Vorticity organization in the outer layer of turbulent channels with disturbed walls

The vortex clusters in the turbulent outer region of rough- and smooth-walled channels, and their associated velocity structures, are compared using data from numerical experiments at friction Reynolds numbers Re(tau) <= 674. The results indicate that the roughness of the wall does not affect their properties, particularly the existence of wall-detached and wall-attached populations, and the self-similar size distribution of the latter. The average flow field conditioned to the attached clusters reveals similar conical structures of low streamwise velocity for the rough- and smooth-walled cases, which eventually grow into the global modes previously identified from spectral analysis. We conclude that the vortex clusters of the turbulent outer region either originate away from the wall, or quickly forget their origin, in agreement with Townsend's similarity hypothesis.