Discussion of the impact of pressure fluctuations on local scouring

We aim at discussing the potential impact of pressure fluctuations on the local scour process around a bridge pier by estimating the force F_B acting on the sediment layer due to these fluctuations. To do so, we apply highly resolved large-eddy simulations of the flow around a wall-mounted cylinder at three moderate Reynolds numbers. To provide the required grid resolution in the region of interest around the cylinder, the grid is refined by locally embedded grids. Instantaneous fluctuations of the pressure coefficient C_p are up to two orders of magnitudes larger than the friction coefficient C_f in significant regions around the cylinder. Furthermore, regions of low pressure are located in close vicinity to regions of high pressure, which implies strong pressure gradients and fluctuations in time. Pressure gradients in the fluid phase induce velocity fields in the sediment layer. We estimate the time scales by which such a velocity field reaches quasi-steady conditions and the resulting volume forces on the sediment. Furthermore, we discuss a practical experimental setup of the flow around a bridge pier. For the assumptions made in this study, the estimated forces due to pressure gradients cannot overcome the gravity force acting on a sediment grain. Nevertheless, the estimated forces are of the same order of magnitude than forces stabilizing the sediment layer and should be taken into account in future models of scour evolution.