Numerical Investigation of Near Bed Turbulence on a Stationary Stone

A large-eddy simulation of the flow field around a stationary stone in a fully developed turbulent channel flow with a Reynolds number of was performed. The instantaneous near bed turbulence was simulated in order to gain a comprehensive understanding of pressure force and shear stress exerting on the entire stone surface. Here, we introduce the first numerical attempt to analyse entrainment conditions of a single stationary stone and obtain qualitative as well as quantitative results of the near bed flow and pressure field around the stone. A hybrid mesh was used to discretize the domain. A large-eddy-simulation with the Smagorinsky sub-grid scale method was utilized for the solution of the three-dimensional incompressible Navier-Stokes equations. The Smagorinsky model coefficients were calibrated using high resolution spatio-temporal experimental data obtained from four-dimensional particle tracking velocimetry. Convectional drag and lift formulas were employed for the computation of force coefficients. Total forces surrounding the stone were computed and a mean drag coefficient of C_d = 0.521 was determined along the streamwise direction. The pressure distribution between the bottom part and topside of the stone was depicted. It was found that the gradient in between always generates a directional force which is constantly lifting the stone towards the center of the bulk. Forces in lateral direction were observed to be non-symmetric due to the different converging angles in the wake region.