Wall-modeled large-eddy simulation of an atmospheric boundary layer encountering a smooth-to-rough transition. Here the transition is the smooth salt flat that feeds into a rough dune field at White Sands national Park. 

The first video displays vortex structures using then Q-Criterion, and colored in grayscale by values of the Reynolds shear-stress, a marker of turbulence production. Darker values indicate more intense values. Here, we have zoomed in on a set of dunes towards the center of the dune field.

The second video demonstrates the impact of the roughness transition on the velocity profile. The center-cut plane shows the instantaneous streamwise velocity, with the flow going from right-to-left. The thin shear layer of the flow seen upstream of the transition grows downstream as the flow encounters the dunes, indicating the formation of the internal boundary layer. On the dunes, the magnitude of the wall shear-stress is shown,