Fluids

27 pages, 14405 KiB

Open AccessArticle

Features of Motion and Heat Transfer of Swirling Flows in Channels of Complex Geometry

by Sergey Dmitriev, Alexey Sobornov and Andrey Kurkin

Fluids 2024, 9(12), 293; https://doi.org/10.3390/fluids9120293 - 10 Dec 2024

The computational and experimental study results of swirling single-phase coolant motion and heat transfer for the standard operation parameters of a nuclear power plant are presented. The experimental model is a vertical heat exchanger of a “pipe in a pipe” type with the [...] Read more.

The computational and experimental study results of swirling single-phase coolant motion and heat transfer for the standard operation parameters of a nuclear power plant are presented. The experimental model is a vertical heat exchanger of a “pipe in a pipe” type with the countercurrent movement of coolants. Six different swirlers (three with a constant twist pitch and three with a variable pitch) were considered. The heat exchanger temperature field was measured at various combinations of coolant flow rates, and a channel pressure drop for each swirl was determined. Computational studies were performed using the Omega-based Reynolds stress model and SST model with a correction for curvature streamlines. A good agreement between numerical and experimental data was obtained. Based on the velocity and temperature fields, swirling flow motion features in channels with a variable swirl pitch were discovered. For each intensifier, the effectiveness criterion in comparison with a pipe channel was determined. Full article

(This article belongs to the Special Issue Modelling Flows in Pipes and Channels)

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19 pages, 10536 KiB

Open AccessFeature PaperArticle

Numerical Study of Laminar Unsteady Circular and Square Jets in Crossflow in the Low Velocity Ratio Regime

by Francisco C. Martins and José C. F. Pereira

Fluids 2024, 9(12), 292; https://doi.org/10.3390/fluids9120292 - 10 Dec 2024

Abstract

The unsteady three-dimensional flow interactions in the near field of square and circular jets issued normally to a crossflow were predicted by direct numerical simulations, aiming to investigate the effect of the nozzle cross-section on the vortical structures formed in this region. The [...] Read more.

The unsteady three-dimensional flow interactions in the near field of square and circular jets issued normally to a crossflow were predicted by direct numerical simulations, aiming to investigate the effect of the nozzle cross-section on the vortical structures formed in this region. The analysis focuses on jets in crossflow with moderate Reynolds numbers (

R e_{j} = 200

and

R e_{j} = 300

) based on the jet velocity the characteristic length of the nozzle and low jet-to-cross-flow velocity ratios,

0.25 \leq R \leq 1.4

, where the jets are absolutely unstable. In this regime, the flow becomes periodic and laminar, and three distinct wake flow configurations were identified: (1) symmetric shedding of hairpin vortices at

R e_{j} = 200

; (2) the formation of toroidal vortices as the legs of hairpin vortices merge and the vortices roll up at

R e_{j} = 300

and

R \leq 0.67

; (3) asymmetric shedding of hairpin vortices in the square jet at

R e_{j} = 300

and

R \geq 0.9

, where higher-frequency hairpin vortex shedding combines with a low-frequency spanwise oscillation in the counter-rotating vortex pair. The dynamics of each of these flow states were analyzed. Power spectral density plots show a measurable increase in the shedding frequencies in

R e_{j} = 300

jets with R, and that these frequencies are consistently larger in circular jets. Full article

(This article belongs to the Special Issue Flow Control Techniques: Advances in Flow System Analysis, Modeling and Applications)

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21 pages, 28976 KiB

Open AccessArticle

Interaction of the Shock Train Leading Edge and Filamentary Plasma in a Supersonic Duct

by Loren C. Hahn, Philip A. Lax, Scott C. Morris and Sergey B. Leonov

Fluids 2024, 9(12), 291; https://doi.org/10.3390/fluids9120291 - 7 Dec 2024

Abstract

Quasi-direct current (Q-DC) filamentary electrical discharges are used to control the shock train in a back-pressured Mach 2 duct flow. The coupled interaction between the plasma filaments and the shock train leading edge (STLE) is studied for a variety of boundary conditions. Electrical [...] Read more.

Quasi-direct current (Q-DC) filamentary electrical discharges are used to control the shock train in a back-pressured Mach 2 duct flow. The coupled interaction between the plasma filaments and the shock train leading edge (STLE) is studied for a variety of boundary conditions. Electrical parameters associated with the discharge are recorded during actuation, demonstrating a close correlation between the STLE position and dynamics. High-speed self-aligned focusing schlieren (SAFS) and high frame-rate color camera imaging are the primary optical diagnostics used to study the flowfield and plasma morphology. Shock tracking and plasma characterization algorithms are employed to extract time-resolved quantitative data during shock–plasma interactions. Four distinct shock–plasma interaction types are identified and outlined, revealing a strong dependence on the spacing between the uncontrolled STLE and discharge electrodes and a moderate dependence on flow parameters. Full article

(This article belongs to the Special Issue High Speed Flows, 2nd Edition)

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35 pages, 2694 KiB

Open AccessReview

Synthetic Jet Actuators for Active Flow Control: A Review

by Howard H. Ho, Ali Shirinzad, Ebenezer E. Essel and Pierre E. Sullivan

Fluids 2024, 9(12), 290; https://doi.org/10.3390/fluids9120290 - 6 Dec 2024

Abstract

A synthetic jet actuator (SJA) is a fluidic device often consisting of a vibrating diaphragm that alters the volume of a cavity to produce a synthesized jet through an orifice. The cyclic ingestion and expulsion of the working fluid leads to a zero-net [...] Read more.

A synthetic jet actuator (SJA) is a fluidic device often consisting of a vibrating diaphragm that alters the volume of a cavity to produce a synthesized jet through an orifice. The cyclic ingestion and expulsion of the working fluid leads to a zero-net mass-flux and the transfer of linear momentum to the working fluid over an actuation cycle, leaving a train of vortex structures propagating away from the orifice. SJAs are a promising technology for flow control applications due to their unique features, such as no external fluid supply or ducting requirements, short response time, low weight, and compactness. Hence, they have been the focus of many research studies over the past few decades. Despite these advantages, implementing an effective control scheme using SJAs is quite challenging due to the large parameter space involving several geometrical and operational variables. This article aims to explain the working mechanism of SJAs and provide a comprehensive review of the effects of SJA design parameters in quiescent conditions and cross-flow. Full article

(This article belongs to the Special Issue Flow Control Techniques: Advances in Flow System Analysis, Modeling and Applications)

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14 pages, 2453 KiB

Open AccessArticle

Simplified Approach to Evaluate Cavitation Intensity Based on Time Information on Imposed Pressure in Liquid

by Hiroyuki Kawashima, Hiroyuki Kogawa, Masatoshi Futakawa and Nobuatsu Tanaka

Fluids 2024, 9(12), 289; https://doi.org/10.3390/fluids9120289 - 6 Dec 2024

Abstract

Cavitation damage is an important research topic in fluid–structure interactions, such as those being studied using the mercury target for the pulsed neutron source at the Materials Life Science Experimental Facility/Japan Proton Accelerator Complex. Hence, the estimation of cavitation damage (cavitation intensity) is [...] Read more.

Cavitation damage is an important research topic in fluid–structure interactions, such as those being studied using the mercury target for the pulsed neutron source at the Materials Life Science Experimental Facility/Japan Proton Accelerator Complex. Hence, the estimation of cavitation damage (cavitation intensity) is required from the perspective of structural integrity. The results of previous studies suggest that the maximum radii of cavitation bubbles immediately prior to collapse are related to cavitation intensity. Therefore, we propose a method for estimating the maximum radius from the time information by measuring the vibrations of structure walls that are induced by collapsing cavitation bubbles in a confined liquid. In this study, we used a magnetic impact testing machine to experimentally investigate the cavitation bubble dynamics, directly observe the bubble collapsing behavior, and measure the induced vibration. We experimentally confirmed that the time information is useful in the estimation of the maximum radii of bubbles. Moreover, we theoretically derived a simple evaluation formula to estimate the maximum radius from the time responses of the imposed pressure in a confined liquid in a structure. Full article

(This article belongs to the Special Issue Cavitation and Bubble Dynamics)

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19 pages, 11272 KiB

Open AccessArticle

A Chamfered Anchor Impeller Design for Enhanced Efficiency in Agitating Viscoplastic Fluids

by Amine Benmoussa and José C. Páscoa

Fluids 2024, 9(12), 288; https://doi.org/10.3390/fluids9120288 - 5 Dec 2024

Abstract

In industrial mixing processes, impeller design, rotational speed, and mixing conditions play a crucial role in determining process efficiency, product quality, and energy consumption. Optimizing the performance of stirring systems for non-Newtonian fluids is essential for achieving better results. This study examines the [...] Read more.

In industrial mixing processes, impeller design, rotational speed, and mixing conditions play a crucial role in determining process efficiency, product quality, and energy consumption. Optimizing the performance of stirring systems for non-Newtonian fluids is essential for achieving better results. This study examines the hydrodynamic and thermal performance of stirring systems for viscoplastic fluids, utilizing close-clearance anchor impellers with chamfered angles of 22.5°, 45°, and 67.5° in cylindrical, flat-bottom and unbaffled vessels. Through a comprehensive comparative analysis between standard and chamfered impeller designs, the study evaluates their efficacy in overcoming yield stress, enhancing flow dynamics, and improving thermal homogeneity. The effects of Reynolds number and yield stress on the hydrodynamic and thermal states are analyzed. The results indicate that the 67.5° chamfered impeller significantly improves flow distribution and minimizes dead zones, particularly in critical areas between the anchor blades and vessel walls, where mixing stagnation typically occurs. It also enhances vertical mixing by promoting a broader shear spread along the vessel height and a more uniform temperature distribution. These insights contribute to the development of more efficient agitation systems, applicable across various industries handling complex fluids. Full article

(This article belongs to the Special Issue Industrial CFD and Fluid Modelling in Engineering, 2nd Edition)

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16 pages, 5224 KiB

Open AccessArticle

Large Eddy Simulation (LES) of Hydrogen Jet Flames and Finite Element Analysis of Thermal Barrier Coating

by Alon Davidy

Fluids 2024, 9(12), 287; https://doi.org/10.3390/fluids9120287 - 5 Dec 2024

Abstract

A jet flame occurs when the release of flammable gas or liquid ignites, resulting in a long, intense, and highly directional flame. This type of fire is commonly associated with industrial incidents involving pipelines, storage tanks, and other pressurized equipment. Jet fires are [...] Read more.

A jet flame occurs when the release of flammable gas or liquid ignites, resulting in a long, intense, and highly directional flame. This type of fire is commonly associated with industrial incidents involving pipelines, storage tanks, and other pressurized equipment. Jet fires are a significant concern in the oil and gas industry due to the handling and processing of large volumes of flammable hydrocarbons under pressure. The new computational method presented here includes several aspects of hydrogen jet flame accidents and their mitigation: the CFD simulation of a hydrogen jet flame using the HyRAM code and Fire Dynamics Simulator (FDS) software 5.0 using a large eddy simulation (LES) turbulence model; the calculation of the gaseous mixture’s thermo-physical properties using the GASEQ thermochemical code; the calculation of convective and radiative heat fluxes using empirical correlation; and a heat transfer simulation on the pipe thermal barrier coating (TBC) using COMSOL Multiphysics software 4.2a during the heating phase. This method developed for the ceramic blanket was validated successfully against the previous experimental results obtained by Gravit et al. It was shown that a jet fire’s maximum temperature obtained using FDS software was similar to that obtained using GASEQ thermochemical software 0.79 and HyRAM software. The TBC’s surface temperature reached 1945 °C. The stainless steel’s maximal temperature reached 165.5 °C. There was a slight decrease in UTS at this temperature. Full article

(This article belongs to the Special Issue Analytical and Computational Fluid Dynamics of Combustion and Fires)

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20 pages, 8620 KiB

Open AccessArticle

Evaporation Dynamics and Dosimetry Methods in Numerically Assessing MDI Performance in Pulmonary Drug Delivery

by Mohamed Talaat, Xiuhua Si and Jinxiang Xi

Fluids 2024, 9(12), 286; https://doi.org/10.3390/fluids9120286 - 5 Dec 2024

Abstract

Metered dose inhalers (MDIs) play a crucial role in managing respiratory diseases, but their effectiveness depends on whether the intended dose is delivered to the target, which can be influenced by various factors. Accurate assessment of MDI performance is crucial for optimizing MDI [...] Read more.

Metered dose inhalers (MDIs) play a crucial role in managing respiratory diseases, but their effectiveness depends on whether the intended dose is delivered to the target, which can be influenced by various factors. Accurate assessment of MDI performance is crucial for optimizing MDI delivery and ensuring drug efficacy. This study numerically examined the role of evaporation dynamics and dosimetry methods in assessing the efficiency of MDI delivery to different regions in a mouth–lung model extending to the eleventh generation (G11) of lung bifurcations. The experimentally determined spray exit speed, applied dose, and droplet size distribution were implemented as the initial/boundary conditions. Large eddy simulations (LES) were used to resolve the transient inhalation flows, and a chemical species model was applied to simulate vapor and temperature variations in the airflow. A multi-component model was used to consider the heat and mass transfer between the droplets and the airflow. The model was validated against literature data and applied to evaluate the impact of evaporation on pulmonary drug delivery using MDI, in comparison to inert particles. Three methods were used to quantify deposition, which were based on the droplet count, the droplet mass, and the drug carried by the droplets. The results demonstrate that evaporation notably alters the spray droplet size distribution and subsequent deposition patterns. Compared to inert particles, evaporation led to significantly more droplets ranging from 1–5 µm entering the pulmonary region. For a given region, large discrepancies were observed in the deposition fraction (DF) using different dosimetry methods. In the lower lung, the count-based DF (33.9%) and mass-based DF (2.4%) differed by more than one order of magnitude, while the drug-based DF fell between them (20.5%). This large difference highlights the need to include evaporation in predictive dosimetry, as well as to use the appropriate method to quantify the delivery efficiency of evaporating droplets. Full article

(This article belongs to the Special Issue Respiratory Flows)

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16 pages, 7953 KiB

Open AccessFeature PaperArticle

The Bottleneck in the Scalar Dissipation Rate Spectra: Dependence on the Schmidt Number

by Paolo Orlandi

Fluids 2024, 9(12), 285; https://doi.org/10.3390/fluids9120285 - 4 Dec 2024

Abstract

The mean dissipation rate of turbulent energy reaches a constant value at high Taylor–Reynolds numbers (

R_{λ}

). This value is associated with the well-scaling dissipation spectrum in Kolmogorov units, where the maximum corresponds to the bottleneck peak. Even the scalar dissipation [...] Read more.

The mean dissipation rate of turbulent energy reaches a constant value at high Taylor–Reynolds numbers (

R_{λ}

). This value is associated with the well-scaling dissipation spectrum in Kolmogorov units, where the maximum corresponds to the bottleneck peak. Even the scalar dissipation rate at the high

R_{λ}

considered in the present direct numerical simulations attains a constant value as

S c

increases. In this scenario, the maximum of the scalar dissipation spectra reaches its peak within the bottleneck, starting at

S c > 0.5

. A qualitative explanation for the formation of the two bottlenecks is related to the blockage of energy transfer from large to small scales in the inertial ranges. Within the bottleneck, the self-similar, ribbon-like structures transition into the rod-like structures characteristic of the exponential decay range. Investigating the viscous dependence of the bottleneck’s amplitude may be aided by examining the evolution of a passive scalar. As

S c

decreases, the scalar spectra undergo changes across the wave number k range. The bottleneck is dismantled, and at very low

S c

values, the spectrum tends towards Batchelor’s theoretical prediction, diminishing proportionally to

k^{- 17 / 3}

. To comprehend the flow structures responsible for the bottleneck, visualizations of

θ \nabla^{2} θ

and probability density functions at various

S c

values are presented and compared with those of

u_{i} \nabla^{2} u_{i}

. The numerical method employed for generating three-dimensional spectra and quantities such as energy and scalar variance dissipation in physical space must be accurate, particularly in resolving small scales. This paper additionally demonstrates that the second-order finite difference scheme conserving kinetic energy and scalar variance in the inviscid limit in viscous simulations accurately predicts the exponential decay range in one-dimensional and three-dimensional turbulent kinetic energy and scalar variance spectra. Full article

(This article belongs to the Special Issue Future Trends and Challenges in High Performance Computing for Turbulence)

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24 pages, 5576 KiB

Open AccessArticle

On the Numerical Investigation of Two-Phase Evaporative Spray Cooling Technology for Data Centre Applications

by Ning Gao, Syed Mughees Ali and Tim Persoons

Fluids 2024, 9(12), 284; https://doi.org/10.3390/fluids9120284 - 29 Nov 2024

Abstract

Two-phase evaporative spray cooling technology can significantly reduce power consumption in data centre cooling applications. However, the literature lacks an established methodology for assessing the overall performance of such evaporation systems in terms of the water-energy nexus. The current study develops a Lagrangian–Eulerian [...] Read more.

Two-phase evaporative spray cooling technology can significantly reduce power consumption in data centre cooling applications. However, the literature lacks an established methodology for assessing the overall performance of such evaporation systems in terms of the water-energy nexus. The current study develops a Lagrangian–Eulerian computational fluid dynamics (CFD) modelling approach to examine the functionality of these two-phase evaporative spray cooling systems. To replicate a modular system, a hollow spray cone nozzle with Rosin–Rammler droplet size distribution is simulated in a turbulent convective natural-air environment. The model was validated against the available experimental data from the literature. Parametric studies on geometric, flow, and climatic conditions, namely, domain length, droplet size, water mass flow rate, temperature, and humidity, were performed. The findings indicate that at elevated temperatures and low humidity, evaporation results in a bulk temperature reduction of up to 12 °C. A specific focus on the climatic conditions of Dublin, Ireland, was used as an example to optimize the evaporative system. A new formulation for the coefficient of performance (COP) is established to assess the performance of the system. Results showed that doubling the injector water mass flow rate improved the evaporated mass flow rate by 188% but reduced the evaporation percentage by 28%, thus reducing the COP. Doubling the domain length improved the temperature drop by 175% and increased the relative humidity by 160%, thus improving the COP. The COP of the evaporation system showed a systematic improvement with a reduction in the droplet size and the mass flow rate for a fixed domain length. The evaporated system COP improves by two orders of magnitude (~90 to 9500) with the reduction in spray Sauter mean diameter (SMD) from 292 μm to 8–15 μm. Under this reduction, close to 100% evaporation rate was achieved in comparison to only a 1% evaporation rate for the largest SMD. It was concluded that the utilization of a fine droplet spray nozzle provides an effective solution for the reduction in water consumption (97% in our case) for data centres, whilst concomitantly augmenting the proportion of evaporation. Full article

(This article belongs to the Special Issue Evaporation, Condensation and Heat Transfer)

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18 pages, 3782 KiB

Open AccessArticle

Active Displacement of a Unique Diatom–Ciliate Symbiotic Association

by Yonara Garcia, Felipe M. Neves, Flavio R. Rusch, Leandro T. De La Cruz, Marina E. Wosniack, J. Rudi Strickler, Marcos G. E. da Luz and Rubens M. Lopes

Fluids 2024, 9(12), 283; https://doi.org/10.3390/fluids9120283 - 29 Nov 2024

Abstract

Adaptive movement in response to individual interactions represents a fundamental evolutionary solution found by both unicellular organisms and metazoans to avoid predators, search for resources or conspecifics for mating, and engage in other collaborative endeavors. Displacement processes are known to affect interspecific relationships, [...] Read more.

Adaptive movement in response to individual interactions represents a fundamental evolutionary solution found by both unicellular organisms and metazoans to avoid predators, search for resources or conspecifics for mating, and engage in other collaborative endeavors. Displacement processes are known to affect interspecific relationships, especially when linked to foraging strategies. Various displacement phenomena occur in marine plankton, ranging from the large-scale diel vertical migration of zooplankton to microscale interactions around microalgal cells. Among these symbiotic interactions, collaboration between the centric diatom Chaetoceros coarctatus and the peritrich ciliate Vorticella oceanica is widely known and has been recorded in several studies. Here, using 2D and 3D tracking records, we describe the movement patterns of the non-motile, chain-forming diatoms (C. coarctatus) carried by epibiotic ciliates (V. oceanica). The reported data on the Chaetoceros–Vorticella association illustrated the consortium’s ability to generate distinct motility patterns. We established that the currents generated by the attached ciliates, along with the variability in the contraction and relaxation of ciliate stalks in response to food concentration, resulted in three types of trajectories for the consortium. The characteristics of these distinct paths were determined using robust statistical methods, indicating that the different displacement behaviors allowed the consortium to adequately explore distributed resources and remain within the food-rich layers provided in the experimental containers. A simple mechanical–stochastic model was successfully applied to simulate the observed displacement patterns, further supporting the proposed mechanisms of collective response to the environment. Full article

(This article belongs to the Special Issue Biological Fluid Dynamics, 2nd Edition)

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14 pages, 7159 KiB

Open AccessArticle

Experimental Investigation of Anisotropic Invariants in Streams with Rigid Vegetation and 3D Bedforms

by Kourosh Nosrati, Ali Rahm Rahimpour, Hossein Afzalimehr, Mohammad Nazari-Sharabian and Moses Karakouzian

Fluids 2024, 9(12), 282; https://doi.org/10.3390/fluids9120282 - 28 Nov 2024

Abstract

The presence of vegetation in submerged conditions and bedforms are a reality in coarse-bed streams. However, this reality has not been well investigated in the literature, despite being a major challenge for natural stream restoration. In order to control many unknown factors affecting [...] Read more.

The presence of vegetation in submerged conditions and bedforms are a reality in coarse-bed streams. However, this reality has not been well investigated in the literature, despite being a major challenge for natural stream restoration. In order to control many unknown factors affecting prototype scale, this experimental study has been conducted in a laboratory flume, considering 3D bedforms. The results of this study show that 3D bedforms with submerged vegetation elements may change all estimations from 3D to 2D forms near the bed due to the change in roughness. This will change the classic determinations of resistance to flow and sediment transport via Reynolds stress and turbulent flow and may lead to more-affordable complex hydraulic process modeling. Full article

(This article belongs to the Special Issue Environmental Hydraulics, Turbulence and Sediment Transport, 3rd Edition)

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27 pages, 2703 KiB

Open AccessReview

Indoor Air Quality Control for Airborne Diseases: A Review on Portable UV Air Purifiers

by Shriram Sankurantripati and Florent Duchaine

Fluids 2024, 9(12), 281; https://doi.org/10.3390/fluids9120281 - 26 Nov 2024

Abstract

The spread of airborne diseases such as COVID-19 underscores the need for effective indoor air quality control. This review focuses on ventilation strategies and portable air purifiers as key mitigation solutions. Ventilation systems, including natural and mechanical approaches, can reduce pathogen concentrations by [...] Read more.

The spread of airborne diseases such as COVID-19 underscores the need for effective indoor air quality control. This review focuses on ventilation strategies and portable air purifiers as key mitigation solutions. Ventilation systems, including natural and mechanical approaches, can reduce pathogen concentrations by improving airflow. However, combining ventilation with portable air purifiers, particularly those using HEPA filters, ESP filters, and UV-C radiation, can enhance Indoor air quality. While HEPA and ESP filters focus on trapping airborne particles, UV-C radiation can inactivate pathogens by disrupting their RNA. A review of UV air purifiers reveals a lack of studies on their efficacy and effectiveness in real-world settings. A thorough investigation into the performance of this mitigation solution is necessary, focusing on varying key factors, such as purifier placement, airflow dynamics, and UV dosage, to ensure optimal effectiveness. High-fidelity computational methods are essential in accurately assessing these factors, as informed by the physics of airborne transmission. Such advanced computations are necessary to determine the viability of portable UV air purifiers in mitigating airborne transmission in enclosed environments such as hospitals and public spaces. Integrating advanced air purification technologies with proper ventilation can improve safety in indoor environments and prevent future disease-related outbreaks. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2024)

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31 pages, 9275 KiB

Open AccessArticle

Hybrid CFD PINN FSI Simulation in Coronary Artery Trees

by Nursultan Alzhanov, Eddie Y. K. Ng and Yong Zhao

Fluids 2024, 9(12), 280; https://doi.org/10.3390/fluids9120280 - 25 Nov 2024

Abstract

This paper presents a novel hybrid approach that integrates computational fluid dynamics (CFD), physics-informed neural networks (PINN), and fluid–structure interaction (FSI) methods to simulate fluid flow in stenotic coronary artery trees and predict fractional flow reserve (FFR) in areas of stenosis. The primary [...] Read more.

This paper presents a novel hybrid approach that integrates computational fluid dynamics (CFD), physics-informed neural networks (PINN), and fluid–structure interaction (FSI) methods to simulate fluid flow in stenotic coronary artery trees and predict fractional flow reserve (FFR) in areas of stenosis. The primary objective is to utilize a 1D PINN model to accurately predict outlet flow conditions, effectively addressing the challenges of measuring or estimating these conditions within complex arterial networks. Validation against traditional CFD methods demonstrates strong accuracy while embedding physics-based training to ensure compliance with fundamental fluid dynamics principles. The findings indicate that the hybrid CFD PINN FSI method generates realistic outflow boundary conditions crucial for diagnosing stenosis, requiring minimal input data. By seamlessly integrating initial conditions established by the 1D PINN into FSI simulations, this approach enables precise assessments of blood flow dynamics and FFR values in stenotic regions. This innovative application of 1D PINN not only distinguishes this methodology from conventional data-driven models that rely heavily on extensive datasets but also highlights its potential to enhance our understanding of hemodynamics in pathological states. Ultimately, this research paves the way for significant advancements in non-invasive diagnostic techniques in cardiology, improving clinical decision making and patient outcomes. Full article

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35 pages, 2186 KiB

Open AccessArticle

Optimising Physics-Informed Neural Network Solvers for Turbulence Modelling: A Study on Solver Constraints Against a Data-Driven Approach

by William Fox, Bharath Sharma, Jianhua Chen, Marco Castellani and Daniel M. Espino

Fluids 2024, 9(12), 279; https://doi.org/10.3390/fluids9120279 - 25 Nov 2024

Abstract

Physics-informed neural networks (PINNs) have emerged as a promising approach for simulating nonlinear physical systems, particularly in the field of fluid dynamics and turbulence modelling. Traditional turbulence models often rely on simplifying assumptions or closed numerical models, which simplify the flow, leading to [...] Read more.

Physics-informed neural networks (PINNs) have emerged as a promising approach for simulating nonlinear physical systems, particularly in the field of fluid dynamics and turbulence modelling. Traditional turbulence models often rely on simplifying assumptions or closed numerical models, which simplify the flow, leading to inaccurate flow predictions or long solve times. This study examines solver constraints in a PINNs solver, aiming to generate an understanding of an optimal PINNs solver with reduced constraints compared with the numerically closed models used in traditional computational fluid dynamics (CFD). PINNs were implemented in a periodic hill flow case and compared with a simple data-driven approach to neural network modelling to show the limitations of a data-driven model on a small dataset (as is common in engineering design). A standard full equation PINNs model with predicted first-order stress terms was compared against reduced-boundary models and reduced-order models, with different levels of assumptions made about the flow to monitor the effect on the flow field predictions. The results in all cases showed good agreement against direct numerical simulation (DNS) data, with only boundary conditions provided for training as in numerical modelling. The efficacy of reduced-order models was shown using a continuity only model to accurately predict the flow fields within 0.147 and 2.6 percentage errors for streamwise and transverse velocities, respectively, and a modified mixing length model was used to show the effect of poor assumptions on the model, including poor convergence at the flow boundaries, despite a reduced solve time compared with a numerically closed equation set. The results agree with contemporary literature, indicating that physics-informed neural networks are a significant improvement in solve time compared with a data-driven approach, with a novel proposition of numerically derived unclosed equation sets being a good representation of a turbulent system. In conclusion, it is shown that numerically unclosed systems can be efficiently solved using reduced-order equation sets, potentially leading to a reduced compute requirement compared with traditional solver methods. Full article

(This article belongs to the Special Issue Future Trends and Challenges in High Performance Computing for Turbulence)

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22 pages, 11690 KiB

Open AccessReview

The Potential of Machine Learning Methods for Separated Turbulent Flow Simulations: Classical Versus Dynamic Methods

by Stefan Heinz

Fluids 2024, 9(12), 278; https://doi.org/10.3390/fluids9120278 - 25 Nov 2024

Abstract

Feasible and reliable predictions of separated turbulent flows are a requirement to successfully address the majority of aerospace and wind energy problems. Existing computational approaches such as large eddy simulation (LES) or Reynolds-averaged Navier–Stokes (RANS) methods have suffered for decades from well-known computational [...] Read more.

Feasible and reliable predictions of separated turbulent flows are a requirement to successfully address the majority of aerospace and wind energy problems. Existing computational approaches such as large eddy simulation (LES) or Reynolds-averaged Navier–Stokes (RANS) methods have suffered for decades from well-known computational cost and reliability issues in this regard. One very popular approach to dealing with these questions is the use of machine learning (ML) methods to enable improved RANS predictions. An alternative is the use of minimal error simulation methods (continuous eddy simulation (CES), which may be seen as a dynamic ML method) in the framework of partially or fully resolving simulation methods. Characteristic features of the two approaches are presented here by considering a variety of complex separated flow simulations. The conclusion is that minimal error CES methods perform clearly better than ML-RANS methods. Most importantly and in contrast to ML-RANS methods, CES is demonstrated to be well applicable to cases not involved in the model development. The reason for such superior CES performance is identified here: it is the ability of CES to properly account for causal relationships induced by the structure of separated turbulent flows. Full article

(This article belongs to the Special Issue Machine Learning and Artificial Intelligence in Fluid Mechanics)

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27 pages, 15508 KiB

Open AccessArticle

Impact of a Near-Surface Plasma Region on the Bow Shock Wave and Aerodynamic Characteristics of a High-Speed Model in Xenon

by Olga A. Azarova, Tatiana A. Lapushkina and Oleg V. Kravchenko

Fluids 2024, 9(12), 277; https://doi.org/10.3390/fluids9120277 - 23 Nov 2024

Abstract

The main objective of this study is to demonstrate the active influence on the location of the bow shock wave, as well as on the parameters of an aerodynamic body, of a gas discharge organized near the frontal surface, between the body and [...] Read more.

The main objective of this study is to demonstrate the active influence on the location of the bow shock wave, as well as on the parameters of an aerodynamic body, of a gas discharge organized near the frontal surface, between the body and the bow shock wave. The research is carried out using both experimental and numerical methods at the freestream Mach number M = 6.8. The working gas is xenon. It is shown that the location of the steady bow shock wave, along with the current and power of the discharge, is associated with the change in the adiabatic index of the plasma created by the discharge, which, in turn, is determined by plasma parameters such as the degrees of nonequilibrium and the degree of ionization. It is shown that the adiabatic index with the power supplied to the impact zone in the range of 30–120 kW can both increase and decrease in the range of 1.25–1.288. A study of the discharge-created plasma zone is conducted, and the correspondence between the gas discharge current and power and the average parameters in the plasma zone created by the discharge are presented. A good agreement between the numerical and experimental data is shown. The results obtained can be useful in the development of control systems for high-speed flows based not only on the effects of heating but also on the impact of plasma parameters. Full article

(This article belongs to the Special Issue High Speed Flows, 2nd Edition)

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14 pages, 2506 KiB

Open AccessArticle

Investigation of Nonlinear Relations Among Flow Profiles Using Artificial Neural Networks

by Shiming Yuan, Caixia Chen, Yong Yang and Yonghua Yan

Fluids 2024, 9(12), 276; https://doi.org/10.3390/fluids9120276 - 23 Nov 2024

Abstract

This study investigated the ability of artificial neural networks (ANNs) to resolve the nonlinear dynamics inherent in the behavior of complex fluid flows, which often exhibit multifaceted characteristics that challenge traditional analytical or numerical methods. By employing flow profile pairs that are generated [...] Read more.

This study investigated the ability of artificial neural networks (ANNs) to resolve the nonlinear dynamics inherent in the behavior of complex fluid flows, which often exhibit multifaceted characteristics that challenge traditional analytical or numerical methods. By employing flow profile pairs that are generated through high-fidelity numerical simulations, encompassing both the one-dimensional benchmark problems and the more intricate three-dimensional boundary layer transition problem, this research convincingly demonstrates that neural networks possess a remarkable capacity to effectively capture the discontinuities and the subtle wave characteristics that occur at small scales within complex fluid flows, thereby showcasing their robustness in handling intricate fluid dynamics phenomena. Furthermore, even in the context of challenging three-dimensional problems, this study reveals that the average velocity profiles can be predicted with a high degree of accuracy, utilizing a limited number of input profiles during the training phase, which underscores the efficiency and efficacy of the model in understanding complex systems. The findings of this study significantly underscore the immense potential that artificial neural networks, along with deep learning methodologies, hold in advancing our comprehension of the fundamental physics that govern complex fluid dynamics systems, while concurrently demonstrating their applicability across a variety of flow scenarios and their capacity to yield insightful revelations regarding the nonlinear relationships that exist among diverse flow parameters, thus paving the way for future research in this critical area of study. Full article

(This article belongs to the Special Issue Future Trends and Challenges in High Performance Computing for Turbulence)

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12 pages, 1555 KiB

Open AccessArticle

A Simple Mathematical Model to Predict the Pressure Drop for Transport of Deformable Particles in Homogeneous Porous Media

by Víctor Matías-Pérez, Simón López-Ramírez, Elizbeth Franco-Urresti and Carlos G. Aguilar-Madera

Fluids 2024, 9(12), 275; https://doi.org/10.3390/fluids9120275 - 22 Nov 2024

Abstract

The transport of deformable particles (TDPs) through porous media has been of considerable interest due to the multiple applications found in industrial and medical processes. The adequate design of these applications has been mainly achieved through experimental efforts, since TDPs through porous media [...] Read more.

The transport of deformable particles (TDPs) through porous media has been of considerable interest due to the multiple applications found in industrial and medical processes. The adequate design of these applications has been mainly achieved through experimental efforts, since TDPs through porous media are challenging to model because of the mechanical blockage of the pore throat due to size exclusion, deformation in order to pass through the pore throat under the driven pressure, and breakage under strong extrusion. In this work, based on the diffusivity equation and considering the TDP as a complex fluid whose viscosity and density depend on the local pressure, a simple but accurate theoretical model is proposed to describe the pressure behavior under steady- and unsteady-state flow conditions. Assuming a linear pressure dependence of the viscosity and density of the TDPs, valid for moderate pressure changes, the solution of the mathematical model yields a quantitative correlation between the pressure evolution and the parameters compressibility, viscosity coefficient, elastic modulus, particle size, and friction factor. The predictions of the model agree with experiments and allow the understanding of transport of deformable particles through a porous media. Full article

(This article belongs to the Special Issue Multiphase Flow for Industry Applications)

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