Numerical simulation on wind fields over complex terrain using nested grids

In order to develop an overall efficient and accurate model of simulating an unsteady three-dimensional airflow over complex terrain with characteristic length scales on the order of kilometers, we have been examining the large-eddy simulation (LES) technique using a finite-difference method (FDM). These LES codes are referred to as the RIAM-COMPACT (Research institute for Applied Mechanics, Kyushu University, Computational JPrediction of Airflow over Complex Terrain), and are based on two grid systems and corresponding variable arrangement: one is an orthogonal staggered grid; the other is a generalized curvilinear collocated grid. In this paper, using the RIAM-COMPACT based on a generalized curvilinear collocated grid, we have performed the calculations of a non-stratified airflow over real complex terrain. This area covers the new campus of Kyushu University. To investigate more clearly the influence of topography on the airflow over the new campus area, we employed nested grids: the outer grid domain is 9.75km × 4.65km × 1.46km with horizontal grid spacing of 50m and the inner grid domain is 5km × 2.7km × 1.46km with horizontal grid spacing of 25m. The numerical results were compared with the wind tunnel experiment, and airflow characteristics due to the topographic effect, such as the wind speed-up and the separated flow, were successively simulated.