Large-eddy simulation of turbulent airflow over complex terrain

In order to develop an overall efficient and accurate method of predicting an unsteady three-dimensional airflow over a complex terrain with characteristic length scales on the order of kilometers, we recently developed the CFD codes referred to as the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain). In this paper, we carried out the calculation of turbulent airflow over a real complex terrain in a horizontal region of 9.5 km×5 km with a relatively fine spatial resolution of 50 m. This area covers the new campus of Kyushu University. In order to generate instantaneous velocity fluctuations in an approaching flow, an unsteady flow field in the driver unit is directly calculated. The numerical results obtained by RIAM-COMPACT demonstrated that the changes induced on the wind field by the topographic effect, such as the local wind acceleration and the flow separation, were successfully simulated. We emphasized the following features as regards the wind field over the new campus area under the influence of the west wind. The wind field in the central region of the new campus area is strongly influenced by the wake region generated behind Mt. Hiyama (244 m). The airflows moving around the side of Mt. Hiyama exhibit relatively small fluctuations. When these winds reach Mt. Ishigateke (99 m), they are locally accelerated at the peak. Consequently, a local speed-up effect is confirmed in the southern part of the new campus area.