An experimental and numerical study of channel flow with rough walls

A fully turbulent channel flow with smooth and rod-roughened walls has been investigated using hot-wire anemometry and direct numerical simulations (DNS). The mean flow follows the law of the wall for both surfaces and the velocity defect suggests that the outer layer is very little affected by the roughness. The Reynolds stresses appear to be very similar for the two surface geometries outside y approximate to 5k, where k is the roughness height. A quadrant analysis shows that the structural differences close to the wall extend somewhat further out. The turbulence structure is further investigated using stress ratios and the anisotropy tensor, which corroborate the findings from the Reynolds stresses. Many of the recent investigations on boundary layers seem to find large differences between smooth and rough wall data in the outer layer also. A tentative explanation for the apparent dependence on flow type of the surface roughness effects is given.