| This thesis presents a numerical study of the flow around a cylinder mounted on a channel bed at the initial and final stages of the erosion and deposition process using eddy resolving techniques (Large Eddy Simulation and Detached Eddy Simulation).The study provides a better understanding of the mechanisms that govern the interaction of the approach flow with the bridge pier and the channel bed and the effect of pier shape. In particular, the study allow to determine the distributions of bed shear stress and pressure fluctuations at the bed at the start of the erosion process, based on which scour protection methods can be proposed.For cylinders of circular section, the flat bed case was studied numerically under a wide range of cylinder Reynolds numbers (900< ReD<10 6), while the deformed bed case (the bathymetry corresponding to equilibrium scour conditions was obtained from experiment) was studied at 104< ReD<2*105.Comparison of simulations conducted at ReD-104 and ReD-105-106 allowed investigation of scale effects. The effects of the shape of the cylinder were investigated by comparing the flow past a cylinder of circular section and the flow past a large-aspect-ratio rectangular cylinder placed perpendicular to the incoming flow.The investigation confirmed the findings of previous experimental studies of flow past surface-mounted bluff bodies that the large turbulence amplification inside the HV region is due to the presence of low-frequency aperiodic bi-modal oscillations of the primary necklace vortex. This large amplification explains why the HV system is so effective in removing sediment around the cylinder base.Of particular interest was the study of the characteristics of the large-scale vortex shedding behind the cylinder, dynamics of the legs of the necklace vortices, the eddies shed in the DSLs, and the eddies present in the recirculation region behind the cylinder, the interactions among these eddies, and the changes in their dynamics with the Reynolds number, the shape of the bed around the cylinder and the shape of the cylinder. These interactions are controlling, to a large extent, the scouring mechanisms around the cylinder during the erosion and deposition process. |
