Numerical simulation of scour process around bridge piers in cohesive soil

Abstract

This thesis is concerned with numerical simulation of scour process in cohesive soil around cylindrical bridge piers. The numerical method is a multi-block chimera Reynolds-Averaged Navier-Stokes (RANS) method in a general body-fitted curvilinear coordinate system with an incorporation of a scour rate equation. The chimera technique is employed to connect the overlapped grids together and transfer infor mation across the grid boundaries. The turbulence quantities are evaluated using a two-layer turbulence model. At each time step in computation, the streambed shear stress distribution is calculated, and the scour rate at each bed point is determined through the scour rate equation. Scour depth increase is calculated by multiplying the scour rate by the time increment. Numerical simulations of scour process around a model cylindrical pier mounted in porcelain were conducted. The numerical solutions successfully captured the important flow features such as flow separations and eddies around the cylinder. The variations of the streambed bathymetry and the shear stress distribution with time were calculated. The computed time histories of scour depth were in good agreement with the experimental results. A scour process around a full-scale cylindrical pier was computed as well to present the numerical method's capability to calculate bridge pier scour in flood conditions. By means of numerical computation, a parameter study was also carried out to study the influences of the critical shear stress and the slope of scour rate curve on scour process. It is found that the effects of the critical shear stress are significant. The final scour depth and the time needed to reach it increase with decreasing critical shear stress. The slope of the time to reach the final scour depth. The steeper the slope is, the more quickly the final scour depth is attained. Finally, the dependence of the maximum streambed shear stress around a cylinder on the Reynolds number was investigated through numerical computations. An empirical formula of the maximum streambed shear stress was obtained based on the computational results.