Numerical study on turbulent boundary layers over two-dimensional hills — Effects of surface roughness and slope

The characteristics of turbulent boundary layers over hilly terrain depend strongly on the slope and surroundings, especially the upstream roughness conditions. In this study, Large-eddy simulation (LES) is systematically performed to study the turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, and two kinds of upstream roughness condition corresponding to two kinds of velocity profile are considered. A wind tunnel experiment is carried out to investigate the turbulent flow around the hills in addition to providing experimental data to validate the numerical study. An auxiliary simulation is performed to simulate the turbulence to be imposed at the inlet boundary of simulation of the flow over hills. The effects of roughness elements are modeled by adding an external force to the governing equations of the fluids. The results show that the proposed numerical method, which strengthens the simulation of the effects of inflow turbulence and roughness, provides reasonably good results. A large-scale separation bubble is established on the leeside of the steep hill. The reattachment point extends further downstream when the hill is rough. No stable separation bubble is found on the low hill.

► A numerical model is proposed to simulate the boundary layer over a hill.
► The necessity of modeling inflow turbulence and surface roughness is stressed.
► Separation bubble is established at the steep hill.
► The reattachment point extends further downstream when the hill becomes rough.