Eddy Simulation Of Turbulent Flow Research Articles

Multiscale subgrid models of large eddy simulation for turbulent flows

Purpose – The purpose of this paper is to propose three modified subgrid-scale (SGS) eddy-viscosity models to improve their original eddy-viscosity models (the Smagorinsky model (SM), the mixed-scale model (MSM), and the wall-adapted local eddy-viscosity model (WALE)) in the simulation of turbulent flows in near-wall region. Design/methodology/approach – The subgrid viscosity is related to the norm of strain rate tensor of the smallest resolved scales, instead of the norm of the resolved strain rate tensor of the large scales. Findings – All the SGS viscosity of the modified eddy-viscosity models (small-large model, modified MSM, and modified WALE) is closer to y+3 behavior than those of the original eddy-viscosity models (SM, MSM, and WALE) near the wall. Originality/value – The norm of strain rate tensor of the smallest scales used in eddy-viscosity models, instead of the norm of strain rate tensor, makes the eddy viscosity in near-wall region approach to zero in a physical sense.

Detached eddy simulation of turbulent flow around square and circular cylinders on Cartesian cut cells

Square and circular cylinders in three-dimensional turbulent flows are studied numerically using the LES and DES turbulence models. One aim of the present study is to implement the LES and DES turbulence models in a cell-centered finite volume method (FVM) developed for solving the Navier–Stokes equations on Cartesian cut cells. The Cartesian cut cell approach is known to be robust for problems in geometrically complex domains with fixed or moving boundaries. For the purpose of validating the present numerical model, the current flow past fixed square and circular cylinders at moderate Reynolds numbers is tested first. Comparison of the computed results with experimental data reveals that the DES models are superior to the conventional LES and RANS models. The second aim of the present study is to assess the performance of different RANS based DES turbulence models. By means of the comparison of results obtained with the 0-equation mixing-length, 1-equation S–A and 2-equation k–ω based DES models for the flow over the same circular cylinder, some recommendations are proposed. According to the present study, in terms of accuracy the 1-equation S–A based DES model is very promising. Beside this, if the computational cost is the main concern, the 0-equation mixing-length based DES model might be an ideal option, achieving a good balance between accuracy and efficiency.

Dynamic optimization methodology based on subgrid-scale dissipation for large eddy simulation

A dynamic procedure based on subgrid-scale dissipation is proposed for large eddy simulation of turbulent flows. In the new method, the model coefficients are determined by minimizing the square error of the resolved dissipation rate based on the Germano identity. A dynamic two-term mixed model is tested and evaluated both a priori and a posteriori in simulations of homogeneous and isotropic turbulence. The new dynamic procedure proves to be more effective to optimize the model coefficients as compared with traditional method. The corresponding dynamic mixed model can predict the physical quantities more accurately than traditional dynamic mixed model.

Entropy Filtered Density Function for Large Eddy Simulation of Turbulent Flows

A methodology termed entropy filtered density function (En-FDF) is developed to account for entropy transport in large eddy simulation of turbulent flows. The filtered entropy transport equation includes several unclosed terms which are the subgrid scale entropy flux and the filtered entropy generation resulting from irreversibilities in heat conduction, mass diffusion, chemical reaction and viscous dissipation. The En-FDF contains the complete statistical information about entropy, velocity, scalar and turbulent frequency fields and thus, provides closure for all the unclosed terms in the filtered transport equations. An exact transport equation is derived for the En-FDF which contains the effects of convection along with chemical reaction and its associated entropy generation in closed forms. The En-FDF transport is modeled by a set of stochastic differential equations. The numerical solution procedure is based on a hybrid finite difference/Monte Carlo method in which the large eddy simulation filtered transport equations are solved by the finite difference, and the stochastic differential equations are solved by a Lagrangian Monte Carlo procedure. This methodology is applied for large eddy simulation of a turbulent shear layer involving transport of passive scalars and predictions are compared against the data generated by direct numerical simulation. The En-FDF predictions show favorable agreement with the direct numerical simulation data.

Detached eddy simulation of turbulent flow in isolated street canyons of different aspect ratios

Air quality management in urban areas requires the use of advanced modeling tools, able to predict and evaluate the pollution level under different traffic and meteorological conditions. In the present paper, the Artificial Compressibility version of the Characteristic Based Split algorithm (AC–CBS) was used to assess the performance of the Spalart–Allmaras based Detached Eddy Simulation (SA–DES) model, for the calculation of incompressible turbulent flow in different urban street canyon configurations. To our knowledge, the DES version of the SA turbulence model was applied in this work for the first time for the simulation of turbulent flow in a street canyon. The proposed DES model was able to accurately reproduce the turbulent characteristics of the flow compared with results from real street canyon experiments, wind tunnel experiments, and also to that obtained with RANS simulations. These results are very similar to the ones obtained from Large Eddy Simulation (LES) of street canyons flow reported in some recent publications, but with the potential characteristic of reduced computational costs. The DES approach is very promising for the simulation of transient turbulent flows in urban areas when complex three–dimensional domains are considered. The performance of the DES model evaluated for the mean dimensionless streamwise velocity profiles was comparable to that of Reynolds–Averaged Navier–Stokes RANS approach if referred to Hit Rate (HR) validation metric, and even better if referred to Factor of two observation (FAC2) validation metric. An accurate reproduction of the turbulent flow is crucial for urban pollutant dispersion simulations, since the distribution of the pollutant concentrations could differ by order of magnitude in the different points of the street canyon. DES approach results were able to accurately predict the unsteadiness characteristic of the flow, and to reproduce some minor vortex structures, which were not observed in the RANS cases, that will lead to a more accurate reproduction of the pollutant concentrations.

P076 角柱を設置した平行平板間流れと熱伝達のLES解析(メカボケーション学生研究発表セッション)

P076 角柱を設置した平行平板間流れと熱伝達のLES解析(メカボケーション学生研究発表セッション)

Detached Eddy Simulation of Turbulent Flow in Stirred Tank Reactor

The difficulties associated with the use of the standard LES (Large Eddy Simulation) models, particularly in near-wall regions, has lead to the development of hybrid models that attempt to combine the best aspects of RANS (Reynolds Averaged Navier Stokes) and LES methodologies in a single solution strategy. An example of a hybrid technique is the detached-eddy simulation (DES). In this paper, application of DES model is made in the analysis of turbulent flow generated in impeller driven (Rushton Impeller) stirred tank reactor in order to understand the flow behavior especially around the impeller region and near the wall of the tank. The simulated result is compared with Experimental results of Wu and Patterson [1]. DES is found more accurate against the experimental result in predicting the turbulent flow of velocity profile than the LES model which ultimately increases its interest in turbulent flow analysis.

Unsteady Characteristics of Sediment Transport Under Non-harmonic Waves

Sediment transport under waves is one of the key dynamic processes of coastal sediment motion. Previous studies for this issue are mainly based on linear wave theory and quasi-steady simplification. However, waves in real circumstance are always irregular, generally with a certain degree of asymmetry and/or velocity-leaning especially in shallow water. It is envisaged that sediment movement under non-harmonic waves is different from sinusoidal waves. Based on large eddy simulation of turbulent flows in wave boundary layer, a modified method is proposed to calculate sediment transport rate under non-harmonic waves. It is further used to explore the influences of flow acceleration and the phase lead between the free stream velocity and bed shear stress on sediment transport rate in different phases of one wave cycle for forth-leaning. The net transport rate is found to increase with the degree of asymmetry and velocity-leaning index. Both the acceleration and the phase lead have a great influence on average transport rate in each phase.

Large eddy simulation of turbulent flow around a wall mounted hemisphere

Turbulent air flow around a wall-mounted hemisphere is investigated numerically for Reynolds numbers equal to 36,000 and 64,000. Large eddy simulation (LES) is used to simulate the flow field and results are compared with wind tunnel measurements and computations from RNG k–ε modeling. The shortcoming of the RANS model is reported and the LES advantages are highlighted for modeling of such external flows around a bluff body. Various subgrid scale models are considered and compared for LES simulation. Using the numerical simulation results, streamlines at different sections of the flow domain are presented and pressure and velocity distribution at different locations over the hemisphere are plotted and compared. It is shown that the LES model can be properly used to obtain the complex flow field behind the hemisphere where the RANS model fails to predict the flow field accurately. Among subgrid scale models the prediction of kinetic energy turbulence model shows better agreement with the experimental data.

A residual based eddy viscosity model for the large eddy simulation of turbulent flows

A new eddy viscosity model for the large eddy simulation of turbulent flows is developed. The eddy viscosity is proportional to the product of the magnitude of the fine scale velocity and the grid size. The residual based variational multiscale formulation is used to obtain a computationally inexpensive approximation for the fine scale velocity field. It is noted that the fine scale velocity field, and hence the eddy viscosity, are proportional to the residual of the resolved scale solution. As a result the eddy viscosity automatically vanishes whenever or wherever the coarse scale solution is accurate. In that regard this model is inherently dynamic and does not require the application of a dynamic procedure. The performance of this model is tested in predicting the decay of incompressible and compressible homogeneous turbulence. In both cases it is found that it is more accurate than the dynamic version of the Smagorinsky-type eddy viscosity model. In addition, it is easier to implement as it does not require a dynamic procedure in order to determine model parameters.

Swirling-strength based large eddy simulation of turbulent flow around single square cylinder at low Reynolds numbers

In view of the fact that large scale vortices play the substantial role of momentum transport in turbulent flows, large eddy simulation (LES) is considered as a better simulation model. However, the sub-grid scale (SGS) models reported so far have not ascertained under what flow conditions the LES can lapse into the direct numerical simulation. To overcome this discrepancy, this paper develops a swirling strength based the SGS model to properly model the turbulence intermittency, with the primary characteristics that when the local swirling strength is zero, the local sub-grid viscosity will be vanished. In this paper, the model is used to investigate the flow characteristics of zero-incident incompressible turbulent flows around a single square cylinder (SC) at a low Reynolds number range Re ∈ [103, 104]. The flow characteristics investigated include the Reynolds number dependence of lift and drag coefficients, the distributions of time-spanwise averaged variables such as the sub-grid viscosity and the logarithm of Kolmogorov micro-scale to the base of 10 at Re = 2 500 and 104, the contours of spanwise and streamwise vorticity components at t = 170. It is revealed that the peak value of sub-grid viscosity ratio and its root mean square (RMS) values grow with the Reynolds number. The dissipation rate of turbulent kinetic energy is larger near the SC solid walls. The instantaneous factor of swirling strength intermittency (FSI) exhibits some laminated structure involved with vortex shedding.

Evaluation of scale resolving turbulence generation methods for Large Eddy Simulation of turbulent flows

Large Eddy Simulation (LES) has become an attractive simulation method even for technical processes and it usually provides space and time resolved fluctuations of a significant portion of the spectrum. However, in contrast to a RANS simulation an accurate LES requires the definition of suitable initial and boundary conditions, which includes turbulent structures with physically sound spatial and temporal correlations. Such turbulent structures are usually generated artificially at the boundary. Three different algorithms for generating turbulent fluctuations are evaluated in the present work. The investigated methods are Filtered noise [1], Diffused noise [2] and an Inverse Fourier approach [3,4]. These techniques were developed for generating inflow data for LES and have already been used in published research [5–15], e.g. for investigating turbulent combustion processes. In the present work the turbulent statistics i.e. energy spectra and velocity correlations as well as derived quantities such as turbulent kinetic energy and subgrid scale viscosity are investigated in more detail in a comparative fashion for the generated turbulent velocity fields. As a simple test case, the decay of turbulence in a cubical box, is considered here to provide information on the initially generated turbulence as well as its temporal evolution. The results are analyzed in detail and are compared to experimental data. Turbulence fluctuations generated by Filtered noise and Diffused noise lead to similar results. The resulting energy spectra and velocity correlations agree generally well with experimental data despite some discrepancies at very early times after initialization. The Inverse Fourier approach yielded good agreement at all times, but at increased computational cost. In addition, the implementation of Filtered noise and Diffused noise might be easier for most cases of practical interest. In particular, the Diffused noise approach can be used for the generation of inhomogeneous turbulence on arbitrary grids.

LES Study of Flow Between Shrouded Co-rotating Disks

Large eddy simulations of turbulent flow between shrouded co-rotating disks, representing a simplified model of a hard disk drive, are performed. The computation domain surrounds a complete disk and is bounded at top and bottom by half a disk. Therefore, it is possible to compute the fluctuating pressure field surrounding the middle disk. Also, the influence of the shroud geometry is taken into account by comparing a flat shroud wall and a wall with rib chambers. In the flat shroud case, the fluctuating pressure on the upper- and lower-surface of a disk indicates a strong correlation with fluid motion travelling across the disk-tip clearance region. However, in the ribbed shroud case the organized flow structure that is observed in the flat shroud case disappears and the fluctuating pressure acting on the surface of the disk is remarkably diminished.

Large eddy simulation of turbulent flow and heat transfer in a square duct with unstable natural convection on the cross section

In this paper, large eddy simulations with the dynamic Smagorinsky eddy viscosity model are performed for a fully developed turbulent flow and heat transfer in a square duct with unstable natural convection on the cross section at a turbulent Reynolds number of 400, a Prandtl number of 0.7 and different Grashof numbers from 105 to 5×107. The MPI parallel programming language is implemented to speed up the calculations. The influences of the buoyancy force on the mean flow and heat transfer, turbulent intensity and Reynolds stresses are analyzed. The results show that the mean velocity decreases while the subgrid viscosity, turbulent intensity and heat transfer increase obviously with the increase of Grashof number. The turbulent intensity in the streamwise direction decreases considerably in the near-wall region while the turbulent intensity in the spanwise direction increases clearly in almost whole regions with an increase in Grashof number. The spatial distribution of Reynolds stresses are mainly influenced by the thermal buoyancy force. The turbulence production rate and the buoyancy force production term of Reynolds stress component 〈v′v′〉 increase with the increase of Grashof number.

Statistical behavior of the orthogonal subgrid scale stabilization terms in the finite element large eddy simulation of turbulent flows

Numerical simulations have proved that Variational Multiscale Methods (VMM) perform well as pure numerical large eddy simulation (LES) models. In this paper we focus on the orthogonal subgrid scale (OSS) finite element method and make an analysis of the statistical behavior of its stabilization terms in the quasi static approximation. This is done by resorting to results from classical statistical fluid mechanics concerning two point velocity, pressure and combined correlation functions of various orders. Given a fine enough mesh with characteristic element size h in the inertial subrange of a turbulent flow, it is shown that the rate of transfer of subgrid kinetic energy provided by the OSS stabilization terms does not depend on h and that it equals the molecular physical dissipation rate (up to a dimensionless constant that only depends on the finite element shapes) for a proper redesign of the standard parameters of the formulation. This is a noteworthy fact taking into account that the subgrid stabilization terms do not arise from physical considerations, but from the mathematical necessity to allow equal interpolation for the pressure and velocity fields, as well as to control convection. Therefore, the obtained results contribute somehow to the line of reasoning supporting that pure numerical approaches (i.e., without introducing additional physical models) could probably suffice in the LES simulation of turbulent flows.

Large eddy simulation study of turbulent flow around smooth and rough domes

Large eddy simulation of turbulent flow around smooth and rough hemispherical domes was conducted. The roughness of the rough dome was generated by a special approach using quadrilateral solid blocks placed alternately on the dome surface. It was shown that this approach is capable of generating the roughness effect with a relative success. The subgrid-scale model based on the transport of the subgrid turbulent kinetic energy was used to account for the small scales effect not resolved by large eddy simulation. The turbulent flow was simulated at a subcritical Reynolds number based on the approach free stream velocity, air properties, and dome diameter of 1.4 × 105. Profiles of mean pressure coefficient, mean velocity, and its root mean square were predicted with good accuracy. The comparison between the two domes showed different flow behavior around them. A flattened horseshoe vortex was observed to develop around the rough dome at larger distance compared with the smooth dome. The separation phenomenon occurs before the apex of the rough dome while for the smooth dome it is shifted forward. The turbulence-affected region in the wake was larger for the rough dome.

Large eddy simulation of turbulent flow and mass transfer in far-field of swirl diffusers

Herein, the turbulent flow structures, mass transfer and ventilations characteristics in the far-field of swirl diffusers are numerically studied at different swirl angles (45°≤α≤65°). The results are obtained via a 2nd order finite difference scheme and large eddy simulation (LES) with the dynamic Smagorinsky procedure in sub-grid-scale modeling. Accordingly, the instantaneous and average velocity distributions, transient evolution of the mass transfer, and the overall and local air qualities are investigated in this paper. Also, three main secondary-flow structures are observed in the simulations: a bubble vortex breakdown (α≤45°), a toroidal recirculation zone on the top corners of the room (α≤50°), and a toroidal central recirculation zone attached to the floor (far-field). However, the jet type alternates from the open swirl to Coanda jet in the range 50°≤α≤55°, where both types exhibit enhanced ventilation characteristics. In this range, the open swirl jet with the highest swirl angle is more effective in discharging the aged air and also shows improved local air quality in the central region of the room (at α=50°). On the other hand, the Coanda jet with the least swirl angle is more effective in mixing the fresh air throughout the domain and improves the average air quality (at α=55°).

A dynamic sub-grid scale model for large eddy simulation of turbulent flows in a lid-driven cubical cavity

This work undertakes a numerical study of turbulent incompressible flows in lid-driven cubical cavities using Large Eddy Simulation and two sub-grid scale models, i.e., the WALE (Wall-Adapting Local Eddy-viscosity) model and the corresponding dynamic sub-grid model (DSGS). In the process of using DSGS, an optimal value of constant CW of the WALE model was determined for a pre-set Reynolds number Re=104. The computed numerical results showed very good agreement with those Direct Numerical Simulation (DNS) results and with the experimental measurements found in the literature. Optimal values of CW were determined afterwards with the DSGS model and they were proposed for the analysis of higher Reynolds number turbulent flows. At the end, a power law correlation between CW and Re was proposed for the range 104⩽Re⩽3×104.

Analytical insights into the partially integrated transport modeling method for hybrid Reynolds averaged Navier-Stokes equations-large eddy simulations of turbulent flows

The basis of the partially integrated transport modeling (PITM) method was introduced by Schiestel and Dejoan [“Towards a new partially integrated transport model for coarse grid and unsteady turbulent flow simulations,” Theor. Comput. Fluid Dyn. 18, 443 (2005)]10.1007/s00162-004-0155-z and Chaouat and Schiestel [“A new partially integrated transport model for subgrid-scale stresses and dissipation rate for turbulent developing flows,” Phys. Fluids 17, 065106 (2005)]10.1063/1.1928607. This method provides a continuous approach for hybrid RANS-LES (Reynolds averaged Navier-Stokes equations-large eddy simulations) simulations with seamless coupling between RANS and LES regions. The main ingredient of the method is the new dissipation-rate equation that can be applied as a subfilter scale turbulence model. Then, it becomes easy to convert almost any usual RANS transport model into a subfilter scale model. In particular, the method can be applied to two equation models and to stress transport models as well. In the derivation of the method, the partial integration technique allows to keep a link between the spectral space and the physical space of the resulting model. The physical turbulent processes involving the production, dissipation, and flux transfer of the turbulent energy are introduced in the equations. The present work, after recalling the main building steps of the PITM method, brings further insight into the physical interpretation of the method, its underlying hypotheses and its internal acting mechanisms. In particular, the finiteness of the coefficients used in the dissipation-rate equation is discussed in detail from a theoretical point of view. Then, we consider the analytical example of self-similar turbulent flow for analyzing the dissipation-rate equation. From an analytical solution obtained by Taylor series expansions taking into account the Kovasznay hypothesis for evaluating the transfer term, we compute the functional coefficients \documentclass[12pt]{minimal}\begin{document}$c_{{\epsilon }_2}$\end{document}cε2 and \documentclass[12pt]{minimal}\begin{document}$c_{sfs{\epsilon }_2}$\end{document}csfsε2 used in RANS and LES methodologies, respectively, and we demonstrate that both coefficients take on finite values when the Reynolds number goes to infinity. Finally, after briefly mentioning some flow illustrations to get a real appraisal of the PITM method in its capabilities to simulate unsteady flows on relatively coarse grids with a sufficient accuracy for engineering computations, we study the coefficient \documentclass[12pt]{minimal}\begin{document}$c_{sfs{\epsilon }_2}$\end{document}csfsε2 through one chosen example.

Large Eddy Simulation of Turbulent Flow and Mixing Time in a Gas–Liquid Stirred Tank

Mixing time is a key parameter relevant to the scale-up and design of agitated reactors. Although there have been many published papers on mixing times in stirred tanks predicted by computational fluid dynamics (CFD), there are few reports on the large eddy simulation (LES) based prediction of the mixing time in a gas liquid stirred tank. In this work, an LES method based on an Eulerian-Eulerian model is presented for predicting the mixing time in a gas liquid stirred tank agitated by a Rushton turbine. In order to verify the simulated results, mixing time experiments were carried out using a conductivity technique. In the present LES, the Smagorinsky subgrid scale model was used to model the effect of subgrid scale on the resolved scales. The concentration distributions and operating parameters such as feed positions, impeller speeds, and gas flow rates on the mixing time were examined. It is shown that the predicted concentration distributions of tracers are more irregular and realistic by using LES. Also, the mixing time decreases with the increase of impeller speed. However, with increasing gas flow rate, the mixing time first increases and then levels off. The predicted mixing time by the LES method shows good agreement with the measured values.