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Fluids, Volume 4, Issue 3 (September 2019) – 62 articles

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Open AccessArticle
Closed-Form Non-Stationary Solutionsfor Thermo and Chemovibrational Viscous Flows
Fluids 2019, 4(3), 175; https://doi.org/10.3390/fluids4030175 - 19 Sep 2019
Cited by 1 | Viewed by 542
Abstract
A class of closed-form exact solutions for the Navier–Stokes equation written in the Boussinesq approximation is discussed. Solutions describe the motion of a non-homogeneous reacting fluid subjected to harmonic vibrations of low or finite frequency. Inhomogeneity of the medium arises due to the [...] Read more.
A class of closed-form exact solutions for the Navier–Stokes equation written in the Boussinesq approximation is discussed. Solutions describe the motion of a non-homogeneous reacting fluid subjected to harmonic vibrations of low or finite frequency. Inhomogeneity of the medium arises due to the transversal density gradient which appears as a result of the exothermicity and chemical transformations due to a reaction. Ultimately, the physical mechanism of fluid motion is the unequal effect of a variable inertial field on laminar sublayers of different densities. We derive the solutions for several problems for thermo- and chemovibrational convections including the viscous flow of heat-generating fluid either in a plain layer or in a closed pipe and the viscous flow of fluid reacting according to a first-order chemical scheme under harmonic vibrations. Closed-form analytical expressions for fluid velocity, pressure, temperature, and reagent concentration are derived for each case. A general procedure to derive the exact solution is discussed. Full article
(This article belongs to the Special Issue Coupled Flow and Heat or Mass Transport)
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Open AccessArticle
Recycled Cellulose Aerogels from Paper Waste for a Heat Insulation Design of Canteen Bottles
Fluids 2019, 4(3), 174; https://doi.org/10.3390/fluids4030174 - 18 Sep 2019
Cited by 4 | Viewed by 748
Abstract
Exercising in a tropical climate with constant high temperatures and high humidity increases the risk of heatstroke for active people who frequently train outdoors. For these active persons, a cooling source of water nearby can be essential, and this is usually carried in [...] Read more.
Exercising in a tropical climate with constant high temperatures and high humidity increases the risk of heatstroke for active people who frequently train outdoors. For these active persons, a cooling source of water nearby can be essential, and this is usually carried in canteen bottles. However, commercially available water canteen bottles have limited thermal insulation capability to keep the liquid content cooled for the required period. This work proposed an engineering solution to enhance the heat insulation performance of water canteen bottles, using recycled cellulose aerogels made from paper waste for the first time as an insulating layer. Recycled cellulose aerogels wrapped around the water canteen bottle provides excellent thermal insulation performance, while not adding significant weight to the bottle. The temperature of the ice slurry in the canteen bottle was measured periodically over four hours with a mercury thermometer. The effects of the static and dynamic conditions on the temperature rate were also quantified. A 1.5 cm thickness of 1.0 wt.% recycled cellulose aerogel wrapped around the canteen bottle can provide an excellent thermal insulation performance with the lowest rise in temperature, achieving a low final temperature of the ice slurry content of 3.5 °C after 4 h. This result is much better than that provided by available commercial bottles under the same conditions. Full article
(This article belongs to the Special Issue Coupled Flow and Heat or Mass Transport)
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Open AccessReview
Wormlike Micellar Solutions, Beyond the Chemical Enhanced Oil Recovery Restrictions
Fluids 2019, 4(3), 173; https://doi.org/10.3390/fluids4030173 - 17 Sep 2019
Cited by 2 | Viewed by 844
Abstract
While traditional oil recovery methods are limited in terms of meeting the overall oil demands, enhanced oil recovery (EOR) techniques are being continually developed to provide a principal portion of our energy demands. Chemical EOR (cEOR) is one of the EOR techniques that [...] Read more.
While traditional oil recovery methods are limited in terms of meeting the overall oil demands, enhanced oil recovery (EOR) techniques are being continually developed to provide a principal portion of our energy demands. Chemical EOR (cEOR) is one of the EOR techniques that shows an efficient oil recovery factor in a number of oilfields with low salinity and temperature ranges. However, the application of cEOR under the harsh conditions of reservoirs where most of today’s crude oils come from remains a challenge. High temperatures, the presence of ions, divalent ions, and heterogeneous rock structures in such reservoirs restrict the application of cEOR. Polymer solutions, surfactants, alkaline-based solutions, and complex multi-components of them are common chemical displacing fluids that failed to show successful recovery results in hostile conditions for various reasons. Wormlike micellar solutions (WMS) are viscoelastic surfactants that possess advantageous characteristics for overcoming current cEOR challenges. In this study, we first review the major approaches and challenges of commonly used chemical agents for cEOR applications. Subsequently, we review special characteristics of WMS that make them promising materials for the future of cEOR. Full article
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Open AccessArticle
Influence of Toroidal Flow on Stationary Density of Collisionless Plasmas
Fluids 2019, 4(3), 172; https://doi.org/10.3390/fluids4030172 - 16 Sep 2019
Viewed by 687
Abstract
Starting from the given passive particle equilibrium particle cylindrical profiles, we built self-consistent stationary conditions of the Maxwell-Vlasov equation at thermodynamic equilibrium with non-flat density profiles. The solutions to the obtained equations are then discussed. It appears that the presence of an azimuthal [...] Read more.
Starting from the given passive particle equilibrium particle cylindrical profiles, we built self-consistent stationary conditions of the Maxwell-Vlasov equation at thermodynamic equilibrium with non-flat density profiles. The solutions to the obtained equations are then discussed. It appears that the presence of an azimuthal (poloidal) flow in the plasma can ensure radial confinement, while the presence of a longitudinal (toroidal) flow can enhance greatly the confinement. Moreover in the global physically reasonable situation, we find that no unstable point can emerge in the effective integrable Hamiltonian of the individual particles, hinting at some stability of the confinement when considering a toroidal geometry in the large aspect ratio limit. Full article
(This article belongs to the Special Issue Modelling of Plasma Flow)
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Open AccessArticle
Assessment of Solution Algorithms for LES of Turbulent Flows Using OpenFOAM
Fluids 2019, 4(3), 171; https://doi.org/10.3390/fluids4030171 - 12 Sep 2019
Cited by 2 | Viewed by 1216
Abstract
We validate and test two algorithms for the time integration of the Boussinesq form of the Navier—Stokes equations within the Large Eddy Simulation (LES) methodology for turbulent flows. The algorithms are implemented in the OpenFOAM framework. From one side, we have implemented an [...] Read more.
We validate and test two algorithms for the time integration of the Boussinesq form of the Navier—Stokes equations within the Large Eddy Simulation (LES) methodology for turbulent flows. The algorithms are implemented in the OpenFOAM framework. From one side, we have implemented an energy-conserving incremental-pressure Runge–Kutta (RK4) projection method for the solution of the Navier–Stokes equations together with a dynamic Lagrangian mixed model for momentum and scalar subgrid-scale (SGS) fluxes; from the other side we revisit the PISO algorithm present in OpenFOAM (pisoFoam) in conjunction with the dynamic eddy-viscosity model for SGS momentum fluxes and a Reynolds Analogy for the scalar SGS fluxes, and used for the study of turbulent channel flows and buoyancy-driven flows. In both cases the validity of the anisotropic filter function, suited for non-homogeneous hexahedral meshes, has been studied and proven to be useful for industrial LES. Preliminary tests on energy-conservation properties of the algorithms studied (without the inclusion of the subgrid-scale models) show the superiority of RK4 over pisoFoam, which exhibits dissipative features. We carried out additional tests for wall-bounded channel flow and for Rayleigh–Bènard convection in the turbulent regime, by running LES using both algorithms. Results show the RK4 algorithm together with the dynamic Lagrangian mixed model gives better results in the cases analyzed for both first- and second-order statistics. On the other hand, the dissipative features of pisoFoam detected in the previous tests reflect in a less accurate evaluation of the statistics of the turbulent field, although the presence of the subgrid-scale model improves the quality of the results compared to a correspondent coarse direct numerical simulation. In case of Rayleigh–Bénard convection, the results of pisoFoam improve with increasing values of Rayleigh number, and this may be attributed to the Reynolds Analogy used for the subgrid-scale temperature fluxes. Finally, we point out that the present analysis holds for hexahedral meshes. More research is need for extension of the methods proposed to general unstructured grids. Full article
(This article belongs to the Special Issue Multiscale Turbulent Transport) Printed Edition available
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Open AccessArticle
Slug Translational Velocity for Highly Viscous Oil and Gas Flows in Horizontal Pipes
Fluids 2019, 4(3), 170; https://doi.org/10.3390/fluids4030170 - 12 Sep 2019
Cited by 2 | Viewed by 792
Abstract
Slug translational velocity, described as the velocity of slug units, is the summation of the maximum mixture velocity in the slug body and the drift velocity. Existing prediction models in literature were developed based on observation from low viscosity liquids, neglecting the effects [...] Read more.
Slug translational velocity, described as the velocity of slug units, is the summation of the maximum mixture velocity in the slug body and the drift velocity. Existing prediction models in literature were developed based on observation from low viscosity liquids, neglecting the effects of fluid properties (i.e., viscosity). However, slug translational velocity is expected to be affected by the fluid viscosity. Here, we investigate the influence of high liquid viscosity on slug translational velocity in a horizontal pipeline of 76.2-mm internal diameter. Air and mineral oil with viscosities within the range of 1.0–5.5 Pa·s were used in this investigation. Measurement was by means of a pair of gamma densitometer with fast sampling frequencies (up to 250 Hz). The results obtained show that slug translational velocity increases with increase in liquid viscosity. Existing slug translational velocity prediction models in literature were assessed based on the present high viscosity data for which statistical analysis revealed discrepancies. In view of this, a new empirical correlation for the calculation of slug translational velocity in highly viscous two-phase flow is proposed. A comparison study and validation of the new correlation showed an improved prediction performance. Full article
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Open AccessArticle
Naut Your Everyday Jellyfish Model: Exploring How Tentacles and Oral Arms Impact Locomotion
Fluids 2019, 4(3), 169; https://doi.org/10.3390/fluids4030169 - 10 Sep 2019
Cited by 1 | Viewed by 1370
Abstract
Jellyfish are majestic, energy-efficient, and one of the oldest species that inhabit the oceans. It is perhaps the second item, their efficiency, that has captivated scientists for decades into investigating their locomotive behavior. Yet, no one has specifically explored the role that their [...] Read more.
Jellyfish are majestic, energy-efficient, and one of the oldest species that inhabit the oceans. It is perhaps the second item, their efficiency, that has captivated scientists for decades into investigating their locomotive behavior. Yet, no one has specifically explored the role that their tentacles and oral arms may have on their potential swimming performance. We perform comparative in silico experiments to study how tentacle/oral arm number, length, placement, and density affect forward swimming speeds, cost of transport, and fluid mixing. An open source implementation of the immersed boundary method was used (IB2d) to solve the fully coupled fluid–structure interaction problem of an idealized flexible jellyfish bell with poroelastic tentacles/oral arms in a viscous, incompressible fluid. Overall tentacles/oral arms inhibit forward swimming speeds, by appearing to suppress vortex formation. Nonlinear relationships between length and fluid scale (Reynolds Number) as well as tentacle/oral arm number, density, and placement are observed, illustrating that small changes in morphology could result in significant decreases in swimming speeds, in some cases by upwards of 80–90% between cases with or without tentacles/oral arms. Full article
(This article belongs to the Special Issue Advances in Biological Flows and Biomimetics)
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Open AccessArticle
Unstable Density-Driven Flow in Fractured Porous Media: The Fractured Elder Problem
Fluids 2019, 4(3), 168; https://doi.org/10.3390/fluids4030168 - 09 Sep 2019
Cited by 2 | Viewed by 800
Abstract
The Elder problem is one of the well-known examples of an unstable density-driven flow (DDF) and solute transport in porous media. The goal of this research is to investigate the influence of fracture networks on this benchmark problem due to the great importance [...] Read more.
The Elder problem is one of the well-known examples of an unstable density-driven flow (DDF) and solute transport in porous media. The goal of this research is to investigate the influence of fracture networks on this benchmark problem due to the great importance of the fractured heterogeneity effect on unstable DDF. For this aim, the fractured Elder problem is solved using COMSOL Multiphysics, which is a finite element method simulator. Uniform and orthogonal fracture networks are embedded to analyze free convective flow and development of unstable salt plumes. The results indicate that the mesh sensitivity of the fractured Elder problem is greater than the homogeneous case. Furthermore, it has been shown that in the fractured cases, the onset of instability and free convection occur with lower critical Rayleigh number, which means that fracture networks have a destabilizing effect. Also, we examined the structural properties of fracture networks that control convective flow patterns, and the simulation results show that the strength of convection and instability at the beginning of the intrusion is proportional to the aperture size of the fractures. Moreover, the increase of the fracture’s density leads different modes of transient convective modes, until a specific fracture density after which the transient convective modes become similar to the homogenous case. Full article
(This article belongs to the Special Issue Convective Instability in Porous Media, Volume II)
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Open AccessArticle
Heat Convective Effects on Turbulence and Airflow inside an B767 Aircraft Cabin
Fluids 2019, 4(3), 167; https://doi.org/10.3390/fluids4030167 - 08 Sep 2019
Viewed by 564
Abstract
Thermal plumes generated by human bodies can affect the temperature and humidity of the surrounding environment. An experimental study investigated the effects of thermal plumes formed by aircraft passengers on airflow and turbulence characteristics inside aircraft-cabins. An 11-row, wide-body B767 cabin mockup was [...] Read more.
Thermal plumes generated by human bodies can affect the temperature and humidity of the surrounding environment. An experimental study investigated the effects of thermal plumes formed by aircraft passengers on airflow and turbulence characteristics inside aircraft-cabins. An 11-row, wide-body B767 cabin mockup was used with actual seats, air diffusers and cabin profile. Thermal manikins were used simulating passengers in the cabin. Tracer gas and air speed inside the cabin were measured while the heat from the manikins was turned on and off to help understand the effects of the thermal heat released by the manikins. Results showed that tracer gas distribution were more uniformly and equally distributed around the release source and the air speed fluctuation were lower under cooler environments when the thermal manikins were turned off. Heated environments increased the values of turbulence kinetic energy and the turbulence intensity levels. However, the effects on the turbulence intensity were less significant compared to the turbulence kinetic energy. On the other hand, the dissipation rates were higher for unheated cases in the front and back sections of the mockup cabin. The relative uncertainty for tracer gas sampling ranged between ±5–14% for heated manikins versus ±8–17% for unheated manikins. Higher uncertainty levels accompanied the turbulence measurements due to the highly chaotic nature of the flow inside the cabin. Full article
(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)
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Open AccessArticle
The Influence of Bubbles on Foamed Cement Viscosity Using an Extended Stokesian Dynamics Approach
Fluids 2019, 4(3), 166; https://doi.org/10.3390/fluids4030166 - 06 Sep 2019
Cited by 3 | Viewed by 779
Abstract
We want to study the influence of bubbles on the viscosity of suspensions with a computational approach that also accounts for the arrangement of the bubbles due to shearing flow. This requires a large number of bubbles to properly simulate and requires a [...] Read more.
We want to study the influence of bubbles on the viscosity of suspensions with a computational approach that also accounts for the arrangement of the bubbles due to shearing flow. This requires a large number of bubbles to properly simulate and requires a large amount of computational resources. Here we develop a set of equations to define the viscosity ratio from the simulation results to show the influence of the bubbles on the viscosity as a function of the volume fraction. One application of this work has been used to study a specific type of cement that has bubbles injected into the slurry while it is still fluid. The bubbles are added to reduce the density but they also improve the properties of the cement with the increase in viscosity. We show that the computed results match the few experimental results that have been reported. Full article
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Open AccessFeature PaperArticle
Spatial Positioning and Operating Parameters of a Rotary Bell Sprayer: 3D Mapping of Droplet Size Distributions
Fluids 2019, 4(3), 165; https://doi.org/10.3390/fluids4030165 - 05 Sep 2019
Cited by 9 | Viewed by 1492
Abstract
In this study, we evaluated the fundamental physical behavior during droplet formation and flow from a rotary bell spray in the absence of an electrostatic field. The impact of a wide range of operating parameters of the rotary bell sprayer, such as flow [...] Read more.
In this study, we evaluated the fundamental physical behavior during droplet formation and flow from a rotary bell spray in the absence of an electrostatic field. The impact of a wide range of operating parameters of the rotary bell sprayer, such as flow rates, rotational speeds, and spatial positioning, on droplet sizes and size distributions using a three-dimensional (3-D) mapping was studied. The results showed that increasing the rotational speed caused the Sauter mean diameter of the droplets to decrease while increasing flow rate increased the droplet sizes. The rotational speed effect, however, was dominant compared to the effect of flow rate. An increase in droplet size radially away from the cup was noted in the vicinity of the cup, nevertheless, as the lateral distances from the cup and rotational speed were increased, the droplet sizes within the flow field became more uniform. This result is of importance for painting industries, which are looking for optimal target distances for uniform painting appearance. Furthermore, the theoretical formulation was validated with experimental data, which provides a wider range of applicability in terms of environment and parameters that could be tested. This work also provides an abundance of measurements, which can serve as a database for the validation of future droplet disintegration simulations. Full article
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Open AccessArticle
An Explicit Meshless Point Collocation Solver for Incompressible Navier-Stokes Equations
Fluids 2019, 4(3), 164; https://doi.org/10.3390/fluids4030164 - 03 Sep 2019
Cited by 2 | Viewed by 638
Abstract
We present a strong form, meshless point collocation explicit solver for the numerical solution of the transient, incompressible, viscous Navier-Stokes (N-S) equations in two dimensions. We numerically solve the governing flow equations in their stream function-vorticity formulation. We use a uniform Cartesian embedded [...] Read more.
We present a strong form, meshless point collocation explicit solver for the numerical solution of the transient, incompressible, viscous Navier-Stokes (N-S) equations in two dimensions. We numerically solve the governing flow equations in their stream function-vorticity formulation. We use a uniform Cartesian embedded grid to represent the flow domain. We discretize the governing equations using the Meshless Point Collocation (MPC) method. We compute the spatial derivatives that appear in the governing flow equations, using a novel interpolation meshless scheme, the Discretization Corrected Particle Strength Exchange (DC PSE). We verify the accuracy of the numerical scheme for commonly used benchmark problems including lid-driven cavity flow, flow over a backward-facing step and unbounded flow past a cylinder. We have examined the applicability of the proposed scheme by considering flow cases with complex geometries, such as flow in a duct with cylindrical obstacles, flow in a bifurcated geometry, and flow past complex-shaped obstacles. Our method offers high accuracy and excellent computational efficiency as demonstrated by the verification examples, while maintaining a stable time step comparable to that used in unconditionally stable implicit methods. We estimate the stable time step using the Gershgorin circle theorem. The stable time step can be increased through the increase of the support domain of the weight function used in the DC PSE method. Full article
(This article belongs to the Special Issue Recent Numerical Advances in Fluid Mechanics) Printed Edition available
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Open AccessArticle
Energy Transfer in Incompressible Magnetohydrodynamics: The Filtered Approach
Fluids 2019, 4(3), 163; https://doi.org/10.3390/fluids4030163 - 02 Sep 2019
Viewed by 575
Abstract
We develop incompressible magnetohydrodynamic (IMHD) energy budget equations with a spatial filtering kernel and estimate the scaling of the structure functions. The Politano-Pouquet law is recovered as an upper bound on the scale-to-scale energy transfer. The primary result of this work is the [...] Read more.
We develop incompressible magnetohydrodynamic (IMHD) energy budget equations with a spatial filtering kernel and estimate the scaling of the structure functions. The Politano-Pouquet law is recovered as an upper bound on the scale-to-scale energy transfer. The primary result of this work is the relation of the scaling of IMHD invariants. It can be produced by hypothesizing a scale-independent energy transfer rate. These results have relevance in plasma regimes where the approximations of IMHD are justified. We measure structure functions with solar wind data and find support for the relations. Full article
(This article belongs to the Special Issue Modelling of Plasma Flow)
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Open AccessArticle
Modeling the Excess Velocity of Low-Viscous Taylor Droplets in Square Microchannels
Fluids 2019, 4(3), 162; https://doi.org/10.3390/fluids4030162 - 02 Sep 2019
Cited by 2 | Viewed by 929
Abstract
Microscopic multiphase flows have gained broad interest due to their capability to transfer processes into new operational windows and achieving significant process intensification. However, the hydrodynamic behavior of Taylor droplets is not yet entirely understood. In this work, we introduce a model to [...] Read more.
Microscopic multiphase flows have gained broad interest due to their capability to transfer processes into new operational windows and achieving significant process intensification. However, the hydrodynamic behavior of Taylor droplets is not yet entirely understood. In this work, we introduce a model to determine the excess velocity of Taylor droplets in square microchannels. This velocity difference between the droplet and the total superficial velocity of the flow has a direct influence on the droplet residence time and is linked to the pressure drop. Since the droplet does not occupy the entire channel cross-section, it enables the continuous phase to bypass the droplet through the corners. A consideration of the continuity equation generally relates the excess velocity to the mean flow velocity. We base the quantification of the bypass flow on a correlation for the droplet cap deformation from its static shape. The cap deformation reveals the forces of the flowing liquids exerted onto the interface and allows estimating the local driving pressure gradient for the bypass flow. The characterizing parameters are identified as the bypass length, the wall film thickness, the viscosity ratio between both phases and the C a number. The proposed model is adapted with a stochastic, metaheuristic optimization approach based on genetic algorithms. In addition, our model was successfully verified with high-speed camera measurements and published empirical data. Full article
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Open AccessArticle
Statistical Structure and Deviations from Equilibrium in Wavy Channel Turbulence
Fluids 2019, 4(3), 161; https://doi.org/10.3390/fluids4030161 - 27 Aug 2019
Cited by 2 | Viewed by 967
Abstract
The structure of turbulent flow over non-flat surfaces is a topic of major interest in practical applications in both engineering and geophysical settings. A lot of work has been done in the fully rough regime at high Reynolds numbers where the effect on [...] Read more.
The structure of turbulent flow over non-flat surfaces is a topic of major interest in practical applications in both engineering and geophysical settings. A lot of work has been done in the fully rough regime at high Reynolds numbers where the effect on the outer layer turbulence structure and the resulting friction drag is well documented. It turns out that surface topology plays a significant role on the flow drag especially in the transitional roughness regime and therefore, is hard to characterize. Survey of literature shows that roughness function depends on the interaction of roughness height, flow Reynolds number, and topology shape. In addition, if the surface topology contains large enough scales then it can impact the outer layer dynamics and in turn modulate the total frictional force. Therefore, it is important to understand the mechanisms underlying drag increase from systematically varied surface undulations in order to better interpret quantifications based on mean statistics such as roughness function. In this study, we explore the mechanisms that modulate the turbulence structure over a two-dimensional (2D) sinusoidal wavy surface with a fixed amplitude, but varying slopes that are sufficiently small to generate only intermittent flow separation. To accomplish this, we perform a set of highly resolved direct numerical simulations (DNS) to model the turbulent flow between two infinitely wide 2D wavy plates at a friction Reynolds number, R e τ = 180 , which represents modest scale separation. We pursue two different but related flavors of analysis. The first one adopts a roughness characterization flavor of such wavy surfaces. The second one focuses on understanding the nonequilibrium near-surface turbulence structure and their impact on roughness characterization. Analysis of the different statistical quantifications show strong dependence on wave slope for the roughness function indicating drag increase due to enhanced turbulent stresses resulting from increased production of vertical velocity variance from the surface undulations. Full article
(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)
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Open AccessArticle
Parameters and Branching Auto-Pulses in a Fluid Channel with Active Walls
Fluids 2019, 4(3), 160; https://doi.org/10.3390/fluids4030160 - 26 Aug 2019
Viewed by 581
Abstract
We present numerical solutions of the semi-phenomenological model of self-propagating fluid pulses (auto-pulses) in the channel branching into two thinner channels, which simulates branching of a hypothetical artificial artery. The model is based on the lubrication theory coupled with elasticity and has the [...] Read more.
We present numerical solutions of the semi-phenomenological model of self-propagating fluid pulses (auto-pulses) in the channel branching into two thinner channels, which simulates branching of a hypothetical artificial artery. The model is based on the lubrication theory coupled with elasticity and has the form of a single nonlinear partial differential equation with respect to the displacement of the elastic wall as a function of the distance along the channel and time. The equation is solved numerically using the 1D integrated radial basis function network method. Using homogeneous boundary conditions on the edges of space domain and continuity condition at the branching point, we obtained and analyzed solutions in the form of auto-pulses penetrating through the branching point from the thick channel into the thin channels. We evaluated magnitudes of the phenomenological coefficients responsible for the active motion of the walls in the model. Full article
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Open AccessArticle
CFD Julia: A Learning Module Structuring an Introductory Course on Computational Fluid Dynamics
Fluids 2019, 4(3), 159; https://doi.org/10.3390/fluids4030159 - 23 Aug 2019
Cited by 6 | Viewed by 4789
Abstract
CFD Julia is a programming module developed for senior undergraduate or graduate-level coursework which teaches the foundations of computational fluid dynamics (CFD). The module comprises several programs written in general-purpose programming language Julia designed for high-performance numerical analysis and computational science. The paper [...] Read more.
CFD Julia is a programming module developed for senior undergraduate or graduate-level coursework which teaches the foundations of computational fluid dynamics (CFD). The module comprises several programs written in general-purpose programming language Julia designed for high-performance numerical analysis and computational science. The paper explains various concepts related to spatial and temporal discretization, explicit and implicit numerical schemes, multi-step numerical schemes, higher-order shock-capturing numerical methods, and iterative solvers in CFD. These concepts are illustrated using the linear convection equation, the inviscid Burgers equation, and the two-dimensional Poisson equation. The paper covers finite difference implementation for equations in both conservative and non-conservative form. The paper also includes the development of one-dimensional solver for Euler equations and demonstrate it for the Sod shock tube problem. We show the application of finite difference schemes for developing two-dimensional incompressible Navier-Stokes solvers with different boundary conditions applied to the lid-driven cavity and vortex-merger problems. At the end of this paper, we develop hybrid Arakawa-spectral solver and pseudo-spectral solver for two-dimensional incompressible Navier-Stokes equations. Additionally, we compare the computational performance of these minimalist fashion Navier-Stokes solvers written in Julia and Python. Full article
(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics) Printed Edition available
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Open AccessArticle
Impact of Longitudinal Acceleration and Deceleration on Bluff Body Wakes
Fluids 2019, 4(3), 158; https://doi.org/10.3390/fluids4030158 - 18 Aug 2019
Cited by 2 | Viewed by 932
Abstract
This study investigated the unsteady acceleration aerodynamics of bluff bodies through the study of a channel mounted square cylinder undergoing free-stream acceleration of ±20 ms−2 with Reynolds numbers spanning 3.2 × 104 to 3.6 × 105. To achieve this, [...] Read more.
This study investigated the unsteady acceleration aerodynamics of bluff bodies through the study of a channel mounted square cylinder undergoing free-stream acceleration of ±20 ms−2 with Reynolds numbers spanning 3.2 × 104 to 3.6 × 105. To achieve this, a numerical simulation was created with a commercial finite volume unstructured computational fluid dynamics code, which was first validated using Improved Delayed Detached Eddy Simulation against experimental and direct numerical simulated results. Then, the free stream conditions were subjected to a periodic velocity signal where data were recorded and ensemble averaged over at least 30 distinct acceleration and deceleration data points. This enabled the comparison of body forces and flow field variations among accelerating, steady and decelerating free-stream conditions. Body force analysis determined that decelerating and accelerating drag forces varied −47% and 44%, respectively, in comparison to steady free-stream conditions. In addition, several differences were also observed and explored such as near-body flow structures, wake dynamics, Kármán vortices and vorticity production during the aforementioned conditions. The primary interest of this study was for the future application towards road vehicles for predictive dynamic modeling and aerodynamic development. Full article
(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)
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Open AccessArticle
Modelling of Self-Ignition in Spark-Ignition Engine Using Reduced Chemical Kinetics for Gasoline Surrogates
Fluids 2019, 4(3), 157; https://doi.org/10.3390/fluids4030157 - 17 Aug 2019
Cited by 1 | Viewed by 1027
Abstract
A numerical and experimental investigation in to the role of gasoline surrogates and their reduced chemical kinetic mechanisms in spark ignition (SI) engine knocking has been carried out. In order to predict autoignition of gasoline in a spark ignition engine three reduced chemical [...] Read more.
A numerical and experimental investigation in to the role of gasoline surrogates and their reduced chemical kinetic mechanisms in spark ignition (SI) engine knocking has been carried out. In order to predict autoignition of gasoline in a spark ignition engine three reduced chemical kinetic mechanisms have been coupled with quasi-dimensional thermodynamic modelling approach. The modelling was supported by measurements of the knocking tendencies of three fuels of very different compositions yet an equivalent Research Octane Number (RON) of 90 (ULG90, PRF90 and 71.5% by volume toluene blended with n-heptane) as well as iso-octane. The experimental knock onsets provided a benchmark for the chemical kinetic predictions of autoignition and also highlighted the limitations of characterisation of the knock resistance of a gasoline in terms of the Research and Motoring octane numbers and the role of these parameters in surrogate formulation. Two approaches used to optimise the surrogate composition have been discussed and possible surrogates for ULG90 have been formulated and numerically studied. A discussion has also been made on the various surrogates from the literature which have been tested in shock tube and rapid compression machines for their autoignition times and are a source of chemical kinetic mechanism validation. The differences in the knock onsets of the tested fuels have been explained by modelling their reactivity using semi-detailed chemical kinetics. Through this work, the weaknesses and challenges of autoignition modelling in SI engines through gasoline surrogate chemical kinetics have been highlighted. Adequacy of a surrogate in simulating the autoignition behaviour of gasoline has also been investigated as it is more important for the surrogate to have the same reactivity as the gasoline at all engine relevant p T conditions than having the same RON and Motored Octane Number (MON). Full article
(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion) Printed Edition available
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Open AccessArticle
Temperature Error Reduction of DPD Fluid by Using Partitioned Runge-Kutta Time Integration Scheme
Fluids 2019, 4(3), 156; https://doi.org/10.3390/fluids4030156 - 17 Aug 2019
Cited by 1 | Viewed by 844
Abstract
This study puts emphasis on reducing the temperature error of dissipative particle dynamics (DPD) fluid by directly applying a minimal-stage third-order partitioned Runge-Kutta (PRK3) method to the time integration, which does not include any of additional governing equations and change in the DPD [...] Read more.
This study puts emphasis on reducing the temperature error of dissipative particle dynamics (DPD) fluid by directly applying a minimal-stage third-order partitioned Runge-Kutta (PRK3) method to the time integration, which does not include any of additional governing equations and change in the DPD thermostat formulation. The error is estimated based on the average values of both kinetic and configurational temperatures. The result shows that the errors in both temperatures errors are greatly reduced by using the PRK3 scheme as comparing them to those of previous studies. Additionally, the comparison among three different PRK3 schemes demonstrates our recent findings that the symplecticity conservation of the system is important to reduce the temperature error of DPD fluid especially for large time increments. The computational efficiencies are also estimated for the PRK3 scheme as well as the existing ones. It was found from the estimation that the simulation using the PRK3 scheme is more than twice as efficient as those using the existing ones. Finally, the roles of both kinetic and configurational temperatures as error indicators are discussed by comparing them to the velocity autocorrelation function and the radial distribution function. It was found that the errors of these temperatures involve different characteristics, and thus both temperatures should be taken into account to comprehensively evaluate the numerical error of DPD. Full article
(This article belongs to the Special Issue Coupled Flow and Heat or Mass Transport)
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Open AccessArticle
Uncertainty Quantification of Non-Dimensional Parameters for a Film Cooling Configuration in Supersonic Conditions
Fluids 2019, 4(3), 155; https://doi.org/10.3390/fluids4030155 - 10 Aug 2019
Cited by 1 | Viewed by 942
Abstract
In transonic high-pressure turbine stages, oblique shocks originating from vane trailing edges impact the suction side of each adjacent vane. High-pressure vanes are cooled to tolerate the combustor exit-temperature levels, then it is highly probable that shock impingement will occur in proximity to [...] Read more.
In transonic high-pressure turbine stages, oblique shocks originating from vane trailing edges impact the suction side of each adjacent vane. High-pressure vanes are cooled to tolerate the combustor exit-temperature levels, then it is highly probable that shock impingement will occur in proximity to a row of cooling holes. The presence of such a shock, together with the inevitable manufacturing deviations, alters the location of the shock impingement and of the performance parameters of each cooling hole. The present work provides a general description of the aero-thermal field that occurs on the rear suction side of a cooled vane. Computational Fluid Dynamics (CFD) is used to evaluate the deterministic response of the selected configurations in terms of adiabatic effectiveness, discharge coefficient, blowing ratio, density ratio, and momentum ratio. Turbulence is modelled by using both the Shear Stress Transport method (SST) and the Reynolds Stress Model (RSM) implemented in ANSYS® FLUENT®. The obtained results are compared with the experimental data obtained by the Institut für Thermische Strömungsmaschinen in Karlsruhe. Two uncertainty quantification methodologies based on Hermite polynomials and Padè–Legendre approximants are used to consider the probability distribution of the geometrical parameters and to evaluate the response surfaces for the system response quantities. Trailing-edge and cooling-hole diameters have been considered to be aleatory unknowns. Uncertainty quantification analysis allows for the assessment of the mutual effects on global and local parameters of the cooling device. Obtained results demonstrate that most of the parameters are independent by the variation of the aleatory unknowns while the standard deviation of the blowing ratio associated with the hole diameter uncertainty is around 12%, with no impact by the trailing-edge thickness. No relevant advantages are found using either SST model or RSM in combination with Hermite polynomials and Padè–Legendre approximants. Full article
(This article belongs to the Special Issue Turbomachinery Flow Analysis)
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Open AccessFeature PaperArticle
Modeling Flow and Pressure Fields in Porous Media with High Conductivity Flow Channels and Smart Placement of Branch Cuts for Variant and Invariant Complex Potentials
Fluids 2019, 4(3), 154; https://doi.org/10.3390/fluids4030154 - 09 Aug 2019
Cited by 5 | Viewed by 962
Abstract
A long overdue distinction between so-called variant and invariant complex potentials is proposed here for the first time. Invariant complex potentials describe physical flows where a switch of the real and imaginary parts of the function will still describe the same type of [...] Read more.
A long overdue distinction between so-called variant and invariant complex potentials is proposed here for the first time. Invariant complex potentials describe physical flows where a switch of the real and imaginary parts of the function will still describe the same type of physical flow (but only rotated by π/2). Such invariants can be formulated with Euler’s formula to depict the same flow for any arbitrary orientation with respect to the coordinate system used. In contrast, variant complex potentials, when swapping their real and imaginary parts, will result in two fundamentally different physical flows. Next, we show that the contour integrals of the real and imaginary part of simple variant and invariant complex potentials generally do not generate any discernable branch cut problems. However, complex potentials due to the multiple superpositions of simple flows, even when invariant, may involve many options for selecting the branch cut locations. Examples of such branch cut choices are given for so-called areal doublets and areal dipoles, which are powerful tools to describe the streamlines and pressure fields for flow in porous media with enhanced permeability flow channels. After a discussion of the branch cut solutions, applications to a series of synthetic and field examples with enhanced permeability flow channels are given with examples of the streamline and pressure field solutions. Full article
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Open AccessArticle
Wind Turbine Wake Modeling in Accelerating Wind Field: A Preliminary Study on a Two-Dimensional Hill
Fluids 2019, 4(3), 153; https://doi.org/10.3390/fluids4030153 - 09 Aug 2019
Viewed by 831
Abstract
Complex terrain can influence wind turbine wakes and wind speed profiles in a wind farm. Consequently, predicting the performance of wind turbines and energy production over complex terrain is more difficult than it is over flat terrain. In this preliminary study, an engineering [...] Read more.
Complex terrain can influence wind turbine wakes and wind speed profiles in a wind farm. Consequently, predicting the performance of wind turbines and energy production over complex terrain is more difficult than it is over flat terrain. In this preliminary study, an engineering wake model, that considers acceleration on a two-dimensional hill, was developed based on the momentum theory. The model consists of the wake width and wake wind speed. The equation to calculate the rotor thrust, which is calculated by the wake wind speed profiles, was also formulated. Then, a wind-tunnel test was performed in simple flow conditions in order to investigate wake development over a two-dimensional hill. After this the wake model was compared with the wind-tunnel test, and the results obtained by using the new wake model were close to the wind-tunnel test results. Using the new wake model, it was possible to estimate the wake shrinkage in an accelerating two-dimensional wind field. Full article
(This article belongs to the Special Issue Turbomachinery Flow Analysis)
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Open AccessArticle
On the Bound Wave Phase Lag
Fluids 2019, 4(3), 152; https://doi.org/10.3390/fluids4030152 - 09 Aug 2019
Cited by 2 | Viewed by 816
Abstract
More than three decades ago, it was noted that the ocean infragravity bound wave increasingly lags behind the forcing short-wave groups when propagating towards the shore. To date, the most recent theoretical prediction of this so-called phase lag remained a first-order approximation in [...] Read more.
More than three decades ago, it was noted that the ocean infragravity bound wave increasingly lags behind the forcing short-wave groups when propagating towards the shore. To date, the most recent theoretical prediction of this so-called phase lag remained a first-order approximation in terms of depth variations. Here, a new semi-analytical solution is proposed which does not rely on this approximation. Strong agreement is obtained when the new solution is compared with high-resolution laboratory data involving both bichromatic and random wave conditions. This newly proposed theoretical phase lag is then extensively compared with the former one, highlighting an increasing discrepancy between the two solutions as the relative bottom slope increases. The four influencing parameters, namely the bottom slope, the water depth, the incident short-wave peak period and the incident group period, are shown to impact, each in a specific way, the bound wave phase lag. While the latter is seen to increase with lower water depths and/or with higher short-wave peak periods, both the bottom slope and the group period can affect the phase lag in a different manner. Indeed, steeper bed slopes induce lower phase lags in shallow water but higher ones in deep water, while higher group periods induce higher phase lags for gentle slopes but lower ones for steep slopes. Full article
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Open AccessArticle
Modeling and Analysis of the Effects of Noise Barrier Shape and Inflow Conditions on Highway Automobiles Emission Dispersion
Fluids 2019, 4(3), 151; https://doi.org/10.3390/fluids4030151 - 08 Aug 2019
Cited by 2 | Viewed by 803
Abstract
Recent research has suggested that noise barriers have significant impacts on near-road automobile emissions reduction. T-shaped noise barriers have better performance on reducing noise than others, however, their effects on automobile emissions reduction are not clear. In this research, commercial software ANSYS® [...] Read more.
Recent research has suggested that noise barriers have significant impacts on near-road automobile emissions reduction. T-shaped noise barriers have better performance on reducing noise than others, however, their effects on automobile emissions reduction are not clear. In this research, commercial software ANSYS®Fluent 19.2 (Ansys Inc., Canonsburg, PA, USA) was applied to simulate the noise barrier shape and different inflow wind shear condition effects on highway automobiles emission dispersion. Various Reynolds Averaged Navier-Stokes (RANS) models were tested. The realizable k-ε turbulence model was selected to simulate the turbulent flow caused by fast moving vehicles on highway based on the comparison results. A non-reacting species transport model was applied to simulate emission dispersion. Results showed that the T-shaped barrier was able to help reduce highway automobiles emission concentration in downstream areas more than the rectangular barrier. An optimized range of the T-shape was proposed; under the inflow condition without wind shear, the noise barrier shape effects on automobiles emission reduction were not significant. Full article
(This article belongs to the Special Issue Computational Simulation of Pollution Dispersion)
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Open AccessFeature PaperArticle
Laser-Plasma Accelerated Protons: Energy Increase in Gas-Mixtures Using High Mass Number Atomic Species
Fluids 2019, 4(3), 150; https://doi.org/10.3390/fluids4030150 - 08 Aug 2019
Cited by 1 | Viewed by 765
Abstract
The idea of using a gas-mixture comprising atoms with a high mass number in order to increase proton energies in laser induced plasma acceleration at critical density is investigated by means of 2D PIC (Particle-In-Cell) simulations. Comparing and discussing the case of a [...] Read more.
The idea of using a gas-mixture comprising atoms with a high mass number in order to increase proton energies in laser induced plasma acceleration at critical density is investigated by means of 2D PIC (Particle-In-Cell) simulations. Comparing and discussing the case of a pure hydrogen plasma and that of a plasma containing higher mass number species with a small percentage of hydrogen, we demonstrate that the mixture enhances the energies of the accelerated protons. We also show that using a gas-mixture introduces the possibility of using the densities ratio in order to change the relative acceleration of the species. Full article
(This article belongs to the Special Issue Modelling of Plasma Flow)
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Open AccessArticle
Investigation of Hydrodynamically Dominated Membrane Rupture, Using Smoothed Particle Hydrodynamics–Finite Element Method
Fluids 2019, 4(3), 149; https://doi.org/10.3390/fluids4030149 - 03 Aug 2019
Cited by 3 | Viewed by 1091
Abstract
The rupturing process of a membrane, located between two fluids at the center of a three-dimensional channel, is numerically investigated. The smoothed particle hydrodynamics (SPH) and the finite element method (FEM) are used, respectively, for modeling the fluid and solid phases. A range [...] Read more.
The rupturing process of a membrane, located between two fluids at the center of a three-dimensional channel, is numerically investigated. The smoothed particle hydrodynamics (SPH) and the finite element method (FEM) are used, respectively, for modeling the fluid and solid phases. A range of pressure differences and membrane thicknesses are studied and two different rupturing processes are identified. These processes differ in the time scale of the rupture, the location of the rupture initiation, the level of destruction and the driving mechanism. Full article
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Open AccessArticle
Turbulence Modeling Effects on the CFD Predictions of Flow over a Detailed Full-Scale Sedan Vehicle
Fluids 2019, 4(3), 148; https://doi.org/10.3390/fluids4030148 - 01 Aug 2019
Cited by 3 | Viewed by 1249
Abstract
In today’s road vehicle design processes, Computational Fluid Dynamics (CFD) has emerged as one of the major investigative tools for aerodynamics analyses. The age-old CFD methodology based on the Reynolds Averaged Navier–Stokes (RANS) approach is still considered as the most popular turbulence modeling [...] Read more.
In today’s road vehicle design processes, Computational Fluid Dynamics (CFD) has emerged as one of the major investigative tools for aerodynamics analyses. The age-old CFD methodology based on the Reynolds Averaged Navier–Stokes (RANS) approach is still considered as the most popular turbulence modeling approach in automotive industries due to its acceptable accuracy and affordable computational cost for predicting flows involving complex geometries. This popular use of RANS still persists in spite of the well-known fact that, for automotive flows, RANS turbulence models often fail to characterize the associated flow-field properly. It is even true that more often, the RANS approach fails to predict correct integral aerodynamic quantities like lift, drag, or moment coefficients, and as such, they are used to assess the relative magnitude and direction of a trend. Moreover, even for such purposes, notable disagreements generally exist between results predicted by different RANS models. Thanks to fast advances in computer technology, increasing popularity has been seen in the use of the hybrid Detached Eddy Simulation (DES), which blends the RANS approach with Large Eddy Simulation (LES). The DES methodology demonstrated a high potential of being more accurate and informative than the RANS approaches. Whilst evaluations of RANS and DES models on various applications are abundant in the literature, such evaluations on full-car models are relatively fewer. In this study, four RANS models that are widely used in engineering applications, i.e., the realizable k ε two-layer, Abe–Kondoh–Nagano (AKN) k ε low-Reynolds, SST k ω , and V2F are evaluated on a full-scale passenger vehicle with two different front-end configurations. In addition, both cases are run with two DES models to assess the differences between the flow predictions obtained using RANS and DES. Full article
(This article belongs to the Special Issue Turbulence and Transitional Modeling of Aerodynamic Flows)
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Open AccessReview
Instability and Convection in Rotating Porous Media: A Review
Fluids 2019, 4(3), 147; https://doi.org/10.3390/fluids4030147 - 01 Aug 2019
Cited by 4 | Viewed by 858
Abstract
A review on instability and consequent natural convection in rotating porous media is presented. Taylor-Proudman columns and geostrophic flows exist in rotating porous media just the same as in pure fluids. The latter leads to a tendency towards two-dimensionality. Natural convection resulting from [...] Read more.
A review on instability and consequent natural convection in rotating porous media is presented. Taylor-Proudman columns and geostrophic flows exist in rotating porous media just the same as in pure fluids. The latter leads to a tendency towards two-dimensionality. Natural convection resulting from density gradients in a gravity field as well as natural convection induced by density gradients due to the centripetal acceleration are being considered. The former is the result of gravity-induced buoyancy, the latter is due to centripetally-induced buoyancy. The effect of Coriolis acceleration is also discussed. Linear stability analysis as well as weak nonlinear solutions are being derived and presented. Full article
(This article belongs to the Special Issue Convective Instability in Porous Media, Volume II)
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Open AccessArticle
Numerical Investigation of Pressure Influence on the Confined Turbulent Boundary Layer Flashback Process
Fluids 2019, 4(3), 146; https://doi.org/10.3390/fluids4030146 - 01 Aug 2019
Cited by 2 | Viewed by 865
Abstract
Boundary layer flashback from the combustion chamber into the premixing section is a threat associated with the premixed combustion of hydrogen-containing fuels in gas turbines. In this study, the effect of pressure on the confined flashback behaviour of hydrogen-air flames was investigated numerically. [...] Read more.
Boundary layer flashback from the combustion chamber into the premixing section is a threat associated with the premixed combustion of hydrogen-containing fuels in gas turbines. In this study, the effect of pressure on the confined flashback behaviour of hydrogen-air flames was investigated numerically. This was done by means of large eddy simulations with finite rate chemistry as well as detailed chemical kinetics and diffusion models at pressures between 0.5 bar and 3 bar. It was found that the flashback propensity increases with increasing pressure. The separation zone size and the turbulent flame speed at flashback conditions decrease with increasing pressure, which decreases flashback propensity. At the same time the quenching distance decreases with increasing pressure, which increases flashback propensity. It is not possible to predict the occurrence of boundary layer flashback based on the turbulent flame speed or the ratio of separation zone size to quenching distance alone. Instead the interaction of all effects has to be accounted for when modelling boundary layer flashback. It was further found that the pressure rise ahead of the flame cannot be approximated by one-dimensional analyses and that the assumptions of the boundary layer theory are not satisfied during confined boundary layer flashback. Full article
(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion) Printed Edition available
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