropriate means to simultaneously account for the relative influences of inertia and polymeric viscoelastic effects. The use of such a power law dependence for&nbsp;Re&nbsp;additionally gives&nbsp;P&prime;&nbsp;values which are dominated by&nbsp;ReEI&nbsp;values when the Weissenberg number Wi is greater than the elastic instability transition onset value. The experimental conditions associated with this value correspond to the change from inertia domination (with buoyance influences) to polymeric viscoelastic domination which occurs for shear rates in the vicinity of 11 to 12 s&minus;1. For Weissenberg numbers greater than the onset value, Nusselt numbers associated with H = 5 mm are generally the highest values measured, with magnitudes that steadily increase with&nbsp;&gamma;&#729;. Associated Nusselt numbers become as high as about 3.0, whereas zero-shear rate values (obtained with zero rotation) are in the vicinity of 1.0. At lower Weissenberg number magnitudes (below the transition onset value), Nusselt numbers cover a wide range of values as experimental conditions and configuration are varied, as a consequence of the complicated and simultaneous influences of inertia, buoyancy, and dilute polymer presence.

]]>Fluids doi: 10.3390/fluids8090257

Authors: Fardausur Rahaman Abd Alhamid Rafea Sarhan Jamal Naser

In this work, a three-dimensional CFD model for the gas&ndash;solid flow of two different particle sizes in a CFB riser coupled with a kinetic theory (KT) has been developed. The properties of the solid phases are calculated using the proposed multi-particle kinetic theory. The CFD model is implemented in the commercial CFD software CFX4.4. In the current model, one gas phase and two solid phases are used. However, the model is generalised for one carrier phase and N number of solid phases to enable a realistic particle size distribution in the system. The momentum, volume fraction and granular temperature equations are solved for each individual solid phase and implemented into the CFD model through user-defined functions (UDFs). The k-&epsilon; turbulence model is used in simulating the circulating fluidised bed model. For verification, simulation results obtained with the new KT model were compared with experimental data, and then the model was used for further analysis. It was found that the proposed multi-particle model can be used to calculate the properties of gas&ndash;solid systems with particles of different sizes and/or densities, removing the assumptions of previous models that required all the particles to be of an equal mass, size and density.

]]>Fluids doi: 10.3390/fluids8090256

Authors: Leidy J. Cerón-Martínez Andrés M. Hurtado-Benavides Alfredo Ayala-Aponte Liliana Serna-Cock Diego F. Tirado

Valorizing agri-food industrial waste is essential for a circular economy, yielding high-value products, waste reduction, technological solutions, employment opportunities, and enhanced food security. This work shows the valorization of seeds generated as residues from the agri-food industries of guava pera (Psidium guajava) and Tommy Atkins mango (Mangifera indica L.), through extraction with supercritical carbon dioxide (scCO2). After the optimization of the initial solid condition of the raw material (i.e., particle size and moisture content), scCO2 pressure and temperature were established through the response surface methodology (RSM) to obtain an oily extract with the highest content in bioactive compounds of commercial relevance, as well as with a high antioxidant capacity. The total amount of oily extract in guava and mango seeds was 14% and 9%, respectively, while the maximum recovery of supercritical extract was 95% from guava seeds at 38 MPa and 50 &deg;C, and 88% from mango seeds at 37 MPa and 63 &deg;C. Bioactive fractions rich in squalene, &gamma;-tocopherol, &alpha;-tocopherol, campesterol, &beta;-sitosterol, and stigmasterol were obtained. The best supercritical extraction conditions, in terms of the bioactive fractions richest in minor compounds, were at 17 MPa and 50 &deg;C for guava seeds and at 23 MPa and 63 &deg;C for mango seeds. At these conditions, the highest antioxidant capacities were also found for the extracts. Thus, these bioactive fractions could be used in a variety of products in the cosmetic, food, pharmaceutical, and medical activities due to the beneficial properties of the identified compounds in health as antioxidants, anti-inflammatories, and cholesterol reducers.

]]>Fluids doi: 10.3390/fluids8090255

Authors: Michalina Ślemp Andrzej Miniewicz

Marangoni bursting describes the spontaneous spread of a droplet of a binary mixture of alcohol/water deposited on a bath of oil, followed by its fast spontaneous fragmentation into a large number of smaller droplets in a self-similar way. Several papers have aimed to describe the physical phenomena underlying this spectacular phenomenon, in which two opposite effects, solutal and thermal Marangoni stresses, play competitive roles. We performed investigations of the Marangoni bursting phenomenon, paying attention to the surface temperature changes during bursting and after it. Fragmentation instabilities were monitored using a thermal camera for various initial alcohol/water compositions and at different stages of the process. We uncovered the role of thermocapillary Marangoni flows within the more viscous oil phase that are responsible for outward and inward shrinking of the periphery circle at the final stage of the phenomenon, enabling a more comprehensive understanding of the thermal Marangoni effect. Simulations of the Marangoni thermocapillary effect in an oil bath by solving coupled Navier&ndash;Stokes and heat transport equations using the COMSOL Multiphysics software platform support our experimental observations.

]]>Fluids doi: 10.3390/fluids8090254

Authors: Marco Hostettler Raphael Grüter Simon Stingelin Flavio De Lorenzi Rudolf M. Fuechslin Cyrill Jacomet Stephan Koll Dirk Wilhelm Gernot K. Boiger

Peristaltic pump technology is widely used wherever relatively low, highly accurately dosed volumetric flow rates are required and where fluid contamination must be excluded. Thus, typical fields of application include food, pharmaceuticals, medical technology, and analytics. In certain cases, when applied in conjunction with polymer-based tubing material, supplied peristaltic flow rates are reported to be significantly lower than the expected set flow rates. Said flow rate reductions are related to (i) the chosen tube material, (ii) tube material fatigue effects, and (iii) the applied pump frequency. This work presents a fast, dynamic, multiphysics, 1D peristaltic pump solver, which is demonstrated to capture all qualitatively relevant effects in terms of peristaltic flow rate reduction within linear peristaltic pumps. The numerical solver encompasses laminar fluid dynamics, geometric restrictions provided by peristaltic pump operation, as well as viscoelastic tube material properties and tube material fatigue effects. A variety of validation experiments were conducted within this work. The experiments point to the high degree of quantitative accuracy of the novel software and qualify it as the basis for elaborating an a priori drive correction.

]]>Fluids doi: 10.3390/fluids8090253

Authors: Sung-Jun Yoo Shori Yamauchi Hyungyu Park Kazuhide Ito

Highway buses are used in a wide range of commuting services and in the tourist industry. The demand for highway bus transportation has dramatically increased in the recent post-pandemic world, and airborne transmission risks may increase alongside the demand for highway buses, owing to a higher passenger density in bus cabins. We developed a numerical prediction method for the spatial distribution of airborne transmission risks inside bus cabins. For a computational fluid dynamics analyses, targeting two types of bus cabins, sophisticated geometries of bus cabins with realistic heating, ventilation, and air-conditioning were reproduced. The passengers in bus cabins were reproduced using computer-simulated persons. Airflow, heat, and moisture transfer analysis were conducted based on computational fluid dynamics, to predict the microclimate around passengers and the interaction between the cabin climate and passengers. Finally, droplet dispersion analysis using the Eulerian&ndash;Lagrangian method and an investigation of the spatial distribution of infection/spread risks, assuming SARS-CoV-2 infection, were performed. Through parametric analyses of passive and individual countermeasures to reduce airborne infection risks, the effectiveness of countermeasures for airborne infection was discussed. Partition installation as a passive countermeasure had an impact on the human microclimate, which decreased infection risks. The individual countermeasure, mask-wearing, almost completely prevented airborne infection.

]]>Fluids doi: 10.3390/fluids8090252

Authors: Rim Elfahem Bastien Bouchet Boussad Abbes Guillaume Polidori Fabien Beaumont

This study aims to investigate the impact of body heat loss on the thermal and aerodynamic conditions in a whole-body cryotherapy chamber. The underlying hypothesis is that the heat generated by the human body alters the thermal and aerodynamic environment inside the cabin. A numerical study was conducted to test this hypothesis and analyze the thermodynamic exchanges between the human body and the cabin during a 3 min whole-body cryotherapy session. The computational fluid dynamics (CFD) approach was used to study the unsteady heat transfer between the human body and the interior of the cryotherapy cabin. A thermal boundary condition, based on a mathematical model developed from experimental data, was applied to simulate skin cooling kinetics over time. The post-processing of the 3D results, including temperature, velocity fields, and thermal flux maps at the body surface, provided insight into the thermo-convective mechanisms involved in a whole-body cryotherapy session. The study found that body heat loss significantly affects the temperature fields inside the cabin, leading to global modifications of the aeraulic and thermal conditions. These findings suggest that cryotherapy protocols may need to be adjusted or the cabin set temperature optimized to enhance the therapeutic benefits.

]]>Fluids doi: 10.3390/fluids8090251

Authors: Alessandro Corvaglia Massimo Rundo Sara Bonati Manuel Rigosi

Partial electrification of hydraulic circuits to achieve energy savings requires an increase in the angular speed of the positive displacement pumps, with the risk of incomplete filling. In this context, the paper focuses on developing a computational fluid dynamics (CFD) model using SimericsMP+ for two external gear pumps, namely helical and spur type gears. The objective of this study is the analysis of the phenomena occurring on the suction side under conditions of incomplete filling at high speeds. Both CFD models have been validated by conducting experimental tests for measuring the flow rate delivered at various inlet pressures and angular speeds. The experimental results confirm the model&rsquo;s capability to accurately detect the operating conditions at which the delivered flow rate starts to decrease due to the partial filling of the inter-teeth chambers. Furthermore, this paper investigates the effects of certain geometrical modifications to the spur gear pump. Specifically, the influence of the gear&rsquo;s width-to-diameter ratio is studied, revealing that a lower ratio leads to slightly better filling. Conversely, increasing the inlet port diameter results in no improvement. Based on this study, the modelling approach appears to be accurate enough to serve as design tool for optimizing pumps to improve their filling capability.

]]>Fluids doi: 10.3390/fluids8090249

Authors: Christos Manopoulos Dimitrios Mathioulakis

A novel device of flow rate augmentation is proposed and experimentally examined in a horizontal valveless closed loop pump using a time-dependent stenosis (convergent&ndash;divergent channel) in contrast with the commonly used taper tubes of constant opening as flow rectifiers. The stenosis, being a part of the flexible tube of the pump, is formed by a semi-cylindrical surface attached to a compression spring of adjustable pretension compressing the tube against a flat plate. Located at either side of the pump pincher, the shape of the stenosis changes in time, without any external power source, as a function of the fluid pressure and the pretension of the spring. The spring pretension is adjusted by a trial-and-error procedure aiming for net flow rate maximization for each pinching frequency. For the examined pitching frequencies (5 Hz to 11 Hz, for which net flow rate is maximized) and for compression ratios 38% to 75%, the maximum net flow rate was found to be 720% of the non-stenosis case. Important parameters for flow enhancement were found to be the stenosis location along the loop, its opening, the compression ratio at the pincher area and the pinching frequency.

]]>Fluids doi: 10.3390/fluids8090250

Authors: Hassan Ali Ghazwani Khairuddin Sanaullah Afrasyab Khan

High-speed gas/vapour jets injected into a cross-moving sonic liquid signifies a vital phenomenon which bears useful applications in environmental and energy processes. In the present experimental study, a pulsating jet of supersonic steam was injected into cross-flowing water. Circulation zones of opposite vorticity owing to the interaction between the steam jet and cross-water flow were found. However, a large circulation appeared in front of the nozzle exit. Also, most small circulation regions were observed at higher water-flow rates (&gt;2 m3/s). Among the prime mixing variables (i.e., turbulence kinetic energy (TKE) and Reynolds shear stress (RSS)), the RSS estimations backed a small diffusive phenomenon within a region far from the nozzle exit. Further information extracted from the PIV images indicated the existence of Kelvin&ndash;Helmholtz (KH) instabilities. The counter-rotating vortex pairs (CVPs) appeared to be significant in the region close to the nozzle exit, and they exhibited leeward side folds. Moreover, the effects of the operating conditions on the pressure recovery and mixing efficiency as well as the penetration and the separation height were evaluated to determine the optimisation of the phenomenon. By applying extreme difference analysis, the mixing efficiency was found as the most influential parameter.

]]>Fluids doi: 10.3390/fluids8090248

Authors: Paolo Orlandi Sergio Pirozzoli

Traditionally, Fourier spectra have been employed to gain a deeper understanding of turbulence flow structures. The investigation of isotropic forced turbulence with passive scalars offers a straightforward means to examine the disparities between velocity and passive scalar spectra. This flow configuration has been extensively studied in the past, encompassing a range of Reynolds and Schmidt numbers. In this present study, direct numerical simulations (DNS) of this flow are conducted at sufficiently high Reynolds numbers, enabling the formation of a wide inertial range. The primary focus of this investigation is to quantitatively assess the variations in scalar spectra with the Schmidt number (Sc). The spectra exhibit a transition from a k&minus;5/3 scaling for low Sc to a k&minus;4/3 scaling for high Sc. The emergence of the latter power law becomes evident at Sc = 2, with its width expanding as Sc increases. To gain further insights into the underlying flow structures, a statistical analysis is performed by evaluating quantities aligned with the principal axes of the strain field. The study reveals that enstrophy is primarily influenced by the vorticity aligned with the intermediate principal strain axis, while the scalar gradient variance is predominantly controlled by the compressive strain. To provide a clearer understanding of the differences between enstrophy and scalar gradient variance, joint probability density functions (PDFs) and visualizations of the budget terms for both quantities are presented. These visualizations serve to elucidate the distinctions between the two and offer insights into their respective behaviors.

]]>Fluids doi: 10.3390/fluids8090247

Authors: Mateus D. Bacelar Hugo C. M. G. Ferreira Rajai S. Alassar André B. Lopes

We present a rare exact solution of the Navier&ndash;Stokes equations for the Hagen&ndash;Poiseuille flow through a quarter-elliptic tube. Utilizing the separation of variables method, we derive the solution and report expressions for both the volumetric flow rate and the friction factor&ndash;Reynolds number product.

]]>Fluids doi: 10.3390/fluids8090246

Authors: Gabriele Muzzioli Fabrizio Paltrinieri Luca Montorsi Massimo Milani

This paper proposes a CFD methodology for the simulation of the slipper&rsquo;s dynamics of a swash-plate axial piston unit under actual operating conditions. The study considers a typical slipper design, including a vented groove at the swash-plate interface. The dynamic fluid&ndash;body interaction (DFBI) model is exploited to find the instantaneous position of the slipper, while the morphing approach is adopted to cope with the corresponding mesh distortion. A modular approach is adopted to ensure high-quality mesh on the entire slipper surface and sliding interfaces provide the fluid dynamic connection between neighboring regions. The external forces acting on the slipper are included by means of user-defined lookup tables with the simulation estimating the lift force induced by fluid compression. Moreover, the force produced by the metal-to-metal contact between the slipper and the swash plate is modeled through a specific tool of the software. The pressure signal over an entire revolution of the pump is taken as an input of the simulation and a variable time step is used to manage the high-pressure gradients occurring in the regions of inner and outer dead points of the piston. The weakly compressible characteristic of the fluid is considered by a specific pressure-dependent density approach, and the two-equation eddy-viscosity k-&omega; SST (shear stress transport) model is used to assess the turbulent behavior of the flow. Furthermore, the transitional model predicts the onset of transition, thus solving different equations depending on whether the flow enters a laminar or turbulent regime. In conclusion, the proposed methodology investigates the motion of the slipper in response to several external forces acting on the component. The numerical results are discussed in terms of variable clearance height, pressure distribution within the gap, and lift forces acting on the slipper under specific pump operations.

]]>Fluids doi: 10.3390/fluids8090245

Authors: Mirza M. Shah

The author&rsquo;s published correlations for subcooled boiling in channels are further studied and developed in this work. The areas explored include choice of equivalent diameters for annuli and partially heated channels, effects of flow direction, micro-gravity, and orientation of heated surface. A new correlation is developed, which is a modification of the author&rsquo;s earlier correlation. The author&rsquo;s previous correlations and the new correlation are compared with a very wide range of test data for round tubes, rectangular channels, and annuli. Several other correlations are also compared with the same data. The authors&rsquo; correlations provide good agreement with data, the new correlation giving the least deviation. The data included hydraulic diameters from 0.176 to 22.8 mm, reduced pressure from 0.0046 to 0.922, subcooling from 0 to 165 K, mass flux from 59 to 31,500 kgm&minus;2s&minus;1, all flow directions, and terrestial to micro gravity. The new correlation has mean absolute deviation (MAD) of 13.3% with 2270 data points from 49 sources. Correlations by others had MAD of 18% to 116%. The results are presented and discussed.

]]>Fluids doi: 10.3390/fluids8090244

Authors: Francesco Orlandi Gabriele Muzzioli Massimo Milani Fabrizio Paltrinieri Luca Montorsi

The geometric complexity and high-pressure gradients that characterize the design of the flow field of gear pumps make it very difficult to obtain an accurate CFD simulation of the component. Usually, assumptions are made both in terms of geometrical features and physics being included in the analysis. The contact between the teeth, which is a key factor for the correct functioning of these pumps, represents a critical challenge in 3D CFD simulations, mainly due to the intrinsic limits of the dynamic meshing techniques that can hardly effectively manage a zero or close to zero gap point forming during gear rotation. The geometric complexity and high-pressure gradients that characterize the gear pump flow field make a CFD analysis quite difficult, and the contact between the gear teeth is usually avoided, thus being an extremely important feature. In this paper, a gear pump composed of inlet and outlet pipes was considered, and the contact between the gear&rsquo;s teeth was modeled in two different ways, one where it is effectively implemented and one where it is avoided using distancing and a proper casing modification. Herein, a new methodology is proposed for the application of the dynamic mesh method in the Simcenter STAR-CCM+ environment using an adaptive remeshing technique. The proposed methodology is compared with the alternative overset meshing method available in the software. The new meshing method is implemented using a user-routing that reproduces the real geometry of the gears while rotating during the pump operation, with teeth contact included. The routine is optimized in order to limit the additional computation and time needed for the remeshing process. The results that can be obtained using the two meshing approaches for the gear pump are compared in terms of computational effort and the accuracy of the results. The two methods showed opposite results in almost all the reported results, with the overset being more precise in the radial pressure evaluation and the dynamic being more reliable in the cavitation/aeration extension cloud.

]]>Fluids doi: 10.3390/fluids8090243

Authors: Taíse Toniazzo João Paulo Fabi

Pectin is a versatile polysaccharide produced mainly from natural food sources and agro-industrial wastes, adding value to these by-products. For food applications, it is necessary that pectin first interacts with water for technical purposes. As a food additive, pectin acts as a solution thickener and gelling agent for food formulation, even in concentrations of less than 1 (g/100 mL or g/100 g), and it is sufficient to influence food products&rsquo; stability, rheology, texture, and sensory properties. Therefore, this review paper attempts to discuss the versability of pectin use, focusing on food application. It starts by showing the chemical structure, the sources&rsquo; potential, thickening, and gelling mechanisms and concludes by showing the main applications to the food sector and its rheological properties.

]]>Fluids doi: 10.3390/fluids8090242

Authors: Saïf ed-Dîn Fertahi Tarik Belhadad Anass Kanna Abderrahim Samaouali Imad Kadiri Ernesto Benini

This critical review delves into the impact of Computational Fluid Dynamics (CFD) modeling techniques, specifically 2D, 2.5D, and 3D simulations, on the performance and vortex dynamics of Darrieus turbines. The central aim is to dissect the disparities apparent in numerical outcomes derived from these simulation methodologies when assessing the power coefficient (Cp) within a defined velocity ratio (&lambda;) range. The examination delves into the prevalent turbulence models shaping Cp values, and offers insightful visual aids to expound upon their influence. Furthermore, the review underscores the predominant rationale behind the adoption of 2D CFD modeling, attributed to its computationally efficient nature vis-&agrave;-vis the more intricate 2.5D or 3D approaches, particularly when gauging the turbine&rsquo;s performance within the designated &lambda; realm. Moreover, the study meticulously curates a compendium of findings from an expansive collection of over 250 published articles. These findings encapsulate the evolution of pivotal parameters, including Cp, moment coefficient (Cm), lift coefficient (Cl), and drag coefficient (Cd), as well as the intricate portrayal of velocity contours, pressure distributions, vorticity patterns, turbulent kinetic energy dynamics, streamlines, and Q-criterion analyses of vorticity. An additional focal point of the review revolves around the discernment of executing 2D parametric investigations to optimize Darrieus turbine efficacy. This practice persists despite the emergence of turbulent flow structures induced by geometric modifications. Notably, the limitations inherent to the 2D methodology are vividly exemplified through compelling CFD contour representations interspersed throughout the review. Vitally, the review underscores that gauging the accuracy and validation of CFD models based solely on the comparison of Cp values against experimental data falls short. Instead, the validation of CFD models rests on time-averaged Cp values, thereby underscoring the need to account for the intricate vortex patterns in the turbine&rsquo;s wake&mdash;a facet that diverges significantly between 2D and 3D simulations. In a bid to showcase the extant disparities in CFD modeling of Darrieus turbine behavior and facilitate the selection of the most judicious CFD modeling approach, the review diligently presents and appraises outcomes from diverse research endeavors published across esteemed scientific journals.

]]>Fluids doi: 10.3390/fluids8090241

Authors: Marcus S. Elliott Jonathan S. Cole Ross W. Blair Gary H. Menary

A three-dimensional, transient computational fluid dynamics analysis was conducted on an idealised geometry of a coronary artery fitted with representative geometries of an Absorb bioresorbable vascular scaffold (BVS) or a Xience drug-eluting stent (DES) in order to identify and compare areas of disturbed flow and potential risk sites. A non-Newtonian viscosity model was used with a transient velocity boundary condition programmed with user-defined functions. At-risk areas were quantified in terms of several parameters linked to restenosis: wall shear stress, time-averaged wall shear stress, oscillatory shear index, particle residence time, and shear rate. Results indicated that 71% of the BVS stented surface area had time-averaged wall shear stress values under 0.4 Pa compared to 45% of the DES area. Additionally, high particle residence times were present in 23% and 8% of the BVS and DES areas, respectively, with risk areas identified as being more prominent in close proximity to crowns and link struts. These results suggest an increased risk for thrombosis and neointimal hyperplasia for the BVS compared to the DES, which is in agreement with the outcomes of clinical trials. It is intended that the results of this study may be used as a pre-clinical tool to aid in the design of bioresorbable coronary stents.

]]>Fluids doi: 10.3390/fluids8090240

Authors: Jairo Eduardo Leiva Mateus Marco Antonio Reyes Huesca Federico Méndez Lavielle Enrique Geffroy Aguilar

The formation of flow-induced, oriented structures in two-phase systems, as in this study, is a phenomenon of considerable interest to the scientific and industrial sectors. The main difficulty in understanding the formation of bands of droplets is the simultaneous interplay of physicochemical, hydrodynamic, and mechanical effects. Additionally, banded structure materials frequently show multiple length scales covering several decades as a result of complex time-dependent stress fields. Here, to facilitate understanding a subset of these structures, we studied water in oil emulsions and focused on the effects of three variables specifically: the confinement factor&nbsp;(Co=2R/H), the viscosity ratio (p), and the applied shear rate (&gamma;&#729;). The confinement (Co) is the ratio between the drop&rsquo;s diameter (2R) and the separation of (the gap between) the circular rotating disks (H) containing the emulsion. We carried out (a) observations of the induced structure under different simple shear rates, as well as (b) statistical and morphological analysis of these bands. At low shear rates, the system self-assembles into bands along the direction of the flow and stacked normal to the velocity gradient direction. At higher shear rates is possible to observe bands normal to the vorticity direction. Here, we show that a detailed analysis of the dynamics of the band structures is amenable, as well as measurements of flow field anomalies simultaneously observed. The local emulsion viscosity varies in time, increasing in regions of higher droplet concentration and subsequently inducing velocity components perpendicular to the main flow direction. Thus, the emulsion morphology evolves and changes macroscopically. A relatively plausible explanation is attributed to the competitive effects of coalescence and the rupture of drops, where p values less than one predominate coalescence.

]]>Fluids doi: 10.3390/fluids8090239

Authors: Soo-Jin Jeong Sang-Jin Lee Seong-Joon Moon

Accurate evaluation of thermo-fluid dynamic characteristics in tanks is critically important for designing liquid hydrogen tanks for small-scale hydrogen liquefiers to minimize heat leakage into the liquid and ullage. Due to the high costs, most future liquid hydrogen storage tank designs will have to rely on predictive computational models for minimizing pressurization and heat leakage. Therefore, in this study, to improve the storage efficiency of a small-scale hydrogen liquefier, a three-dimensional CFD model that can predict the boil-off rate and the thermo-fluid characteristics due to heat penetration has been developed. The prediction performance and accuracy of the CFD model was validated based on comparisons between its results and previous experimental data, and a good agreement was obtained. To evaluate the insulation performance of polyurethane foam with three different insulation thicknesses, the pressure changes and thermo-fluid characteristics in a partially liquid hydrogen tank, subject to fixed ambient temperature and wind velocity, were investigated numerically. It was confirmed that the numerical simulation results well describe not only the temporal variations in the thermal gradient due to coupling between the buoyance and convection, but also the buoyancy-driven turbulent flow characteristics inside liquid hydrogen storage tanks with different insulation thicknesses. In the future, the numerical model developed in this study will be used for optimizing the insulation systems of storage tanks for small-scale hydrogen liquefiers, which is a cost-effective and highly efficient approach.

]]>Fluids doi: 10.3390/fluids8090238

Authors: Michael Gerard Connolly Malachy J. O’Rourke Alojz Ivankovic

This comprehensive study focused on the standard taxi sign used in Ireland and its impact on drag production, fuel expenses, and CO2 emissions. Experimental analysis revealed that the conventional taxi sign significantly increased drag, especially when mounted on streamlined vehicles such as saloon cars, due to flow separation issues on the rear roof and rear windshield. Longitudinal reorientation of the sign offered a 14-fold reduction in drag increase compared to the traditional placement. It was found that positioning the sign in the middle of the roof offered the greatest fuel efficiency. Furthermore, the study estimated that implementing longitudinal repositioning on all Irish taxi signs could save drivers approximately EUR 832 per year and reduce national CO2 emissions by a substantial 22,464 tonnes annually. Comparative analyses with international taxi signs demonstrated that the Irish sign had significantly larger drag contributions, emphasizing the need for improved aerodynamics. To address the inherent drag issue, the study explored novel appendable devices and proposed alternative taxi sign designs. Among the tested solutions, a magnet-mounted front ramp proved the most effective, reducing total drag by nearly 30%. Additionally, a motorized flip-up taxi sign design demonstrated a remarkable 40% reduction in drag. Finally, a newly proposed taxi sign design, featuring longitudinal positioning and pointed triangular front and rear faces, resulted in a minimal 4.3% increase in vehicle drag compared to the baseline car.

]]>Fluids doi: 10.3390/fluids8080237

Authors: Rakesh Basavegowda Krishnappa S. Gowreesh Subramanya Abhijit Deshpande Bharatesh Chakravarthi

This paper presents a study on flow hydrodynamics for single-channel serpentine flow field (SCSFF) and cross-split serpentine flow field configurations (CSSFF) for different geometric dimensions of channel and rib width ratios with electrode intrusion over varying compression ratios (CRs) in an all-iron redox flow battery. Pressure drops (&Delta;p) measured experimentally across a cell active area of 131 cm2 for different electrolyte flow rates were numerically validated. A computational fluid dynamics study was conducted for detailed flow analyses, velocity magnitude contours, flow distribution, and uniformity index for the intrusion effect of a graphite felt electrode bearing a thickness of 6 mm with a channel compressed to varying percentages of 50%, 60%, and 70%. Experimental pressure drops (&Delta;p) over the numerical value resulted in the maximum error approximated to 4%, showing good agreement. It was also reported that the modified version of the cross-split serpentine flow field, model D, had the lowest pressure drop, &Delta;p, of 2223.4 pa, with a maximum uniformity index at the electrode midplane of 0.827 for CR 50%, across the active cell area. The pressure drop (&Delta;p) was predominantly higher for increased compression ratios, wherein intrusion phenomena led to changes in electrochemical activity; it was found that the velocity distribution was quite uniform for a volumetric uniformity index greater than 80% in the felt.

]]>Fluids doi: 10.3390/fluids8080236

Authors: Benet Eiximeno Carlos Tur-Mongé Oriol Lehmkuhl Ivette Rodríguez

The noise radiated by the flow around a cylinder in the subcritical regime at ReD=1&times;104 and at a subsonic Mach number of M=0.5 is here studied. The aerodynamic sound radiated by a cylinder has been studied with a wide range of Reynolds numbers, but there are no studies about how the Mach number affects the acoustic field in the subsonic regime. The flow field is resolved by means of large-eddy simulations of the compressible Navier&ndash;Stokes equations. For the study of the noise propagation, formulation 1C of the Ffowcs Williams&ndash;Hawkings analogy is used. The fluid flow results show good agreement when comparing the surface pressure coefficient, the recirculation length, the vortex shedding frequency and the force coefficients against other studies performed under similar conditions. The dynamic mode decomposition of the pressure fluctuations is used to relate them with the far-field noise. It is shown that, in contrast to what happens for low Mach numbers, quadrupoles have a significant impact mainly in the observers located in the streamwise direction. This effect leads to a global monopole directivity pattern as the shear fluctuations compensate for the lower value of the aeolian tone away from the cross-stream direction.

]]>Fluids doi: 10.3390/fluids8080235

Authors: Hamidreza Abbaszadeh Reza Norouzi Veli Sume Alban Kuriqi Rasoul Daneshfaraz John Abraham

This study investigates the effects of gate openings and different sill widths on the sluice gate&rsquo;s energy dissipation and discharge coefficient (Cd). The physical model of the sills includes rectangular sills of different dimensions. The results show that the gate opening size is inversely related to the Cd for a gate without a sill. In addition, increasing the gate opening size for a given discharge decreases the relative energy dissipation, and increasing the Froude number increases the relative energy dissipation. The results also show that the Cd and relative energy dissipation decrease when the width of the sill is decreased, thus increasing the total area of the flux flowing through the sluice gate and vice versa. According to the experimental results, the relative energy dissipation and the Cd of the sluice gate are larger for all sill widths than without the sill. Finally, non-linear polynomial relationships are presented based on dimensionless parameters for predicting the relative energy dissipation and outflow coefficient.

]]>Fluids doi: 10.3390/fluids8080234

Authors: Oisín McCay Rajesh Nimmagadda Syed Mughees Ali Tim Persoons

Effective natural-convection-cooled heat sinks are vital to the future of electronics cooling due to their low energy demand in the absence of an external pumping agency in comparison to other cooling methods. The present numerical study was carried out with ANSYS Fluent and aimed at identifying a more-effective fin design for enhancing heat transfer in natural convection applications for a fixed base-plate size of 100 mm &times; 100 mm under an applied heat flux of 4000 W/m2. The Rayleigh number used in the present study lied within the range of 2.6 &times; 106 to 4.5 &times; 106. Initially, a baseline case with rectangular fins was considered in the present study, and it was optimized with respect to fin spacing. This optimized baseline case was then validated against the semi-empirical correlation from the scientific literature. Upon good agreement, the validated model was used for comparative analysis of different heat sink configurations with rectangular, trapezoidal, curved, and angled fins by constraining the surface area of the heat transfer. The optimized fin spacing obtained for the baseline case was also used for the other heat sink configurations, and then, the fin designs were further optimized for better performance. However, for the angled fin case, the optimized configuration found in the scientific literature was adopted in the present study. The proposed novel curved fin design with a shroud showed a 4.1% decrease in the system&rsquo;s thermal resistance with an increase in the heat transfer coefficient of 4.4% when compared to the optimized baseline fin case. The obtained results were further non-dimensionalized with the proposed scaling in terms of the baseline case for the two novel heat sink cases (trapezoidal, curved).

]]>Fluids doi: 10.3390/fluids8080232

Authors: Wassim Abdel Nour Joseph Jabbour Damien Serret Philippe Meliga Elie Hachem

This paper assesses the feasibility of performing topology optimization of laminar incompressible flows governed by the steady-state Navier&ndash;Stokes equations using anisotropic mesh adaptation to achieve a high-fidelity description of all fluid&ndash;solid interfaces. The present implementation combines an immersed volume method solving stabilized finite element formulations cast in the variational multiscale (VMS) framework and level-set representations of the fluid&ndash;solid interfaces, which are used as an a posteriori anisotropic error estimator to minimize interpolation errors under the constraint of a prescribed number of nodes in the mesh. Numerical results obtained for several two-dimensional problems of power dissipation minimization show that the optimal designs are mesh-independent (although the convergence rate does decreases as the number of nodes increases), agree well with reference results from the literature, and provide superior accuracy over prior studies solved on isotropic meshes (fixed or adaptively refined).

]]>Fluids doi: 10.3390/fluids8080233

Authors: Charles Brissot Léa Cailly-Brandstäter Elie Hachem Rudy Valette

The integration of phase change phenomena through an interface is a numerical challenge that requires proper attention. Solutions to properly ensure mass and energy conservation were developed for finite difference and finite volume methods, but not for Finite Element methods. We propose a Finite Element phase change model based on an Eulerian framework with a Continuous Surface Force (CSF) approach. It handles both momentum and energy conservation at the interface for anisotropic meshes in a light an efficient way. To do so, a model based on the Level Set method is developed. A thick interface is considered to fit with the CSF approach. To properly compute the energy conservation, heat fluxes are extended through this interface thanks to the resolution of a transport equation. A dedicated pseudo compressible Navier&ndash;Stokes solver is added to compute velocity jumps with a source term at the interface in the velocity divergence equation. Several 1D and 2D benchmarks are considered with increasing complexity to highlight the performances of each feature of the framework. This stresses the capacity of the model to properly tackle phase change problems.

]]>Fluids doi: 10.3390/fluids8080231

Authors: Miguel Alberto Manna Anouchah Latifi

In this study, the evolution of surface water solitary waves under the action of Jeffreys&rsquo; wind&ndash;wave amplification mechanism in shallow water is analytically investigated. The analytic approach is essential for numerical investigations due to the scale of energy dissipation near coasts. Although many works have been conducted based on the Jeffreys&rsquo; approach, only some studies have been carried out on finite depth. We show that nonlinearity, dispersion, and anti-dissipation are the dominating phenomena, obeying an anti-diffusive and fully nonlinear Serre&ndash;Green&ndash;Naghdi (SGN) equation. Applying an appropriate perturbation method, the current research yields a Korteweg&ndash;de Vries&ndash;Burger-type equation (KdV-B), combining weak nonlinearity, dispersion, and anti-dissipation. This derivation is novel. We show that the continuous transfer of energy from wind to water results in the growth over time of the KdV-B soliton&rsquo;s amplitude, velocity, acceleration, and energy, while its effective wavelength decreases. This phenomenon differs from the classical results of Jeffreys&rsquo; approach and is due to finite depth. In this study, it is shown that expansion and breaking occur in finite time. These times are calculated and expressed with respect to soliton- and wind-appropriate parameters and values. The obtained values are measurable in experimental facilities. A detailed analysis of the breaking time is conducted with regard to various criteria. By comparing these times to the experimental results, the validity of these criteria are examined.

]]>Fluids doi: 10.3390/fluids8080230

Authors: Ilya Starodumov Ksenia Makhaeva Andrey Zubarev Ivan Bessonov Sergey Sokolov Pavel Mikushin Dmitri Alexandrov Vasiliy Chestukhin Felix Blyakhman

This mainly theoretical work is devoted to the study of the contribution of the cell-free layer (CFL) near the vessel wall to hemodynamics in a large coronary artery with stenosis to assess the relevance of CFL modeling to the needs of interventional cardiology. An Euler&ndash;Euler model considering blood as a two-component fluid with a discrete phase of erythrocytes and a liquid plasma phase was applied to a simple 2d vessel with 65% stenosis. It was found that both the CFL thickness and the local contribution of the CFL thickness to hemodynamics are inhomogeneous along the vessel. The effects of CFL on the velocity profiles, vortex formation, hematocrit, viscosity, and wall shear stresses in the area of stenosis were determined. To demonstrate the significance of CFL modeling for prognostic purposes, the same hemodynamic conditions, analyzed using a one-component model, were also considered. A comparison analysis showed that the existence of CFL resulted in a significant overestimation (up to over 100%) of the main hemodynamic characteristics of the flow obtained using the model based on the Carreau equation.

]]>Fluids doi: 10.3390/fluids8080229

Authors: Yi Liang Cheng Wang Pengtao Sun

In this paper, an interface-fitted fictitious domain finite element method is developed for the simulation of fluid&ndash;rigid particle interaction problems in cases of rotated particles with small displacement, where an interface-fitted mesh is employed for the discrete scheme to capture the fluid&ndash;rigid particle interface accurately, thereby improving the solution accuracy near the interface. Moreover, a linearization and decoupling process is presented to release the constraint between velocities of fluid and rigid particles in the finite element space, and to make the developed numerical method easy to be implemented. Our numerical experiments are carried out using two different moving interface-fitted meshes; one is obtained by a rotational arbitrary Lagrangian&ndash;Eulerian (ALE) mapping, and the other one through a local smoothing process among interface-cut elements. A unified velocity is defined in the entire domain based on the fictitious domain method, making it easier to develop an interface-fitted mesh generation algorithm in a fixed domain. Both show that the proposed method has a good performance in accuracy for simulating a neutrally buoyant particle in plane shear flow. This approach can be easily extended to fluid&ndash;structure interaction problems involving fluids in different states and structures in different shapes with large displacements or deformations.

]]>Fluids doi: 10.3390/fluids8080228

Authors: Alexandra Grekova Svetlana Strelova Anton Lysikov Mikhail Tokarev

Adsorption energy storage is a promising resource-saving technology that allows the rational use of alternative heat sources. One of the most important parts of the adsorption heat accumulator is the adsorber heat exchanger. The parameters of heat transfer in this unit determine how fast heat from an alternative energy source, such as the Sun, will be stored. For the design of adsorption heat accumulators, plate fin heat exchangers are mainly used. In this paper, the procedure for the estimation of the global heat transfer coefficient for the adsorber heat exchanger depending on its geometry is considered. The heat transfer coefficient for a LiCl/SiO2 sorbent flat layer under conditions of heat storage stage was measured. Based on these data, the global heat transfer coefficients for a number of industrial heat exchangers were theoretically estimated and experimentally measured for the adsorption cycle of daily heat storage. It was shown that theoretically obtained values are in good agreement with the values of the global heat transfer coefficients measured experimentally. Thus, the considered technique makes it possible to determine the most promising geometry of the plate fin heat exchanger for a given adsorption heat storage cycle without complicated experiments.

]]>Fluids doi: 10.3390/fluids8080227

Authors: Karthick Rajkumar Eike Tangermann Markus Klein

This study aims to facilitate a physical understanding of resonating cavity flows with efficient numerical treatments of turbulence. It reinforces the efficiency and affordability of scale-adaptive numerical techniques for simulating open cavity flows with a separated shear layer consisting of a wide range of flow scales. Visualization of the resonant modes occurring due to the acoustic feedback loop aids in a better understanding of large-scale flow oscillations. Under this scope, scale-adaptive simulation (SAS) based on the k-&omega; SST RANS model with different turbulence treatments has been studied for an open cavity configuration with a length-to-depth (L/D) ratio of 5.7 featuring Mach number (Ma) 0.8 and Reynolds number (Re) 12&times;106. It is shown that the essential cavity flow physics has been captured using the SAS approach with more than 90% improved computational efficiency compared to commonly used hybrid RANS-LES approaches. In addition, wall-modeled SAS when supplemented with an artificial forcing concept to trigger the model provides very good spectral estimates comparable with hybrid RANS-LES results. Following the validation of numerical approaches, the directional dependence of the cavity resonance is investigated under asymmetric flow conditions, and spanwise interference of waves due to the lateral walls of the cavity has been observed.

]]>Fluids doi: 10.3390/fluids8080226

Authors: Anna Samnioti Eirini Maria Kanakaki Sofianos Panagiotis Fotias Vassilis Gaganis

Sour gas in hydrocarbon reservoirs contains significant amounts of H2S and smaller amounts of CO2. To minimize operational costs, meet air emission standards and increase oil recovery, operators revert to acid gas (re-)injection into the reservoir rather than treating H2S in Claus units. This process requires the pressurization of the acid gas, which, when combined with low-temperature conditions prevailing in subsurface pipelines, often leads to the formation of hydrates that can potentially block the fluid flow. Therefore, hydrates formation must be checked at each pipeline segment and for each timestep during a flow simulation, for any varying composition, pressure and temperature, leading to millions of calculations that become more intense when transience is considered. Such calculations are time-consuming as they incorporate the van der Walls&ndash;Platteeuw and Langmuir adsorption theory, combined with complex EoS models to account for the polarity of the fluid phases (water, inhibitors). The formation pressure is obtained by solving an iterative multiphase equilibrium problem, which takes a considerable amount of CPU time only to provide a binary answer (hydrates/no hydrates). To accelerate such calculations, a set of classifiers is developed to answer whether the prevailing conditions lie to the left (hydrates) or the right-hand (no hydrates) side of the P-T phase envelope. Results are provided in a fast, direct, non-iterative way, for any possible conditions. A set of hydrate formation &ldquo;yes/no&rdquo; points, generated offline using conventional approaches, are utilized for the classifier&rsquo;s training. The model is applicable to any acid gas flow problem and for any prevailing conditions to eliminate the CPU time of multiphase equilibrium calculations.

]]>Fluids doi: 10.3390/fluids8080225

Authors: Callen Schwefler Peyton Nienaber Hans C. Mayer

An inverted bottle empties in a time Te,0 through a process called &ldquo;glugging&rdquo;, whereby gas and liquid compete at the neck (of diameter DN). In contrast, an open-top container empties in a much shorter time Te through &ldquo;jetting&rdquo; due to the lack of gas&ndash;liquid competition. Experiments and theory demonstrate that, by introducing a perforation (diameter dp), a bottle empties through glugging, jetting, or a combination of the two. For a certain range of dp/DN, the perforation increases the emptying time, and a particular value of dp/DN is associated with a maximum emptying time Te,max. We show that the transition from jetting to glugging is initiated by the jet velocity reaching a low threshold, thereby allowing a slug of air entry into the neck that stops jetting and starts the glugging. Once initiated, the glugging proceeds as though there is no perforation. Experimental results covered a range of E&ouml;tv&ouml;s numbers from Eo&sim; 20&ndash;200 (equivalent to a range of DN/Lc&sim; 4&ndash;15, where Lc is the capillary length). The phenomenon of bottle emptying with a perforation adds to the body of bottle literature, which has already considered the influence of shape, inclination, liquid properties, etc.

]]>Fluids doi: 10.3390/fluids8080224

Authors: Jahrul M Alam

This article investigates the applications of wavelet transforms and machine learning methods in studying turbulent flows. The wavelet-based hierarchical eddy-capturing framework is built upon first principle physical models. Specifically, the coherent vortex simulation method is based on the Taylor hypothesis, which suggests that the energy cascade occurs through vortex stretching. In contrast, the adaptive wavelet collocation method relies on the Richardson hypothesis, where the self-amplification of the strain field and a hierarchical breakdown of large eddies drive the energy cascade. Wavelet transforms are computational learning architectures that propagate the input data across a sequence of linear operators to learn the underlying nonlinearity and coherent structure. Machine learning offers a wealth of data-driven algorithms that can heavily use statistical concepts to extract valuable insights into turbulent flows. Supervised machine learning needs &ldquo;perfect&rdquo; turbulent flow data to train data-driven turbulence models. The current advancement of artificial intelligence in turbulence modeling primarily focuses on accelerating turbulent flow simulations by learning the underlying coherence over a low-dimensional manifold. Physics-informed neural networks offer a fertile ground for augmenting first principle physics to automate specific learning tasks, e.g., via wavelet transforms. Besides machine learning, there is room for developing a common computational framework to provide a rich cross-fertilization between learning the data coherence and the first principles of multiscale physics.

]]>Fluids doi: 10.3390/fluids8080223

Authors: Marcelo V. Flamarion Roberto Ribeiro-Jr Diogo L. S. S. Vianna Alex M. Sato

This paper concerns the interaction between solitary waves on the surface of an ideal fluid and a localized external force, which models a moving disturbance on the free surface or an obstacle moving at the bottom of a channel. Previous works have investigated this interaction under the assumption that the external force moves with variable speed and constant acceleration. However, in this paper we adopt a different approach and consider the scenario in which the external force moves with variable speed and non-constant acceleration. Using the Whitham equation framework, we investigate numerically trapped waves excited by a periodic external force. Our experiments reveal regimes in which solitary waves are spontaneously generated and trapped for large times at the external force. In addition, we compare the results predicted by the Whitham equation with those of the Korteweg&ndash;de Vries equation.

]]>Fluids doi: 10.3390/fluids8080222

Authors: Dimitrios G. Koubogiannis Marios Vasileios N. Benetatos

A micro-energy harvesting device proposed in the literature was numerically studied. It consists of two bluff bodies in a micro-channel and a flexible diaphragm at its upper wall. Vortex shedding behind bodies induces pressure fluctuation causing vibration of the diaphragm that converts mechanical energy to electrical by means of a piezoelectric membrane. Research on enhancing vortex shedding was justified due to the low power output of the device. The amplitude and frequency of the unsteady pressure fluctuation on the diaphragm were numerically predicted. The vortex shedding severity was mainly assessed in terms of pressure amplitude. The CFD model set-up was described in detail, and appropriate metrics to assess the energy harvesting potential were defined. Several 2D cases were simulated to study the effect of the inlet Reynolds number and channel blockage ratio on the prospective performance of the device. Furthermore, the critical blockage ratio leading to the vortex shedding suppression was sought. A higher inlet velocity for a constant blockage ratio was found to enhance vortex shedding and the pressure drop. Great blockage ratio values but lower than the critical ones seemed to provide great pressure amplitudes at the expense of a moderate pressure drop. There is evidence that the field is fruitful for further research and relevant directions were provided.

]]>Fluids doi: 10.3390/fluids8080221

Authors: René Rodríguez-Rivera Ignacio Carvajal-Mariscal Hilario Terres-Peña Mauricio De la Cruz-Ávila Jorge E. De León-Ruiz

This study presents a comprehensive assessment of the hydrodynamic performance of a novel pipe network with tessellated geometry and allometric scales. Numerical simulations were used to evaluate flow behaviour and pressure drop. The comparison geometry featured a Parallel Pipe Pattern (PPP), while the proposed design employed a Rhombic Tessellation Pattern (RTP). Steady-state simulations were conducted under identical boundary conditions, examining water mass flows ranging from 0.01 to 0.06 kg/s. The results revealed RTP significant advantages over the PPP. The RTP, integrated with a fractal tree pattern, demonstrated remarkable capabilities in achieving uniform flow distribution and maintaining laminar flow regimes across the mass flow rates. Additionally, exhibited an average reduction in pressure drop of 92% resulting in improved efficiency. The Reynolds number at PPP inlet was 5.4 times higher than in the RTP, explaining the considerably higher pressure drop. At a mass flow rate of 0.06 kg/s, the PPP experienced a pressure drop of up to 3.43 kPa, while the RTP&rsquo;s pressure drop was only 0.350 kPa, highlighting a remarkable decrease of 91.5%. These findings underscore the RTP superior performance in minimizing pressure drop, making it suitable for accommodating higher mass flow rates, thus highlighting its exceptional engineering potential.

]]>Fluids doi: 10.3390/fluids8080220

Authors: Jianyong Yin Yongxue Zhang Dehong Gong Lei Tian Xianrong Du

A bubble&rsquo;s motion is strongly influenced by the boundaries of tip structures, which correspond to the bubble&rsquo;s size. In the present study, the dynamic behaviors of a cavitation bubble near a conical tip structure are investigated experimentally and numerically. A series of experiments were carried out to analyze the bubble&rsquo;s shape at different relative cone distances quantitatively. Due to the crucial influence of the phase change on the cavitation bubble&rsquo;s dynamics over multiple cycles, a compressible two-phase model taking into account the phase change and heat transfer implemented in OpenFOAM was employed in this study. The simulation results regarding the bubble&rsquo;s radius and shape were validated with corresponding experimental photos, and a good agreement was achieved. The bubble&rsquo;s primary physical features (e.g., shock waves, liquid jets, high-pressure zones) were well reproduced, which helps us understand the underlying mechanisms. Meanwhile, the latent damage was quantified by the pressure load at the cone apex. The effects of the relative distance &gamma; and cone angle &theta; on the maximum temperature, pressure peaks, and bubble position are discussed and summarized. The results show that the pressure peaks during the bubble&rsquo;s collapse increase with the decrease in &gamma;. For a larger &gamma;, the first minimum bubble radius increases while the maximum temperature decreases as &theta; increases; the pressure peak at the second final collapse is first less than that at the first final collapse and then much greater than that one. For a smaller &gamma;, the pressure peaks at different &theta; values do not vary very much.

]]>Fluids doi: 10.3390/fluids8080219

Authors: Josef Hasslberger

Following the relative success of mixed models in the Large Eddy Simulation of complex turbulent flow configurations, an alternative formulation is suggested here which incorporates the concept of a local subgrid activity sensor. The general idea of mixed models is to combine the advantages of structural models (superior alignment properties), usually of the scale similarity type, and functional models (superior stability), usually of the eddy viscosity type, while avoiding their disadvantages. However, the key question is the mathematical realization of this combination, and the formulation in this work accounts for the local level of underresolution of the flow. The justification and evaluation of the newly proposed mixed model is based on a priori and a posteriori analysis of homogeneous isotropic turbulence and laminar&ndash;turbulent transition in the Taylor&ndash;Green vortex, respectively. The suggested model shows a robust and accurate behavior for the cases investigated. In particular, it outperforms the separate structural and functional base models as well as the simulation without an explicit subgrid-scale model.

]]>Fluids doi: 10.3390/fluids8080218

Authors: Arash Ghahraman Gyula Bene

This study presents a comprehensive analysis of the second-order perturbation theory applied to the Navier&ndash;Stokes equations governing free surface flows. We focus on gravity&ndash;capillary surface waves in incompressible viscous fluids of finite depth over a flat bottom. The amplitude of these waves is regarded as the perturbation parameter. A systematic derivation of a nonlinear-surface-wave equation is presented that fully takes into account dispersion, while nonlinearity is included in the leading order. However, the presence of infinitely many over-damped modes has been neglected and only the two least-damped modes are considered. The new surface-wave equation is formulated in wave-number space rather than real space and nonlinear terms contain convolutions making the equation an integro-differential equation. Some preliminary numerical results are compared with computational-modelling data obtained via open source CFD software OpenFOAM.

]]>Fluids doi: 10.3390/fluids8080217

Authors: Alfredo M. Abuchar-Curi Oscar E. Coronado-Hernández Jairo Useche Verónica J. Abuchar-Soto Argemiro Palencia-Díaz Duban A. Paternina-Verona Helena M. Ramos

The outlet angle and shape of impeller blades are important parameters in centrifugal pump design. There is a lack of detailed studies related to double curvature impellers in centrifugal pumps in the current literature; therefore, an experimental and numerical analysis of double curvature impellers was performed. Six impellers were made and then assessed in a centrifugal pump test bed and simulated via 3D CFD simulation. The original impeller was also tested and simulated. One of the manufactured impellers had the same design as the original, and the other five impellers had a double curvature. Laboratory tests and simulations were conducted with three rotation speeds: 1400, 1700, and 1900 RPM. Head and performance curve equations were obtained for the pump&ndash;engine unit based on the flow of each impeller for the three rotation speeds. The results showed that a double curvature impeller improved pump head by approximately 1 m for the range of the study and performance by about 2% when compared to basic impeller. On the other hand, it was observed that turbulence models such as k-&epsilon; and SST k-&omega; reproduced similar physical and numerical results.

]]>Fluids doi: 10.3390/fluids8080216

Authors: Max Quissek Uladzimir Budziankou Sebastian Pollak Thomas Lauer

Computational fluid dynamics (CFD) are an essential tool for the development of diesel engine aftertreatment systems using selective catalytic reduction (SCR) to reduce nitrous oxides (NOx). In urea-based SCR, liquid urea&ndash;water solution (UWS) is injected into the hot exhaust gas, where it transforms into gaseous ammonia. This ammonia serves as a reducing agent for NOx. CFD simulations are used to predict the ammonia distribution in the exhaust gas at the catalyst inlet. The goal is to achieve the highest possible uniformity to realize homogeneous NOx reduction across the catalyst cross section. The current work focuses on the interaction of UWS droplets with the hot walls of the exhaust system. This is a crucial part of the preparation of gaseous ammonia from the injected liquid UWS. Following experimental investigations, a new impingement model is described based on the superposition of four basic impingement behaviors, each featuring individual secondary droplet characteristics. The droplet&ndash;wall heat transfer, depending on surface temperature and impingement behavior, is also calculated using a newly parameterized model. Applying the presented approach, the cooling of a steel plate from intermittent spray impingement is simulated and compared to measurements. The second validation case is the distribution of ammonia at the catalyst inlet of an automotive SCR system. Both applications show good agreement and demonstrate the quality of the new model.

]]>Fluids doi: 10.3390/fluids8070215

Authors: Dustin Steven Weaver Sanja Mišković

This paper presents the results of simulations of particle-laden air&ndash;solid jet flow in long straight tubes using CFD-DEM, along with an analysis of coarse-graining. Although previous studies have used CFD-DEM for similar flows, these have typically been in a dilute flow regime where uncoupled simulations can be used effectively. However, fully coupled simulations can introduce issues, necessitating validation studies to ensure that all coupling parameters are effectively used and that the physics is accurately represented. This paper validated the simulations against two different experimental studies, with fluid Reynolds numbers between 10,000 and 40,000 and Stokes numbers between 5.6 and 50. Interestingly, the profiles of the mean particle velocity exhibited fewer discrepancies as the Stokes number increased, but more discrepancies for the root-mean-squared velocity compared to the experiments. The particle number flux was consistent with the experiments after the nozzle exit. Coarse-graining was also applied to the same simulations, achieving relatively accurate results. However, as expected, the scaling of contact collision frequencies, forces, and stresses could not be achieved, meaning that coarse-graining may be useful for comparing designs or operating parameters on an industrial scale, but falls short when measuring the total energy dissipation of one experiment.

]]>Fluids doi: 10.3390/fluids8070214

Authors: Dimitrios Kolokotronis Srikrishna Sahu Yannis Hardalupas Alex M. K. P. Taylor Akira Arioka

It is well established that spray characteristics from automotive injectors depend on, among other factors, whether cavitation arises in the injector nozzle. Bulk cavitation, which refers to the cavitation development distant from walls and thus far from the streamline curvature associated with salient points on a wall, has not been thoroughly investigated experimentally in injector nozzles. Consequently, it is not clear what is causing this phenomenon. The research objective of this study was to visualize cavitation in three different injector models (designated as Type A, Type B, and Type C) and quantify the liquid flow field in relation to the bulk cavitation phenomenon. In all models, bulk cavitation was present. We expected this bulk cavitation to be associated with a swirling flow with its axis parallel to that of the nozzle. However, liquid velocity measurements obtained through particle image velocimetry (PIV) demonstrated the absence of a swirling flow structure in the mean flow field just upstream of the nozzle exit, at a plane normal to the hypothetical axis of the injector. Consequently, we applied proper orthogonal decomposition (POD) to analyze the instantaneous liquid velocity data records in order to capture the dominant coherent structures potentially related to cavitation. It was found that the most energetic mode of the liquid flow field corresponded to the expected instantaneous swirling flow structure when bulk cavitation was present in the flow.

]]>Fluids doi: 10.3390/fluids8070213

Authors: Nicolas Hafen Jan E. Marquardt Achim Dittler Mathias J. Krause

Wall-flow filters are applied in the exhaust treatment of internal combustion engines for the removal of particulate matter (PM). Over time, the pressure drop inside those filters increases due to the continuously introduced solid material, which forms PM deposition layers on the filter substrate. This leads to the necessity of regenerating the filter. During such a regeneration process, fragments of the PM layers can potentially rearrange inside single filter channels. This may lead to the formation of specific deposition patterns, which affect a filter&rsquo;s pressure drop, its loading capacity and the separation efficiency. The dynamic formation process can still not consistently be attributed to specific influence factors, and appropriate calculation models that enable a quantification of respective factors do not exist. In the present work, the dynamic rearrangement process during the regeneration of a wall-flow filter channel is investigated. As a direct sequel to the investigation of a static deposition layer in a previous work, the present one additionally investigates the dynamic behaviour following the detachment of individual layer fragments as well as the formation of channel plugs. The goal of this work is the extension of the resolved particle methodology used in the previous work via a discrete method to treat particle&ndash;particle and particle&ndash;wall interactions in order to evaluate the influence of the deposition layer topology, PM properties and operating conditions on dynamic rearrangement events. It can be shown that a simple mean density methodology represents a reproducible way of determining a channel plug&rsquo;s extent and its average density, which agrees well with values reported in literature. The sensitivities of relevant influence factors are revealed and their impact on the rearrangement process is quantified. This work contributes to the formulation of predictions on the formation of specific deposition patterns, which impact engine performance, fuel consumption and service life of wall-flow filters.

]]>Fluids doi: 10.3390/fluids8070212

Authors: Dimitris Drikakis Filippos Sofos

The significant growth of artificial intelligence (AI) methods in machine learning (ML) and deep learning (DL) has opened opportunities for fluid dynamics and its applications in science, engineering and medicine. Developing AI methods for fluid dynamics encompass different challenges than applications with massive data, such as the Internet of Things. For many scientific, engineering and biomedical problems, the data are not massive, which poses limitations and algorithmic challenges. This paper reviews ML and DL research for fluid dynamics, presents algorithmic challenges and discusses potential future directions.

]]>Fluids doi: 10.3390/fluids8070211

Authors: Bernard Xavier Tchomeni Kouejou Alfayo Anyika Alugongo

Accurate prediction of the dynamic behavior of coupled shafts in a fluid medium is crucial to accurately estimate equipment life and enable safe operation. However, this task is far from trivial due to the vibrations induced by the highly nonlinear nature of the machine system. This paper presents an experimental analysis of a cardan shaft under the influence of viscous hydrodynamic forces. An experimental setup was created using a cardan shaft rig installed in a plexiglas tank, with a self-aligned crack simulator supporting the driveshaft for crack extraction. Adequate instrumentation was used to measure the rotor&rsquo;s fluctuation under industrial viscous fluid at various motor speeds. By analyzing the changes of unwanted high vibration, the obtained results demonstrated that the characteristics of the cracks in the fluid medium can be efficiently extracted from multiple tests using the wavelet synchrosqueezing transform and energy spectrum. This latter aspect, in particular, implies that the responses that can be observed in practice are highly sensitive to the values of the system parameters: average flow velocity, mass eccentricity, and shaft stiffness, among others. Finally, the study provides conclusions on practical applications for the reliable identification of cracks in a viscous fluid to validate the recently published theoretical study.

]]>Fluids doi: 10.3390/fluids8070210

Authors: Aldo Saul Laguna-Canales Guillermo Urriolagoitia-Sosa Beatriz Romero-Ángeles Miguel Martinez-Mondragon Miguel Angel García-Laguna Martin Ivan Correa-Corona Daniel Maya-Anaya Guillermo Manuel Urriolagoitia-Calderón

In motorsports, the correct design of every device that constitutes a vehicle is a significant task for engineers because the car&rsquo;s efficiency on the track depends on making it competitive. However, the physical integrity of the pilot is also at stake, since a bad vehicle design can cause serious mishaps. To achieve the correct development of a front wing for a single-seater vehicle, it is necessary to adequately simulate the forces that are generated on a car to evaluate its performance, which depends on the aerodynamic forces of the front wing that are present due to its geometry. This work provided a new design and evaluation through the numerical analysis of three new front wings for single-seater vehicles that comply with the regulations issued by the International Automobile Federation (FIA) for the 2022 season. Additionally, a 3D-printed front wing prototype was developed to be evaluated in an experimental study to corroborate the results obtained through computer simulations. A wind tunnel experiment test was performed to validate the numerically simulated data. Also, we developed a numerical simulation and characterization of three front wings already used in Formula One from a previous season (the end of the 2021 season). This work defined how these devices perform, and in the same way, it identified how their evolution over time has provided them with substantial benefits and greater efficiency. All the numerical simulations were carried out by applying the Finite Volume Method, allowing us to obtain the values of the aerodynamic forces that act on the front wing. Also, it was possible to establish a comparison between the three newly designed proposals from the most aerodynamic advantages to produce a prototype and perform an experimental test. The results of the experimental test showed similarity to those of the numerical analyses, making it clear that the methodology followed during the development of the work was correct. In addition, the mechanical designs carried out to develop the front wing can be considered ideal, because the results showed that the front wing could be competitive, and applying it caused a downforce to be favored that prevented the car from being thrown off the track. Additionally, the results indicate this is an effective proposal for use in a single-seater vehicle and that the design methodology delivers optimal results.

]]>Fluids doi: 10.3390/fluids8070209

Authors: Nastaran Rezaee John Aunna Jamal Naser

In this study, a numerical investigation of the Marangoni flow in foam fractionation was conducted, with a specific focus on the film of micro-foams in both the interior and exterior regions. A three-dimensional node&ndash;film&ndash;plateau border system was employed to model the system, utilizing time-dependent mass conservation equations. The study emphasized the influence of the surfactant concentration in the foam fractionation column and the mobility of the air&ndash;liquid interface on the Marangoni velocity within the film. The results indicated that higher surfactant concentration in the reflux column resulted in a significant increase in Marangoni velocities. Furthermore, a mobile interface enhanced the Marangoni flow, whereas a rigid interface reduced its intensity. The behaviour of the Marangoni flow was explored in both interior and exterior foams, revealing distinct characteristics. The presence of a wall in the exterior foam altered the flow dynamics, leading to a reduced Marangoni velocity compared to interior films.

]]>Fluids doi: 10.3390/fluids8070208

Authors: Jonathan Viquerat Elie Hachem

The coupling of deep reinforcement learning to numerical flow control problems has recently received considerable attention, leading to groundbreaking results and opening new perspectives for the domain. Due to the usually high computational cost of fluid dynamics solvers, the use of parallel environments during the learning process represents an essential ingredient to attain efficient control in a reasonable time. Yet, most of the deep reinforcement learning literature for flow control relies on on-policy algorithms, for which the massively parallel transition collection may break theoretical assumptions and lead to suboptimal control models. To overcome this issue, we propose a parallelism pattern relying on partial-trajectory buffers terminated by a return bootstrapping step, allowing a flexible use of parallel environments while preserving the on-policiness of the updates. This approach is illustrated on a CPU-intensive continuous flow control problem from the literature.

]]>Fluids doi: 10.3390/fluids8070207

Authors: Ángel Antonio Rodríguez-Sevillano María Jesús Casati-Calzada Rafael Bardera-Mora Lucía Ballesteros-Grande Lucía Martínez-García-Rodrigo Alejandra López-Cuervo-Alcaraz Jaime Fernández-Antón Juan Carlos Matías-García Estela Barroso-Barderas

This article aims to highlight the importance of utilizing flow visualization techniques in the preliminary analysis of streamlined and bluff bodies, especially the potential use of visualization with ink in a water channel as a tool for a preliminary approach during fluid mechanics analysis. According to this, the study compares the results obtained using a classical flow visualization technique, ink injection in water, with those obtained through the employment of a laser-based measurement and visualization technique, called PIV, in a low-speed wind tunnel. The article briefly presents both techniques and highlights the importance of flow visualization in the analysis of aerodynamic bodies. This study focuses on flow over NACA airfoils at extremely low Reynolds numbers, which is of great interest for application in extraterrestrial atmospheres, such as on Mars. After obtaining images of the flow streamlines over the NACA 0018 airfoil, the results of both techniques were compared. The results indicated that there was good agreement between the visualization with the water channel and the PIV results, suggesting that the results obtained in the water channel represented a scientifically valid approximation and an essential complement to computational models that require experimental validation.

]]>Fluids doi: 10.3390/fluids8070206

Authors: Yuhan Wei Chunning Ji Dekui Yuan Liqun Song Dong Xu

A harbor seal&rsquo;s whisker is able to sense the trailing vortices of marine organisms due to its unique three-dimensional wavy shape, which suppresses the vibrations caused by its own vortex-shedding, while exciting large-amplitude and synchronized vibrations in a wake flow. This provides insight into the development of whisker-inspired sensors, which have broad applications in the fields of ocean exploration and marine surveys. However, the harbor seal&rsquo;s whisker may lose its vibration suppression ability when the angle of attack (AoA) of the incoming flow is large. In order to explore the flow-induced vibration (FIV) features of a harbor seal&rsquo;s whisker at various angles of attack (&theta;=0&ndash;90&#8728;), this study experimentally investigates the effect of AoA on the vibration response of a whisker model in a wide range of reduced velocities (Ur = 3&ndash;32.2) and the Reynolds number, Re = 400&ndash;7000, in a circulating water flume. Meanwhile, for the sake of comparison, the FIV response of an elliptical cylinder with the same equivalent diameters is also presented. The results indicate that an increase in AoA enhances the vibration amplitude and expands the lock-in range for both the whisker model and the elliptical cylinder. The whisker model effectively suppresses vibration responses at &theta;=0&#8728; due to its unique three-dimensional wavy shape. However, when &theta;&ge;30&#8728;, the wavy surface structure gradually loses its suppression ability, resulting in large-amplitude vibration responses similar to those of the elliptical cylinder. For &theta; = 30&#8728; and 45&#8728;, the vibration responses of the whisker model and the elliptical cylinder undergo three vibration regimes, i.e., vortex-induced vibration, transition response, and turbulent-induced vibration, with the increasing Ur. However, at &theta; = 60&#8728; and 90&#8728;, the vortex-shedding gradually controls the FIV response, and only the vortex-induced vibration is observed.

]]>Fluids doi: 10.3390/fluids8070205

Authors: Hassan A. S. Ghazwani Khairuddin Sanaullah Afrasyab Khan

The hydrodynamics of steam&ndash;water two-phase flows under the effects of shearing, swirling, and large-scale discrete boundary conditions were investigated on an experimental basis. The steam was injected in a swirling configuration into concurrently flowing water. Both the steam and water were injected at gauge pressures of 1.0 and 2.0 bars, whereas the swirling was caused by a propeller moving at rotational speeds of 60 and 300 rpm. The ensembled normalized amplitudes of the velocity fluctuations across a layer defined by spatial positions along the radial and axial directions inhibited the swirling steam&ndash;water two-phase flows. This ensembled nature and the normalized amplitudes of the velocity fluctuations were investigated under the action of the above-stated operating conditions, and it was found that the swirling steam&ndash;water two-phase body of the flow showed proportionally varying percentages of ensembled-normalized amplitudes of the velocity fluctuations.

]]>Fluids doi: 10.3390/fluids8070204

Authors: Victor Kozlov Stanislav Subbotin Ivan Karpunin

The oscillation of the liquid interface in axisymmetric Hele-Shaw cells (conical and flat) is experimentally studied. The cuvettes, which are thin conical layers of constant thickness and flat radial Hele-Shaw cells, are filled with two immiscible liquids of similar densities and a large contrast in viscosity. The axis of symmetry of the cell is oriented vertically; the interface without oscillations is axially symmetric. An oscillating pressure drop is set at the cell boundaries, due to which the interface performs radial oscillations in the form of an oscillating &ldquo;tongue&rdquo; of a low-viscosity liquid, periodically penetrating into a more viscous liquid. An increase in the oscillation amplitude leads to the development of a system of azimuthally periodic structures (fingers) at the interface. The fingers grow when the viscous liquid is forced out of the layer and reach their maximum in the phase of maximum displacement of the interface. In the reverse course, the structures decrease in size and, at a certain phase of oscillations, take the form of small pits directed toward the low-viscosity fluid. In a conical cell, a bifurcation of period doubling with an increase in amplitude is found; in a flat cell, it is absent. A slow azimuthal drift of finger structures is found. It is shown that the drift is associated with the inhomogeneity of the amplitude of fluid oscillations in different radial directions. The fingers move from the region of a larger to the region of a lower amplitude of the interface oscillations.

]]>Fluids doi: 10.3390/fluids8070203

Authors: Aditya Suryadi Tan Thomas Sattel Richard Subianto

Magnetorheological (MR) dampers have a virtue over conventional dampers, where their damping properties can be adjusted using a magnetic field. However, MR dampers have been barely implemented in small vibratory systems, in which the modal mass and stiffness are relatively small. This is due to two major reasons, namely its high parasitic damping force and big moving mass. When such an MR damper is installed in a small vibratory system, the system&lsquo;s default damping ratio is increased and therefore its dynamic is reduced. Here, a new concept of an MR damper utilizing the porous medium and shear operating mode together with an external non-moving electromagnet is proposed. This combination results in an MR damper with a low parasitic damping force and a small moving mass. For comparison purposes, a benchmark MR damper with comparable geometry is constructed. The proposed MR damper possesses a passive friction force that is 8&times; smaller and OFF-state passive viscous damping that is 10&ndash;20&times; smaller than the benchmark MR damper. An investigation of the proposed MR damper performance in a test vibratory system shows almost no reduction of the system dynamic. Therefore, this proposed MR damper configuration can be suitable for applications in small vibratory systems.

]]>Fluids doi: 10.3390/fluids8070202

Authors: Abraham Medina Diego Benjamin García Abel López Villa Benjamin Castillo-Morales Georgiy Polupan

Recently, in a previous study, we experimentally showed the existence of an optimal injected steam mass flow rate, per unit length, &#981;opt, which produces the maximal recovery of condensate in a preformed steam chamber with an elliptical cross section of a horizontal semi-major axis. Mutatis mutandis, in this work, we present experimental studies in preformed steam chambers: one elliptical and the other circular. In both cases, we also found the existence of unique optimal values. These configurations try to recreate the steam condensation at a given time-lapse, as it would occur during the growth stage of the steam-assisted gravity drainage (SAGD) process: a method used in the recovery of heavy and extra-heavy oil from homogeneous reservoirs. Finding the optimal mass flow rates in the actual recovery process could be useful in the design of optimized SAGD processes.

]]>Fluids doi: 10.3390/fluids8070201

Authors: Amir Akbarzadeh Iman Borazjani

Currently, most wind turbine performance analyses and simulations are performed assuming constant pitch and yaw angles during each rotation. Nevertheless, induced vibration or rotor imbalance can affect the pitch or yaw angle within each rotation. In this study, the effects of low-amplitude sinusoidal pitch angle oscillations of the blade on the performance of a wind turbine was investigated numerically by comparing it against the baseline (without pitch variations). Large eddy simulations were performed in which the motion of blades was handled by the curvilinear immersed boundary (CURVIB) method. The grid resolution was increased near the moving immersed boundaries using dynamic overset grids to resolve rotating blades. It was found that low-amplitude (up to 3 degrees) sinusoidal oscillations in the pitch angle negligibly affected the mean torque but increased its fluctuations and created distinct features in the wake of the turbine. In fact, the turbine&rsquo;s mean torque at wind speed of 15 m/s decreases from 1245 N.m to 1223 N.m, while its fluctuation (standard deviation) increases from 2.85 N.m to 7.94 N.m, with a dynamic pitch of 0.5 degrees and frequency of 3.6 Hz.

]]>Fluids doi: 10.3390/fluids8070200

Authors: Max Koch Werner Lauterborn Christiane Lechner Robert Mettin

A bubble expanding and collapsing near a solid boundary develops a liquid jet toward the boundary. The jet leaves a torus bubble and induces vortices in the liquid that persist long after the bubble oscillations have ceased. The vortices are studied numerically in axial symmetry and compared to experiments in the literature. The flow field is visualized with different methods: vorticity with superimposed flow-direction arrows for maps at a time instant and colored-liquid-layer flow-field maps (dye advection) for following the complete long-term fluid flow up to a chosen time since bubble generation. Bubbles with equal energy&mdash;maximum radius in a free liquid Rmax&infin;= 500 &micro;m&mdash;are studied for different distances Dinit from the solid boundary. The interval of normalized distances D* = Dinit/Rmax&infin; from 0.4 to 1.8 is covered. Two types of vortices were reported in experiments, one moving toward the solid boundary and one moving away from it. This finding is reproduced numerically with higher resolution of the flow field and in more detail. The higher detail reveals that the two types of vortices have different rotation directions and coexist with individually varying vorticity amplitude throughout the interval studied. In a quite narrow part of the interval, the two types change their strength and extent with the result of a reversal of the dominating rotational direction of the fluid flow. Thereby, the experimentally found transition interval could be reproduced and refined. It is interesting to note that in the vortex transition interval, the erosion of a solid surface is strongly augmented.

]]>Fluids doi: 10.3390/fluids8070198

Authors: Weichen Hong Huidan Yu Jun Chen John Talamantes Dave M. Rollins Xin Fang Jianyun Long Chenke Xu Alan P. Sawchuk

Vascular disease is the leading cause of morbidity and mortality and a major cause of disability for Americans, and arterial stenosis is its most common form in systemic arteries. Hemodynamic characterization in a stenosed arterial system plays a crucial role in the diagnosis of its lesion severity and the decision-making process for revascularization, but it is not readily available in the current clinical measurements. The newly emerged image-based computational hemodynamics (ICHD) technique provides great potential to characterize the hemodynamics with fine temporospatial resolutions in realistic human vessels, but medical data is rather limited for validation requirements. We present an image-based experimental hemodynamics (IEHD) technique through a mock circulation loop (MCL) to bridge this critical gap. The MCL mimics blood circulation in human stenosed systemic arterial systems that can be either 3D-printed silicone, artificial, or cadaver arteries and thus enables in vitro measurement of hemodynamics. In this work, we focus on the development and validation of the MCL for the in vitro measurement of blood pressure in stenosed silicone arteries anatomically extracted from medical imaging data. Five renal and six iliac patient cases are studied. The pressure data from IEHD were compared with those from ICHD and medical measurement. The good agreements demonstrate the reliability of IEHD. We also conducted two parametric studies to demonstrate the medical applicability of IEHD. One was the cardiovascular response to MCL parameters. We found that blood pressure has a linear correlation with stroke volume and heart rate. Another was the effect of arterial stenosis, characterized by the volumetric reduction (VR) of the arterial lumen, on the trans-stenotic pressure gradient (TSPG). We parametrically varied the stenosis degree and measured the corresponding TSPG. The TSPG-VR curve provides a critical VR that can be used to assess the true hemodynamic severity of the stenosis. Meanwhile, the TSPG at VR = 0 can predict the potential pressure improvement after revascularization. Unlike the majority of existing MCLs that are mainly used to test medical devices involving heart function, this MCL is unique in its specific focus on pressure measurement in stenosed human systemic arteries. Meanwhile, rigorous hemodynamic characterization through concurrent IEHD and ICHD will significantly enhance our current understanding of the pathophysiology of stenosis and contribute to advancements in the medical treatment of arterial stenosis.

]]>Fluids doi: 10.3390/fluids8070199

Authors: Gordon Gilja Robert Fliszar Antonija Harasti Manousos Valyrakis

High-frequency velocimeters used for flow measurements during laboratory experiments allow the user to select the range for several operation parameters to set up the instrument for optimal velocity measurement. The discrepancies between velocity measurements collected with different instrument configurations can be significant, depending on the flume bed configuration and boundary conditions. The aim of this paper is to quantify the differences in flow velocity profiles measured with Acoustic Doppler Velocimeter Profilers (ADVPs) configured using a combination of profiling parameters: Ping Algorithm (PA), Transmit Pulse Size (TPS), and Cell Size (CS). Whereas in Part I of this research, the goal was to identify the optimal probe configuration for downstream measurement of the complex hydraulic structure (pier protected with riprap) based on a match of the flow rate with measurements from other instruments, in this paper, effect of distinct probe configuration on velocity profile and turbulent kinetic energy (TKE) is demonstrated. Differences between ADVPs&rsquo; configurations were analyzed through sensitivity analysis with the intention to calculate and compare any discrepancies in the velocity measurements for all the three measured velocity components: streamwise u, spanwise v and vertical w collected on two characteristic flume cross-sections. The results show that each parameter change has a significant effect on the measured values of each velocity component when compared to the Target Configuration (TC). The largest root-mean-square-error (RMSE) is observed when TPS is changed, followed by CS and PA. Absolute RMSE calculated for TPS change from 4 mm to 1 mm is, on average, 6.30 cm/s, 0.90 cm/s, and 0.82 cm/s for velocity components u, v and w, respectively. Absolute RMSE calculated for CS change from 1 mm to 4 mm is, on average, 4.49 cm/s, 0.88 cm/s, and 0.71 cm/s for velocity components u, v and w, respectively. Absolute RMSE calculated for PA change from Adaptive to Max interval is, on average, 4.04 cm/s, 0.63 cm/s, and 0.68 cm/s for velocity components u, v and w, respectively. For a change in all parameters, RMSE is greater for the cross-section downstream of the pier than for the approach cross-section: on average, 90%, 57% and 54% for a change in the PA, TPS, and CS, respectively.

]]>Fluids doi: 10.3390/fluids8070197

Authors: Nikolay Bukharin Mouhammad El Hassan

Ejectors are simple mechanical devices with no moving parts which convert the pressure energy of a motive fluid to kinetic energy and generate suction of the secondary fluid. The ability to recover waste heat, to operate using solar power and the ability to use geothermal energy make ejector-based systems attractive in different industrial applications. The main challenge of ejector-based refrigeration systems is their relatively low coefficient of performance (COP). In order to increase the ejector performance, two chemically distinct fluids can be used in the refrigeration cycle. It is suggested that a higher molecular mass be used for the motive fluid to improve the entrainment ratio of the binary fluid ejector (BFE) and thus the system COP. Inert gas combinations of argon&ndash;helium and krypton&ndash;air are studied in this paper using computational fluid dynamics (CFDs) and experimental measurements. All CFD cases were axisymmetric and the appropriate turbulence model was selected based on experimental validation. Specifically, the entrainment ratio and the static pressure along the ejector wall were measured to validate the CFD predictions. It was found that the molar entrainment ratio was significantly higher in argon&ndash;helium compared to krypton&ndash;air. The static pressure measurements along the wall, in addition, exhibited good agreement with the results obtained via computational fluid dynamics (CFDs).

]]>Fluids doi: 10.3390/fluids8070196

Authors: Safia Ihsan Ali David Patton Kimberley A. Myers Julio Garcia

Tetralogy of Fallot (TOF) is the most prevalent cyanotic congenital heart defect (CHD) that alters normal blood flow through the heart and accounts for 10% of all CHD. Pulmonary stenosis and regurgitation are common in adults who have undergone TOF repair (rTOF) and can impact the load on the right ventricle, blood flow pressure, and pulmonary hemodynamics. Pressure mapping, obtained through 4D-flow magnetic resonance imaging (4D-flow MRI), has been applied to identify abnormal heart hemodynamics in CHD. Hence, the aim of this research was to compare pressure drop and relative pressures between patients with repaired TOF (rTOF) and healthy volunteers. An in vitro validation was performed, followed by an in vivo validation. We hypothesized that pressure drop is a more stable pressure mapping method than relative pressures to detect altered hemodynamics. A total of 36 subjects, 18 rTOF patients and 18 controls underwent cardiac MRI scans and 4D-flow MRI. Pressure drops and relative pressures in the MPA were higher in rTOF patients compared to the controls (p &lt; 0.05). Following the in vitro validation, pressure drops proved to be a more stable pressure mapping method than relative pressures, as the flow loses its laminarity and becomes more turbulent. In conclusion, this study demonstrated that flow hemodynamics in rTOF can exhibit altered pressure maps. Pressure mapping can help provide further insight into rTOF patients&rsquo; hemodynamics to improve patient care and clinical decisions.

]]>Fluids doi: 10.3390/fluids8070195

Authors: Nicholas J. Ward

For a few decades, machine learning has been extensively utilized for turbulence research. The goal of this work is to investigate the reconstruction of turbulence from minimal or lower-resolution datasets as inputs using reduced-order models. This work seeks to effectively reconstruct high-resolution 3D turbulent flow fields using unsupervised physics-informed deep learning. The first objective of this study is to reconstruct turbulent channel flow fields and verify these with respect to the statistics. The second objective is to compare the turbulent flow structures generated from a GAN with a DNS. The proposed deep learning algorithm effectively replicated the first- and second-order statistics of turbulent channel flows of Re&tau;= 180 within a 2% and 5% error, respectively. Additionally, by incorporating physics-based corrections to the loss functions, the proposed algorithm was also able to reconstruct &lambda;2 structures. The results suggest that the proposed algorithm can be useful for reconstructing a range of 3D turbulent flows given computational and experimental efforts.

]]>Fluids doi: 10.3390/fluids8070194

Authors: Aleandro Saez Maurizio Manzo Marco Ciarcià

After one year of operation, the Ingenuity rotorcraft and the Perseverance rover continue their exploration missions on Mars. Succeeding the technology demonstration phase, by proving its flight capabilities, Ingenuity transitioned to a new mission stage to explore modes of flight exploration for future scouting missions. This research intends to analyze, using numerical simulations, the aerodynamic conditions such as those experienced by Ingenuity during its flight missions. For this work, ANSYS Fluent software was used to simulate the flow around the cambered plates, and a three-equation intermittency (&gamma;) shear stress transport turbulence model with compressible formulation was implemented. The influence of the camber and its position for the cambered plates were explored, and a sensitivity analysis with respect to the Mach number was performed. The objective of this project was to determine the optimal configuration to produce the optimal lift-to-drag ratio for the range of analysis. The results were in line with the ones shown by NASA (OVERFLOW). Moreover, this analysis showed the ANSYS Fluent applicability for assessing aerodynamic surfaces for unmanned aerial systems operating at low density and low Reynolds number regimes.

]]>Fluids doi: 10.3390/fluids8070193

Authors: Pietro Mazzei Emma Frosina Adolfo Senatore

This research presents a comparison between two numerical approaches developed and later compared for studying External Gear Pumps (EGPs). Models have been developed for studying pumps with helical gears. Firstly, a three-dimensional (3D) CFD numerical model has been built using a commercial code. Then, a new tool called EgeMATor MP+, completely developed by the authors and capable of completely simulating this pump&rsquo;s typologies is presented. Thanks to different subroutines developed in different interconnected environments, this tool can fully analyze those pumps, starting from the drawing. Both numerical approaches have been detailed, highlighting their strengths and weaknesses and the tweaking required to reach more accurate results. Both numerical models have been set up with the same boundary conditions to obtain a more accurate comparison. Comparisons have been performed using tests performed on a commercial pump taken as reference, focusing on steady-state volumetric performance as well as the transient features of the outlet port pressure oscillations. The comparison of the (Q,p) characteristics showed that the 3D CFD numerical model has a slightly better accuracy, but both models have errors that fall into the uncertainty range of the experimental measurements. In addition, the pressure ripples comparison verified good agreements, where also the double flank behavior of the pump is predicted. While comparing the two simulation approaches, the paper highlights the limits and strengths of each one of the two approaches. In particular, it is shown how both models can match the experimental results considering proper assumptions. The paper constitutes a unique contribution to the field of numerical simulation of EGPs and represents a useful reference to designers looking for suitable methods for simulating existing or novel design solutions.

]]>Fluids doi: 10.3390/fluids8070192

Authors: Somaya Younoussi Abdeslem Ettaouil

The present work aims to study the aerodynamic characteristics of a newly designed three-bladed horizontal-axis wind turbine (HAWT) using the Computational Fluid Dynamic (CFD) method. The blade geometry is designed using an improved Blade Element Momentum (BEM) method to be similar in size to the Ampair300 wind turbine. The shear stress transport (SST) transition turbulence model closure is utilized to solve the steady state three-dimensional Reynolds Averaged Navier-Stokes (RANS) equations. The Ansys Fluent CFD solver is used to solve the problem. Then, a comparison between the two turbines&rsquo; operating conditions is conducted by monitoring the pressure coefficient, pressure contours and velocity vectors at five different radial positions. The analysis of the Tip Speed Ratio (TSR) effects on the turbine efficiency and on the flow behavior on the blade and in the near wake is carried out. For 8 m/s wind speed, the optimum pitch angle is also investigated, and the results are prepared against each TSR.

]]>Fluids doi: 10.3390/fluids8070191

Authors: Ilya Vladyko Nikolay Miskiv Vladimir Serdyukov Aleksandr Nazarov Anton Surtaev

Spray cooling is a highly effective method of heat removal that has broad practical applications, including use in modern cooling systems designed for microelectronics and microchips. It is known that spray cooling performance is influenced by a huge number of factors. This experimental research is devoted to the study of the influence of a liquid flow rate in the range of 15.1&ndash;24.2 cm3/s, heat flux up to 6.4 MW/m2, and nozzle-to-surface distance on the heat transfer rate in non-boiling mode and the distribution of the local temperature of the heat exchange surface during spray cooling. It is shown that the heat transfer coefficient weakly depends on the heat flux for all studied nozzle-to-surface distances. It is demonstrated that the nozzle-to-surface distance has a significant influence on the heat transfer and the temperature distributionon the heating surface during spray cooling in non-boiling mode. At the same time, there is an optimal distance at which the maximum heat transfer rate and uniformity of the temperature are achieved. Criteria and a ratio for determining the optimal distance from the spray nozzle to the heated surface are proposed.

]]>Fluids doi: 10.3390/fluids8070190

Authors: Baran Teoman Andrei Potanin Piero M. Armenante

In this work, the roles of the orifice shape and off-bottom clearance of the dip tube on the discharge flow rate of a complex fluid from a dispensing bottle and on the resulting residual &ldquo;heel&rdquo; volume left in the bottle were investigated. Particle Image Velocimetry (PIV) was used to monitor the discharge rate and the heel. The dip tube clearance and the orifice shape both affected the formation of the heel. Dip tubes provided with a flat cut orifice not only resulted in a smaller heel compared to angled cut orifices but also generated a higher flow rate at constant suction pressure. Reducing the dip tube clearance produced smaller heel volumes irrespective of the shape of the dip tube orifice. The results of this work were validated using the velocity contour maps obtained by PIV and, separately, with the heel profiles obtained from the PIV raw images.

]]>Fluids doi: 10.3390/fluids8070189

Authors: Maria Antonietta Boniforti Roberto Magini Tania Orosco Salinas

Flow diverter stents (FDS) are increasingly used for the treatment of complex intracranial aneurysms such as fusiform, giant, or wide-neck aneurysms. The primary goal of these devices is to reconstruct the diseased vascular segment by diverting blood flow from the aneurysm. The resulting intra-aneurysmal flow reduction promotes progressive aneurysm thrombosis and healing of the disease. In the present study, a numerical investigation was performed for modeling blood flow inside a patient-specific intracranial aneurysm virtually treated with FDS. The aim of the study is to investigate the effects of FDS placement prior to the actual endovascular treatment and to compare the effectiveness of devices differing in porosity. Numerical simulations were performed under pulsatile flow conditions, taking into account the non-Newtonian behavior of blood. Two possible post-operative conditions with virtual stent deployment were simulated. Hemodynamic parameters were calculated and compared between the pre-operative (no stent placement) and post-operative (virtual stent placement) aneurysm models. FDS placement significantly reduced intra-aneurysmal flow velocity and increased the Relative Residence Time (RRT) on the aneurysm, thus promoting thrombus formation within the dilatation and aneurysm occlusion. The results highlighted an increase in the effectiveness of FDS as its porosity increased. The proposed analysis provides pre-operative knowledge on the impact of FDS on intracranial hemodynamics, allowing the selection of the most effective treatment for the specific patient.

]]>Fluids doi: 10.3390/fluids8070187

Authors: Yue Chen Qichao Wang Hongbing Xiong Lijuan Qian

Vapor bubbles are widely concerned in many industrial applications. The deformation and collapse of a vapor bubble near a free surface after being heated and raised from the bottom wall are investigated in this paper. On the basis of smoothed particle hydrodynamics (SPH) and the van der Waals (VDW) equation of state, a numerical model of fluid dynamics and phase change was developed. The effects of fluid dynamics were considered, and the phase change of evaporation and condensation between liquid and vapor were discussed. Quantitative and qualitative comparisons between our numerical model and the experimental results were made. After verification, the numerical simulation of bubbles with the effects of the shear viscosity &eta;s and the heating distance L were taken into account. The regularity of the effect of the local Reynolds number (Re) and the Ohnesorge number (Oh) on the deformation of vapor bubbles is summarized through a further analysis of several cases, which can be summarized into four major patterns as follows: umbrella, semi-crescent, spheroid, and jet. The results show that the Re number has a great influence on the bubble deformation of near-wall bubbles. For Re &gt; 1.5 &times; 102 and Oh &lt; 3 &times; 10&minus;4, the shape of the bubble is umbrella; for Re &lt; 5 &times; 100 and Oh &gt; 10&minus;3, the bubble is spheroidal; and for 5 &times; 100 &lt; Re &lt; 1.5 &times; 102, 3 &times; 10&minus;4 &lt; Oh &lt; 10&minus;3, the bubble is semi-crescent. For liquid-surface bubbles, the Re number effect is small, and when Oh &gt; 5 &times; 10&minus;3, the shape of the bubble is jet all the time; there is no obvious difference in the bubble deformation, but the jet state is more obvious as the Re decreases. Finally, the dynamic and energy mechanisms behind each mode are discussed. The bubble diameter, bubble symmetry coefficient, and rising velocity were analyzed during their whole processes of bubble growth and collapse.

]]>Fluids doi: 10.3390/fluids8070188

Authors: Ngo Minh Tuan Tran The Long Tran Bao Ngoc

Lubrication and cooling in hard machining is an urgent and growing concern. The use of a suitable cooling lubrication condition is a crucial factor and has a great influence on the machining efficiency and the machined surface quality in hard machining. Among the proposed technological solutions, minimum quantity lubrication (MQL) using nano-cutting oils is a novel solution and its effectiveness has been proven for hard turning. This work aims to investigate the influence of MQL technological parameters using MoS2 nano-cutting oil including nanoparticle concentration, air pressure, and air flow rate on surface roughness and the resultant cutting force in hard turning using CBN inserts. Box-Behnken optimal experimental design and ANOVA analysis were used to study the influence of the input parameters and determine the optimal values. The results present the influence of the survey parameters and provide technological guides for specific objective functions for further sustainable studies on MQL hard turning using nano-cutting oil.

]]>Fluids doi: 10.3390/fluids8070186

Authors: Anh Viet Pham Kazuaki Inaba

High-aspect-ratio (HAR) rectangular jets have attracted attention in automobile air conditioning (A/C) systems and turbulent jet applications owing to their excellent air delivery and mixing and attractive interior design. Active flow control (AFC) of rectangular jets using plasma actuators (PAs) has proven to be a promising technique because the actuator is simple, has low energy consumption, and can create flow features without interference. This research aims to understand the interaction between PAs and flow from a HAR rectangular nozzle using hot-wire anemometry, particle image velocimetry, and theoretical studies. Understanding how PAs affect the flow is beneficial for designing air vents to fit automobile A/C systems and various engineering applications by recreating the flow features with other AFC techniques and actuators. The combination of periodic excitation and vectoring effects transfers the flow&rsquo;s mean energy to organized structures&mdash;known as spanwise vortexes&mdash;as large as 6 mm. The interaction between these coherent structures and the dissipative environment compresses the vortexes, resulting in the flow converging on the spanwise&ndash;streamwise (X&ndash;Z) plane and diverging on the transverse&ndash;streamwise (X&ndash;Y) plane. HAR rectangular jet flow features controlled by PAs can be predicted for specific cases by calculating the Strouhal number based on PA operating parameters.

]]>Fluids doi: 10.3390/fluids8060185

Authors: Min-Gyun Park Hyun-Su Kang Youn-Jea Kim

Fog interferes with traffic flow and causes major accidents. In foggy conditions, traffic accident death rates are higher than in other weather conditions. Research on fog dissipation technology is needed to reduce the incidence of accidents caused by fog. There are various artificial methods to remove fog. In this study, two methods of natural dissipation by gravity sedimentation and a solid carbon dioxide seeding fog dissipation mechanism were compared and analyzed in cold fog conditions. Solid carbon dioxide was selected as the fog dissipation particle. In this experiment, solid carbon dioxide seeding with three different values of weight (500 g, 1000 g, and 1500 g) was considered. This is because fog particles can be supercooled and fog can be removed. A light detection and ranging (LiDAR) sensor were used to quantitatively check the effect of improving visibility when solid carbon dioxide was seeded in the fog. The LiDAR sensor detects the surrounding environment through distance measurements by emitting lasers and processing the laser responses. A camera was used to visually observe the phenomenon occurring inside the calorimetric chamber. As a result, the fog dissipation mechanism using solid carbon dioxide seeding under cold fog conditions was proven to be effective in improving the visible distance compared with natural dissipation.

]]>Fluids doi: 10.3390/fluids8060184

Authors: Nick Schneider Simon Köhler Jens von Wolfersdorf

Spectral proper orthogonal decomposition (SPOD) has seen renewed interest in recent years due to its unique ability to decouple organised motion at different timescales from large datasets with limited available information. This paper investigated the unsteady components of the flow field within a simplified turbine centre frame (TCF) model by applying SPOD to experimental, time-resolved flow speed data captured by particle image velocimetry (PIV). It was observed that conventional methods failed to capture the two significant active bands in the power spectrum predicted by preliminary hot wire anemometry measurements. Therefore, a modification to the SPOD procedure, which employs subsampling of the time sequence recorded in the experiment to artificially lower the PIV data acquisition frequency, was developed and successfully deployed to analyse the TCF flow field. The two dynamically active bands were identified in the power spectra, resulting in a closer match to the preceding analyses. Within these bands, SPOD&rsquo;s ability to capture spatial coherence was leveraged to detect several plausible coherent, fluctuating structures in two perpendicular planes. A partial three-dimensional reconstruction of the flow phenomena suggested that both bands were associated with a distinct mode of organised motion, each contributing a significant percentage of the system&rsquo;s total fluctuating energy.

]]>Fluids doi: 10.3390/fluids8060183

Authors: Pranshul Sardana Mohammadreza Zolfaghari Guilherme Miotto Roland Zengerle Thomas Brox Peter Koltay Sabrina Kartmann

The reliable non-contact dispensing of droplets in the pico- to microliter range is a challenging task. The dispensed drop volume depends on various factors such as the rheological properties of the liquids, the actuation parameters, the geometry of the dispenser, and the ambient conditions. Conventionally, the rheological properties are characterized via a rheometer, but this adds a large liquid overhead. Fluids with different Ohnesorge number values produce different spatiotemporal motion patterns during dispensing. Once the Ohnesorge number is known, the ratio of viscosity and surface tension of the liquid can be known. However, there exists no mathematical formulation to extract the Ohnesorge number values from these motion patterns. Convolutional neural networks (CNNs) are great tools for extracting information from spatial and spatiotemporal data. The current study compares seven different CNN architectures to classify five liquids with different Ohnesorge numbers. Next, this work compares the results of various data cleaning conditions, sampling strategies, and the amount of data used for training. The best-performing model was based on the ECOmini-18 architecture. It reached a test accuracy of 94.2% after training on two acquisition batches (a total of 12,000 data points).

]]>Fluids doi: 10.3390/fluids8060182

Authors: Jana Hoffmann Daniel A. Weiss

Transonic planar flows around a circular cylinder are investigated numerically for laminar and turbulent flow conditions with Reynolds numbers of 50&le;ReD&le;300 and 8890&le;ReD&le; 80,000 and free stream Mach numbers in the range of 0.2&le;Ma&infin;&le;2. A commercially available CFD tool is used and validated for this purpose. The results show that the flow phenomena occurring can be grouped into eight regimes. Compared to the incompressible flow regimes, several new phenomena can be found. In contrast, at higher Ma&infin; of 0.6&le;Ma&infin;&le;0.8 vortices in the wake of the cylinder are suppressed for ReD=50. In some cases, Ma&infin;=0.8 and ReD&ge;300, &lambda;-shocks are formed in the near cylinder wake. For supersonic Ma&infin;, two different phenomena are observed. Beside the well-known oblique and detached shocks, for 50&le;ReD&le;300 a wake with instabilities is formed downstream of the cylinder. Furthermore, the temporal mean drag coefficient C&macr;D, the Strouhal number Str, as well as the critical Mach number Macrit are calculated from the simulation results and are interpreted.

]]>Fluids doi: 10.3390/fluids8060181

Authors: John V. Shebalin

We continue our study of the transition of ideal, homogeneous, incompressible, magnetohydrodynamic (MHD) turbulence from non-equilibrium initial conditions to equilibrium using long-time numerical simulations on a 1283 periodic grid. A Fourier spectral transform method is used to numerically integrate the dynamical equations forward in time. The six runs that previously went to near equilibrium are here extended into equilibrium. As before, we neglect dissipation as we are primarily concerned with behavior at the largest scale where this behavior has been shown to be essentially the same for ideal and real (forced and dissipative) MHD turbulence. These six runs have various combinations of imposed rotation and mean magnetic field and represent the five cases of ideal, homogeneous, incompressible, and MHD turbulence: Case I (Run 1), with no rotation or mean field; Case II (Runs 2a and 2b), where only rotation is imposed; Case III (Run 3), which has only a mean magnetic field; Case IV (Run 4), where rotation vector and mean magnetic field direction are aligned; and Case V (Run 5), which has non-aligned rotation vector and mean field directions. Statistical mechanics predicts that dynamic Fourier coefficients are zero-mean random variables, but largest-scale coherent magnetic structures emerge and manifest themselves as Fourier coefficients with very large, quasi-steady, mean values compared to their standard deviations, i.e., there is &lsquo;broken ergodicity.&rsquo; These magnetic coherent structures appeared in all cases during transition to near equilibrium. Here, we report that, as the runs were continued, these coherent structures remained quasi-steady and energetic only in Cases I and II, while Case IV maintained its coherent structure but at comparatively low energy. The coherent structures that appeared in transition in Cases III and V were seen to collapse as their associated runs extended into equilibrium. The creation of largest-scale, coherent magnetic structure appears to be a dynamo process inherent in ideal MHD turbulence, particularly in Cases I and II, i.e., those cases most pertinent to planets and stars. Furthermore, the statistical theory of ideal MHD turbulence has proven to apply at the largest scale, even when dissipation and forcing are included. This, along with the discovery and explanation of dynamically broken ergodicity, is essentially a solution to the &lsquo;dynamo problem&rsquo;.

]]>Fluids doi: 10.3390/fluids8060180

Authors: Werner Eßl Georg Reiss Peter Raninger Werner Ecker Nadine Körbler Eva Gerold Helmut Antrekowitsch Jolanta Klocek Thomas Krivec

Multi-droplet impingement is a fundamental aspect inherent to all kinds of technical spray processes which typically aim at enhancing the convective exchange of reagents or heat at the impinged surface. In this paper, the impingement of multiple droplets onto a structured surface is investigated by a comprehensive CFD model, which resolves the dynamics of the individual droplets and the film on a micro-scale level based on the Volume of Fluid (VOF) method. The considered surface topology includes cavities and is typical for protective masks used in the spray etching of Printed Circuit Boards (PCBs). The agitation of the liquid film in terms of the convective mass transfer rates across virtual horizontal evaluation planes is studied and the influence of film height and droplet impaction velocity is elaborated. Passive tracer tracking is employed to investigate the release and re-entrainment of fluid at the surface cavities. Two modes of mass exchange between the cavities and the main flow upon droplet impingement are identified, which are central inflow accompanied by lateral outflow (1) and lateral inflow with outflow at the opposing side (2). A statistical analysis of the allocation of tracer particles shows that high impaction velocities and low film heights correlate with an enhanced decay of tracer particles within the cavities. The susceptibility to re-entrainment is also reduced by high impaction velocities, whereas increased film heights are found to promote re-entrainment.

]]>Fluids doi: 10.3390/fluids8060179

Authors: Arseniy Berezin Anastasia Perepelkina Anton Ivanov Vadim Levchenko

Grid refinement is used to reduce computing costs while maintaining the precision of fluid simulation. In the lattice Boltzmann method (LBM), grid refinement often uses interpolated values. Here, we developed a method in which interpolation in space and time is not required. For this purpose, we used the moment matching condition and rescaled the nonequilibrium part of the populations, thereby developing a recalibration procedure that allows for the transfer of information between different LBM stencils in the simulation domain. Then, we built a nonuniform lattice that uses stencils with different shapes on the transition. The resulting procedure was verified by performing benchmarks with the 2D Poisselle flow and the advected vortex. It is suggested that grids with adaptive geometry can be built with the proposed method.

]]>Fluids doi: 10.3390/fluids8060178

Authors: Fei Sun Hong Ji Shengqing Yang Chen Li

Raising the working speed of hydraulic pumps to maximize the efficient matching range of electric motors is one of the possible ways to achieve energy efficiency in electric machinery. By means of a simulation method verified with subsequent experiments in terms of filling efficiency, this paper first analyzed the suction capacity of crescent-type internal gear pumps with different geometric parameters at high speed, and the gear pair that is more suitable for high-speed operation was obtained. Subsequently, as the more significant contributions, two pairing solutions of a non-positive displacement pump and an internal gear pump were proposed to pressurize the inlet of the gear pump to keep it from cavitating. In the compact design solution, the inclined-holes type and axial-flow blade pumps share the same speed as the hydraulic pump, while the decentralized layout solution allows for flexible adjustment of the centrifugal impeller-type pump speed to maximize the filling capability. The final simulation results show that, with the help of inclined-holes type and centrifugal impeller type pumps, the filling efficiency of the internal gear pump at 6000 rpm can be improved by 3.59% and 5.84%, respectively, while the axial-flow blades pump fails to eliminate cavitation regardless of speed. Moreover, when the hydraulic pump works at 6000 rpm, the centrifugal impeller speed needs to be set above 2500 rpm to make sense.

]]>Fluids doi: 10.3390/fluids8060177

Authors: Daniel Costero Federico Piscaglia

In computations of unsteady flow problems by the arbitrary Lagrangian&ndash;Eulerian (ALE) method, the introduction of the grid velocity in the transport terms of the governing equations is not a sufficient condition for conservativeness if topology changes in the dynamic mesh are present and the number of mesh cells changes. We discuss an extension to second-order time differencing schemes (Implicit Euler and Crank&ndash;Nicolson) in the finite volume framework, to achieve second-order time-accuracy of the solution. Numerical experiments are given to illustrate the effectiveness of the presented method.

]]>Fluids doi: 10.3390/fluids8060176

Authors: Chen Xu Wei Zhang

This study investigated the diffusion impact on the chemical perturbation of NOx and O3 caused by the streamer and leader parts of a blue jet in the low stratosphere (18&ndash;30 km), using the coupling of a detailed stratospheric chemistry model and a typical diffusion model. The study found that diffusion significantly impacted the evolution of chemical perturbations at both short-term and long-term time scales after the blue jet discharge, with changes in NOx and O3 concentrations observed at different altitudes (18&ndash;28 km). At 18 km, the concentrations of NOx and N2O that account for diffusion start to decrease after 1 s, whereas those without diffusion remain at their peak concentrations. Meanwhile, O3 is slowly destroyed with less NOx, rather than dropping to an unrealistic low value immediately after the discharge without diffusion. The perturbation caused by the blue jet discharge disappears within a few tens of seconds at 18 km when molecular diffusion is considered. At 30 km, the chemical perturbation from four point sources was observed through changes in NO2 concentrations. However, the total concentration of NO2 perturbed by the streamer part discharge at the given surface was negligible when considering diffusion. Overall, this study provided a useful model tool for a more accurate assessment of the chemical effects of individual blue jets.

]]>Fluids doi: 10.3390/fluids8060175

Authors: Masih Zolghadr Seyed Mohammad Ali Zomorodian Abazar Fathi Ravi Prakash Tripathi Neda Jafari Darshan Mehta Parveen Sihag Hazi Mohammad Azamathulla

The causes of many bridge failures have been reported to be local scour around abutments. This study examines roughening elements as devices with which to intercept the downflow responsible for the formation of the principal vortex, which is what triggers local scour around abutments. Two vertical wall abutments with different widths were examined under four different hydraulic conditions in a clear-water regime. Elements with different thicknesses (t) and protrusions (P) with the same dimensions, (P = t = 0.05 L, 0.1 L, 0.2 L, and 0.3 L, where L is the length of the abutment) and with varying depths of installation (Z) were considered. Elements were installed in two positions: between the sediment surface and water elevation and buried within the sediment. To determine the optimum depth of installation, one element was first installed on the sediment surface, and the number of elements was increased in each subsequent test. The results show that installing elements between water surface elevation and the sediment&rsquo;s initial level did not show any defined trend on scour depth reduction. However, the optimum installation depth of the elements is 0.6&ndash;0.8 L below the initial bed level. Moreover, the roughening elements with thickness and protrusion of P = t = 0.2 L resulted in the most effective protection of the foundation. The best arrangement, (P = t = 0.2 L and Z = &gt;0.6&ndash;0.8 L) reduced the maximum scour depth by up to 30.4% and 32.8% for the abutment with smaller and larger widths, respectively.

]]>Fluids doi: 10.3390/fluids8060174

Authors: Yupeng Sun Hafiz Muhammad Adeel Hassan Joe Alexandersen

Stacked plate heat exchangers are widely used in thermal energy storage systems and a comprehensive and accurate analysis is necessary for their application and optimization. The fluid flow distribution between the plates is important to ensure even and full usage of the thermal energy storage potential. However, due to the complex topography of the plate surface, it would be computationally expensive to simulate the flow distribution in the multiple channels using a full three-dimensional model, so this work applies a reduced-dimensional model to significantly reduce the computational cost of the simulation and provides a comprehensive analysis of the effect of the internal structure on the internal flow distribution. The work extends a previously presented model to consider transient flow and a multichannel height distribution strategy to allow for simulating multiple channels between stacks of plates. Based on fully-developed flow assumptions, the three-dimensional model is reduced to a planar model, thus obtaining simulation results with satisfactory accuracy at a significantly lower computational cost. The model is verified by a three-dimensional simulation of a sliced two-channel model representing the considered system. The reduced-dimensional model gives similar results to the three-dimensional model for different geometrical and physical parameters. Lastly, the extended reduced-dimensional model is used to simulate the flow of a full two-channel model and the influence of the plate topography on the internal flow distribution is investigated through a comprehensive parametric analysis. The analysis shows that the complex topography of the plate surface eliminates the variation in inlet velocity and significantly changes the internal fluid flow, eventually resulting in a consistent velocity distribution.

]]>Fluids doi: 10.3390/fluids8060173

Authors: Arash Ghahraman Gyula Bene

Viscous linear surface waves are studied at arbitrary wavelength, layer thickness, viscosity, and surface tension. We find that in shallow enough fluids no surface waves can propagate. This layer thickness is determined for some fluids, water, glycerin, and mercury. Even in any thicker fluid layers, propagation of very short and very long waves is forbidden. When wave propagation is possible, only a single propagating mode exists for a given horizontal wave number. In contrast, there are two types of non-propagating modes. One kind of them exists at all wavelength and material parameters, and there are infinitely many such modes for a given wave number, distinguished by their decay rates. The other kind of non-propagating mode that is less attenuated may appear in zero, one, or two specimens. We notice the presence of two length scales as material parameters, one related to viscosity and the other to surface tension. We consider possible modes for a given material on the parameter plane layer thickness versus wave number and discuss bifurcations among different mode types. Motion of surface particles and time evolution of surface elevation is also studied at various parameters in glycerin, and a great variety of behaviour is found, including counterclockwise surface particle motion and negative group velocity in wave propagation.

]]>Fluids doi: 10.3390/fluids8060172

Authors: Denis Kuimov Maxim Minkin Alexandr Yurov Alexandr Lukyanov

Cavitation, as a unique technology for influencing liquid substances, has attracted much attention in the oil refining industry. The unique capabilities of cavitation impact can initiate the destruction of molecular compounds in the liquid. At the same time with a large number of successful experimental studies on the treatment of liquid hydrocarbon raw materials, cavitation has not been introduced in the oil refining industry. Often the impossibility of implementation is based on the lack of a unified methodology for assessing the intensity and threshold of cavitation creation. The lack of a unified methodology does not allow for predicting the intensity and threshold of cavitation generation in different fluids and cavitation-generating devices. In this review, the effect of cavitation on various rheological properties and fractional composition of liquid hydrocarbons is investigated in detail. The possibility of using the cavitation number as a single parameter for evaluating the intensity and threshold of cavitation generation is analyzed, and the limitations of its application are evaluated. The prospects of introducing the technology into the industry are discussed and a new vision of calculating the analog of cavitation numbers based on the analysis of the mutual influence of feedstock parameters and geometry of cavitators on each other is presented.

]]>Fluids doi: 10.3390/fluids8060171

Authors: Ian Adams Julian Simeonov Samuel Bateman Nathan Keane

We have developed and tested a numerical model for turbulence resolving simulations of dense mud&ndash;water mixtures in oscillatory bottom boundary layers, based on a low Stokes number formulation of the two-phase equations. The resulting non-Boussinesq equation for the fluid momentum is coupled to a transport equation for the mud volumetric concentration, giving rise to a volume-averaged fluid velocity that is non-solenoidal, and the model was implemented as a new compressible flow solver. An oscillating pressure gradient force was implemented in the correction step of the standard semi-implicit method for pressure linked equations (SIMPLE), for consistency with the treatment of other volume forces (e.g., gravity). The flow solver was further coupled to a new library for Bingham plastic materials, in order to model the rheological properties of dense mud mixtures using empirically determined concentration-dependent yield stress and viscosity. We present three direct numerical simulation tests to validate the new MudMixtureFoam solver against previous numerical solutions and experimental data. The first considered steady flow of Bingham plastic fluid with uniform concentration around a sphere, with Bingham numbers ranging from 1 to 100 and Reynolds numbers ranging from 0.1 to 100. The second considered the development of turbulence in oscillatory bottom boundary layer flow, and showed the formation of an intermittently turbulent layer with peak velocity perturbations exceeding 10 percent of the freestream flow velocity and occurring at a distance from the bottom comparable to the Stokes boundary layer thickness. The third considered the effects of density stratification due to resuspended sediment on turbulence in oscillatory bottom boundary layer flow with a bulk Richardson number of 1&times;10&minus;4 and a Stokes&ndash;Reynolds number of 1000, and showed the formation of a lutocline between 20 and 40 Stokes boundary layer depths. In all cases, the new solver produced excellent agreement with the previous results.

]]>Fluids doi: 10.3390/fluids8060170

Authors: Raed Alrdadi Michael H. Meylan

In this study, a numerical simulation of fluid flow through railway ballast in the time domain is presented, providing a model for unsteady-state flow. It is demonstrated that the position of the free surface with respect to time can also be used to solve the steady flow case. The effect of ballast fouling is included in the model to capture the realistic behavior of railway ballast, which is critical to understanding the impact of flooding. A thorough comparison with a range of previous studies, including theoretical and experimental approaches, is made, and very close agreement is obtained. The significant impact of ballast fouling on fluid flow and its potential consequences for railway infrastructure are highlighted by the simulation. Valuable insights into the behavior of water flow through porous media and its relevance to railway ballast management are offered by this study.

]]>Fluids doi: 10.3390/fluids8060169

Authors: Victor Bolobov Yana Martynenko Sergey Yurtaev

Reduction of energy expenditures required for various technological processes is a pressing issue in today&rsquo;s economy. One of the ways to solve this issue in regard to liquefied natural gas (LNG) storage is the recovery of its vapours from LNG tanks using an ejector system. In that respect, studies on the outflow of the real gas through the nozzle, the main element of the ejector, and identifying differences from the ideal gas outflow, are of high relevance. Particularly, this concerns the determination of the discharge coefficient &micro; as the ratio of the actual flowrate to the ideal one, taking into account the energy losses at gas outflow through the nozzle. The discharge coefficient values determined to date for various nozzle geometries are, as a rule, evaluated empirically and contradictory in some cases. The authors suggest determining the discharge coefficient by means of an experiment. This paper includes &micro; values determined using this method for the critical outflow of air to atmosphere through constrictor nozzles with different outlet diameters (0.003 m; 0.004 m; 0.005 m) in the pressure range at the nozzle inlet of 0.5&ndash;0.9 MPa. The obtained results may be used for the design of an ejector system for the recovery of the boil-off gas from LNG tanks, as well as in other fields of industry, for the design of technical and experimental devices with nozzles for various applications.

]]>Fluids doi: 10.3390/fluids8060168

Authors: Bulat Yakupov Ivan Smirnov

The acoustic cavitation of fluids, as well as related physical and chemical phenomena, causes a variety of effects that are highly important in technological processes and medicine. Therefore, it is important to be able to control the conditions that allow cavitation to begin and progress. However, the accurate prediction of acoustic cavitation is dependent on a complex relationship between external influence parameters and fluid characteristics. The multiparameter problem restricts the development of successful theoretical models. As a result, it is critical to identify the most important parameters influencing the onset of the cavitation process. In this paper, the ultrasonic frequency, hydrostatic pressure, temperature, degassing, density, viscosity, volume, and surface tension of a fluid were investigated using machine learning to determine their significance in predicting acoustic cavitation strength. Three machine learning models based on support vector regression (SVR), ridge regression (RR), and random forest (RF) algorithms with different input parameters were trained. The results showed that the SVM algorithm performed better than the other two algorithms. The parameters affecting the active cavitation nuclei, namely hydrostatic pressure, ultrasound frequency, and outgassing degree, were found to be the most important input parameters influencing the prediction of the cavitation threshold. Other parameters have a minor impact when compared to the first three, and their role can be compensated for by alternative variables. The further development of the obtained results provides a new way to optimize and improve existing theoretical models.

]]>Fluids doi: 10.3390/fluids8060167

Authors: Anastasia S. Vanina Alexander V. Sychev Anastasia I. Lavrova Pavel V. Gavrilov Polina L. Andropova Elena V. Grekhnyova Tatiana N. Kudryavtseva Eugene B. Postnikov

Studying transport processes in the brain&rsquo;s extracellular space is a complicated problem when considering the brain&rsquo;s tissue. Tests of corresponding physical and mathematical problems, as well as the need for materials with cheap but realistic properties to allow for testing of drug delivery systems, lead to the development of artificial phantom media, one kind of which is explored in this work. We report results from quantifying the spread of a standard contrast agent used in clinical computed tomography, Iopromide, in samples of collagen-based hydrogels. Its pure variant as well as samples supplied with lipid and surfactant additives were explored. By comparing to solutions of the diffusion equation which reproduce these data, the respective diffusion coefficients were determined. It was shown that they are relevant to the range typical for living tissue, grow with elevation in the lipid content and diminish with growth in surfactant concentration.

]]>Fluids doi: 10.3390/fluids8060166

Authors: Sergey E. Yakush Yuli D. Chashechkin Andrey Y. Ilinykh Vladislav A. Usanov

The impingement of a short-duration water jet on a pool of molten Rose&rsquo;s metal is studied experimentally herein. Short-duration water jet impacting on the free surface of a molten metal pool with a temperature of 300 &deg;C are generated with a pneumatic water delivery system, with two-camera high-speed video registration. A total of 14 experimental series, each containing 5 repeated tests, are performed for a water volume of 0.2&ndash;1 mL and a jet impact velocity of 4.1&ndash;9.0 m/s. The cavity development in the melt layer is studied, with the main stages described herein. Despite the significantly higher density of melt in comparison with water, the cavity can reach the melt pool bottom; furthermore, its further collapse results in the formation of a central jet rising to the height of a few centimeters. The maximum height of the central jet is shown to depend linearly on the total momentum of the water jet, and a semi-logarithmic correlation is found for the maximum diameter of the cavity. Repeatability analysis is performed within each experimental series, and the relative standard deviation for the melt splash height is shown to be from 8.8% to 26.8%. The effects of the pool depth, the vessel shape, and the water temperature are weaker in the range of the experimental parameters used here.

]]>Fluids doi: 10.3390/fluids8060165

Authors: Victor L. Mironov Sergey V. Mironov

We present the theoretical description of plane Couette flow based on the previously proposed equations of vortex fluid, which take into account both the longitudinal flow and the vortex tubes rotation. It is shown that the considered equations have several stationary solutions describing different types of laminar flow. We also discuss the simple model of turbulent flow consisting of vortex tubes, which are moving chaotically and simultaneously rotating with different phases. Using the Boussinesq approximation, we obtain an analytical expression for the stationary profile of mean velocity in turbulent Couette flow, which is in good agreement with experimental data and results of direct numerical simulations. Our model demonstrates that near-wall turbulence can be described by a coordinates-independent coefficient of eddy viscosity. In contrast to the viscosity of the fluid itself, this parameter characterizes the turbulent flow and depends on Reynolds number and roughness of the channel walls. Potentially, the proposed model can be considered as a theoretical basis for the experimental measurement of the eddy viscosity coefficient.

]]>Fluids doi: 10.3390/fluids8060164

Authors: Mouh Assoul Abdelouahab El jaouahiry Jamila Bouchgl Mourad Echchadli Saïd Aniss

We investigate the effect of horizontal quasi-periodic oscillation on the stability of two superimposed immiscible fluid layers confined in a horizontal Hele-Shaw cell. To approximate real oscillations, a quasi-periodic oscillation with two incommensurate frequencies is considered. Thus, the linear stability analysis leads to a quasi-periodic oscillator, with damping, which describes the evolution of the amplitude of the interface. Two types of quasi-periodic instabilities occur: the low-wavenumber Kelvin-Helmholtz instability and the large-wavenumber resonances. We mainly show that, for equal amplitudes of the superimposed accelerations, and for a low irrational frequency ratio, there is competition between several resonance modes allowing a very large selection of the wavenumber from lower to higher values. This is a way to control the sizes of the waves. Furthermore, increasing the frequency ratio has a stabilizing effect for both types of instability whose thresholds are found to correspond to quasi-periodic solutions using the frequency spectrum. For a ratio of the two superimposed displacement amplitudes equal to unity and less than unity, the number of resonances and competition between their modes also become significant for the intermediate values of the ratio of frequencies. The effects of other physical and geometrical parameters, such as the damping coefficient, density ratio, and heights of the two fluid layers, are also examined.

]]>Fluids doi: 10.3390/fluids8060163

Authors: Stoyan Nedeltchev

This article focuses on the prediction of the small bubble holdups (assuming the existence of two major bubble classes) in two bubble columns (0.289 m in ID and 0.102 m in ID), operated with organic liquids under various conditions (including high temperature and pressure). A new correction factor has been established in the existing model for the prediction of the gas holdups in the homogeneous regime. The correction parameter is a single function of the E&ouml;tv&ouml;s number (gravitational forces to surface tension forces), which characterizes the bubble shape. In addition, the behavior of small bubble holdups in 1-butanol (selected as a frequently researched alcohol) aerated with nitrogen, in a smaller BC (0.102 m in ID), at various operating pressures, is presented and discussed. The ratio of small bubble holdup to overall gas holdup, as a function of superficial gas velocity and operating pressure, has been investigated. All small bubble holdups in this work have been measured by means of the dynamic gas disengagement technique.

]]>Fluids doi: 10.3390/fluids8060162

Authors: Yuxing Lin Ebrahim Kadivar Ould el Moctar

In this work, we experimentally investigated the cavitation effects on the hydrodynamic behavior of a circular cylinder at different cavitating flows. We analyzed the cavitation dynamics behind the circular cylinder using a high-speed camera and also measured the associated hydrodynamic forces on the circular cylinder using a load cell. We studied the cavitation dynamics around the cylinder at various types of the cavitating regimes such as cloud cavitation, partial cavitation and cavitation inception. In addition, we analyzed the cavitation dynamics at three different Reynolds numbers: 1 &times; 105, 1.25 &times; 105 and 1.5 &times; 105. The results showed that the hydrodynamics force on the circular cylinder can be increased with the formation of the cavitation behind the cylinder compared with the cylinder at cavitation inception regime. The three-dimensional flow caused complex cavitation behavior behind the cylinder and a strong interaction between vortex structures and cavity shedding mechanism. In addition, the results revealed that the effects of the Reynolds number on the cavitation dynamics and amplitude of the shedding frequency is significant. However the effects of the cavitation number on the enhancement of the amplitude of the shedding frequency in the cavitating flow with a constant velocity is slightly higher than the effects of Reynolds number on the enhancement of the amplitude of the shedding frequency at a constant cavitation number.

]]>Fluids doi: 10.3390/fluids8050161

Authors: Jiangbo Wu Yao Lv Yongqing He Xiaoze Du Jie Liu Wenyu Zhang

Erythrocyte enrichment is needed for blood disease diagnosis and research. DLD arrays with an I-shaped pillar (I-pillar) sort erythrocytes in a unique, accurate, and low-reagent method. However, the existing I-shaped pillar DLD arrays for erythrocyte sorting have the drawbacks of higher flow resistance and more challenging fabrication. A two-dimensional erythrocyte simulation model and the arbitrary Lagrangian&ndash;Euler equations at the cell&ndash;fluid boundary were built based on the fluid&ndash;solid coupling method to investigate the influencing factors of the erythrocyte flow path in an I-pillar DLD array and find its optimization method. Three different sizes of I-pillars were built and multiple sets of corresponding arrays were constructed, followed by finite element simulations to separately investigate the effects of these arrays on the induction of erythrocyte motion paths. This work demonstrates the motion paths of erythrocyte models in a series of I-pillar arrays with different design parameters, aiming to summarize the variation modes of erythrocyte motion paths, which in turn provides some reference for designing and optimizing the pillar size and array arrangement methods for I-pillar array DLD chips.

]]>Fluids doi: 10.3390/fluids8050159

Authors: Abdulaziz Al Baraikan Krzysztof Czechowicz Paul D. Morris Ian Halliday Rebecca C. Gosling Julian P. Gunn Andrew J. Narracott Gareth Williams Pankaj Garg Maciej Malawski Frans van de Vosse Angela Lungu Dan Rafiroiu David Rodney Hose

Acting upon clinical patient data, acquired in the pathway of percutaneous intervention, we deploy hierarchical, multi-stage, data-handling protocols and interacting low- and high-order mathematical models (chamber elastance, state-space system and CFD models), to establish and then validate a framework to quantify the burden of ischaemia. Our core tool is a compartmental, zero-dimensional model of the coupled circulation with four heart chambers, systemic and pulmonary circulations and an optimally adapted windkessel model of the coronary arteries that reflects the diastolic dominance of coronary flow. We guide the parallel development of protocols and models by appealing to foundational physiological principles of cardiac energetics and a parameterisation (stenotic Bernoulli resistance and micro-vascular resistance) of patients&rsquo; coronary flow. We validate our process first with results which substantiate our protocols and, second, we demonstrate good correspondence between model operation and patient data. We conclude that our core model is capable of representing (patho)physiological states and discuss how it can potentially be deployed, on clinical data, to provide a quantitative assessment of the impact, on the individual, of coronary artery disease.

]]>Fluids doi: 10.3390/fluids8050160

Authors: Manal Alotaibi Shoug Alotaibi Ruud Weijermars

Gaussian solutions of the diffusion equation can be applied to visualize the flow paths in subsurface reservoirs due to the spatial advance of the pressure gradient caused by engineering interventions (vertical wells, horizontal wells) in subsurface reservoirs for the extraction of natural resources (e.g., water, oil, gas, and geothermal fluids). Having solved the temporal and spatial changes in the pressure field caused by the lowered pressure of a well&rsquo;s production system, the Gaussian method is extended and applied to compute and visualize velocity magnitude contours, streamlines, and other relevant flow attributes in the vicinity of well systems that are depleting the pressure in a reservoir. We derive stream function and potential function solutions that allow instantaneous modeling of flow paths and pressure contour solutions for transient flows. Such analytical solutions for transient flows have not been derived before without time-stepping. The new closed-form solutions avoid the computational complexity of time-stepping, required when time-dependent flows are modeled by superposing steady-state solutions using complex analysis methods.

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