Roughness effects on near-wall turbulence modelling for open-channel flows

Accurate modelling of near-wall turbulence influenced by roughness is crucial in hydraulic engineering. This study presents modifications on the k-epsilon turbulence model for open-channel flows over rough bed based on reliable particle image velocimetry data. Experiments have been conducted for different roughness heights. Some near-wall turbulence characteristics (e.g. bursting events) are identified and the profiles of mean streamwise velocity, Reynolds shear stress and turbulence intensities are validated with direct numerical simulation data. Experimental results confirm that the coefficient c(mu) involved in the k-epsilon model varies with roughness Reynolds number k(s)(+) in outer region, and with both k(s)(+) and y(+) (normalized distance from the wall) in near-wall region. A new damping function is reconstructed, and the profile of turbulent kinetic energy close to the wall is analytically obtained. The present modifications accounting for wall roughness effects are further implemented into the k-epsilon model and validation shows that the prediction accuracy for the near-wall flow has been improved.