Roughness effects in turbulent channel flow

DNS of turbulent channel flow at Reτ = 400 is performed. Both walls are roughened by square rods. The height of the roughness elements is smaller by a factor of 3 than in all earlier computer simulations. The turbulent flow is in the upper end of the transitional regime, where viscous drag is negligible. Substantial changes in all turbulence statistics are only seen within the roughness sublayer, where detailed information is provided. Hence, smooth-wall scaling applies in the outer layer. The anomalous turbulence features observed in the near-wall region are due to the blocking effect of the roughness elements and the intense shear layers emanating from the rods.