Direct numerical simulation of three-dimensional turbulent rough channels: parameterization and flow physics

Direct numerical simulations of the three-dimensional flow past rough surfaces with elements of different shapes are performed to create a database. Our main interest is in finding a new parameterization for turbulent rough flows, which, so far, has been based on the concept of equivalent sand grain height or on the net separation between k and d type roughnesses. The new parameterization permits us to find a simple expression for the roughness function and the root mean square of the normal velocity fluctuation at the plane of the crests. We also wish to find statistical quantities characterizing the effects of the different rough surfaces: one is the ratio between mean flow and turbulence time scales (Sq/epsilon), the other is the helicity density. Passive scalar visualizations evince a reduction of the wall streak coherence, and the absence of a signature of the rough surfaces on the passive scalar distribution. The tendency towards a flow isotropy near the roughness has been explained also through Sq/epsilon.