The Horseshoe Vortex System Around a Circular Bridge Pier on Equilibrium Scoured Bed

Large eddy simulation (LES) is used to investigate the horseshoe vortex (HV) system around a circular bridge pier located on scoured bed corresponding to equilibrium conditions. The deformed bed bathymetry is obtained from experiments. Simulation is performed for two different inflow boundary conditions, one in which the upstream flow is fully turbulent including the turbulent fluctuations obtained from a precalculated LES simulation in a periodic channel (Re=18,000) and one in which the mean streamwise velocity profile corresponding to the first simulation is used (Re=5000, no turbulent fluctuations). The former case corresponds exactly to the experimental conditions, the latter serves to highlight the effect of incoming turbulence on the HV system. The dynamics of the instantaneous coherent structures associated with the HV system around the cylinder and the spectral content of the flow in this region are analyzed. Special attention is given to the interaction between the legs of the horseshoe vortices and the flow behind the cylinder and to the distribution of the bed shear stress. The statistics of the flow are calculated. It is found that the bed shear stress fluctuations around the local mean values can be very high, especially in the scour and near wake regions. The mean flow fields show that for the given scour bathymetry and flow conditions, the mean HV system contains one primary eddy situated fairly close to the cylinder. Inside this eddy the mean pressure fluctuations and resolved kinetic energy levels are very high. Maximum mean bed shear stress levels are observed beneath this eddy as well as close to the small corner vortex at the base of the cylinder.