Resolved Turbulent Stresses Research Articles

A dynamic forcing scheme incorporating backscatter for hybrid simulation

In this paper, a dynamic forcing scheme incorporating backscatter is proposed in order to remove the artificial buffer layer in a hybrid Reynolds-averaged Navier-Stokes (RANS)/large-eddy simulation (LES) approach. In contrast to previous forcing techniques, the proposed forcing is determined dynamically from the flow field itself, and does not require any extraction of turbulent fields from reference direct numerical simulation (DNS) or high-resolution LES databases. Transport equations for the resolved turbulent stresses and kinetic energy are introduced to investigate the effects of dynamic forcing on reduction of the thickness and impact of the artificial buffer layer. The proposed forcing model has been tested in the context of turbulent channel flows with Reynolds numbers Reτ = 650 and 1020 (based on the wall friction velocity and half channel height). In order to validate the hybrid RANS/LES approach, flow statistics obtained from the simulations have been thoroughly compared against the available DNS data.

Large-eddy simulation of turbulent flows in a heated streamwise rotating channel

In this paper, large-eddy simulation has been performed to investigate the impacts of the centrifugal and Coriolis forces on a heated plane channel flow subjected to streamwise system rotations. In order to study the rotation effects, a variety of rotation numbers Roτ ranging from 0 to 15 have been tested in conjunction with a fixed low Reynolds number Reτ=300. The subgrid-scale effects on the budget balance of turbulent stresses and heat fluxes have been investigated, and the physical mechanisms underlying the transport of resolved turbulent stresses under the influence of streamwise system rotations have been thoroughly analyzed. Numerical simulations were performed using two dynamic subgrid-scale stress models and two dynamic subgrid-scale heat flux models; namely, the conventional dynamic model (DM) and an advanced dynamic nonlinear model (DNM) for closure of the filtered momentum equation, and the conventional dynamic eddy thermal diffusivity model (DEDM) and an advanced dynamic full linear tensor thermal diffusivity model (DFLTDM) for closure of the filtered thermal energy equation. The predictive performances of the DM and DNM have been carefully compared in conjunction with the same subgrid-scale heat flux model DFLTDM. In order to validate the numerical approach, turbulent statistics obtained from the simulations have been compared with the available direct numerical simulation data.

Large-eddy simulation of turbulent heat convection in a spanwise rotating channel flow

In this paper, we investigate the effects of the Coriolis force in a heated plane channel flow subjected to spanwise rotation using the method of large-eddy simulation. We present both the general and simplified transport equations for the resolved turbulent stresses, which are essential for understanding the unique pattern of turbulent kinetic energy production in a rotating system. Numerical simulations are performed using primarily two dynamic subgrid-scale stress models and one dynamic subgrid-scale heat flux model; namely, the conventional dynamic model (DM) and a novel dynamic nonlinear model (DNM) for closure of the filtered momentum equation, and an advanced dynamic full linear tensor thermal diffusivity model (DFLTDM) for closure of the filtered thermal energy equation. The turbulent flow field studied herein is characterized by a Reynolds number Re τ = 150 and various rotation numbers Ro τ ranging from 0 to 7.5. In order to validate the LES approach, turbulent statistics obtained from the simulations are thoroughly compared with the available experimental results and direct numerical simulation (DNS) data. A detailed comparative study has been conducted in order to evaluate the performance of the DM and DNM in terms of their prediction of characteristic features of the velocity and temperature fields and their capability of reflecting both forward and backward scatter of kinetic energy between the filtered and subgrid scales.

A formulation for near-wall RANS/LES coupling

A near-wall eddy-viscosity formulation for LES is presented. A RANS-like eddy-viscosity corrected with the resolved turbulent stress is imposed in the near-wall region. The RANS eddy-viscosity is obtained from a resolved LES of channel flow at Re τ = 395 and stored in a look-up table. When used with a wall stress model, this technique enables LES to be performed on coarse grids. Results are presented for channel flow at several Reynolds numbers up to Re τ = 10,000. Various issues concerning the numerical behavior of the method are discussed.

An eddy-viscosity based near-wall treatment for coarse grid large-eddy simulation

An eddy-viscosity-based near-wall treatment is proposed to enable large-eddy simulations (LES) to be performed on coarse grids. This formulation consists of imposing wall stress boundary conditions and an eddy viscosity in the near-wall region. The wall stress and eddy viscosity have a Reynolds-averaged Navier-Stokes-like character and are obtained from an averaged velocity profile of a resolved LES of channel flow at Reτ=395. Both are tabulated and are used for the instantaneous quantities. The tabulated eddy viscosity is further corrected using the resolved turbulent stress. Numerical results for flow in a channel at several Reynolds numbers ranging from Reτ=395 to Reτ=10000 are presented.

On Taylor-series expansions of residual stress

Although subgrid-scale models of similarity type are insufficiently dissipative for practical applications to large-eddy simulation, in recently published a priori analyses, they perform remarkably well in the sense of correlating highly against exact residual stresses. Here, Taylor-series expansions of residual stress are exploited to explain the observed behavior and “success” of similarity models. Specifically, the first few terms of the exact residual stress τkl are obtained in (general) terms of the Taylor coefficients of the grid filter. Also, by expansion of the test filter, a similar expression results for the resolved turbulent stress tensor Lkl in terms of the Taylor coefficients of both the grid and test filters. Comparison of the expansions for τkl and Lkl yields the grid- and test-filter dependent value of the constant cL in the scale-similarity model of Liu et al. [J. Fluid Mech. 275, 83 (1994)]. Until recently, little attention has been given to issues related to the convergence of such expansions. To this end, we re-express the convergence criterion of Vasilyev et al. [J. Comput. Phys. 146, 82 (1998)] in terms of the transfer function and the cutoff wave number of the filter. As a rule of thumb, the less dissipative the filter (e.g., the higher the cutoff), the faster the rate of convergence.

Large-Eddy Simulation of Turbulent Obstacle Flow Using a Dynamic Subgrid-Scale Model

We apply the dynamic subgrid-scale model to a large-eddy simulation of turbulent channel flow with a square rib mounted on one wall. The Reynolds number Re is 3.21 x 10 3 based on the mean velocity above the obstacle and the obstacle's height. Near-wall structures are resolved with the no-slip boundary condition. The results show better agreement with direct numerical simulation than large-eddy simulation with a fixed model constant, verifying the value of the dynamic subgrid-scale model for simulating complex turbulent flows. ARGE-EDDY simulation (LES) is an accurate method of simulating complex turbulent flows in which the large flow structures are computed while small scales are modeled. The rationale behind this method is based on two observations: most of the turbulent energy is in the large structures, and the small scales are more isotropic and universal. Therefore, LES may be more general and less geometry dependent than Reynolds-averaged modeling, although it comes at higher cost. Even though LES has been used by many investigators, most research has been limited to flows with simple geometry. In engineering applications, however, one encounters more complicated geometries. Here we shall consider a rectangular parallelepiped mounted on a flat surface. Related flows are those over surfaces protruding from submarines (conning towers or control fins), wind flows around buildings, and airflows over computer chips, among others. The most distinctive features associated with these flows are three dimensionality, flow separation due to protruding surfaces, and large-scale unsteadiness. As a model flow, we consider a plane channel flow in which a two-dimensional obstacle is mounted on one surface (see Fig. 1). This relatively simple geometry contains flow separation and reattachment. Flow in this geometry has been studied by Tropea and Gackstatter1 for low Re and Werner and Wengle2 and Dimaczek et al.3 for high Re, among others. Recently, Germano et al. 4 suggested a dynamic subgridscale model (DSGSM) in which the model coefficient is dynamically computed as computation progresses rather than specified a priori. This approach is based on an algebraic identity between the subgrid-scale stresses at two different filter levels and the resolved turbulent stresses. They applied the model to transitional and fully turbulent channel flows and showed that the model contributes nothing in laminar flow and exhibits the correct asymptotic behavior in the nearwall region of turbulent flows without an ad hoc damping function. This is a significant improvement over conventional subgrid-scale modeling.

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. This feature is responsible for the correct near-wall behavior of the model coefficient. In the near-wall region the eddy viscosity is reduced to account for the energy transfer from small to large scales that may occur locally.

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.