Statistics of the turbulent boundary layers over 3D cube-roughened walls

Abstract

Direct numerical simulations (DNSs) of turbulent boundary layers (TBLs) over three-dimensional (3D) cube-roughened walls were performed and the turbulent characteristics in the inner and outer layers were statistically analyzed. The spanwise spacing was varied over p(z)/k = 2, 3, 4, and 6 (p(z) is the spanwise spacing between cubes and k is the height of the roughness) to examine the effects of the roughness spacing on the TBLs. The form drag (C-p) reached a maximum at p(z)/k = 3, whereas the skin-friction drag (C-f) reached a minimum at the same extent. The Reynolds stresses in the outer region were shown to increase with increasing p(z)/k, and similar behavior was observed in the wall-normal velocity fluctuations at the roughness crest (v(w)(+)). The properties of the turbulence in the inner and outer layers were found to be well represented by the roughness density (lambda(p)).