The effect of surface roughness on flow structures in a neutrally stratified planetary boundary layer flow

Abstract

The effects of surface roughness on the structures of a neutrally stratified planetary boundary layer flow are investigated by the large-eddy simulation technique. Our numerical model, which assumes horizontal periodicity, shows that the growth of an internal boundary layer (IBL) in response to an abrupt change of surface roughness (either smooth-to-rough transition or rough-to-smooth transition) obeys the 4/5th power of the time, similar to that along the downwind fetch. A sudden increase or decrease in the surface shear stress during the transition is also observed. A quadrant analysis shows that during the transition, ejections and sweeps are altered significantly. Flow visualization further illustrates that the distribution density and the strength of coherent vortical structures and ejection eddies increase substantially during the smooth-to-rough transition. Conversely, these parameters decrease in the rough-to-smooth transition. The mean velocity profile has an inflection point at the IBL top, but the coherent vortical motions and ejection eddies affected by the change of the roughness are inside the IBL, suggesting that this inflection point is more static than dynamic. We also compare the quasi-steady coherent flow structures of different surface roughness values after the transition period. Streak spacing appears to increase with increasing surface roughness. Ejection eddies and vortical structures increase in scale as well as in strength as the surface roughness increases. The correlation between drag coefficient and flow structures in boundary layer flows is discussed.