Author(s): Thorsten Stoesser; Wolfgang Rodi; Jochen Frohlich
Keywords: Large Eddy Simulation; Turbulence; Rough beds; Open-channel flow; Coherent structures; Low-speed streaks; Vortex amalgamation
Abstract: The paper presents results of a Large Eddy Simulation (LES) of the flow in an open channel where the channel bed is roughened with one layer of spheres. The roughness height k, which corresponds to the sphere diameter d is 0. 23 of the channel depth. The Reynolds number Reτ, based on the average friction velocity uτ and the channel depth h is approximately 2820. The flow configuration was selected to correspond to laboratory experiments of Detert (2005), which are currently in progress. Mean streamwise velocities from the LES are compared with the measured data and the distributions of the calculated turbulence intensities are evaluated by comparing them with empirical relationships for flow over rough walls suggested by Nezu (1977). The occurrence of low- and high-speed streaks is examined and their spanwise spacing is quantified. Moreover, sweeps and ejections are shown to occur as well as the amalgamation process i. e. ejection of fluid into the outer layer associated with vortex growth.