DNS of turbulent channel flows with boundary roughened with

A method to simulate the effects of a roughened surface on a turbulent boundary layer is introduced. The method is easy to implement, does not increase the numerical overhead of the code and affects the mean velocity in an \textit{a priori} predictable way. A single parameter $k$ is sufficient to fully characterize the roughness. The procedure has been tested in turbulent channel flows at $Re_\tau=1000$, with roughness heights $k^+$ spanning the transitional regime. The properties of the rough flow agree well with experimental data.