The application of CFD techniques on the wind field simulation over steep mountains: A method study

Two technical issues related to the application of Computational Fluid Dynamics (CFD) on the wind simulation over steep terrains are discussed in this paper. The first issue is on how to appropriately set the top height of the simulation domain, and the second one is on how to appropriately set the inlet flow boundary conditions. The observed radial velocity data collected from a Doppler radar that is deployed to the northeast of the Hong Kong International Airport are used to validate the CFD simulation results. For the first issue, some previous studies on environmental fluid dynamics or structure wind engineering pointed out that a higher top height, such as 5 times of the vertical height of the obstacle, will lead to a better simulation since a higher top can help to avoid negative impacts of an inappropriately low top boundary on the flow field around the obstacles. However, in the current study, it is showed that if the focus area of the study is in a low elevation area, it is unnecessary to set a high domain top height. The numerical tests show that for a mountain with an elevation of around 1000 m, the difference in the simulated wind fields in low layers is not significant between the two tests in which the top heights of domain are set to be 3 times and 6 times of the obstacle height respectively. For the second issue, three numerical tests are carried out to compare the performances of three methods, namely the single profile (SP) method, the zero-dimensional interpolation (ZDI) method, and the two-dimensional interpolation (TDI) method. The results show that applying the SP to the whole inlet boundary is not sufficient to describe the inlet information, and the simulation results are the worst among the three ones. Surprisingly, though the TDI method can provide higher resolution data on the inlet boundary, the simulation results obtained by the TDI method are generally poorer than those obtained by the ZDI method. This study shows that CFD can provide better simulation results on vortex/wave shedding processes triggered by steep terrains compared to mesoscale models. When using CFD on wind simulation over a complex terrain, a "practical oriented" setting on physical models, parameters and boundary conditions is encouraged to use in order to get an optimized compromise between the simulation accuracy and the computation load.