Direct Numerical Simulation of Turbulent Flow over Random Roughness*

Direct numerical simulations (DNS) are performed to study the effects of a random rough wall on turbulent channel flow at $Re_{\tau}=400$ and $Re_{\tau}=600$. The surface geometry matches the experiments of Flack and Schultz (private communication). The skin friction coefficient of the DNS shows good agreement with the experimental results. The mean velocity profiles and velocity fluctuations are compared to smooth channel flow. Pressure fluctuations are more intense in the roughness layer. The mean momentum budget is examined. It is seen that roughness causes the transition from viscous to inertial dominance to occur closer to the wall compared to a smooth wall.