Research

17 pages, 4894 KiB

Open AccessArticle

Evaluation of Turbulence Models in Unsteady Separation

by Claire Yeo MacDougall, Ugo Piomelli and Francesco Ambrogi

Fluids 2023, 8(10), 273; https://doi.org/10.3390/fluids8100273 - 07 Oct 2023

Viewed by 1600

Unsteady separation is a phenomenon that occurs in many flows and results in increased drag, decreased lift, noise emission, and loss of efficiency or failure in flow devices. Turbulence models for the steady or unsteady Reynolds-averaged Navier–Stokes equations (RANS and URANS, respectively) are [...] Read more.

Unsteady separation is a phenomenon that occurs in many flows and results in increased drag, decreased lift, noise emission, and loss of efficiency or failure in flow devices. Turbulence models for the steady or unsteady Reynolds-averaged Navier–Stokes equations (RANS and URANS, respectively) are commonly used in industry; however, their performance is often unsatisfactory. The comparison of RANS results with experimental data does not clearly isolate the modeling errors, since differences with the data may be due to a combination of modeling and numerical errors, and also to possible differences in the boundary conditions. In the present study, we use high-fidelity large-eddy simulation (LES) results to carry out a consistent evaluation of the turbulence models. By using the same numerical scheme and boundary conditions as the LES, and a grid on which grid convergence was achieved, we can isolate modeling errors. The calculations (both LES and RANS) are carried out using a well-validated, second-order-accurate code. Separation is generated by imposing a freestream velocity distribution, that is modulated in time. We examined three frequencies (a rapid, flutter-like oscillation, an intermediate one in which the forcing and the flow have the same timescales, and a quasi-steady one). We also considered three different pressure distributions, one with alternating favorable and adverse pressure gradients (FPGs and APGs, respectively), one oscillating between an APG and a zero-pressure gradient (ZPG), and one with an oscillating APG. All turbulence models capture the general features of this complex unsteady flow as well or better than in similar steady cases. The presence, during the cycle, of times in which the freestream pressure-gradient is close to zero affects significantly the model performance. Comparing our results with those in the literature indicates that numerical errors due to the type of discretization and the grid resolution are as significant as those due to the turbulence model. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

18 pages, 6968 KiB

Open AccessArticle

On the Composite Velocity Profile in Zero Pressure Gradient Turbulent Boundary Layer: Comparison with DNS Datasets

by Antonios Liakopoulos and Apostolos Palasis

Fluids 2023, 8(10), 260; https://doi.org/10.3390/fluids8100260 - 25 Sep 2023

Cited by 2 | Viewed by 1396

Abstract

Data obtained by direct numerical simulations (DNS) of the Zero-Pressure-Gradient Turbulent Boundary Layer were analyzed and compared to a mathematical model of the mean velocity profile (MVP) in the range 1000 ≤ Re_θ ≤ 6500. The mathematical model is based on the [...] Read more.

Data obtained by direct numerical simulations (DNS) of the Zero-Pressure-Gradient Turbulent Boundary Layer were analyzed and compared to a mathematical model of the mean velocity profile (MVP) in the range 1000 ≤ Re_θ ≤ 6500. The mathematical model is based on the superposition of an accurate description of the inner law and Coles’ wake function with appropriately chosen parameters. It is found that there is excellent agreement between the mathematical model and the DNS data in the inner layer when the Reynolds number based on momentum thickness, Re_θ, is greater than 1000. Furthermore, there is very good agreement over the entire boundary layer thickness, when Re_θ is greater than 2000. The diagnostic functions Ξ and Γ based on DNS data are examined and their characteristics are discussed in relation to the existence of a logarithmic layer or a power law behavior of the MVP. The diagnostic functions predicted by the mathematical model are also presented. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

16 pages, 10239 KiB

Open AccessArticle

Flow-Induced Forces for a Group of One Large and Several Small Structures in the Sheared Turbulent Flow

by Henry Francis Annapeh and Victoria Kurushina

Fluids 2023, 8(5), 158; https://doi.org/10.3390/fluids8050158 - 17 May 2023

Viewed by 936

Abstract

Evaluating the hydrodynamic force fluctuations acting on each structure in a group of subsea objects of different cross-section shapes, sizes and relative positions represents a challenge due to the sensitivity of the vortex shedding process, especially for a variety of sheared flows. The [...] Read more.

Evaluating the hydrodynamic force fluctuations acting on each structure in a group of subsea objects of different cross-section shapes, sizes and relative positions represents a challenge due to the sensitivity of the vortex shedding process, especially for a variety of sheared flows. The present study uses the numerical 2D computational fluid dynamics model to estimate the flow-induced forces on a group of small circular and D-shaped cylinders in the linear and parabolic sheared flow, which are placed in proximity to a larger structure of the squared cross-section. This allows us to evaluate loads, which are affected by the presence of subsea equipment located on the seabed. The average Reynolds number of the considered linear flow profile is 3900, while the parabolic flow profile has the maximum Reynolds number of 3900. The k-ω SST turbulence model is used for simulations. The work demonstrates the effect of the cross-sectional shape of smaller cylinders on hydrodynamic coefficients, explores the effect from the spacing in between the structures and highlights differences between loads in the linearly sheared and parabolic flow. The results obtained show that the presence of the squared cylinder notably influences the mean drag coefficient on the first cylinder, for both circular and D-shaped cylinders. The parabolic sheared flow profile in this series leads to the highest mean drag and the highest amplitudes of the fluctuating drag and lift coefficients. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

28 pages, 10996 KiB

Open AccessArticle

Suppression of the Spatial Hydrodynamic Instability in Scale-Resolving Simulations of Turbulent Flows Inside Lined Ducts

by Mikhail Shur, Mikhail Strelets and Andrey Travin

Fluids 2023, 8(4), 134; https://doi.org/10.3390/fluids8040134 - 17 Apr 2023

Viewed by 1064

Abstract

This paper addresses one of the major obstacles arising in the high-fidelity scale-resolving simulations of turbulent flows inside ducts with the walls covered by acoustic liners in order to attenuate the sound radiated from the duct. It consists of the development of spatial [...] Read more.

This paper addresses one of the major obstacles arising in the high-fidelity scale-resolving simulations of turbulent flows inside ducts with the walls covered by acoustic liners in order to attenuate the sound radiated from the duct. It consists of the development of spatial hydrodynamic (convective) instability over the treated walls at the low values of the acoustic resistance of the liner. For reasons that remain unclear, the growth rate of this instability and its effect on sound propagation through the duct is strongly overestimated by the CFD simulations using the macroscopic concept of the locally reacting acoustic impedance. A new damping volume source term (“body force”) is proposed, whose introduction into the momentum equation resolves this issue by means of artificially suppressing the instability while remaining within the framework of the computationally efficient model of the impedance wall, i.e., without trying to simulate the liner microscopically. Examples are presented of the application of the developed methodology to the flows in the grazing impedance tubes with two different liners. They suggest that the proposed form of the damping source term can be considered universal and that the suppression of the hydrodynamic instability ensured by this term is not accompanied by any significant distortion of the propagation of the sound waves and the turbulence statistics, except for a very narrow near-wall region. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

9 pages, 508 KiB

Open AccessArticle

Turbulence via Intermolecular Potential: Viscosity and Transition Range of the Reynolds Number

by Rafail V. Abramov

Fluids 2023, 8(3), 101; https://doi.org/10.3390/fluids8030101 - 18 Mar 2023

Cited by 2 | Viewed by 2227

Abstract

Turbulence in fluids is an ubiquitous phenomenon, characterized by spontaneous transition of a smooth, laminar flow to rapidly changing, chaotic dynamics. In 1883, Reynolds experimentally demonstrated that, in an initially laminar flow of water, turbulent motions emerge without any measurable external disturbance. To [...] Read more.

Turbulence in fluids is an ubiquitous phenomenon, characterized by spontaneous transition of a smooth, laminar flow to rapidly changing, chaotic dynamics. In 1883, Reynolds experimentally demonstrated that, in an initially laminar flow of water, turbulent motions emerge without any measurable external disturbance. To this day, turbulence remains a major unresolved phenomenon in fluid mechanics; in particular, there is a lack of a mathematical model where turbulent dynamics emerge naturally from a laminar flow. Recently, we proposed a new theory of turbulence in gases, according to which turbulent motions are created in an inertial gas flow by the mean field effect of the intermolecular potential. In the current work, we investigate the effect of viscosity in our turbulence model by numerically simulating the air flow at normal conditions in a straight pipe for different values of the Reynolds number. We find that the transition between laminar and turbulent flow in our model occurs, without any deliberate perturbations, as the Reynolds number increases from 2000 to 4000. As the simulated flow becomes turbulent, the decay rate of the time averaged Fourier spectrum of the kinetic energy in our model approaches Kolmogorov’s inverse five-thirds law. Both results are consistent with experiments and observations. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

16 pages, 3891 KiB

Open AccessArticle

A Method for Choosing the Spatial and Temporal Approximations for the LES Approach

by Sergei Bakhne and Vladimir Sabelnikov

Fluids 2022, 7(12), 376; https://doi.org/10.3390/fluids7120376 - 07 Dec 2022

Cited by 6 | Viewed by 1315

Abstract

Analysis and optimization of the hybrid upwind-central numerical methods for modern versions of large eddy simulations (LESs) are presented herein. Optimization was performed based on examination of the characteristics of the spatial and temporal finite-volume approximations of the convective terms of filtered Navier–Stokes [...] Read more.

Analysis and optimization of the hybrid upwind-central numerical methods for modern versions of large eddy simulations (LESs) are presented herein. Optimization was performed based on examination of the characteristics of the spatial and temporal finite-volume approximations of the convective terms of filtered Navier–Stokes equations. A method for selecting level of subgrid viscosity that corresponds to the chosen numerical scheme and makes it possible to obtain an extended inertial interval of the energy spectrum is proposed. A series of LESs of homogeneous isotropic turbulence decay were carried out, and the optimal values of the subgrid model constants included in the hybrid shear stress transport delay detached eddy simulation (SST-DDES) method were determined. A procedure for generating a consistent initial field of subgrid parameters for these simulations is described. The three-stage explicit Runge–Kutta method was demonstrated to be sufficient for stable time integration, while the popular explicit midpoint method was not. The slope of the energy spectrum was shown to be almost independent of the central-difference scheme order and of the mesh spacing when the correct numerical method was applied. Numerical viscosity was found to become much greater than subgrid viscosity when the upwind scheme contributed about 10% or more to the convective flux approximation. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

15 pages, 6866 KiB

Open AccessArticle

An Investigation of Scale-Resolving Turbulence Models for Supersonic Retropropulsion Flows

by Gabriel Nastac and Abdelkader Frendi

Fluids 2022, 7(12), 362; https://doi.org/10.3390/fluids7120362 - 23 Nov 2022

Viewed by 1642

Abstract

Characterization of unsteady loads is critical for the development of control systems for next-generation air vehicles. Both Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) methods are prohibitively expensive, and existing Reynolds-Averaged Navier-Stokes (RANS) approaches have been shown to be inadequate in [...] Read more.

Characterization of unsteady loads is critical for the development of control systems for next-generation air vehicles. Both Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) methods are prohibitively expensive, and existing Reynolds-Averaged Navier-Stokes (RANS) approaches have been shown to be inadequate in predicting both mean and unsteady loads. In recent years, scale-resolving methods, such as Partially Averaged Navier-Stokes (PANS) and Detached Eddy Simulation (DES), have been gaining acceptance and filling the gap between RANS and LES. In this study, we focus on a new variant of the PANS method, namely blended PANS or BPANS, which was shown to perform well in the incompressible regime for both wall-bounded and free shear flows. In this paper, we extend BPANS to compressible supersonic flows by adding a compressibility correction, leading to a new model called BPANS CC. The new model is tested using a well-known supersonic mixing layer case, and the results show good agreement with experimental data. The model is then used on a complex supersonic retropropulsion case and the results are in good agreement with experimental data. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

11 pages, 11104 KiB

Open AccessArticle

Computational Fluid Dynamics Model for Analysis of the Turbulent Limits of Hydrogen Combustion

by Ivan Yakovenko, Alexey Kiverin and Ksenia Melnikova

Fluids 2022, 7(11), 343; https://doi.org/10.3390/fluids7110343 - 01 Nov 2022

Cited by 2 | Viewed by 1649

Abstract

This paper presents a novel numerical approach for assessing the turbulent limits of hydrogen combustion. In the framework of this approach, the premixed combustion is studied numerically in the externally generated turbulent field with defined parameters. Two-dimensional calculations are carried out for hydrogen–air [...] Read more.

This paper presents a novel numerical approach for assessing the turbulent limits of hydrogen combustion. In the framework of this approach, the premixed combustion is studied numerically in the externally generated turbulent field with defined parameters. Two-dimensional calculations are carried out for hydrogen–air mixtures of different compositions, and all the possible modes of near-limit combustion are reproduced. Among these modes are: combustion in the form of spatially separated individual kernels and combustion in the form of kernels with subsequent quenching. The critical conditions between the mentioned two modes correspond to the turbulent limits of hydrogen combustion, which are necessary for the evaluation of the hazardous risks related to hydrogen explosions. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

24 pages, 15115 KiB

Open AccessArticle

A Bayesian Nonlinear Reduced Order Modeling Using Variational AutoEncoders

by Nissrine Akkari, Fabien Casenave, Elie Hachem and David Ryckelynck

Fluids 2022, 7(10), 334; https://doi.org/10.3390/fluids7100334 - 20 Oct 2022

Cited by 3 | Viewed by 1879

Abstract

This paper presents a new nonlinear projection based model reduction using convolutional Variational AutoEncoders (VAEs). This framework is applied on transient incompressible flows. The accuracy is obtained thanks to the expression of the velocity and pressure fields in a nonlinear manifold maximising the [...] Read more.

This paper presents a new nonlinear projection based model reduction using convolutional Variational AutoEncoders (VAEs). This framework is applied on transient incompressible flows. The accuracy is obtained thanks to the expression of the velocity and pressure fields in a nonlinear manifold maximising the likelihood on pre-computed data in the offline stage. A confidence interval is obtained for each time instant thanks to the definition of the reduced dynamic coefficients as independent random variables for which the posterior probability given the offline data is known. The parameters of the nonlinear manifold are optimized as the ones of the decoder layers of an autoencoder. The parameters of the conditional posterior probability of the reduced coefficients are the ones of the encoder layers of the same autoencoder. The optimization of both sets of the encoder and the decoder parameters is obtained thanks to the application of a variational Bayesian method, leading to variational autoencoders. This Reduced Order Model (ROM) is not a regression model over the offline pre-computed data. The numerical resolution of the ROM is based on the Chorin projection method. We apply this new nonlinear projection-based Reduced Order Modeling (ROM) for a 2D Karman Vortex street flow and a 3D incompressible and unsteady flow in an aeronautical injection system. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

13 pages, 1402 KiB

Open AccessArticle

Freestream Turbulence Effects on the Aerodynamics of an Oscillating Square Cylinder at the Resonant Frequency

by Yongxin Chen, Kamal Djidjeli and Zheng-Tong Xie

Fluids 2022, 7(10), 329; https://doi.org/10.3390/fluids7100329 - 16 Oct 2022

Cited by 1 | Viewed by 1643

Abstract

Flow past a bluff body in freestream turbulence can substantially change the flow behaviour compared to that in smooth inflow. This paper presents the study of wake flow and aerodynamics of an oscillating square cylinder at the resonant frequency in freestream turbulence, with [...] Read more.

Flow past a bluff body in freestream turbulence can substantially change the flow behaviour compared to that in smooth inflow. This paper presents the study of wake flow and aerodynamics of an oscillating square cylinder at the resonant frequency in freestream turbulence, with the integral length not greater than the cylinder side and the turbulence intensity not greater than 10%. Large eddy simulations (LES) in the Cartesian grid using the Immersed Boundary Method (IBM) technique embedded in a FVM solver, together with an efficient synthetic turbulent inflow generator implemented in an in-house parallel FORTRAN code are used for the study. The results are compared with those for smooth inflow, and relevant data published in the literature. The key findings are: the freestream turbulence conditions evidently reduces the local turbulent scales and fluctuations in the shear layer compared to in smooth flow, as small scale freestream turbulence breaks down cylinder-generated larger scale eddies and weakens them; but does not evidently affect the vortex shedding frequency, or the length of the recirculation region behind the cylinder. This suggests negligible change of drag coefficient compared to in smooth inflow. Moreover, this is because the vortex shedding is dominated by the forced oscillation at the resonance frequency, and the turbulence intensity is small. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Next-Generation Methods for Turbulent Flows

Share This Special Issue

Special Issue Editor

Special Issue Information

Keywords

Published Papers (10 papers)

Research

Further Information

Guidelines

MDPI Initiatives

Follow MDPI