Reynolds-averaged Navier–Stokes Region Research Articles

Improvement of double-buffer problem in LES–RANS interface region by introducing an anisotropy-resolving subgrid-scale model

The “double-buffer problem” has been regarded as a crucial concern for the strategy behind the hybrid large eddy simulation (LES)/Reynolds-averaged Navier–Stokes (RANS) model (or HLR model, for short). Such models are likely to show unphysical mean-velocity distributions in the LES–RANS interface region, where “super-streak structures” also appear that look like low-speed streaks generated in the near-wall region of wall turbulence. To overcome this difficulty, the stochastic backscatter model, in which the vortex structures in the interface region are divided into smaller scales, holds promise due to the effect of random source term imposed in the momentum equation. Although this method is effective, several parameters must be prescribed and their specification process is arbitrary and ambiguous. An alternative advanced HLR model has been proposed, in which an anisotropy-resolving subgrid-scale (SGS) model was adopted in the LES region as well as a one-equation nonlinear eddy viscosity model in the RANS region. Previous investigations indicated that this HLR model did not exhibit or, at least, largely reduced the “double-buffer problem” in the mean-velocity distribution, with no special treatment being applied. The main purpose of the present study is to reveal why this HLR model improves the predictive performance in the LES–RANS interface region. Specifically, we focus on the role of the extra anisotropic term introduced in the SGS model, finding that it plays an important role in enhancing vortex structures in the interface region, leading to a considerable improvement in model performance.

Reynolds-stress-constrained large-eddy simulation of wall-bounded turbulent flows

AbstractIn the traditional hybrid RANS/LES approaches for the simulation of wall-bounded fluid turbulence, such as detached-eddy simulation (DES), the whole flow domain is divided into an inner layer and an outer layer. Typically the Reynolds-averaged Navier–Stokes (RANS) equations are used for the inner layer, while large-eddy simulation (LES) is used for the outer layer. The transition from the inner-layer solution to the outer-layer solution is often problematic due to the lack of small-scale dynamics in the RANS region. In this paper, we propose to simulate the whole flow domain by large-eddy simulation while enforcing a Reynolds-stress constraint on the subgrid-scale (SGS) stress model in the inner layer. Both the algebraic eddy-viscosity model and the one-equation Spalart–Allmaras (SA) model have been used to constrain the Reynolds stress in the inner layer. In this way, we improve the LES methodology by allowing the mean flow of the inner layer to satisfy the RANS solution while small-scale dynamics is included. We validate the Reynolds-stress-constrained large-eddy simulation (RSC-LES) model by simulating three-dimensional turbulent channel flow and flow past a circular cylinder. Our model is able to predict mean velocity, turbulent stress and skin-friction coefficients more accurately in turbulent channel flow and to estimate the pressure coefficient after separation more precisely in flow past a circular cylinder compared with the pure dynamic Smagorinsky model (DSM) and DES using the same grid resolution. Furthermore, the computational cost of the RSC-LES is almost the same as that of DES.

Wall-Resolved LES and Zonal LES of Round Jet Impingement Heat Transfer on a Flat Plate

Numerical large-eddy simulation (NLES) is performed for a round jet impinging on a flat surface at a Reynolds number of Re = 23,000 for nozzle-to-plate spacings of H/D = 6 and 2, where H is the distance from the nozzle to the plate and D is the jet diameter. The Reynolds number has been set to match the experiments of Cooper et al. (Int J. Heat Mass Transfer, vol. 36, pp. 2675–2684, 1993). Two numerical large-eddy simulation approaches are examined. The first quasi-direct numerical simulation (DNS) approach resolves streaklike structures using fine near-wall grids; the second is the zonal approach of Tucker (Int J. Heat Fluid Flow, vol. 25, pp. 625–635, 2004), which uses the Wolfshtein k–l (Int J. Heat Mass Transfer, vol. 12, pp. 301–318, 1969) Reynolds-averaged Navier-Stokes (RANS) model near the walls and NLES elsewhere. A Hamilton-Jacobi equation is used to match the RANS region to the NLES zone. The use of a Spalart-Allmaras model leads to low levels of turbulent viscosity in the near-wall region. This is also observed when using detached-eddy (DES) when using a volume-based filter. The use of the standard DES filter based on maximum grid spacing prevents jet shear-layer transition. The k–l near-wall model maintains RANS levels of turbulent viscosity in the boundary layer. The results of both the near-wall quasi-DNS and hybrid RANS-NLES methods are generally encouraging.

Improved turbulence generation techniques for hybrid RANS/LES calculations

Hybrid turbulence simulations that use a combination of Reynolds-averaged Navier–Stokes (RANS) equations and large-eddy simulation (LES) are becoming increasingly popular to increase predictive accuracy in complex flow situations without the cost of full large-eddy simulations. In a typical application, the RANS equations are solved in the equilibrium or near-equilibrium regions of the flow, while LES equations are used in regions where the flow is away from equilibrium. A difficulty in performing hybrid simulations stems from the mismatch between resolved turbulent scales between the RANS and LES regions: in the RANS region all turbulent scales are modeled, while in the LES region the energy containing scales are resolved. Interfacing these two regions requires the generation of turbulent eddies capable of supporting the Reynolds stresses in the LES field. Previous work by the authors proposed a combination of synthetic turbulence and a controlled forcing method that was capable of quickly generating these eddies. In this paper, we extend this earlier work by investigating the physical meaning of the model parameters to understand their effect on the flow development. We find that by matching the time-averaging window to the local integral time scale of the flow improved results can be obtained compared with previous work. We also determined the range of the controller parameters that give robust results. The method was applied to non-equilibrium boundary-layer flows far from the calibration case and was found to perform well. This technique gives best results if an outer-flow perturbation drives the flow; when the inner-layer dynamics are critical, less rapid establishment of the turbulence is achieved. Even in the worst result, however, the controller was able to establish a realistic flow within five boundary layer thicknesses of the end of the controlled region.

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.