Large Eddy Simulation of Free Surface Flow Over Square Bars in Laminar, Transitional and Turbulent flows

Results of large-eddy simulations of laminar, transitional and turbulent free surface flow at constant Froude number over spanwise-aligned square bars are analyzed. Two different bar spacing corresponding to transitional and k-type roughness are selected. The water surface deformations include mild undulation in flow over transitional roughness and a distinct standing wave in flow over k-type roughness. The streamwise gradient of streamwise velocity in response to the k-type roughness contributes to the formation of standing wave at the water surface which is not present in transitional roughness. The friction coefficient in laminar and transitional flow are of the same order and significantly larger than that in turbulent flow. The large water surface deformation induces larger dynamic pressure in the bulk of the flow over k-type roughness than transitional roughness at all Reynolds number. In turbulent flows, the root-mean-square fluctuations of spanwise velocity and pressure have similar peak values under water surface over the two roughness types implying negligible effects of water surface deformations on these variables. Other velocity and pressure fluctuations, however, have different peak values under the water surface in flow over the two roughness. Instantaneous pressure fluctuations also undergoes large variations while these variations are larger in flow over k-type roughness. Turbulent and wake kinetic energy have similar distribution as the velocity fluctuations and are modulated under the standing wave. The friction factor decomposition reveals that the contribution of dispersive shear stress increases and that of Reynolds shear stress decreases by increasing bar spacing.