Journal Description

Fluids — Open Access Journal

Fluids (ISSN 2311-5521; CODEN: FLUICM) is an international peer-reviewed open access journal on all aspects of fluids. It is published quarterly online by MDPI. The Portuguese Society of Rheology (SPR) is affiliated with Fluids and the society members receive a discount on the article processing charges.

Open Access—free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: Indexed in the Emerging Sources Citation Index (ESCI) - Web of Science and Inspec (IET) from Vol. 2. Indexed in Scopus from Vol. 3.
Rapid Publication: manuscripts are peer-reviewed and a first decision provided to authors approximately 19.6 days after submission; acceptance to publication is undertaken in 3.8 days (median values for papers published in this journal in the first half of 2020).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.

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Latest Articles

Open AccessArticle

Hydrodynamic Responses of a 6 MW Spar-Type Floating Offshore Wind Turbine in Regular Waves and Uniform Current

Yongsheng Zhao and

Fluids 2020, 5(4), 187; https://doi.org/10.3390/fluids5040187 (registering DOI) - 21 Oct 2020

Abstract

In order to improve the understanding of hydrodynamic performances of spar-type Floating Offshore Wind Turbines (FOWTs), in particular the effect of wave-current-structure interaction, a moored 6MW spar-type FOWT in regular waves and uniform current is considered. The wind loads are not considered at this stage. We apply the potential-flow theory and perturbation method to solve the weakly-nonlinear problem up to the second order. Unlike the conventional formulations in the inertial frame of reference, which involve higher derivatives on the body surface, the present method based on the perturbation method in the non-inertial body-fixed coordinate system can potentially avoid theoretical inconsistency at sharp edges and associated numerical difficulties. A cubic Boundary Element Method (BEM) is employed to solve the resulting boundary-value problems (BVPs) in the time domain. The convective terms in the free-surface conditions are dealt with using a newly developed conditionally stable explicit scheme, which is an approximation of the implicit Crank–Nicolson scheme. The numerical model is firstly verified against three reference cases, where benchmark results are available, showing excellent agreement. Numerical results are also compared with a recent model test, with a fairly good agreement though differences are witnessed. Drag loads based on Morison’s equation and relative velocities are also applied to quantify the influence of the viscous loads. To account for nonlinear restoring forces from the mooring system, a catenary line model is implemented and coupled with the time-domain hydrodynamic solver. For the considered spar-type FOWT in regular-wave and current conditions, the current has non-negligible effects on the motions at low frequencies, and a strong influence on the mean wave-drift forces. The second-order sum-frequency responses are found to be negligibly small compared with their corresponding linear components. The viscous drag loads do not show a strong influence on the motions responses, while their contribution to the wave-drift forces being notable, which increases with increasing wave steepness. Full article

(This article belongs to the Special Issue Wind and Wave Renewable Energy Systems)

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Open AccessArticle

Analysis of Cold Air Recirculation in the Evaporators of Large-Scale Air-Source Heat Pumps Using CFD Simulations

Brice Rogié,

Jonas Kjær Jensen,

Svenn Ole Kjøller Hansen and

Wiebke Brix Markussen

Fluids 2020, 5(4), 186; https://doi.org/10.3390/fluids5040186 - 21 Oct 2020

Abstract

The present study investigates cold air recirculation in the evaporators of large-scale air-source heat pumps. A case study considered a 5 MW air-source heat pump producing heat for district heating. The heat pump comprises 20 horizontal evaporators, where each evaporator is equipped with eight fans. The evaporators were implemented in a CFD model, where the influence of the outdoor wind direction on the recirculation was investigated. Firstly, the air recirculation was analysed with no surrounding obstacles. Secondly, the surrounding building and the real ground topology was included in the CFD model, to analyse their influence on the air recirculation. The results show that recirculation occurs for all wind directions, due to the turbulent behaviour of the flow around the evaporators. The results also show that the presence of a building intensifies the recirculation when it is placed directly upstream of the evaporators due to the presence of vortices in the wake of the building. On the other hand, a ground depression helps to reduce the recirculation by having additional energy dissipation due to the sudden change in the ground direction. Full article

(This article belongs to the Special Issue Recent Numerical Advances in Fluid Mechanics, Volume II)

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Open AccessArticle

Numerical Study of Rotating Thermal Convection on a Hemisphere

Patrick Fischer,

Charles-Henri Bruneau and

Hamid Kellay

Fluids 2020, 5(4), 185; https://doi.org/10.3390/fluids5040185 - 20 Oct 2020

Abstract

Numerical simulations of rotating two-dimensional turbulent thermal convection on a hemisphere are presented in this paper. Previous experiments on a half soap bubble located on a heated plate have been used for studying thermal convection as well as the effects of rotation on a curved surface. Here, two different methods have been used to produce the rotation of the hemisphere: the classical rotation term added to the velocity equation, and a non-zero azimuthal velocity boundary condition. This latter method is more adapted to the soap bubble experiments. These two methods of forcing the rotation of the hemisphere induce different fluid dynamics. While the first method is classically used for describing rotating Rayleigh–Bénard convection experiments, the second method seems to be more adapted for describing rotating flows where a shear layer may be dominant. This is particularly the case where the fluid is not contained in a closed container and the rotation is imposed on only one side of it. Four different diagnostics have been used to compare the two methods: the Nusselt number, the effective computation of the convective heat flux, the velocity and temperature fluctuations root mean square (RMS) generation of vertically aligned vortex tubes (to evaluate the boundary layers) and the energy/enstrophy/temperature spectra/fluxes. We observe that the dynamics of the convective heat flux is strongly inhibited by high rotations for the two different forcing methods. Also, and contrary to classical three-dimensional rotating Rayleigh–Bénard convection experiments, almost no significant improvement of the convective heat flux has been observed when adding a rotation term in the velocity equation. However, moderate rotations induced by non-zero velocity boundary conditions induce a significant enhancement of the convective heat flux. This enhancement is closely related to the presence of a shear layer and to the thermal boundary layer just above the equator. Full article

(This article belongs to the Special Issue Thermal Flows)

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Open AccessArticle

Uncertainty Quantification of Trajectory Clustering Applied to Ocean Ensemble Forecasts

Guilherme S. Vieira,

Irina I. Rypina and

Michael R. Allshouse

Fluids 2020, 5(4), 184; https://doi.org/10.3390/fluids5040184 - 17 Oct 2020

Abstract

Partitioning ocean flows into regions dynamically distinct from their surroundings based on material transport can assist search-and-rescue planning by reducing the search domain. The spectral clustering method partitions the domain by identifying fluid particle trajectories that are similar. The partitioning validity depends on the accuracy of the ocean forecasting, which is subject to several sources of uncertainty: model initialization, limited knowledge of the physical processes, boundary conditions, and forcing terms. Instead of a single model output, multiple realizations are produced spanning a range of potential outcomes, and trajectory clustering is used to identify robust features and quantify the uncertainty of the ensemble-averaged results. First, ensemble statistics are used to investigate the cluster sensitivity to the spectral clustering method free-parameters and the forecast parameters for the analytic Bickley jet, a geostrophic flow model. Then, we analyze an operational coastal ocean ensemble forecast and compare the clustering results to drifter trajectories south of Martha’s Vineyard. This approach identifies regions of low uncertainty where drifters released within a cluster predominantly remain there throughout the window of analysis. Drifters released in regions of high uncertainty tend to either enter neighboring clusters or deviate from all predicted outcomes. Full article

(This article belongs to the Special Issue Lagrangian Transport in Geophysical Fluid Flows)

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Open AccessArticle

SISO Piezo Based Circuit Development for Active Structural Vibration Control

Maurizio Arena and

Massimo Viscardi

Fluids 2020, 5(4), 183; https://doi.org/10.3390/fluids5040183 - 16 Oct 2020

Abstract

This paper deals with the issue of developing a smart vibration control platform following an innovative model-based approach. As a matter of fact, obtaining accurate information on system response in pre-design and design phases may reduce both computational and experimental efforts. From this perspective, a multi-degree-of-freedom (MDOF) electro-mechanical coupled system has been numerically schematized implementing a finite element formulation: a robust simulation tool integrating finite element model (FEM) features with Simulink^® capabilities has been developed. Piezo strain actuation has been modelled with a 2D finite element description: the effects exerted on the structure (converse effect) have been applied as lumped loads at the piezo nodes interface. The sensing (direct effect) has instead been modelled with a 2D piezoelectric constitutive equation and experimentally validated as well. The theoretical study led to the practical development of an integrated circuit which allowed for assessing the vibration control performance. The analysis of critical parameters, description of integrated numerical models, and a discussion of experimental results are addressed step by step to get a global overview of the engineering process. The single mode control has been experimentally validated for a simple benchmark like an aluminum cantilevered beam. The piezo sensor-actuator collocated couple has been placed according to an optimization process based on the maximum stored electrical energy. Finally, a good level of correlation has been observed between the forecasting model and the experimental application: the frequency analysis allowed for characterizing the piezo couple behavior even far from the resonance peak. Full article

(This article belongs to the Special Issue Models and Applications of Acoustic for Fluids)

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Open AccessArticle

On the Effect of Block Roughness in Ogee Spillways with Flip Buckets

Rasoul Daneshfaraz,

Amir Ghaderi,

Aliakbar Akhtari and

Silvia Di Francesco

Fluids 2020, 5(4), 182; https://doi.org/10.3390/fluids5040182 - 16 Oct 2020

Abstract

In this study, the effect of the presence of bed-block roughness in an ogee spillway on energy dissipation and jet length is investigated. A series of experimental and numerical tests were conducted using an ogee spillway with block roughness on the bed without a flip bucket and with a flip bucket at different take-off angles (32 °C and 52 °C). To model the free-flow surface, the volume-of-fluid (VOF) method and turbulence model from RNG k–ε were used. Results indicated that the numerical model is fairly capable of simulating a free-flow surface over an ogee spillway; using block roughness on the spillway chute without a bucket, relative energy dissipation increased by 15.4% compared to that in the spillway with a smooth bed, while for the spillway with 32 °C and 52 °C buckets, it increased by 9.5%. The jet length for a spillway with a flip bucket and roughened bed decreased by 8% to 58% compared to that in a smooth bed. Lastly, the relationships for the estimation of relative energy dissipation and jet length are presented. Full article

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Open AccessArticle

Cross-Shelf Transport Through the Interaction among a Coastal Jet, a Topographic Wave, and Tides

Helga S. Huntley,

Charles W. McMahon,

Joseph J. Kuehl and

A. D. Kirwan, Jr.

Fluids 2020, 5(4), 181; https://doi.org/10.3390/fluids5040181 - 16 Oct 2020

Abstract

Shelf break flows are often characterized by along-isobath jets with cross-shelf currents associated with tides and waves guided by variable topography. Here, we address the question: Can a superposition of such flows produce significant aperiodic cross-shelf transport? To answer this question, we use a barotropic analytic model for the jet based on a similarity solution of the shallow water equations over variable topography, a wave disturbance determined by the topography, and a diurnal tidal disturbance. We use standard Lagrangian methods to assess the cross-shelf transport, presenting the results, however, in a Eulerian frame, so as to be amenable to oceanographic observations. The relative roles of the different flow components in cross-shelf transport are assessed through an extensive parameter study. We find that a superposition of all three flow components can indeed produce consequential background aperiodic transport. An application of the model using recent observations from the Texas Shelf demonstrates that a combination of these background mechanisms can produce significant transport under realistic conditions. Full article

(This article belongs to the Special Issue Lagrangian Transport in Geophysical Fluid Flows)

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Open AccessArticle

Coating Flow Near Channel Exit. A Theoretical Perspective

Roger E. Khayat and

Mohammad Tanvir Hossain

Fluids 2020, 5(4), 180; https://doi.org/10.3390/fluids5040180 - 15 Oct 2020

Abstract

The planar flow of a steady moving-wall free-surface jet is examined theoretically for moderate inertia and surface tension. The method of matched asymptotic expansion and singular perturbation is used to explore the rich dynamics near the stress singularity. A thin-film approach is also proposed to capture the flow further downstream where the flow becomes of the boundary-layer type. We exploit the similarity character of the flow to circumvent the presence of the singularity. The study is of close relevance to slot and blade coating. The jet is found to always contract near the channel exit, but presents a mild expansion further downstream for a thick coating film. We predict that separation occurs upstream of the exit for slot coating, essentially for any coating thickness near the moving substrate, and for a thin film near the die. For capillary number of order one, the jet profile is not affected by surface tension but the normal stress along the free surface exhibits a maximum that strengthens with surface tension. In contrast to existing numerical findings, we predict the existence of upstream influence as indicated by the nonlinear pressure dependence on upstream distance and the pressure undershoot (overshoot) in blade (slot) coating at the exit. Full article

(This article belongs to the Special Issue Thermal Flows)

Open AccessArticle

Influence of Hydrodynamic Conditions on Micromixing in Microreactors with Free Impinging Jets

Rufat Sh Abiev and

Alexey A. Sirotkin

Fluids 2020, 5(4), 179; https://doi.org/10.3390/fluids5040179 - 13 Oct 2020

Abstract

An experimental study and mathematical modeling of micromixing in a microreactor with free impinging jets (MRFIJ) with a diameter of 1 mm was carried out. In the experimental part, the iodide-iodate technique was used (involving parallel competing Villermaux–Dushman reactions with the formation of I₃^–). Theoretical assessment revealed that more than 50% of the introduced energy is dissipated in the jets collision region. Through the use of differentiated sampling, an uneven quality distribution of micro mixing in the central and peripheral zones of the reactor was found: at moderate flow rates (700–1000 mL/min, jets velocity of 15–21 m/s) the micromixing in the central part of reactor is up to 12 times better than that in the periphery. Furthermore, the weight fraction of the probes in the central zones of MRFIJ is reduced with increasing jet velocity; this effect is attributed to a more intense formation of ligaments and droplets upon collision of jets and their secondary mixing on the walls of the apparatus. In terms of the weighted average concentration, the best quality of micromixing in the samples is achieved at a flow rate of 300 mL/min. With an increase in the flow rate (and velocity) of the jets, the dependence of the I₃^– concentration on the flow rate has a nonmonotonic character, which is explained by a change in the nature of the flow in the collision zone of the jets: the transition from the formation of a liquid sheet to the intensive formation of ligaments and drops and secondary mixing of the liquid film formed on the walls of the reactor. The effect of “freshness” of solutions on the concentration of reaction products was studied. Full article

(This article belongs to the Special Issue Recent Advances in Single and Multiphase Flows in Microchannels)

Open AccessArticle

The Influence of Different Unsteady Incident Flow Environments on Drag Measurements in an Open Jet Wind Tunnel

Jochen Wiedemann and

Fluids 2020, 5(4), 178; https://doi.org/10.3390/fluids5040178 - 13 Oct 2020

Abstract

Aerodynamic development for road vehicles is usually carried out in a uniform steady-state flow environment, either in the wind tunnel or in Computational Fluid Dynamics (CFD) simulations. However, out on the road, the vehicle experiences unsteady flow with fluctuating angles of incidence

β

[...] Read more.

β

, caused by natural wind, roadside obstacles, or traffic. In order to simulate such flow fields, the Forschungsinstitut für Kraftfahrwesen und Fahrzeugmotoren Stuttgart (FKFS) swing^® system installed in the quarter scale model wind tunnel can create a variety of time-resolved signals with variable

β

. The static pressure gradient in the empty test section, as well as

c_{D}

values of the Society of Automotive Engineers (SAE) body and the DrivAer model, have been measured under these transient conditions. The

c_{D}

measurements have been corrected using the Two-Measurement Correction method in order to decouple the influence of the unsteady flow from that of the static pressure gradient. The investigation has determined that the static pressure gradient in the empty test section varies greatly with different excitation signals. Thus, it is imperative to apply a

c_{D}

correction for unsteady wind tunnel measurements. The corrected

c_{D}

values show that a higher signal amplitude, as in, signals with large

β

, lead to higher drag forces. The influence of the signal frequency on drag values varies depending on the vehicle geometry and needs to be investigated further in the future. Full article

(This article belongs to the Special Issue Aerodynamics and Aeroacoustics of Vehicles)

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Open AccessArticle

Effects of Cross Level Air Interaction within Multilevel Underground Carparks on Indoor Air Quality

Rafat Al-Waked,

Abdalrahman Yassin,

Abdallah Adwan and

Diala Bani Mostafa

Fluids 2020, 5(4), 177; https://doi.org/10.3390/fluids5040177 - 11 Oct 2020

Abstract

Ventilation for underground carparks is critical to indoor air quality (IAQ) due to carbon monoxide (CO) emissions from cars. The IAQ within a multi-level underground carpark of a shopping mall has been investigated using computational fluid dynamics (CFD) model based on ANSYS-FLUENT (18.1) software. The effects of car engines types, porosity of supply and exhaust air louvers and ventilation flow rates on IAQ were examined. A mesh sensitivity study was conducted and the CFD model was validated against the fully mixed mathematical formulations of IAQ with a maximum difference in values of 1.5 ppm and an error of 3.4%. The results showed that the ventilation system must be operated at ACH value of more than 2.7 in order to meet the required CO concentration of 50 ppm within the carpark and should be based on running cars within each level rather than the parking capacity of each level. Porosity of louvers affected air flow distribution between parking levels and led to higher dilution of CO. Therefore, modelling a multilevel underground carpark requires closer attention to cross level interaction across Ramps which could affect the CO concentration within a given level. Full article

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Open AccessArticle

The Fast Discrete Interaction Approximation Concept

Vladislav Polnikov

Fluids 2020, 5(4), 176; https://doi.org/10.3390/fluids5040176 - 10 Oct 2020

Abstract

Hasselmann and coauthors proposed the discrete interaction approximation (DIA) as the best tool replacing the nonlinear evolution term in a numerical wind–wave model. Much later, Polnikov and Farina radically improved the original DIA by means of location all the interacting four wave vectors, used in the DIA configuration, exactly at the nodes of the numerical frequency–angular grid. This provides a nearly two-times enhancement of the speed of numerical calculation for the nonlinear evolution term in a wind–wave model. For this reason, the proposed version of the DIA was called as the fast DIA (FDIA). In this paper, we demonstrate all details of the FDIA concept for several frequency–angular numerical grids of high-resolution with the aim of active implementation of the FDIA in modern versions of world-wide used wind–wave models. Full article

(This article belongs to the Special Issue Mathematical and Numerical Modeling of Water Waves)

Open AccessArticle

CFD Analysis of Turbulent Fibre Suspension Flow

Vijay Shankar,

Anton Lundberg,

Taraka Pamidi,

Lars-Olof Landström and

Örjan Johansson

Fluids 2020, 5(4), 175; https://doi.org/10.3390/fluids5040175 - 08 Oct 2020

Abstract

A new model for turbulent fibre suspension flow is proposed by introducing a model for the fibre orientation distribution function (ODF). The coupling between suspended fibres and the fluid momentum is then introduced through the second and fourth order fibre orientation tensors, respectively. From the modelled ODF, a method to construct explicit expressions for the components of the orientation tensors as functions of the flow field is derived. The implementation of the method provides a fibre model that includes the anisotropic detail of the stresses introduced due to presence of the fibres, while being significantly cheaper than solving the transport of the ODF and computing the orientation tensors from numerical integration in each iteration. The model was validated and trimmed using experimental data from flow over a backwards facing step. The model was then further validated with experimental data from a turbulent fibre suspension channel flow. Simulations were also carried out using a Bingham viscoplastic fluid model for comparison. The ODF model and the Bingham model performed reasonably well for the turbulent flow areas, and the latter model showed to be slightly better given the parameter settings tested in the present study. The ODF model may have good potential, but more rigorous study is needed to fully evaluate the model. Full article

(This article belongs to the Special Issue Fluid Mechanics of Suspensions and Emulsions)

Open AccessArticle

Heat-Dissipation Performance of Nanocomposite Phase-Change Materials in a Twin-Heat-Source System

Yanxin Li,

Jin Wang,

Li Yang and

Bengt Sundén

Fluids 2020, 5(4), 174; https://doi.org/10.3390/fluids5040174 - 07 Oct 2020

Abstract

In this paper, pure paraffin was mixed with CuO (high thermal conductivity) and Span-80 (as a dispersant). The CuO/paraffin nanocomposite phase-change materials (PCMs) were synthesized with mass fractions of 0.3%, 0.6%, and 1.2%, by a two-step method. Heat-transfer characteristics of the heat-pipe–PCMs module and effects of fan power and heating power on the performance of the cooling module in a twin-heat-source system were studied. For two heat sources under 10 W–10 W (heat source 1 with a power of 10 W and heat source 2 with a power of 10 W), the paraffin wax decreases the evaporator temperature by 14.4%, compared with cases without PCMs. Full article

(This article belongs to the Special Issue Thermophysical Properties of Nanofluids - From Measurement to Interpretation)

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Open AccessArticle

The Effect of the Vadasz Number on the Onset of Thermal Convection in Rotating Bidispersive Porous Media

Florinda Capone and

Roberta De Luca

Fluids 2020, 5(4), 173; https://doi.org/10.3390/fluids5040173 - 06 Oct 2020

Abstract

The onset of thermal convection in uniformly rotating bidispersive horizontal porous layer, uniformly heated from below, is analyzed. A generalized Darcy equation for the macro-phase is considered to take the Vadasz number into account. It is proved that the presence of the Vadasz [...] Read more.

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

Open AccessArticle

Experimental Studies on Thermophysical and Electrical Properties of Graphene–Transformer Oil Nanofluid

Charishma Almeida,

Sohan Paul,

Lazarus Godson Asirvatham,

Stephen Manova,

Rajesh Nimmagadda,

Jefferson Raja Bose and

Somchai Wongwises

Fluids 2020, 5(4), 172; https://doi.org/10.3390/fluids5040172 - 03 Oct 2020

Abstract

The thermophysical and electrical properties of graphene–transformer oil nanofluid at three weight percentage concentrations (0.01%, 0.03%, and 0.05%) were experimentally studied. Experiments conducted to find viscosity, surface tension, density, specific resistance, electrical conductivity, and dielectric dissipation at various temperatures ranging from 20 °C to 90 °C. It was noted that the nanofluid with 0.05% concentration showed an enhancement of 2.5% and 16.6% for density and viscosity, respectively, when compared to transformer oil. In addition, an average reduction in surface tension is noted to be 10.1% for the maximum concentration of nanofluid. Increase in heat load and concentration improves Brownian motion and decreases the cohesive force between these particles, which results in a reduction in surface tension and increases the heat-transfer rate compared to transformer oil. In addition, for the maximum concentration of nanoparticles, the electrical conductivity of nanofluid was observed to be 3.76 times higher than that of the transformer oil at 90 °C. The addition of nanoparticles in the transformer oil decreases the specific resistance and improves the electrical conductivity thereby enhancing the breakdown voltage. Moreover, the thermophysics responsible for the improvement in thermophysical and electrical properties are discussed clearly, which will be highly useful for the design of power transmission/distribution systems. Full article

(This article belongs to the Special Issue Thermophysical Properties of Nanofluids - From Measurement to Interpretation)

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Open AccessArticle

Condensation of an Azeotropic Mixture inside 2.5 mm ID Minitubes

Andrea Diani and

Luisa Rossetto

Fluids 2020, 5(4), 171; https://doi.org/10.3390/fluids5040171 - 02 Oct 2020

Abstract

The ongoing miniaturization of air conditioning and refrigeration systems, in order to limit, as much as possible, the refrigerant charge, calls for smaller and smaller heat exchangers. Besides, the new environmental regulations are calling for new pure refrigerants or refrigerants mixtures with lower values of global warming potentials (GWPs). In this context, this paper analyzes the possible implementation of minitubes during condensation of the azeotropic mixture R513A. Two minitubes are tested: a smooth tube with an inner diameter of 2.5 mm, and a microfin tube with an inner diameter at the fin tip of 2.4 mm. The effects of vapor quality (varied in the range 0.10–0.99), of mass velocity (varied in the range 200–1000 kg m⁻² s⁻¹), and of saturation temperature (30 °C and 40 °C) on the heat transfer coefficient are investigated. The experimental results indicate that the heat transfer coefficient increases as both vapor quality and mass velocity increase, both in the case of the smooth tube and of the microfin tube, but the slope of the heat transfer coefficient trend respect to vapor quality is higher in the case of the microfin tube. The microfin tube shows, on average, heat transfer coefficients are 79% higher than those of the smooth tube under the same working conditions. Since R513A is a possible substitute of R134a, some experimental data during condensation heat transfer are also compared against those for R134a. Finally, the experimental results are compared against values estimated by empirical correlations available in the open literature. Full article

(This article belongs to the Special Issue Recent Advances in Single and Multiphase Flows in Microchannels)

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Open AccessArticle

The Influence of Inflow Swirl on Cavitating and Mixing Processes in a Venturi Tube

Hongbo Shi and

Petr Nikrityuk

Fluids 2020, 5(4), 170; https://doi.org/10.3390/fluids5040170 - 30 Sep 2020

Abstract

A study of the mixing flows (Schmidt number = 10

^{3}

A study of the mixing flows (Schmidt number = 10

^{3}

) in a cavitating Venturi tube that feature linear and swirling flows is presented in this paper. The Large Eddy Simulation (LES) turbulence model, the Schnerr–Sauer cavitation model, and the mixture multiphase model, as implemented in the commercial CFD ANSYS FLUENT 16.2, were employed. The main emphasis is spending on the influence of different inlet swirling ratios on the generation of cavitation and mixing behaviors in a Venturi tube. Four different inflow regimes were investigated for the Reynolds number Re = 19,044, 19,250, 19,622, 21,276: zero swirl, 15% swirl, 25% swirl and 50% swirl velocity relative to the transverse inflow velocity, respectively. The computed velocity and pressure profiles were shown in good agreement with the experiment data from the literature. The predicted results indicate that the imposed swirl flow moves the cavitation bubbles away from throat surfaces toward the throat axis. The rapid mixing between two volumetric components is promoted in the divergent section when the intense swirl is introduced. Additionally, the increase in the swirl ratio from 0.15 to 0.5 leads to a linear increase in the static pressure drop and a nonlinear increase in the vapor production. The reduction in the fluid viscosity ratio from

\frac{μ_{2}}{μ_{1}} = 10

\frac{μ_{2}}{μ_{1}} = 1

generates a high cavitation intensity in the throat of the Venturi tube. However, the changes in the pressure drop and vapor volume fraction are significantly small of pure water flow. Full article

(This article belongs to the Special Issue Cavitating Flows)

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Open AccessArticle

Rheological Properties and Flow Behaviour of Cement-Based Materials Modified by Carbon Nanotubes and Plasticising Admixtures

Gintautas Skripkiunas,

Ekaterina Karpova,

Joana Bendoraitiene and

Irmantas Barauskas

Fluids 2020, 5(4), 169; https://doi.org/10.3390/fluids5040169 - 29 Sep 2020

Abstract

In this study, the rheological properties of cement paste modified by a suspension containing both multi-walled carbon nanotubes (MWCNT) and carboxymethyl cellulose (CMC) (MWCNT/CMC suspension) with different types of plasticising admixtures (Pl), such as lignosulphonate (LS), sulfonated naphthalene formaldehyde condensate (NF), and polycarboxylate ether (PCE) were evaluated. The increase in yield stress and plastic viscosity up to 20% was established in the case of the modification of cement-based mixtures by MWCNT in the dosage up to 0.24% by weight of cement (bwoc) without Pl and with LS and NF. The complex modification of cement paste by MWCNT and PCE increases the yield stress and plastic viscosity from the MWCNT dosage of 0.06% and 0.015% bwoc, respectively. The yield stress and plastic viscosity of cement paste with PCE enhanced by 265% and 107%, respectively, in a MWCNT dosage of 0.12% bwoc. MWCNT do not have a significant influence on the flow behaviour index of cement paste; however, in the case of usage of PCE, the shear thickening effect decreased from a MWCNT dosage of 0.03% bwoc. The significant reduction in the volume coefficient of water bleeding by 99, 100, and 83% was obtained with LS, NF, and PCE, respectively, with an increase in MWCNT dosage up to 0.24% bwoc. Full article

(This article belongs to the Special Issue Flow and Heat Transfer in Non-linear Fluids)

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Open AccessArticle

Flow Pressure Behavior Downstream of Ski Jumps

Agostino Lauria,

Giancarlo Alfonsi and

Ali Tafarojnoruz

Fluids 2020, 5(4), 168; https://doi.org/10.3390/fluids5040168 - 29 Sep 2020

Abstract

Ski jump spillways are frequently implemented to dissipate energy from high-speed flows. The general feature of this structure is to transform the spillway flow into a free jet up to a location where the impact of the jet creates a plunge pool, representing an area for potential erosion phenomena. In the present investigation, several tests with different ski jump bucket angles are executed numerically by means of the OpenFOAM^® digital library, taking advantage of the Reynolds-averaged Navier–Stokes equations (RANS) approach. The results are compared to those obtained experimentally by other authors as related to the jet length and shape, obtaining physical insights into the jet characteristics. Particular attention is given to the maximum pressure head at the tailwater. Simple equations are proposed to predict the maximum dynamic pressure head acting on the tailwater, as dependent upon the Froude number, and the maximum pressure head on the bucket. Results of this study provide useful suggestions for the design of ski jump spillways in dam construction. Full article

(This article belongs to the Special Issue Recent Numerical Advances in Fluid Mechanics, Volume II)

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