IDENTIFICATION OF COHERENT FLOW STRUCTURES IN OPEN-CHANNEL FLOW OVER ROUGH BED USING LARGE EDDY SIMULATION

摘要

In this paper we simulate a rough-wall open channel flow using large eddy simulations (LES) in order to gain insight into the physics of this flow. The channel bed is artificially roughened with a matrix of staggered cubes and the Reynolds number Re is 50,000. The relative submergence of roughness elements is approximately 13, such that the elements are considered to roughen the channel bed, rather to block the flow as present in roughness height to water depth ratios below 5. The quadrant analysis is used for the detection of sweeps, ejections, inward and outward interactions. The results show the dominance of the sweep and ejection events near the boundary layer over the rough wall. Furthermore, with help of the instantaneous streamwise velocity fluctuations the identification and quantification of low and high speed streaks is accomplished. All observations made by the large eddy simulations are in good agreement with the theory and with recent laboratory results.