Effect Of Subgrid-scale Model Research Articles

On the robustness and accuracy of large-eddy simulation in predicting complex internal flow of a gas-turbine combustor

The objective of this study is to evaluate the effects of numerical and model setups on the large-eddy simulation (LES) predictive capability for the internal flow of a propulsion-relevant configuration. The specific focus is placed on assessing the LES technique with lower mesh resolutions, which is of technological relevance to practical industrial design. A set of Riemann flux formulations and commonly used subgrid-scale models are considered in this work to produce a hierarchy of LES setups with different dissipation effects (both numerically and physically). The LES results obtained from different setups are compared qualitatively in terms of the key flow characteristics and evaluated quantitatively against the experimental measurements. The error landscape is generated to reveal the predictive qualities of different LES setups. The study shows that the choice of numerical flux formulation plays a prominent role in governing the general flow patterns, while the effect of subgrid-scale model is mainly manifested in transient flow characteristics, such as vortex breakdown and swirl-induced vortical structures. Based on the error analysis, it is found that lower dissipative LES setup is not always beneficial to the LES accuracy. This is in contrast to the commonly accepted understanding in literature for the LES, which was established solely with canonical flow configurations.

The effect of subgrid-scale modeling on LES of turbulent coaxial jets

Large eddy simulation (LES) is an efficient technique to simulate turbulent flows of practical interest, based on the elimination of flow scales smaller than a characteristic length and direct resolution of the largest scales. There is a variety of subgrid-scale models available in the literature to describe the turbulence in small scales, with different levels of complexity and computational cost. Although many advances have been achieved since the development of the LES technique, there is still no consensus on a definitive subgrid model for generic use in engineering applications. The objective of this work is to analyze the effect of subgrid modeling in LES of turbulent coaxial jets. Simulations were carried out for a test problem using the Smagorinsky, Germano and velocity structure function models, and results were compared with experimental data from the literature. The best results for average properties were found with the Smagorinsky model with an optimal ad hoc coefficient, while the dynamic model of Germano was the one that best described the turbulent features. These results show that this type of comparative analysis is of fundamental importance to obtain reliable results when addressing new problems.

The effect of subgrid-scale models on grid-scale/subgrid-scale energy transfers in large-eddy simulation of incompressible magnetohydrodynamic turbulence

In this work, the accuracy of various models used in large-eddy simulations (LES) of incompressible magnetohydrodynamic (MHD) turbulence is evaluated. Particular attention is devoted to the capabilities of models to reproduce the transfers between resolved grid- and subgrid-scales. The exact global balance of MHD turbulent flows is first evaluated from direct numerical simulation (DNS) database. This balance is controlled by the transfers between scales and between kinetic and magnetic energies. Two cases of forced homogeneous isotropic MHD turbulent flows are considered, with and without injecting large scale helicity. The strong helical case leads to domination of the magnetic energy due to an inverse cascade [A. Brandenburg, Astrophys. J. 550(2), 824 (2001); N. E. Haugen et al., Phys. Rev. E 70(1), 016308 (2004)]. The energy transfers predicted by various models are then compared with the transfer extracted from DNS results. This allows to discriminate models classically used for LES of MHD turbulence. In the non-helical case, the Smagorinsky-like model [M. L. Theobald et al., Phys. Plasmas 1, 3016 (1994)] and a mixed model are able to perform stable LES, but the helical case is a more demanding test and all the models lead to unstable simulations.

Effects of Subgrid Scale Modeling on the Deterministic and Stochastic Turbulent Energetic Distribution in Large-Eddy Simulations of a High-Pressure Turbine Stage

This study focuses on the engine-representative MT1 transonic high-pressure turbine. Simulated by use of wall-modeled large-eddy simulations (LES) with three different subgrid scale (SGS) closures, mean pressure profiles across the blades as well as mean radial profiles at the rotor exit are found to be in good agreement with experimental data with only local differences between models. Unsteady flow features, inherently present in LES, are however affected by SGS modeling. This is evidenced by the relative energetic content of the deterministic to stochastic turbulent contributions evaluated, thanks to the triple decomposition analysis of the simulations. Origins of such differences are found to impact the entire radial distribution of the flow and activity, with deterministic and chaotic contributions distributed differently depending on the SGS model and reference frequency used to extract the deterministic signal. Such flow responses can be attributed to the different SGS capacities to satisfy basic turbulent flow features that translate in different dissipative and turbulent diffusive contributions of the three SGS models.

The effect of subgrid-scale models on the entrainment of a passive scalar in a turbulent planar jet

Classical large-eddy simulation (LES) modelling assumes that the passive subgrid-scale (SGS) models do not influence large-scale quantities, even though there is now ample evidence of this in many flows. In this work, direct numerical simulation (DNS) and large-eddy simulations of turbulent planar jets at Reynolds number ReH = 6000 including a passive scalar with Schmidt number Sc = 0.7 are used to study the effect of several SGS models on the flow integral quantities e.g. velocity and scalar jet spreading rates. The models analysed are theSmagorinsky, dynamic Smagorinsky, shear-improved Smagorinsky and the Vreman. Detailed analysis of the thin layer bounding the turbulent and non-turbulent regions – the so-called turbulent/non-turbulent interface (TNTI) – shows that this region raises new challenges for classical SGS models. The small scales are far from equilibrium and contain a high fraction of the total kinetic energy and scalar variance, but the situation is worse for the scalar than for the velocity field. Both a-priori and a-posteriori (LES) tests show that the dynamic Smagorinsky and shear-improved models give the best results because they are able to accurately capture the correct statistics of the velocity and passive scalar fluctuations near the TNTI. The results also suggest the existence of a critical resolution Δx, of the order of the Taylor scale λ, which is needed for the scalar field. Coarser passive scalar LES i.e. Δx ≥ λ results in dramatic changes in the integral quantities. This fact is explained by the dynamics of the small scales near the jet interface.

The effect of subgrid-scale model on prediction of flow around a surface-mounted finite square cylinder

The present study investigates the effect of the subgrid-scale (SGS) stress model on large eddy simulation of flow over a finite-length square cylinder mounted vertically on a ground plane. The study specifically considers a cylinder of aspect ratio AR = 5 immersed in a thin boundary layer for a Reynolds number of Re = 500 (based on the free stream velocity and cylinder width.) Three different subgrid-scale models are considered: a Smagorinsky model, a dynamic Smagorinsky model and a dynamic nonlinear model. The mean flow structure is similar for all SGS models on both grids, although noticeable differences are observed in the size of the recirculation zone, level of resolved-scale turbulence and vortical structures for different SGS models. The dynamic Smagorinsky model appears to be especially sensitive to the grid resolution.

Effects of subgrid-scale modeling on Lagrangian statistics in large-eddy simulation

The application of large-eddy simulation (LES) to turbulent transport processes requires accurate prediction of the Lagrangian statistics of flow fields. However, in most existing SGS models, no explicit consideration is given to Lagrangian statistics. In this paper, we focus on the effects of SGS modeling on Lagrangian statistics in LES ranging from statistics determining single-particle dispersion to those of pair dispersion and multiparticle dispersion. Lagrangian statistics in homogeneous isotropic turbulence are extracted from direct numerical simulation (DNS) and the LES with a spectral eddy-viscosity model. For the case of longtime single-particle dispersion, it is shown that, compared to DNS, LES overpredicts the time scale of the Lagrangian velocity correlation but underpredicts the Lagrangian velocity fluctuation. These two effects tend to cancel one another leading to an accurate prediction of the longtime turbulent dispersion coefficient. Unlike the single-particle dispersion, LES tends to underestimate significantly the rate of relative dispersion of particle pairs and multiple-particles, when initial separation distances are less than the minimum resolved scale due to the lack of subgrid fluctuations. The overprediction of LES on the time scale of the Lagrangian velocity correlation is further confirmed by a theoretical analysis using a turbulence closure theory.

The effect of subgrid-scale models on the near wall vortices: A priori tests

Direct numerical simulation of a turbulent channel flow at Reτ=180 is used to analyze the resolved enstrophy dissipation caused by the subgrid-scale (SGS) models. The SGS enstrophy dissipation attains a minimum (forward scatter) at about y+≈8 and a backscatter region for y+<6, and is correlated with the high-speed streaks. A priori tests show that in the buffer layer, of all the models considered, the dynamic Smagorinsky, filtered structure function, and scale-similarity models display the smallest amount of resolved enstrophy dissipation, whereas the Smagorinsky and mixed models are the most dissipative.

Large eddy simulation (2D) of spatially developing mixing layer using vortex-in-cell for flow field and filtered probability density function for scalar field

A large eddy simulation based on filtered vorticity transport equation has been coupled with filtered probability density function transport equation for scalar field, to predict the velocity and passive scalar fields. The filtered vorticity transport has been formulated using diffusion-velocity method and then solved using the vortex method. The methodology has been tested on a spatially growing mixing layer using the two-dimensional vortex-in-cell method in conjunction with both Smagorinsky and dynamic eddy viscosity subgrid scale models for an anisotropic flow. The transport equation for filtered probability density function is solved using the Lagrangian Monte-Carlo method. The unresolved subgrid scale convective term in filtered density function transport is modelled using the gradient diffusion model. The unresolved subgrid scale mixing term is modelled using the modified Curl model. The effects of subgrid scale models on the vorticity contours, mean streamwise velocity profiles, root-mean-square velocity and vorticity fluctuations profiles and negative cross-stream correlations are discussed. Also the characteristics of the passive scalar, i.e. mean concentration profiles, root-mean-square concentration fluctuations profiles and filtered probability density function are presented and compared with previous experimental and numerical works. The sensitivity of the results to the Schmidt number, constant in mixing frequency and inflow boundary conditions are discussed. Copyright © 2005 John Wiley & Sons, Ltd.

The effect of subgrid-scale models on the vortices computed from large-eddy simulations

Direct numerical and large-eddy simulations (DNS/LES) of temporal plane jets are carried out in order to analyze the effect of the subgrid-scale (SGS) models on the vortices obtained from LES. The dynamics of the filtered vorticity norm (or filtered enstrophy) is analyzed through the application of a box filter to temporal DNS of turbulent plane jets (Reλ≈100), using a methodology similar to da Silva and Métais [J. Fluid Mech. 473, 103 (2002)]. Special emphasis is placed on the enstrophy SGS dissipation term, which represents the effect of the SGS models on the vortices computed from LES. When the filter is placed in the inertial range region the evolution of the vorticity norm is governed by the enstrophy production and enstrophy SGS dissipation, which represents, in the mean, a sink of resolved enstrophy. Thus the coherent vortices obtained from LES are subjected to an additional (nonviscous) dissipation mechanism. Locally, however, the enstrophy SGS dissipation can be either a sink or a source of resolved vorticity (forward/backward enstrophy cascade), but the forward cascade dominates, in analogy with what happens with the resolved kinetic energy equation. A priori tests are conducted using several SGS models in order to analyze their ability to represent the enstrophy SGS dissipation. The models analyzed are the Smagorinsky, structure function, filtered structure function, dynamic Smagorinsky, gradient, scale similarity, and mixed. It turns out that in terms of spatial localization all the models lead to a good correlation between the “real” and “modeled” enstrophy SGS dissipation. Moreover, all the SGS models, even of eddy-viscosity type, are able to provide enstrophy SGS backscatter. However, in terms of statistical behavior the eddy-viscosity models do not provide enough enstrophy backscatter as the non-eddy-viscosity models. LES are carried out and show that the Smagorinsky, structure function, and mixed models cause excessive vorticity dissipation compared to the other models, and although the enstrophy SGS dissipation affects mainly the smallest resolved scales, it may affect also some low-wave numbers. An estimation of the “vorticity error” and its wave number dependence is given, for each SGS model. Both a priori tests and LES show that the dynamic Smagorinsky and filtered structure function models seem to be the best suited to a correct prediction of the resolved vorticity field.

Large eddy simulations of the flow around a circular cylinder: effects of grid resolution and subgrid scale modeling

Large-eddy simulations of the flow around a circular cylinder at a Reynolds number, based on cylinder diameter and free-stream velocity, <TEX>$Re_D=2{\times}10^4$</TEX> are presented. Three different dynamic subgrid-scale models are used, viz. the dynamic eddy-viscosity model and two different mixed two-parameter models. The sensitivity to grid refinement in the spanwise and radial directions is systematically investigated. For the highest resolution considered, the effects of subgrid-scale modeling are also discussed in detail. In particular, it is shown that SGS modeling has a significant influence on the low-frequency modulations of the aerodynamics loads, which are related to significant changes in the near wake structure.

Large Eddy Simulation of a Road Vehicle with Drag-Reduction Devices

The e ow around an idealized road vehicle at Reynolds numbers up to 10 5 has been simulated using large eddy simulation; the numerical technique is based on the immersed boundary approach, which allows efe cient calculations to be carried out on a Cartesian grid. The effect of the Reynolds number and the wake modie cations produced by drag reduction devices attached to the base have been analyzed and compared with available experimental data. Averaged drag coefe cient and mean velocity proe les are in good agreement with measurement. The effect of subgrid-scale modeling (Smagorinsky and dynamic model ) has also been studied.

Effects of subgrid-scale modeling on time correlations in large eddy simulation

The effects of the unresolved subgrid-scale (SGS) motions on the energy balance of the resolved scales in large eddy simulation (LES) have been investigated actively because modeling the energy transfer between the resolved and unresolved scales is crucial to constructing accurate SGS models. But the subgrid scales not only modify the energy balance, they also contribute to temporal decorrelation of the resolved scales. The importance of this effect in applications including the predictability problem and the evaluation of sound radiation by turbulent flows motivates the present study of the effect of SGS modeling on turbulent time correlations. This paper compares the two-point, two-time Eulerian velocity correlation in isotropic homogeneous turbulence evaluated by direct numerical simulation (DNS) with the correlations evaluated by LES using a standard spectral eddy viscosity. It proves convenient to express the two-point correlations in terms of spatial Fourier decomposition of the velocity field. The LES fields are more coherent than the DNS fields: their time correlations decay more slowly at all resolved scales of motion and both their integral scales and microscales are larger than those of the DNS field. Filtering alone is not responsible for this effect: in the Fourier representation, the time correlations of the filtered DNS field are identical to those of the DNS field itself. The possibility of modeling the decorrelating effects of the unresolved scales of motion by including a random force in the model is briefly discussed. The results could have applications to the problem of computing sound sources in isotropic homogeneous turbulence by LES.

Numerical Analysis of Wall Turbulence Laden with Small Particles by LES (The Effect of SGS Models in the Flow to the Particle Motion).

In this study, the particle-laden flow in a wall turbulence is studied by the Large Eddy Simulation(LES). The results are compared with the Direct Numerical Simulation(DNS)conducted by Rouson et al.(1997). The wall Reynolds number(Reτ)is 180 and the particle diameters are assumed to be smaller than the Kolmogorov length scale. The one-way coupling method is employed for the interactin between the fluid and the particle. The Sub-Grid Scale(SGS)model dependencies on the numerical results are analyzed. Smagorinsky, dynamic SGS and scale similarity modeles are used for the comparison. Although the results simulated by the LES show reasonably good agreement with the DNS results, the difference between the DNS and the LES is larger than that among different SGS models. Further, the stochastic effects of the particle dispersion due to the SGS fluid motion are modeled and investigated. The results show that the stochastic effect is small enough in the present conditions.

Large-eddy simulation of mixing layer using vortex method: Effect of subgrid-scale models on early development

Large-eddy simulation of mixing layer using vortex method: Effect of subgrid-scale models on early development