CFD Techniques for simulation of flow in a scour hole around a bridge pier

The flow field around a bridge pier is complex in detail and the complexity is further increased with the development of a scour hole. As flow-induced scouring around piers can cause bridge failures, a good understanding of the flow field is important to the safe design of the hydraulic structures. The objective of this study is to simulate free-surface flow around a pier in a fixed scour hole, and to further determine shear stress distributions at the channel-bed. Simulations were mainly performed using mesh-based numerical models. The mesh-based numerical model was established using Reynolds averaged momentum and continuity equations in three dimensions, with the k-ɷ model for turbulence closure. The regions around the pier and near the channel-bed were resolved with sufficiently fine mesh so as to capture detailed velocity structures. To explore the appropriate procedures for applying smoothed particle hydrodynamics, a mesh-free model was formulated with kernel approximation of the field variables and particle approximation. The governing equations for dynamic fluid flows for the mesh-free model were written as a set of partial differential equations in Lagrangian description, known as the Navier-Stokes equations. The simulations were carried out under the same geometric and hydraulic conditions as in available laboratory experiments. One of the major findings of this research is that, both models predict a downflow near the upstream nose of the pier which would affect the stability of pier foundations. The mesh-based model results exhibit realistic vortex features around the pier and in the wake region. Another new finding is the occurrence of flow separation and complex vortex stretching confined to the upper water column behind the pier. The predicted bed shear stress and turbulent kinetic energy are shown to compare well with the experimental data. Application of the mesh-free model to the flow in a scour hole around a bridge pier has been successful in generating desired approach flow. Velocity profiles extracted from the results of both models at selected locations in the approach channel, inside the scour hole, are compared. The results presented in this thesis are of practical values for prediction of sediment scour around bridge piers