Fluids

2024

Jump to: 2023, 2022, 2021

12 pages, 1695 KiB

Open AccessArticle

Measuring Turbulent Flows: Analyzing a Stochastic Process with Stochastic Tools

by Evangelos Rozos, Jörg Wieland and Jorge Leandro

Fluids 2024, 9(6), 128; https://doi.org/10.3390/fluids9060128 - 30 May 2024

Viewed by 720

Abstract

Assessing drag force and Reynolds stresses in turbulent flows is crucial for evaluating the stability and longevity of hydraulic structures. Yet, this task is challenging due to the complex nature of turbulent flows. To address this, physical models are often employed. Nonetheless, this [...] Read more.

Assessing drag force and Reynolds stresses in turbulent flows is crucial for evaluating the stability and longevity of hydraulic structures. Yet, this task is challenging due to the complex nature of turbulent flows. To address this, physical models are often employed. Nonetheless, this practice is associated with difficulties, especially in the case of high sampling frequency where the inherent randomness of velocity fluctuations becomes mixed with the measurement noise. This study introduces a stochastic approach, which aims to mitigate bias from measurement errors and provide a probabilistic estimate of extreme stress values. To accomplish this, a simple experimental setup with a hydraulic jump was employed to acquire long-duration velocity measurements. Subsequently, a modified first-order autoregressive model was applied through ensemble simulations, demonstrating the benefits of the stochastic approach. The analysis highlights its effectiveness in estimating the uncertainty of extreme events frequency and minimizing the bias induced by the noise in the high-magnitude velocity measurements and by the limited length of observations. These findings contribute to advancing our understanding of turbulent flow analysis and have implications for the design and assessment of hydraulic structures. Full article

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14 pages, 5562 KiB

Open AccessArticle

Grid Turbulence Measurements with an Acoustic Doppler Current Profiler

by Trygve K. Løken, David Lande-Sudall, Atle Jensen and Jean Rabault

Fluids 2024, 9(3), 60; https://doi.org/10.3390/fluids9030060 - 1 Mar 2024

Viewed by 1514

Abstract

The motivation for this study is to investigate the abilities and limitations of a Nortek Signature1000 acoustic Doppler current profiler (ADCP) regarding fine-scale turbulence measurements. Current profilers offer the advantage of gaining more coherent measurement data than available with point acoustic measurements, and [...] Read more.

The motivation for this study is to investigate the abilities and limitations of a Nortek Signature1000 acoustic Doppler current profiler (ADCP) regarding fine-scale turbulence measurements. Current profilers offer the advantage of gaining more coherent measurement data than available with point acoustic measurements, and it is desirable to exploit this property in laboratory and field applications. The ADCP was tested in a towing tank, where turbulence was generated from a grid towed under controlled conditions. Grid-induced turbulence is a well-studied phenomenon and a good approximation for isotropic turbulence. Several previous experiments are available for comparison and there are developed theories within the topic. In the present experiments, a Nortek Vectrino acoustic Doppler velocimeter (ADV), which is an established instrument for turbulence measurements, was applied to validate the ADCP. It was found that the mean flow measured with the ADCP was accurate within 4% of the ADV. The turbulent variance was reasonably well resolved by the ADCP when large grid bars were towed at a high speed, but largely overestimated for lower towing speed and smaller grid bars. The effective cutoff frequency and turbulent eddy size were characterized experimentally, which provides detailed guidelines for when the ADCP data can be trusted and will allow future experimentalists to decide a priori if the Nortek Signature can be used in their setup. We conclude that the ADCP is not suitable for resolving turbulent spectra in a small-scale grid-induced flow due to the intrinsic Doppler noise and the low spatial and temporal sample resolution relative to the turbulent scales. Full article

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2023

Jump to: 2024, 2022, 2021

17 pages, 5299 KiB

Open AccessArticle

Numerical Simulation of Taylor—Couette—Poiseuille Flow at Re = 10,000

by Andrey Gavrilov and Yaroslav Ignatenko

Fluids 2023, 8(10), 280; https://doi.org/10.3390/fluids8100280 - 19 Oct 2023

Cited by 2 | Viewed by 1863

Abstract

A fully developed turbulent flow in a concentric annulus, Re

= 10,000

,

r_{i} / r_{o} = 0.5

, with an inner rotating cylinder in the velocity range

N = U_{ω} / U_{b} = 0 \div 4

, is [...] Read more.

A fully developed turbulent flow in a concentric annulus, Re

= 10,000

,

r_{i} / r_{o} = 0.5

, with an inner rotating cylinder in the velocity range

N = U_{ω} / U_{b} = 0 \div 4

, is studied via a large-eddy simulation. Also, for comparison, simulations by steady-state, unstatiounary RANS k-

ω

SST (URANS), and Elliptic Blending Model (EBM) were made. The main focus of this study is on the effect of high rotation on the mean flow, turbulence statistics, and vortex structure. Distribution of the tangential velocity and the Reynolds stress tensor change their behaviour at

N > 0.5 \sim 1

. With rotation increases, the production of tangential fluctuation becomes dominant over axial ones and the position of turbulent kinetic energy maximum shifts towards the wall into the buffer zone. URANS and EBM approaches show good agreement with LES in mean flow, turbulent statistics, and integral parameters. The difference in pressure loss prediction between LES and URANS does not exceed 20%, but the average difference is about 11%. The EBM approach underestimates pressure losses up to 9% and on average not more than 5%. Vortex structures are described well by URANS. Full article

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29 pages, 12576 KiB

Open AccessArticle

The Effects of Flexible Cylinder Structural Dynamics to the near Wake Turbulence

by Sharul Sham Dol, Siaw Khur Wee, Tshun Howe Yong and Shaharin Anwar Sulaiman

Fluids 2023, 8(10), 270; https://doi.org/10.3390/fluids8100270 - 1 Oct 2023

Cited by 1 | Viewed by 1557

Abstract

The utilization of a rigid and projecting surface, coupled with an agitator and vortex generator, frequently results in the dissipation of more energy than the production of turbulence that meets the required criteria. By contrast, a passively oscillating flexible protruding surface can generate [...] Read more.

The utilization of a rigid and projecting surface, coupled with an agitator and vortex generator, frequently results in the dissipation of more energy than the production of turbulence that meets the required criteria. By contrast, a passively oscillating flexible protruding surface can generate a greater turbulence level. In the current study, a circular finite cylinder (cantilever) was used as the geometry of the rigid and protruding surface. Both the material and the aspect ratio were varied. Also, a local Reynolds number within the subcritical flow range (10² < Re_D < 10⁵) was considered. The results from the rigid protruding surface (finite cylinder) serve as a validation of the published results and a benchmark for the improvement of the turbulence generated by the flexible protruding surface. The results obtained via an ultrasonic velocity profiler have further demonstrated that the flexible cylinder is capable of generating greater turbulence by examining the turbulence intensity, the turbulence production term and the Reynolds stress. All the flexible cylinders that oscillate show an increase in turbulence production but at different percentages. The cylinders studied in this work ranged from the least structural stiffness (EVA) to moderate (aluminum) and the highest structural stiffness (carbon steel). Through studying the normalized amplitude responses graph for the flexible cylinders, it is found that the oscillating motion does indeed contribute to the increment. A further examination of the results shows that the increase is due to the structural velocity instead of just the oscillating motion. Full article

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25 pages, 14817 KiB

Open AccessArticle

Effect of Schmidt Number on Forced Isotropic Turbulence with Passive Scalars

by Paolo Orlandi and Sergio Pirozzoli

Fluids 2023, 8(9), 248; https://doi.org/10.3390/fluids8090248 - 12 Sep 2023

Viewed by 1318

Abstract

Traditionally, Fourier spectra have been employed to gain a deeper understanding of turbulence flow structures. The investigation of isotropic forced turbulence with passive scalars offers a straightforward means to examine the disparities between velocity and passive scalar spectra. This flow configuration has been [...] Read more.

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

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21 pages, 8944 KiB

Open AccessArticle

Numerical Study of Flow around Two Circular Cylinders in Tandem, Side-By-Side and Staggered Arrangements

by Gracjan M. Skonecki and James M. Buick

Fluids 2023, 8(5), 148; https://doi.org/10.3390/fluids8050148 - 7 May 2023

Cited by 3 | Viewed by 3202

Abstract

Simulations are presented for flow around pairs of circular cylinders at a Reynolds number of 3900. The flow is assumed to be two-dimensional and incompressible in nature and the simulations are performed using a RANS (Reynolds Averaged Navier Stokes) approach with a k [...] Read more.

Simulations are presented for flow around pairs of circular cylinders at a Reynolds number of 3900. The flow is assumed to be two-dimensional and incompressible in nature and the simulations are performed using a RANS (Reynolds Averaged Navier Stokes) approach with a k-ε model. Simulations are performed for three different configurations of the cylinders: A tandem configuration where the line joining the centre of the cylinders is parallel to the mean flow direction; side-by-side, where the centre line is perpendicular to the mean flow direction; and staggered where the centre line is an angle α to the flow direction. Simulation results are presented for cylinder separations ranging from 1.125 to 4 diameters and for values of α between 10° and 60°. The results are presented and discussed in terms of the lift and drag coefficients, the Strouhal number, the vorticity field and the flow regimes observed. The results and flow regimes are also compared to previous observations at lower Reynolds numbers to investigate the Reynolds number dependence of the phenomena. Full article

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18 pages, 6104 KiB

Open AccessArticle

On the Development of an Implicit Discontinuous Galerkin Solver for Turbulent Real Gas Flows

by Edoardo Mantecca, Alessandro Colombo, Antonio Ghidoni, Gianmaria Noventa, David Pasquale and Stefano Rebay

Fluids 2023, 8(4), 117; https://doi.org/10.3390/fluids8040117 - 31 Mar 2023

Cited by 2 | Viewed by 1843

Abstract

The aim of this work is to describe an efficient implementation of cubic and multiparameter real gas models in an existing discontinuous Galerkin solver to extend its capabilities to the simulation of turbulent real gas flows. The adopted thermodynamic models are van der [...] Read more.

The aim of this work is to describe an efficient implementation of cubic and multiparameter real gas models in an existing discontinuous Galerkin solver to extend its capabilities to the simulation of turbulent real gas flows. The adopted thermodynamic models are van der Waals, Peng–Robinson, and Span–Wagner, which differ from each other in terms of accuracy and computational cost. Convective numerical fluxes across elements interfaces are calculated with a thermodynamic consistent linearized Riemann solver, whereas for boundary conditions, a linearized expression of the generalized Riemann invariants is employed. Transport properties are treated as temperature- and density-dependent quantities through multiparameter correlations. An implicit time integration is adopted; Jacobian matrix and thermodynamic derivatives are obtained with the automatic differentiation tool Tapenade. The solver accuracy is assessed by computing both steady and unsteady real gas test cases available in the literature, and the effect of the mesh size and polynomial degree of approximation on the solution accuracy is investigated. A good agreement with experimental and numerical reference data is observed and specific non-classical phenomena are well reproduced by the solver. Full article

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35 pages, 5362 KiB

Open AccessArticle

Laboratory Models of Planetary Core-Style Convective Turbulence

by Emily K. Hawkins, Jonathan S. Cheng, Jewel A. Abbate, Timothy Pilegard, Stephan Stellmach, Keith Julien and Jonathan M. Aurnou

Fluids 2023, 8(4), 106; https://doi.org/10.3390/fluids8040106 - 23 Mar 2023

Cited by 9 | Viewed by 2274

Abstract

The connection between the heat transfer and characteristic flow velocities of planetary core-style convection remains poorly understood. To address this, we present novel laboratory models of rotating Rayleigh–Bénard convection in which heat and momentum transfer are simultaneously measured. Using water (Prandtl number, [...] Read more.

The connection between the heat transfer and characteristic flow velocities of planetary core-style convection remains poorly understood. To address this, we present novel laboratory models of rotating Rayleigh–Bénard convection in which heat and momentum transfer are simultaneously measured. Using water (Prandtl number,

P r ≃ 6

) and cylindrical containers of diameter-to-height aspect ratios of

Γ ≃ 3, 1.5, 0.75

, the non-dimensional rotation period (Ekman number, E) is varied between

10^{- 7} ≲ E ≲ 3 \times 10^{- 5}

and the non-dimensional convective forcing (Rayleigh number,

R a

) ranges from

10^{7} ≲ R a ≲ 10^{12}

. Our heat transfer data agree with those of previous studies and are largely controlled by boundary layer dynamics. We utilize laser Doppler velocimetry (LDV) to obtain experimental point measurements of bulk axial velocities, resulting in estimates of the non-dimensional momentum transfer (Reynolds number,

R e

) with values between

4 \times 10^{2} ≲ R e ≲ 5 \times 10^{4}

. Behavioral transitions in the velocity data do not exist where transitions in heat transfer behaviors occur, indicating that bulk dynamics are not controlled by the boundary layers of the system. Instead, the LDV data agree well with the diffusion-free Coriolis–Inertia–Archimedian (CIA) scaling over the range of

R a

explored. Furthermore, the CIA scaling approximately co-scales with the Viscous–Archimedian–Coriolis (VAC) scaling over the parameter space studied. We explain this observation by demonstrating that the VAC and CIA relations will co-scale when the local Reynolds number in the fluid bulk is of order unity. We conclude that in our experiments and similar laboratory and numerical investigations with

E ≳ 10^{- 7}

,

R a ≲ 10^{12}

,

P r ≃ 7

, heat transfer is controlled by boundary layer physics while quasi-geostrophically turbulent dynamics relevant to core flows robustly exist in the fluid bulk. Full article

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12 pages, 414 KiB

Open AccessArticle

Theoretical Estimates of the Critical Reynolds Number in the Flow around the Sphere on the Basis of Theory of Stochastic Equations and Equivalence of Measures

by Artur V. Dmitrenko and Vladislav M. Ovsyannikov

Fluids 2023, 8(3), 81; https://doi.org/10.3390/fluids8030081 - 23 Feb 2023

Cited by 2 | Viewed by 1699

Abstract

The aim of this investigation is to show the solution for the critical Reynolds number in the flow around the sphere on the basis of theory of stochastic equations and equivalence of measures between turbulent and laminar motions. Solutions obtained by numerical methods [...] Read more.

The aim of this investigation is to show the solution for the critical Reynolds number in the flow around the sphere on the basis of theory of stochastic equations and equivalence of measures between turbulent and laminar motions. Solutions obtained by numerical methods (DNS, LES, RANS) require verification and in this case the theoretical results have special value. For today in the scientific literature, there is J. Talor’s implicit formula connecting the critical Reynolds number with the parameters of the initial fluctuations in the flow around the sphere. Here the derivation of the explicit formula is presented. The results show a satisfactory correspondence of the obtained theoretical dependence for the critical Reynolds number to the experiments in the flow around the sphere. Full article

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22 pages, 7697 KiB

Open AccessArticle

Numerical Study of Flow Downstream a Step with a Cylinder Part 2: Effect of a Cylinder on the Flow over the Step

by Milad Abdollahpour, Paola Gualtieri, David F. Vetsch and Carlo Gualtieri

Fluids 2023, 8(2), 60; https://doi.org/10.3390/fluids8020060 - 10 Feb 2023

Cited by 3 | Viewed by 2206

Abstract

In this study, divided into two parts, the effect on a two-dimensional backward-facing step flow (BFSF) of a cylinder placed downstream of the step was numerically investigated. While in Part 1, the numerical simulations carried out without the cylinder were validated using the [...] Read more.

In this study, divided into two parts, the effect on a two-dimensional backward-facing step flow (BFSF) of a cylinder placed downstream of the step was numerically investigated. While in Part 1, the numerical simulations carried out without the cylinder were validated using the available literature data, in Part 2 the effect of the cylinder was investigated. In the laminar regime, different Reynolds numbers were considered. In the turbulent regime, the effects on the flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step were comparatively studied. When the cylinder was positioned below the step edge mid-plane, flow over the step was not altered by a cylinder. However, in other locations of a cylinder, the added cylinder modified the structure of flow, increasing the skin friction coefficient in the recirculation zone. Furthermore, the pressure coefficient of the bottom wall increased immediately downstream of the cylinder and farther downstream of the reattachment point and remained stable in the flow recovery process. Moreover, the presence of the step significantly influenced the dynamics of the vortex generation and shedding leading to an asymmetric wake distribution. Full article

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20 pages, 3182 KiB

Open AccessArticle

Numerical Study of Flow Downstream a Step with a Cylinder Part 1: Validation of the Numerical Simulations

by Milad Abdollahpour, Paola Gualtieri, David F. Vetsch and Carlo Gualtieri

Fluids 2023, 8(2), 55; https://doi.org/10.3390/fluids8020055 - 3 Feb 2023

Cited by 5 | Viewed by 2320

Abstract

The backward-facing step flow (BFSF) is a classical problem in fluid mechanics, hydraulic engineering, and environmental hydraulics. The nature of this flow, consisting of separation and reattachment, makes it a problem worthy of investigation. In this study, divided into two parts, the effect [...] Read more.

The backward-facing step flow (BFSF) is a classical problem in fluid mechanics, hydraulic engineering, and environmental hydraulics. The nature of this flow, consisting of separation and reattachment, makes it a problem worthy of investigation. In this study, divided into two parts, the effect of a cylinder placed downstream of the step on the 2D flow structure was investigated. In Part 1, the classical 2D BFSF was validated by using OpenFOAM. The BFSF characteristics (reattachment, recirculation zone, velocity profile, skin friction coefficient, and pressure coefficient) were validated for a step-height Reynolds number in the range from 75 to 9000, covering both laminar and turbulent flow. The numerical results at different Reynolds numbers of laminar flow and four RANS turbulence models (standard k-ε, RNG k-ε, standard k-ω, and SST k-ω) were found to be in good agreement with the literature data. In laminar flow, the average error between the numerical results and experimental data for velocity profiles and reattachment lengths and the skin friction coefficient were lower than 8.1, 18, and 20%, respectively. In turbulent flow, the standard k-ε was the most accurate model in predicting pressure coefficients, skin friction coefficient, and reattachment length with an average error lower than 20.5, 17.5, and 6%, respectively. In Part 2, the effect on the 2D flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step was investigated. Full article

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2022

Jump to: 2024, 2023, 2021

7 pages, 1017 KiB

Open AccessCommunication

Sonic Eddy Model of the Turbulent Boundary Layer

by Paul Dintilhac and Robert Breidenthal

Fluids 2022, 7(1), 37; https://doi.org/10.3390/fluids7010037 - 15 Jan 2022

Cited by 1 | Viewed by 2476

Abstract

The effects of Mach number on the skin friction and velocity fluctuations of the turbulent boundary layer are considered through a sonic eddy model. Originally proposed for free shear flows, the model assumes that the eddies responsible for momentum transfer have a rotation [...] Read more.

The effects of Mach number on the skin friction and velocity fluctuations of the turbulent boundary layer are considered through a sonic eddy model. Originally proposed for free shear flows, the model assumes that the eddies responsible for momentum transfer have a rotation Mach number of unity, with the entrainment rate limited by acoustic signaling. Under this assumption, the model predicts that the skin friction coefficient should go as the inverse Mach number in a regime where the Mach number is larger than unity but smaller than the square root of the Reynolds number. The velocity fluctuations normalized by the friction velocity should be the inverse square root of the Mach number in the same regime. Turbulent transport is controlled by acoustic signaling. The density field adjusts itself such that the Reynolds stresses correspond to the momentum transport. In contrast, the conventional van Driest–Morkovin view is that the Mach number effects are due to density variations directly. A new experiment or simulation is proposed to test this model using different gases in an incompressible boundary layer, following the example of Brown and Roshko in the free shear layer. Full article

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18 pages, 3841 KiB

Open AccessArticle

Instability and Transition of a Boundary Layer over a Backward-Facing Step

by Ming Teng and Ugo Piomelli

Fluids 2022, 7(1), 35; https://doi.org/10.3390/fluids7010035 - 14 Jan 2022

Cited by 4 | Viewed by 3976

Abstract

The development of secondary instabilities in a boundary layer over a backward-facing step is investigated numerically. Two step heights are considered,

h / δ_{o}^{*} = 0.5

and

1.0

(where

δ_{o}^{*}

is the displacement thickness at the step location), in [...] Read more.

The development of secondary instabilities in a boundary layer over a backward-facing step is investigated numerically. Two step heights are considered,

h / δ_{o}^{*} = 0.5

and

1.0

(where

δ_{o}^{*}

is the displacement thickness at the step location), in addition to a reference flat-plate case. A case with a realistic freestream-velocity distribution is also examined. A controlled K-type transition is initiated using a narrow ribbon upstream of the step, which generates small and monochromatic perturbations by periodic blowing and suction. A well-resolved direct numerical simulation is performed. The step height and the imposed freestream-velocity distribution exert a significant influence on the transition process. The results for the

h / δ_{o}^{*} = 1.0

case exhibit a rapid transition primarily due to the Kelvin–Helmholtz instability downstream of step; non-linear interactions already occur within the recirculation region, and the initial symmetry and periodicity of the flow are lost by the middle stage of transition. In contrast, case

h / δ_{o}^{*} = 0.5

presents a transition road map in which transition occurs far downstream of the step, and the flow remains spatially symmetric and temporally periodic until the late stage of transition. A realistic freestream-velocity distribution (which induces an adverse pressure gradient) advances the onset of transition to turbulence, but does not fundamentally modify the flow features observed in the zero-pressure gradient case. Considering the budgets of the perturbation kinetic energy, both the step and the induced pressure-gradient increase, rather than modify, the energy transfer. Full article

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2021

Jump to: 2024, 2023, 2022

19 pages, 1971 KiB

Open AccessArticle

A New Anisotropic Four-Parameter Turbulence Model for Low Prandtl Number Fluids

by Giacomo Barbi, Valentina Giovacchini and Sandro Manservisi

Fluids 2022, 7(1), 6; https://doi.org/10.3390/fluids7010006 - 22 Dec 2021

Cited by 4 | Viewed by 2793

Abstract

Due to their interesting thermal properties, liquid metals are widely studied for heat transfer applications where large heat fluxes occur. In the framework of the Reynolds-Averaged Navier–Stokes (RANS) approach, the Simple Gradient Diffusion Hypothesis (SGDH) and the Reynolds Analogy are almost universally invoked [...] Read more.

Due to their interesting thermal properties, liquid metals are widely studied for heat transfer applications where large heat fluxes occur. In the framework of the Reynolds-Averaged Navier–Stokes (RANS) approach, the Simple Gradient Diffusion Hypothesis (SGDH) and the Reynolds Analogy are almost universally invoked for the closure of the turbulent heat flux. Even though these assumptions can represent a reasonable compromise in a wide range of applications, they are not reliable when considering low Prandtl number fluids and/or buoyant flows. More advanced closure models for the turbulent heat flux are required to improve the accuracy of the RANS models dealing with low Prandtl number fluids. In this work, we propose an anisotropic four-parameter turbulence model. The closure of the Reynolds stress tensor and turbulent heat flux is gained through nonlinear models. Particular attention is given to the modeling of dynamical and thermal time scales. Numerical simulations of low Prandtl number fluids have been performed over the plane channel and backward-facing step configurations. Full article

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19 pages, 6765 KiB

Open AccessFeature PaperArticle

Predictions of Vortex Flow in a Diesel Multi-Hole Injector Using the RANS Modelling Approach

by Aishvarya Kumar, Jamshid Nouri and Ali Ghobadian

Fluids 2021, 6(12), 421; https://doi.org/10.3390/fluids6120421 - 23 Nov 2021

Cited by 1 | Viewed by 3025

Abstract

The occurrence of vortices in the sac volume of automotive multi-hole fuel injectors plays an important role in the development of vortex cavitation, which directly influences the flow structure and emerging sprays that, in turn, influence the engine performance and emissions. In this [...] Read more.

The occurrence of vortices in the sac volume of automotive multi-hole fuel injectors plays an important role in the development of vortex cavitation, which directly influences the flow structure and emerging sprays that, in turn, influence the engine performance and emissions. In this study, the RANS-based turbulence modelling approach was used to predict the internal flow in a vertical axis-symmetrical multi-hole (6) diesel fuel injector under non-cavitating conditions. The project aimed to predict the aforementioned vortical structures accurately at two different needle lifts in order to form a correct opinion about their occurrence. The accuracy of the simulations was assessed by comparing the predicted mean axial velocity and RMS velocity of LDV measurements, which showed good agreement. The flow field analysis predicted a complex, 3D, vortical flow structure with the presence of different types of vortices in the sac volume and the nozzle hole. Two main types of vortex were detected: the “hole-to-hole” connecting vortex, and double “counter-rotating” vortices emerging from the needle wall and entering the injector hole facing it. Different flow patterns in the rotational direction of the “hole-to-hole” vortices have been observed at the low needle lift (anticlockwise) and full needle lift (clockwise), due to their different flow passages in the sac, causing a much higher momentum inflow at the lower lift with its much narrower flow passage. Full article

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19 pages, 1256 KiB

Open AccessArticle

A Study of RANS Turbulence Models in Fully Turbulent Jets: A Perspective for CFD-DEM Simulations

by Dustin Steven Weaver and Sanja Mišković

Fluids 2021, 6(8), 271; https://doi.org/10.3390/fluids6080271 - 31 Jul 2021

Cited by 20 | Viewed by 5538

Abstract

This paper presents an analysis of linear viscous stress Favre averaged turbulence models for computational fluid dynamics (CFD) of fully turbulent round jets with a long straight tube geometry in the near field. Although similar work has been performed in the past with [...] Read more.

This paper presents an analysis of linear viscous stress Favre averaged turbulence models for computational fluid dynamics (CFD) of fully turbulent round jets with a long straight tube geometry in the near field. Although similar work has been performed in the past with very relevant solutions, considerations were not given for the issues and limitations involved with coupling between an Eulerian and Lagrangian phase, such as in fully two-way coupled CFD-DEM. These issues include limitations on solution domain, mesh cell size, wall modelling, and momentum coupling between the two phases in relation to the particles size. Therefore, within these considerations, solutions are provided to the Navier–Stokes equations with various turbulence models using a three-dimensional wedge long straight tube geometry for fully developed turbulence flow. Simulations are performed with a Reynolds number of 13,000 and 51,000 using two different tube diameters. It is found that a modified k-

ε

turbulence model achieved the most agreeable results for both the velocity and turbulent flow fields between these two flow regimes, while a modified k-

ω

SST/BSL also provided suitable results. Full article

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