Secondary Currents and Turbulence over Spanwise Heterogeneous Bed Roughness

This paper presents the results of large-eddy simulations (LES) of free surface open-channel flow over a bed with spanwise heterogeneous roughness. The smooth channel bed is roughened via streamwise- orientated rectangular strips. The flows reported on are at moderate Reynolds and Froude numbers and at varying relative submergence (i.e. water-depth-to-strip-height-ratio). Due to the presence of longitudinal strips the flow exhibits significantly different mean flow characteristics than the analogue flow over a smooth channel bed. The strip roughness induces turbulence anisotropy which leads to the formation of strong secondary currents, featuring upflow above the strip and a downflow in the trough between strips. It is found that the relative submergence has a profound influence on the strength of the secondary current which in turn affects the momentum balance, i.e. the greater the submergence the smaller the contribution of secondary currents to the momentum loss.