DNS study of amplitude modulation statistics of turbulent channel flows over rough walls

DNS of long turbulent channel flows over rough walls at friction Reynolds number up to 400 are considered. The walls are hexagonally packed with hemispheres with roughness height h/k=10 and 20 and average spacing between hemispheres from 2 to 4 times the roughness height. A conforming grid approach (unstructured) using spectral finite elements is used to fully resolve the flow with up to 1.6 billion grid points. Analysis of two-point correlation and Hilbert transform applied to the spectrally filtered fluctuating velocity signals is used to study the interaction between large-scale and small-scale turbulence structure. The talk focuses on the effect of roughness parameters on the amplitude modulation accuracy, both for the wall-parallel and wall-normal velocities.