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 $y\,{\approx}\,5k$, 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.

Turbulent channel flow measurements for two different rough surfaces have been compared with a smooth reference case. A range of Reynolds numbers, Reτ∊⟨360,6000⟩, has been investigated using hot-wire anemometry. Reynolds stresses and third-order moments are shown to be very little affected by the substantially different wall conditions outside 5k, where k is the characteristic length scale of the roughness. In this region, a reasonably good collapse with Reynolds number is demonstrated when scaling with friction velocity is used. This contrasts some of the rough-wall investigations previously published for boundary layers and channels with only one rough wall. It is believed that the differences observed are due to the differences in boundary conditions and that symmetrically roughened channel flows and flows in rough-wall pipes may be better candidates for the Townsend’s wall similarity hypothesis than asymmetrical flows.

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