LES study of turbulent flow fields over a three-dimensional steep hill considering the effects of thermal stratification

In this study, large-eddy simulations are performed to elucidate the spatiotemporal characteristics and physical mechanisms of turbulent boundary layers over hilly terrain under stable, neutral, and unstable stratification. The impact of thermal stratification on turbulent flows over a steep three-dimensional hill is clarified through flow patterns and statistical characteristics. Compared to neutral stratification, the separation bubble downstream of the hill crest is reduced under unstable stratification, while it is enlarged under stable stratification. In addition, turbulent eddy motions in the wake region are enhanced in the unstable condition but are suppressed in the stable condition. Both mean velocities and turbulence fluctuations over steep hilly terrain are amplified by unstable stratification and attenuated by stable stratification. The flow characteristics on the hill crest are comprehensively determined by the topography and thermal stratification, whereas the flow dynamics in the hill wake are predominantly influenced by terrain-induced turbulence. Moreover, the mechanisms driving the formation of flow fields over steep hilly topography under different thermal stratification are investigated through force balance analysis using the time-averaged Navier–Stokes equations. The results indicate that turbulence plays a negligible role in the force balance upstream of the hill, while it becomes the dominant factor for the force balances downstream of the hill.