Hybrid RANS/LES applied to complex terrain

Large Eddy Simulation (LES) of the wind in complex terrain is limited by computational cost. The number of computational grid points required to resolve the near-ground turbulent structures (eddies) are very high. The traditional solution to the problem has been to apply a wall function that accounts for the whole near-wall region. Recently, a hybrid method was proposed in which the eddies close to the ground were modelled in a Reynolds-averaged sense (RANS) and the eddies above this region were simulated using LES. The advantage of the approach is the ability to use shallow cells of high aspect ratio in the RANS layer and thereby resolve the mean near-wall velocity profile. The method is applicable to complex terrain and the benefits of traditional LES are kept intact. Using the hybrid method, simulations of the wind over a natural complex terrain near Wellington in New Zealand are demonstrated. 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 method. Additionally, the results from the hybrid method are compared with RANS results. Copyright (C) 2010 John Wiley & Sons, Ltd.