Author(s): Thorsten Stoesser; Jochen Frohlich; Wolfgang Rodi
Keywords: Turbulence; Open-channel flow; Large-eddy simulation; Rough wall; Permeable wall
Abstract: This paper presents results of a Large Eddy Simulation (LES) of turbulent flow in an open channel over and through 3 layers of spheres, which can be regarded as an idealized permeable bed. The setup and boundary conditions are selected analogous to recently completed laboratory experiments. The mean streamwise velocities from the LES are compared to the measured data in order to validate the calculations. Due to a high porosity of the bed there is considerable momentum exchange between the flow above and within the spheres, which produces additional shear stresses and hence leads to velocity and turbulence intensity distributions differing considerably from those over solid beds. The overall agreement between calculated and measured mean streamwise velocities above the bed is very satisfying. Below the roughness interface flow velocities, turbulence intensities and pressure fluctuations are damped exponentially. The damping of pressure fluctuations has also been observed from pressure measurements in a porous gravel bed. Furthermore, coherent flow structures (like sweeps and ejections) are suggested to be the driving mechanism of momentum exchange between the two flow regions.