Large Eddy Simulation Regions Research Articles

Hybrid RANS/LES Method for High Reynolds Numbers, Applied to Atmospheric Flow over Complex Terrain

The use of Large-Eddy Simulation (LES) to predict wall-bounded flows has presently been limited to low Reynolds number flows. Since the number of computational grid points required to resolve the near-wall turbulent structures increase rapidly with Reynolds number, LES has been unattainable for flows at high Reynolds numbers. To reduce the computational cost of traditional LES a hybrid method is proposed in which the near-wall eddies are modelled in a Reynolds-averaged sense. Close to walls the flow is treated with the RANS-equations and this layer act as wall model for the outer flow handled by LES. The well-known high Reynolds number two-equation k - ϵ turbulence model is used in the RANS layer and the model automatically switches to a two-equation k - ϵ subgrid-scale stress model in the LES region. The approach can be used for flow over rough walls.To demonstrate the ability of the proposed hybrid method, simulations of the wind flow over a complex terrain near Wellington in New Zealand are presented. Under certain conditions unsteady flow features have been measured at the site - flow features that could lead to high structural loads on a planned wind farm. These transient flow phenomena are reproduced with the new RANS/LES method. Additionally, the results from the hybrid method are compared with pure RANS results.

Interface conditions for hybrid RANS/LES calculations

Hybrid RANS/LES simulations, in which the attached boundary layers are computed using the Reynolds-averaged Navier–Stokes (RANS) equations, and the non-equilibrium regions of the flow by large-eddy simulation (LES), have received considerable attention over recent years. One issue that may affect (sometimes significantly) the accuracy of the results of hybrid methods is the generation from a smooth RANS field (in which the Reynolds stress is due entirely to the model) of turbulent eddies capable of supporting the Reynolds stresses in the LES region. In this paper, we perform hybrid RANS/LES of turbulent boundary-layer flows in zero, favorable and adverse pressure-gradients. The RANS equations are solved in regions of the flow that are at or near equilibrium, while LES is used elsewhere. Synthetic turbulence and controlled forcing are used to generate turbulent structures in the LES region. When only synthetic turbulence is used at the inflow of the LES region, some distance is required before the proper phase relationships between the modes introduced at the inlet are established; the turbulent statistics are unrealistic in this region, which can extend for 10–20 boundary layer thicknesses. When the forcing is used, this development region is significantly shortened, and realistic statistics are obtained 1–2 boundary-layer thicknesses downstream of the interface. The accuracy of the method, however, was found to depend critically on the accuracy of the RANS solution.

A hybrid RANS/LES simulation of high-Reynolds-number channel flow using additional filtering at the interface

A hybrid method combining large eddy simulation (LES) with the Reynolds-averaged Navier-Stokes (RANS) equation is used to simulate a turbulent channel flow at high Reynolds number. It is known that the mean velocity profile has a mismatch between the RANS and LES regions in hybrid simulations of a channel flow. The velocity mismatch is reproduced and its dependence on the location of the RANS/LES interface and on the type of RANS model is examined in order to better understand its properties. To remove the mismatch and to obtain better velocity profiles, additional filtering is applied to the velocity components in the wall-parallel planes near the interface. The additional filtering was previously introduced to simulate a channel flow at low Reynolds number. It is shown that the filtering is effective in reducing the mismatch even at high Reynolds number. Profiles of the velocity fluctuations of runs with and without the additional filtering are examined to help understand the reason for the mismatch. Due to the additional filtering, the wall-normal velocity fluctuation increases at the bottom of the LES region. The resulting velocity field creates the grid-scale shear stress more efficiently, and an overestimate of the velocity gradient is removed. The dependence of the velocity profile on the grid point number is also investigated. It is found that the velocity gradient in the core region is underestimated in the case of a coarse grid. Attention should be paid not only to the velocity mismatch near the interface but also to the velocity profile in the core region in hybrid simulations of a channel flow at high Reynolds number.