Investigating free-surface turbulent channel flow over rough bed using large-eddy simulation

The flow structure in turbulent free-surface flows are manipulated by both the bed and the water surface. The complexity of fluid flow phenomena occurring in this type of flow increases when roughness elements are attached to the bed. The roughness topography plays a crucial role in modification of flow statistics and structure. For instance, the height of roughness elements, the streamwise and spanwise heterogeneity in their shape and the relative flow submergence all contribute to the formation of various flow structures that modify the momentum and energy transport in the bulk flow and water surface deformations.



In this work large-eddy simulations of free surface turbulent channel flow over a rough bed are performed. The roughness topography consists of hexagonally-packed hemispheres. Two flow depth is considered at similar Froude number but different Reynolds numbers. First and second order flow statistics are investigated near the bed and in the bulk of the flow to evaluate the effects of bed roughness, flow depth and water surface on these flow variables. The effects of bed roughness on the averaged parameters are also represented using the double averaging method in which the Navier-Stokes equations are averaged first temporally and then spatially. Secondary flow is another flow feature in free-surface flows appearing as streamwise-aligned rolls in time-averaged velocity field if the bed roughness topography is heterogeneous in the spanwise direction. This flow is investigated here as well for both flow cases.



The present results show that the effects of Re is more significant than flow depth near the roughness crest. The effects of water surface are similar for both flow depths considered here. The behaviour of the mean streamwise velocity and velocity variances are similar in this work and the differences in their values are attributed to the difference in Re.