Journal Description
Fluids
Fluids
is an international, peer-reviewed, open access journal on all aspects of fluids, published monthly online by MDPI. The Portuguese Society of Rheology (SPR) is affiliated with Fluids and its members receive discounts on the article processing charges.
- Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
- High Visibility: indexed within Scopus, ESCI (Web of Science), Inspec, CAPlus / SciFinder, and other databases.
- Journal Rank: CiteScore - Q2 (Mechanical Engineering)
- Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 20.7 days after submission; acceptance to publication is undertaken in 3.6 days (median values for papers published in this journal in the second half of 2023).
- 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.
Impact Factor:
1.9 (2022);
5-Year Impact Factor:
1.8 (2022)
Latest Articles
Numerical Dissipation Control in High-Order Methods for Compressible Turbulence: Recent Development
Fluids 2024, 9(6), 127; https://doi.org/10.3390/fluids9060127 (registering DOI) - 29 May 2024
Abstract
This comprehensive overview presents our continued efforts in high-order finite difference method (FDM) development for adaptive numerical dissipation control in the long-time integration of direct numerical simulation (DNS), large eddy simulation (LES), and implicit LES (ILES) computations of compressible turbulence for gas dynamics
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This comprehensive overview presents our continued efforts in high-order finite difference method (FDM) development for adaptive numerical dissipation control in the long-time integration of direct numerical simulation (DNS), large eddy simulation (LES), and implicit LES (ILES) computations of compressible turbulence for gas dynamics and MHD. The focus is on turbulence with shock wave numerical simulations using the adaptive blending of high-order structure-preserving non-dissipative methods (classical central, Padé (compact), and dispersion relation-preserving (DRP)) with high-order shock-capturing methods in such a way that high-order shock-capturing methods are active only in the vicinity of shock/shear waves, and high-gradient and spurious high-frequency oscillation regions guided via flow sensors. Any efficient and high-resolution high-order shock-capturing methods are good candidates for the blending of methods procedure. Typically, the adaptive blending of more than one method falls under two camps: hybrid methods and nonlinear filter methods. They are applicable to unstructured finite volume, finite element, discontinuous Galerkin, and spectral element methods. This work represents the culmination of over 20 years of high-order FDM developments and hands-on experience by the authors and collaborators in adaptive numerical dissipation control using the “high order nonlinear filter approach”. Extensions of these FDM versions to curvilinear nonuniform, freestream-preserving moving grids and time-varying deforming grids were also developed. By examining the construction of these two approaches using the high-order multistage type of temporal discretization, the nonlinear filter approach is made more efficient and less CPU-intensive while obtaining similar accuracy. A representative variety of test cases that compare the various blending of high-order methods with standalone standard methods is illustrated. Due to the fact that our nonlinear filter methods are not well known in compressible turbulence with shock waves, the intent of this comprehensive overview is for general audiences who are not familiar with our nonlinear filter methods. For readers interested in the implementation of our methods into their computer code, it is hoped that the long overview will be helpful.
Full article
(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)
Open AccessArticle
Statistical Analysis of Bubble Parameters from a Model Bubble Column with and without Counter-Current Flow
by
P. Kováts and K. Zähringer
Fluids 2024, 9(6), 126; https://doi.org/10.3390/fluids9060126 - 28 May 2024
Abstract
Bubble columns are widely used in numerous industrial processes because of their advantages in operation, design, and maintenance compared to other multiphase reactor types. In contrast to their simple design, the generated flow conditions inside a bubble column reactor are quite complex, especially
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Bubble columns are widely used in numerous industrial processes because of their advantages in operation, design, and maintenance compared to other multiphase reactor types. In contrast to their simple design, the generated flow conditions inside a bubble column reactor are quite complex, especially in continuous mode with counter-current liquid flow. For the design and optimization of such reactors, precise numerical simulations and modelling are needed. These simulations and models have to be validated with experimental data. For this reason, experiments were carried out in a laboratory-scale bubble column using shadow imaging and particle image velocimetry (PIV) techniques with and without counter-current liquid flow. In the experiments, two types of gases—relatively poorly soluble air and well-soluble CO2—were used and the bubbles were generated with three different capillary diameters. With changing gas and liquid flow rates, overall, 108 different flow conditions were investigated. In addition to the liquid flow fields captured by PIV, shadow imaging data were also statistically evaluated in the measurement volume and bubble parameters such as bubble diameter, velocity, aspect ratio, bubble motion direction, and inclination. The bubble slip velocity was calculated from the measured liquid and bubble velocities. The analysis of these parameters shows that the counter-current liquid flow has a noticeable influence on the bubble parameters, especially on the bubble velocity and motion direction. In the case of CO2 bubbles, remarkable bubble shrinkage was observed with counter-current liquid flow due to the enhanced mass transfer. The results obtained for bubble aspect ratio are compared to known correlations from the literature. The comprehensive and extensive bubble data obtained in this study will now be used as a source for the development of correlations needed in the validation of numerical simulations and models. The data are available from the authors on request.
Full article
(This article belongs to the Special Issue Mass Transfer in Multiphase Reactors)
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Open AccessArticle
Effects of Partial Premixing and Coflow Temperature on Flame Stabilization of Lifted Jet Flames of Dimethyl Ether in a Vitiated Coflow Based on Stochastic Multiple Mapping Conditioning Approach
by
Sanjeev Kumar Ghai, Rajat Gupta and Santanu De
Fluids 2024, 9(6), 125; https://doi.org/10.3390/fluids9060125 - 26 May 2024
Abstract
The Reynolds-averaged Navier–Stokes (RANS)-based stochastic multiple mapping conditioning (MMC) approach has been used to study partially premixed jet flames of dimethyl ether (DME) introduced into a vitiated coflowing oxidizer stream. This study investigates DME flames with varying degrees of partial premixing within a
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The Reynolds-averaged Navier–Stokes (RANS)-based stochastic multiple mapping conditioning (MMC) approach has been used to study partially premixed jet flames of dimethyl ether (DME) introduced into a vitiated coflowing oxidizer stream. This study investigates DME flames with varying degrees of partial premixing within a fuel jet across different coflow temperatures, delving into the underlying flame structure and stabilization mechanisms. Employing a turbulence k- model with a customized set of constants, the MMC technique utilizes a mixture fraction as the primary scalar, mapped to the reference variable. Solving a set of ordinary differential equations for the evolution of Lagrangian stochastic particles’ position and composition, the molecular mixing of these particles is executed using the modified Curl’s model. The lift-off height (LOH) derived from RANS-MMC simulations are juxtaposed with experimental data for different degrees of partial premixing of fuel jets and various coflow temperatures. The RANS-MMC methodology adeptly captures LOH for pure DME jets but exhibits an underestimation of flame LOH for partially premixed jet scenarios. Notably, as the degree of premixing escalates, a conspicuous underprediction in LOH becomes apparent. Conditional scatter and contour plots of OH and CH2O unveil that the propagation of partially premixed flames emerges as the dominant mechanism at high coflow temperatures, while autoignition governs flame stabilization at lower coflow temperatures in partially premixed flames. Additionally, for pure DME flames, autoignition remains the primary flame stabilization mechanism across all coflow temperature conditions. The study underscores the importance of considering the degree of premixing in partially premixed jet flames, as it significantly impacts flame stabilization mechanisms and LOH, thereby providing crucial insights into combustion dynamics for various practical applications.
Full article
(This article belongs to the Special Issue Future Trends and Challenges in High Performance Computing for Turbulence)
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Open AccessArticle
Subgrid Turbulent Flux Models for Large Eddy Simulations of Diffusion Flames in Space Propulsion
by
Daniel Martinez-Sanchis, Andrej Sternin, Sagnik Banik, Oskar Haidn and Martin Tajmar
Fluids 2024, 9(6), 124; https://doi.org/10.3390/fluids9060124 - 26 May 2024
Abstract
Subgrid scale models for unresolved turbulent fluxes are investigated, with a focus on combustion for space propulsion applications. An extension to the gradient model is proposed, introducing a dependency on the local burning regimen. The dynamic behaviors of the model’s coefficients are investigated,
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Subgrid scale models for unresolved turbulent fluxes are investigated, with a focus on combustion for space propulsion applications. An extension to the gradient model is proposed, introducing a dependency on the local burning regimen. The dynamic behaviors of the model’s coefficients are investigated, and scaling laws are studied. The discussed models are validated using a DNS database of a high-pressure, turbulent, fuel-rich methane–oxygen diffusion flame. The operating point and turbulence characteristics are selected to resemble those of modern combustors for space propulsion applications to support the future usage of the devised model in this context.
Full article
(This article belongs to the Special Issue Turbulence and Combustion)
Open AccessArticle
Simulation on the Separation of Breast Cancer Cells within a Dual-Patterned End Microfluidic Device
by
Diganta Dutta, Xavier Palmer, Jung Yul Lim and Surabhi Chandra
Fluids 2024, 9(6), 123; https://doi.org/10.3390/fluids9060123 - 25 May 2024
Abstract
Microfluidic devices have long been useful for both the modeling and diagnostics of numerous diseases. In the past 20 years, they have been increasingly adopted for helping to study those in the family of breast cancer through characterizing breast cancer cells and advancing
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Microfluidic devices have long been useful for both the modeling and diagnostics of numerous diseases. In the past 20 years, they have been increasingly adopted for helping to study those in the family of breast cancer through characterizing breast cancer cells and advancing treatment research in portable and replicable formats. This paper adds to the body of work concerning cancer-focused microfluidics by proposing a simulation of a hypothetical bi-ended three-pronged device with a single channel and 16 electrodes with 8 pairs under different voltage and frequency regimes using COMSOL. Further, a study was conducted to examine the frequencies most effective for ACEO to separate cancer cells and accompanying particles. The study revealed that the frequency of EF has a more significant impact on the separation of particles than the inlet velocity. Inlet velocity variations while holding the frequency of EF constant resulted in a consistent trend showing a direct proportionality between inlet velocity and net velocity. These findings suggest that optimizing the frequency of EF could lead to more effective particle separation and targeted therapeutic interventions for breast cancer. This study hopefully will help to create targeted therapeutic interventions by bridging the disparity between in vitro and in vivo models.
Full article
(This article belongs to the Special Issue Physics and Applications of Microfluidics)
Open AccessArticle
N-Symmetric Interaction of N Hetons, II: Analysis of the Case of Arbitrary N
by
Konstantin V. Koshel, Mikhail A. Sokolovskiy, David G. Dritschel and Jean N. Reinaud
Fluids 2024, 9(6), 122; https://doi.org/10.3390/fluids9060122 - 24 May 2024
Abstract
This paper seeks and examines N-symmetric vortical solutions of the two-layer geostrophic model for the special case when the vortices (or eddies) have vanishing summed strength (circulation anomaly). This study is an extension [Sokolovskiy et al. Phys. Fluids 2020, 32, 09660], where
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This paper seeks and examines N-symmetric vortical solutions of the two-layer geostrophic model for the special case when the vortices (or eddies) have vanishing summed strength (circulation anomaly). This study is an extension [Sokolovskiy et al. Phys. Fluids 2020, 32, 09660], where the general formulation for arbitrary N was given, but the analysis was only carried out for . Here, families of stationary solutions are obtained and their properties, including asymptotic ones, are investigated in detail. From the point of view of geophysical applications, the results may help interpret the propagation of thermal anomalies in the oceans.
Full article
(This article belongs to the Collection Advances in Geophysical Fluid Dynamics)
Open AccessArticle
Novel Pour Point Depressants for Crude Oil Derived from Polyethylene Solution in Hexane and Coal Fly Ash
by
Kazim Nadirov, Manap Zhantasov, Tlek Ketegenov, Zhanna Nadirova, Aisulu Batkal, Kaster Kamunur, Gulmira Bimbetova and Rashid Nadirov
Fluids 2024, 9(6), 121; https://doi.org/10.3390/fluids9060121 - 23 May 2024
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Oil transportation becomes much more complicated due to the solidification of paraffins in them at low temperatures and the resulting increase in oil viscosity. To solve this problem, special additives as pour point depressants (PPDs) are used to prevent the agglomeration of paraffin
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Oil transportation becomes much more complicated due to the solidification of paraffins in them at low temperatures and the resulting increase in oil viscosity. To solve this problem, special additives as pour point depressants (PPDs) are used to prevent the agglomeration of paraffin crystals. In this work, 15 PPDs were obtained and tested, consisting of a solution of polyethylene in hexane and also, in some cases, from magnetic nanoparticles (MNPs) extracted from coal fly ash. The most effective result was observed with a mixture of 0.25% polyethylene in hexane and 2% MNPs, which managed to lower the oil’s pour point from 18 °C to −17 °C.
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Open AccessEditorial
Environmental Hydraulics, Turbulence, and Sediment Transport, Second Edition
by
Jaan H. Pu, Manish Pandey and Prashanth Reddy Hanmaiahgari
Fluids 2024, 9(6), 120; https://doi.org/10.3390/fluids9060120 - 22 May 2024
Abstract
Within river systems, the process of bed-forming is intricate, dynamic and is shaped by different factors [...]
Full article
(This article belongs to the Special Issue Environmental Hydraulics, Turbulence and Sediment Transport, 2nd edition)
Open AccessArticle
Circular Fluid Heating—Transient Entropy Generation
by
Fikret Alic
Fluids 2024, 9(5), 119; https://doi.org/10.3390/fluids9050119 - 18 May 2024
Abstract
A technical issue with fluid flow heating is the relatively small temperature increase as the fluid passes through the heating surface. The fluid does not spend enough time inside the heating source to significantly raise its temperature, despite the heating source itself experiencing
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A technical issue with fluid flow heating is the relatively small temperature increase as the fluid passes through the heating surface. The fluid does not spend enough time inside the heating source to significantly raise its temperature, despite the heating source itself experiencing a substantial increase. To address this challenge, the concept of the multiple circular heating of air was developed, forming the basis of this work. Two PTC heaters with longitudinal fins are located within a closed channel inside housing composed of a thermal insulation material. Air flows circularly from one finned surface to another. Analytical modeling and experimental testing were used in the analysis, with established restrictions and boundary conditions. An important outcome of the analysis was the methodology established for the optimization of the geometric and process parameters based on minimizing the transient thermal entropy. In conducting the analytical modeling, the temperature of the PTC heater was assumed to be constant at 150 °C and 200 °C. By removing the restrictions and adjusting the boundary conditions, the established methodology for the analysis and optimization of various thermally transient industrial processes can be applied more widely. The experimental determination of the transient thermal entropy was performed at a much higher air flow rate of 0.005 m3s−1 inside the closed channel. The minimum transient entropy also indicates the optimal time for the opening of the channel, allowing the heated air to exit. The novelty of this work lies in the controlled circular heating of the fluid and the establishment of the minimum transient thermal entropy as an optimization criterion.
Full article
(This article belongs to the Special Issue Evaporation, Condensation and Heat Transfer)
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Open AccessArticle
A Variational Surface-Evolution Approach to Optimal Transport over Transitioning Compact Supports with Domain Constraints
by
Anthony Yezzi
Fluids 2024, 9(5), 118; https://doi.org/10.3390/fluids9050118 - 16 May 2024
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We examine the optimal mass transport problem in between densities with transitioning compact support by considering the geometry of a continuous interpolating support boundary in space-time within which the mass density evolves according to the fluid dynamical framework of Benamou
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We examine the optimal mass transport problem in between densities with transitioning compact support by considering the geometry of a continuous interpolating support boundary in space-time within which the mass density evolves according to the fluid dynamical framework of Benamou and Brenier. We treat the geometry of this space-time embedding in terms of points, vectors, and sets in and blend the mass density and velocity as well into a space-time solenoidal vector field over a compact set . We then formulate a joint optimization for and its support using the shaped gradient of the space-time surface outlining the support boundary . This easily accommodates spatiotemporal constraints, including obstacles or mandatory regions to visit.
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Open AccessArticle
Physical and Numerical Experimentation of Water Droplet Collision on a Wall: A Comparison between PLIC and HRIC Schemes for the VOF Transport Equation with High-Speed Imaging
by
Bruno Silva de Lima, Martin Sommerfeld and Francisco José de Souza
Fluids 2024, 9(5), 117; https://doi.org/10.3390/fluids9050117 - 16 May 2024
Abstract
Liquid films are often found in engineering applications with thicknesses ranging from micrometer scales to large scales with a wide range of applications. To optimize such systems, researchers have dedicated themselves to the development of new techniques. To further contribute to this development,
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Liquid films are often found in engineering applications with thicknesses ranging from micrometer scales to large scales with a wide range of applications. To optimize such systems, researchers have dedicated themselves to the development of new techniques. To further contribute to this development, the objective of this work is to present the results of the collision of water droplets on a wall by means of experimentation and numerical simulations. For physical experimentation, an injector is used to generate a chain of water droplets that collide with the opposite wall, forming a liquid film. Images of the droplets were obtained using two high-speed recording cameras. The results for different droplet sizes and impact angles are presented and the relationship between the momentum parameter and non-dimensional pool size was established. Modeling such processes is a common challenge in engineering, with different techniques having their advantages and limitations. The simulations in this work were run using the volume of fluid method, which consists of solving a transport equation for the volume fraction of each considered fluid. A correlation was found between the surface tension to momentum transport ratio, Scd, and the non-dimensional pool size for different droplet sizes and impact angles. Regions where partial depositions were most likely to occur were found via physical experiments.
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(This article belongs to the Special Issue Multiphase Flow and Granular Mechanics)
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Open AccessArticle
Turbulence and Rossby Wave Dynamics with Realizable Eddy Damped Markovian Anisotropic Closure
by
Jorgen S. Frederiksen and Terence J. O’Kane
Fluids 2024, 9(5), 116; https://doi.org/10.3390/fluids9050116 - 16 May 2024
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The theoretical basis for the Eddy Damped Markovian Anisotropic Closure (EDMAC) is formulated for two-dimensional anisotropic turbulence interacting with Rossby waves in the presence of advection by a large-scale mean flow. The EDMAC is as computationally efficient as the Eddy Damped Quasi Normal
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The theoretical basis for the Eddy Damped Markovian Anisotropic Closure (EDMAC) is formulated for two-dimensional anisotropic turbulence interacting with Rossby waves in the presence of advection by a large-scale mean flow. The EDMAC is as computationally efficient as the Eddy Damped Quasi Normal Markovian (EDQNM) closure, but, in contrast, is realizable in the presence of transient waves. The EDMAC is arrived at through systematic simplification of a generalization of the non-Markovian Direct Interaction Approximation (DIA) closure that has its origin in renormalized perturbation theory. Markovian Anisotropic Closures (MACs) are obtained from the DIA by using three variants of the Fluctuation Dissipation Theorem (FDT) with the information in the time history integrals instead carried by Markovian differential equations for two relaxation functions. One of the MACs is simplified to the EDMAC with analytical relaxation functions and high numerical efficiency, like te EDQNM. Sufficient conditions for the EDMAC to be realizable in the presence of Rossby waves are determined. Examples of the numerical integration of the EDMAC compared with the EDQNM are presented for two-dimensional isotropic and anisotropic turbulence, at moderate Reynolds numbers, possibly interacting with Rossby waves and large-scale mean flow. The generalization of the EDMAC for the statistical dynamics of other physical systems to higher dimension and higher order nonlinearity is considered.
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Open AccessArticle
Characterization Data for the Establishment of Scale-Up and Process Transfer Strategies between Stainless Steel and Single-Use Bioreactors
by
Vincent Bernemann, Jürgen Fitschen, Marco Leupold, Karl-Heinz Scheibenbogen, Marc Maly, Marko Hoffmann, Thomas Wucherpfennig and Michael Schlüter
Fluids 2024, 9(5), 115; https://doi.org/10.3390/fluids9050115 - 16 May 2024
Abstract
The reliable transfer of bioprocesses from single-use bioreactors (SUBs) of different scales to conventional stainless steel stirred-tank bioreactors is of steadily growing interest. In this publication, a scale-up study for SUBs with volumes of 200 L and 2000 L and the transfer to
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The reliable transfer of bioprocesses from single-use bioreactors (SUBs) of different scales to conventional stainless steel stirred-tank bioreactors is of steadily growing interest. In this publication, a scale-up study for SUBs with volumes of 200 L and 2000 L and the transfer to an industrial-scale conventional stainless steel stirred-tank bioreactor with a volume of 15,000 L is presented. The scale-up and transfer are based on a comparison of mixing times and the modeling of volumetric mass transfer coefficients kLa, measured in all three reactors in aqueous PBS/Kolliphor solution. The mass transfer coefficients are compared with the widely used correlation of van’t Riet at constant stirrer tip speeds. It can be shown that a van’t Riet correlation enables a robust and reliable prediction of mass transfer coefficients on each scale for a wide range of stirrer tip speeds and aeration rates. The process transfer from single-use bioreactors to conventional stainless steel stirred-tank bioreactors is proven to be uncritical concerning mass transfer performance. This provides higher flexibility with respect to bioreactor equipment considered for specific processes.
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(This article belongs to the Special Issue Mass Transfer in Multiphase Reactors)
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Open AccessArticle
Impact of Convection Regime on Temperature Distribution in Food Distribution Storage Box
by
Fabien Beaumont, Sébastien Murer, Fabien Bogard and Guillaume Polidori
Fluids 2024, 9(5), 114; https://doi.org/10.3390/fluids9050114 - 14 May 2024
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This study aims to optimize the thermodynamic performance of a cold storage distribution box through the integration of a ventilation system. To achieve this goal, a prototype constructed from expanded polystyrene is developed, incorporating an active ventilation system to ensure cold temperature uniformity.
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This study aims to optimize the thermodynamic performance of a cold storage distribution box through the integration of a ventilation system. To achieve this goal, a prototype constructed from expanded polystyrene is developed, incorporating an active ventilation system to ensure cold temperature uniformity. Thermocouples are integrated into the device to monitor the temporal temperature evolution with and without ventilation. Concurrently, a 2D thermo-aerodynamic investigation is conducted using computational fluid dynamics (CFD). The numerical modeling of the thermodynamic behavior of the cold source employs polynomial laws as input data for the computational code (UDF functions). A comparison between experimental and numerical results reveals the computational code’s accurate prediction of the temporal temperature evolution in the cold storage distribution box, particularly under forced convection conditions. The findings indicate that in the absence of ventilation, thermal exchanges primarily occur through air conduction, whereas with ventilation, exchanges are facilitated by convection. Overall, forced convection induced by the inclusion of a ventilation device enhances thermal transfers and the thermodynamic performance of the cold storage distribution box. Furthermore, air mixing limits thermal stratification, thereby facilitating temperature homogenization.
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Open AccessArticle
Reducing Aerodynamic Drag on Roof-Mounted Lightbars for Emergency Vehicles
by
Michael Gerard Connolly, Malachy J. O’Rourke and Alojz Ivankovic
Fluids 2024, 9(5), 113; https://doi.org/10.3390/fluids9050113 - 11 May 2024
Abstract
This paper investigates the impact of contemporary lightbars on vehicle fuel efficiency with a focus on quantifying their effects on fuel consumption and exploring strategies to improve drag performance through modifications. Simulations showed an 8–11% increase in drag for square-back vehicles, with greater
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This paper investigates the impact of contemporary lightbars on vehicle fuel efficiency with a focus on quantifying their effects on fuel consumption and exploring strategies to improve drag performance through modifications. Simulations showed an 8–11% increase in drag for square-back vehicles, with greater penalties outlined for vehicles with rear-slanting roofs. Given the moderate drag increase, the impact on the driving range, especially for electric vehicles, remains minimal, supporting the continued use of external lightbars. Positioning experiments suggest marginal drag reductions when lowering the lightbar to its lowest position due to additional drag effects that can be caused by the mounting mechanism in its condensed form. Angling the lightbar showed negligible drag increases up to an angle of 2.5 degrees, but beyond that, a 4% increase in drag was observed for every additional 2.5 degrees. Additionally, fitting drag-reducing ramps ahead of the lightbar yielded no significant drag savings. Noise analysis identified that the lightbar’s wake and rear surfaces were responsible for the largest production of noise. The optimal lightbar design was found to incorporate overflow rather than underflow and rear tapering in sync with roof curvature. Appendable clip-on devices for the lightbar, particularly rear clip-ons, demonstrated appreciable drag reductions of up to 2.5%. A final optimised lightbar design produced a minimal 2.8% drag increase when fitted onto an unmarked vehicle, representing a threefold improvement compared with the current generation of lightbars. This study advances the field of lightbar aerodynamics by precisely quantifying drag effects by using highly detailed geometry and examines the significance of optimal positioning, angle adjustment, and appendable clip-on devices in greater depth than any existing published work.
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(This article belongs to the Special Issue Drag Reduction in Turbulent Flows, 2nd Edition)
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Open AccessBrief Report
Gauging Centrifugal Instabilities in Compressible Free-Shear Layers via Nonlinear Boundary Region Equations
by
Omar Es-Sahli, Adrian Sescu and Yuji Hattori
Fluids 2024, 9(5), 112; https://doi.org/10.3390/fluids9050112 - 11 May 2024
Abstract
Curved free shear layers emerge in many engineering problems involving complex flow geometries, such as the flow over a backward-facing step, flows with wall injection in a boundary layer, the flow inside side-dump combustors, or wakes generated by vertical axis wind turbines, among
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Curved free shear layers emerge in many engineering problems involving complex flow geometries, such as the flow over a backward-facing step, flows with wall injection in a boundary layer, the flow inside side-dump combustors, or wakes generated by vertical axis wind turbines, among others. Previous studies involving centrifugal instabilities have mainly focused on wall-flows where Taylor instabilities between two rotating concentric cylinders or Görtler vortices in boundary layers are generated. Curved free shear layer flows, however, have not received sufficient attention, especially in the nonlinear regime. The present work investigates the development of centrifugal instabilities in a curved free shear layer flow in the nonlinear compressible regime. The compressible Navier–Stokes equations are reduced to the nonlinear boundary region equations (BREs) in a high Reynolds number asymptotic framework, wherein the streamwise wavelength of the disturbances is assumed to be much larger than the spanwise and wall-normal counterparts. We study the effect of the freestream Mach number , the shear layer thickness , the amplitude of the incoming disturbance A, and the relative velocity difference across the shear layer on the development of these centrifugal instabilities. Our parametric study shows that, among other things, the kinetic energy of the curved shear layer flow increases with increasing and A decreases with increasing . It was also found that increasing the disturbance amplitude of the incoming disturbance leads to significant growth in the mushroom-like structure’s amplitude and renders the secondary instability structures more prominent, indicating increased mixing for all Mach numbers under consideration.
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(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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Open AccessArticle
Wind Tunnel Experiments on Parallel Blade–Vortex Interaction with Static and Oscillating Airfoil
by
Andrea Colli, Alex Zanotti and Giuseppe Gibertini
Fluids 2024, 9(5), 111; https://doi.org/10.3390/fluids9050111 - 10 May 2024
Abstract
This study aims to experimentally investigate the effects of parallel blade–vortex interaction (BVI) on the aerodynamic performances of an airfoil, in particular as a possible cause of blade stall, since similar effects have been observed in literature in the case of perpendicular BVI.
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This study aims to experimentally investigate the effects of parallel blade–vortex interaction (BVI) on the aerodynamic performances of an airfoil, in particular as a possible cause of blade stall, since similar effects have been observed in literature in the case of perpendicular BVI. A wind tunnel test campaign was conducted reproducing parallel BVI on a NACA 23012 blade model at a Reynolds number of 300,000. The vortex was generated by impulsively pitching a second airfoil model, placed upstream. Measurements of the aerodynamic loads acting on the blade were performed by means of unsteady Kulite pressure transducers, while particle image velocimetry (PIV) techniques were employed to study the flow field over the blade model. After a first phase of vortex characterisation, different test cases were investigated with the blade model both kept fixed at different incidences and oscillating sinusoidally in pitch, with the latter case, a novelty in available research on parallel BVI, representing the pitching motion of a helicopter main rotor blade. The results show that parallel BVI produces a thickening of the boundary layer and can induce local flow separation at incidences close to the stall condition of the airfoil. The aerodynamic loads, both lift and drag, suffer important impulsive variations, in agreement with literature on BVI, the effects of which are extended in time. In the case of the oscillating airfoil, BVI introduces hysteresis cycles in the loads, which are generally reduced. In conclusion, parallel BVI can have a detrimental impact on the aerodynamic performances of the blade and even cause flow separation, which, while not being as catastrophic as in the case of dynamic stall, has relatively long-lasting effects.
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(This article belongs to the Special Issue Flow Visualization: Experiments and Techniques)
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Open AccessArticle
Experimental Investigation of the Effects of Grooves in Fe2O4/Water Nanofluid Pool Boiling
by
Marwa khaleel Rashid, Bashar Mahmood Ali, Mohammed Zorah and Tariq J. Al-Musawi
Fluids 2024, 9(5), 110; https://doi.org/10.3390/fluids9050110 - 8 May 2024
Abstract
In this study, we systematically explored how changing groove surfaces of iron oxide/water nanofluid could affect the pool boiling heat transfer. We aimed to investigate the effect of three types of grooves, namely rectangular, circular, and triangular, on the boiling heat transfer. The
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In this study, we systematically explored how changing groove surfaces of iron oxide/water nanofluid could affect the pool boiling heat transfer. We aimed to investigate the effect of three types of grooves, namely rectangular, circular, and triangular, on the boiling heat transfer. The goal was to improve heat transfer performance by consciously changing surface structure. Comparative analyses were conducted with deionized water to provide valuable insights. Notably, the heat transfer coefficient (HTC) exhibited a significant increase in the presence of grooves. For deionized water, the HTC rose by 91.7% and 48.7% on circular and rectangular grooved surfaces, respectively. Surprisingly, the triangular-grooved surface showed a decrease of 32.9% in HTC compared to the flat surface. On the other hand, the performance of the nanofluid displayed intriguing trends. The HTC for the nanofluid diminished by 89.2% and 22.3% on rectangular and triangular grooved surfaces, while the circular-grooved surface exhibited a notable 41.2% increase in HTC. These results underscore the complex interplay between groove geometry, fluid properties, and heat transfer enhancement in nanofluid-based boiling. Hence, we thoroughly examine the underlying mechanisms and elements influencing these observed patterns in this research. The results provide important insights for further developments in this area by shedding light on how surface changes and groove geometry may greatly affect heat transfer in nanofluid-based pool boiling systems.
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(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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Open AccessArticle
A Numerical Study on the Influence of Riparian Vegetation Patch on the Transportation of Suspended Sediment in a U-Bend Channel Flow
by
Mingyang Wang, Qian Yu, Yuan Xu, Na Li, Jing Wang, Bo Cao, Lu Wang and Eldad J. Avital
Fluids 2024, 9(5), 109; https://doi.org/10.3390/fluids9050109 - 7 May 2024
Abstract
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Bend sections are ubiquitous in natural sandy river systems. This study employs Computational Fluid Dynamics–Discrete Phase Model (CFD-DPM) methodology to analyze particle transport dynamics in U-bend channel flows, focusing on the distinctions between partially vegetated (Case No.1) and non-vegetated (Case No.2) scenarios. The
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Bend sections are ubiquitous in natural sandy river systems. This study employs Computational Fluid Dynamics–Discrete Phase Model (CFD-DPM) methodology to analyze particle transport dynamics in U-bend channel flows, focusing on the distinctions between partially vegetated (Case No.1) and non-vegetated (Case No.2) scenarios. The research aims to unravel the intricate relationships among bending channel-induced secondary flow, vegetation blockage, and particle aggregation, employing both quantitative and qualitative approaches. (I) The key findings reveal that vegetation near the inner walls of curved channels markedly diminishes the intensity of secondary circulation. This reduction in circulation intensity is observed not only within vegetated areas but also extends to adjacent non-vegetated zones. Additionally, the study identifies a close correlation between vertical vortices and particle distribution near the channel bed. While particle distribution generally aligns with the vortices’ margin, dynamic patch-scale eddies near vegetation patches induce deviations, creating wave-like patterns in particle distribution. (II) The application of the Probability Density Function (PDF) provides insights into the radius-wise particle distribution. In non-vegetated channels, particle distribution is primarily influenced by secondary flow and boundary layers. In contrast, the presence of vegetation leads to a complex mixing layer, altering the particle distribution pattern and maximizing PDF values in non-vegetated free flow subzones. (III) Furthermore, the research quantifies spatial–temporal sediment heterogeneity through PDF variance. The findings demonstrate that variance in non-vegetated channels increases towards the outer wall in bending regions. Vegetation-induced turbulence causes higher variance, particularly in the mixing layer subzone, underscoring the significance of eddy size in sediment redistribution. (IV) The study of vertical concentration profiles in vegetated U-bend channels offers additional insights, while secondary flow in non-vegetated channels facilitates upward sediment transport and vegetation presence, although increasing the Turbulent Kinetic Energy (TKE), restricts channel space, and impedes secondary flow, thereby reducing vertical particle suspension. Sediment concentrations are found to be higher in the lower layers of vegetated bends, contrary to the pattern in non-vegetated bends. These findings highlight the complex interplay between vegetation, secondary flow, and sediment transport, illustrating the reduced effectiveness of secondary flow in promoting vertical particle transportation in bending channels due to the vegetation obstruction.
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Open AccessArticle
Study of Orifice Design on Oleo-Pneumatic Shock Absorber
by
Paulo A. S. F. Silva, Ahmed A. Sheikh Al-Shabab, Panagiotis Tsoutsanis and Martin Skote
Fluids 2024, 9(5), 108; https://doi.org/10.3390/fluids9050108 - 3 May 2024
Abstract
Aircraft oil-strut shock absorbers rely on orifice designs to control fluid flow and optimize damping performance. However, the complex nature of cavitating flows poses significant challenges in predicting the influence of orifice geometry on energy dissipation and system reliability. This study presents a
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Aircraft oil-strut shock absorbers rely on orifice designs to control fluid flow and optimize damping performance. However, the complex nature of cavitating flows poses significant challenges in predicting the influence of orifice geometry on energy dissipation and system reliability. This study presents a comprehensive computational fluid dynamics (CFD) analysis of the effects of circular, rectangular, semicircular, and cutback orifice profiles on the internal flow characteristics and damping behavior of oleo-pneumatic shock absorbers. High-fidelity simulations reveal that the rectangular orifice generates higher damping pressures and velocity magnitude than those generated by others designs, while the semicircular shape reduces cavitation inception and exhibits a more gradual pressure recovery. Furthermore, the study highlights the importance of considering both geometric and thermodynamic factors in the design and analysis of cavitating flow systems, as liquid properties and vapor pressure significantly impact bubble growth and collapse behavior. Increasing the orifice length had a negligible impact on damping but moderately raised orifice velocities. This research provides valuable insights for optimizing shock absorber performance across a range of operating conditions, ultimately enhancing vehicle safety and passenger comfort.
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(This article belongs to the Special Issue Turbulent Flow, 2nd Edition)
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