Large Eddy Simulation of open-channel flow over spheres

The paper presents results of several Large Eddy Simulations (LES) of the flow in an open channel where the channel bed is roughened with one or two layers of spheres. The roughness height k, which corresponds to the sphere diameter d is 0.23 of the channel depth. The Reynolds number Re-tau, based on the average friction velocity u(tau) and the channel depth h (distance from the roughness tops of the spheres to the water surface) is approximately 2820. The flow configurations were selected to correspond to recently performed laboratory experiments. 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 [1]. 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. It is shown that these structures occur irrespective of roughness conditions, however further studies and data analysis are needed to evaluate and quantify the effect of porosity on coherent structures.