Ugo Piomelli

A dynamic subgrid‐scale eddy viscosity model

One major drawback of the eddy viscosity subgrid‐scale stress models used in large‐eddy simulations is their inability to represent correctly with a single universal constant different turbulent fields in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model is based on an algebraic identity between the subgrid‐scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid‐scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near‐wall region of a turbulent boundary layer. The results of large‐eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.

Large-eddy simulation: achievements and challenges

Abstract In this paper, the present state of the large-eddy simulation (LES) technique is discussed. Modeling and numerical issues that are under study will be described, and results of state-of-the-art calculations will be presented to highlight the response of the subgrid-scale models to important features of the flow field. Finally, some challenges for research and applications of LES in the near future will be discussed.

Large‐eddy simulation of rotating channel flows using a localized dynamic model

Most applications of the dynamic subgrid‐scale stress model use volume‐ or planar‐averaging to avoid ill‐conditioning of the model coefficient, which may result in numerical instabilities. Furthermore, a spatially‐varying coefficient is mathematically inconsistent with the original derivation of the model. A localization procedure is proposed here that removes the mathematical inconsistency to any desired order of accuracy in time. This model is applied to the simulation of rotating channel flow, and results in improved prediction of the turbulence statistics. The model coefficient vanishes in regions of quiescent flow, reproducing accurately the intermittent character of the flow on the stable side of the channel. Large‐scale longitudinal vortices can be identified, consistent with the observation from experiments and direct simulations. The effect of the unresolved scales on higher‐order statistics is also discussed.

Model consistency in large eddy simulation of turbulent channel flows

Combinations of filters and subgrid scale stress models for large eddy simulation of the Navier–Stokes equations are examined by a priori tests and numerical simulations. The structure of the subgrid scales is found to depend strongly on the type of filter used, and consistency between model and filter is essential to ensure accurate results. The implementation of consistent combinations of filter and model gives more accurate turbulence statistics than those obtained in previous investigations in which the models were chosen independently from the filter. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.

Subgrid-scale backscatter in turbulent and transitional flows

Most subgrid‐scale (SGS) models for large‐eddy simulations (LES) are absolutely dissipative (that is, they remove energy from the large scales at each point in the physical space). The actual SGS stresses, however, may transfer energy to the large scales (backscatter) at a given location. Recent work on the LES of transitional flows [Piomelli et al., Phys. Fluids A 2, 257 (1990)] has shown that failure to account for this phenomenon can cause inaccurate prediction of the growth of the perturbations. Direct numerical simulations of transitional and turbulent channel flow and compressible isotropic turbulence are used to study the backscatter phenomenon. In all flows considered roughly 50% of the grid points were experiencing backscatter when a Fourier cutoff filter was used. The backscatter fraction was less with a Gaussian filter, and intermediate with a box filter in physical space. Moreover, the backscatter and forward scatter contributions to the SGS dissipation were comparable, and each was often much...

High Reynolds number calculations using the dynamic subgrid-scale stress model

The dynamic subgrid‐scale eddy viscosity model has been used in the large‐eddy simulation of the turbulent flow in a plane channel for Reynolds numbers based on friction velocity and channel half‐width ranging between 200 and 2000, a range including values significantly higher than in previous simulations. The computed wall stress, mean velocity, and Reynolds stress profiles compare very well with experimental and direct simulation data. Comparison of higher moments is also satisfactory. Although the grid in the near‐wall region is fairly coarse, the results are quite accurate: the turbulent kinetic energy peaks at y+≂12, and the near‐wall behavior of the resolved stresses is captured accurately. The model coefficient is o(10−3) in the buffer layer and beyond, where the cutoff wave numbers are in the decaying region of the spectra; in the near‐wall region the cutoff wave numbers are nearer the energy‐containing range, and the resolved turbulent stresses become a constant fraction of the resolved stresses....

New approximate boundary conditions for large eddy simulations of wall-bounded flows

Two new approximate boundary conditions have been applied to the large eddy simulation of channel flow with and without transpiration. These new boundary conditions give more accurate results than those previously in use, and allow significant reduction of the required CPU time over simulations in which no‐slip conditions are applied. Mean velocity profiles and turbulence intensities compare well both with experimental data and with the results of resolved simulations. The influence of the approximate boundary conditions remains confined near the point of application and does not affect the turbulence statistics in the core of the flow.

The inner-outer layer interface in large-eddy simulations with wall-layer models

The interaction between the inner and outer layer in large-eddy simulations (LES) that use approximate near-wall treatments is studied.In hybrid Reynolds-averaged Navier–Stokes (RANS)/LES models a transition layer exists between the RANS and LES regions, which has resulted in incorrect prediction of the velocity profiles, and errors of up to 15% in the prediction of the skin friction.Several factors affect this transition layer, but changes we made to the formulation had surprisingly little effect on the mean velocity.In general, it is found that the correct prediction of length- and time-scales of the turbulent eddies in the RANS region is important, but is not the only factor affecting the results.The inclusion of a backscatter model appears to be effective in improving the prediction of the mean velocity profile and skin-friction coefficient.

A priori and a posteriori tests of inflow conditions for large-eddy simulation

Comparisons of inflow conditions for large-eddy simulations of turbulent, wall-bounded flows are carried out. Consistent with previous investigations, it is found that the spectral content of the inflow velocity is important. Inflow conditions based on random-noise, or small-scale eddies only, dissipate quickly. Temporal and spatial filtering of a time series obtained from a separate calculation indicates that it is important to capture eddies of dimensions equal to or larger than the integral length scale of the flow. Three methods for generating inflow velocity fields are tested in a simulation of spatially developing turbulent channel flow. Synthetic turbulence generation methods that introduce realistic length scales are more suitable than uncorrelated random noise, but still require fairly long development lengths before realistic turbulence is established. A recycling method based on the use of turbulent data obtained from a separate calculation, in different flow conditions, was found to result in more rapid transition. A forcing method that includes a control loop also appears to be effective by generating turbulence with the correct Reynolds stresses and correlations within less than ten channel half heights.

Subgrid-Scale Models for Compressible Large-Eddy Simulations ?

Abstract:An a priori study of subgrid-scale (SGS) models for the unclosed terms in the energy equation is carried out using the flow field obtained from the direct simulation of homogeneous isotropic turbulence. Scale-similar models involve multiple filtering operations to identify the smallest resolved scales that have been shown to be the most active in the interaction with the unresolved SGSs. In the present study these models are found to give more accurate prediction of the SGS stresses and heat fluxes than eddy-viscosity and eddy-diffusivity models, as well as improved predictions of the SGS turbulent diffusion, SGS viscous dissipation, and SGS viscous diffusion.