DNS of turbulent flow in a rod-roughened channel

A numerical investigation has been performed to study pressure-driven turbulent flow in a rod-roughened channel at Reynolds number Reτ=400 based on the mean pressure-gradient. Both channel walls were roughened by square rods with a height k of only 1.7% of the channel height or k+=13.6 in wall units. The pitch-to-height ratio was 8, which corresponds to the so-called “k-type” roughness and the resulting flow field was classified as “transitionally rough”. The Reynolds-averaged streamline pattern exhibited two co-rotating vortices which filled the cavity between two consecutive rods. The averaged flow field which separated from one rod did not reattach and a return flow was observed all along the bottom of the cavity. Outside the roughness sublayer, i.e. beyond 5k, no discernible streamwise variation of the mean velocity and second-order statistics could be observed, whereas the correlation between the streamwise and wall-normal mean velocity components persisted somewhat further away from the surface. Inside the roughness sublayer, however, significant differences in the turbulence field between smooth- and rough-wall layers were observed, for instance the high-energy region formed by the shear layer emanating from the crest of the roughness elements. Visualizations of instantaneous flow fields revealed the presence of elongated streaky structures similar to those routinely observed in flows along a smooth surface.