doi:10.1017/S0022112007006465 Printed in the United Kingdom 1 Direct numerical simulation of the turbulent boundary layer over a rod-roughened wall

The effects of surface roughness on a spatially developing turbulent boundary layer (TBL) are investigated by performing direct numerical simulations of TBLs over rough and smooth walls. The Reynolds number based on the momentum thickness was var-ied in the range Reθ =300 ∼ 1400. The roughness elements were periodically arranged two-dimensional spanwise rods, and the roughness height was k=1.5θin, where θin is the momentum thickness at the inlet, which corresponds to k/δ=0.045 ∼ 0.125, δ being the boundary layer thickness. To avoid generating a rough-wall inflow, which is prohibitively difficult, a step change from smooth to rough was placed 80θin downstream from the inlet. The spatially developing characteristics of the rough-wall TBL were examined. Along the streamwise direction, the friction velocity approached a constant value, and self-preserving forms of the turbulent Reynolds stress tensors were obtained. Introduction of the roughness elements affected the turbulent stress not only in the roughness sublayer but also in the outer layer. Despite the roughness-induced increase of the turbulent Reynolds stress tensors in the outer layer, the roughness had only a relatively small effect on the anisotropic Reynolds stress tenso