Theory And Numerical Simulation Of Bridge Pier Scour In Coastal Area

Local scour at piers regardless of river-or sea-crossing bridges undermine the safety of bridges,which has caused common concern.However,the theory and mechanism of water-sediment interaction behind this problem haved not been studied comprehensively,causing the inadequency of predictions of flow,sediment transport and scour.Unlike the fluvial environments,for coastal environments,the hydrodynamics are more complex,e.g.wave transformation and oscillatory flow.In this thesis,the numerical simulation techniques solving big deformation issue in local scour were developed to realize the modelling of coastal bridge piers scour.Meanwhile,the mechanism of wave/current-sediment-scour interaction in the scouring process was studied.The modifications on near-wall turbulence model were conducted based on experimental data measured by particle image velocimetry(PIV)technique;the non-equilibrium sediment transport model for local scouring was established;the relationship between turbulence and sediment transport was obtained based on vortex flow field in front of the pier;the numerical model of wave/current scour was established incorporating the mechanism of water-sediment-morphology interaction;the modeling of flow and scour around a pier located in coastal area was realized for the first time using our numerical model.The main innovative findings and contributions are drawn as following:(1)The modifications on the key parameter cμrelated to eddy viscosity and turbulent kinetic energy k in the k-s turbulence model were performed based on reliable PIV data(validated with direct numerical simulation results)for open-channel flows over rough bed.The relationships between,and k with the roughness Reynolds number ks-were obtained,which is further implemented into the k-ε model.Validation with experimental data shows that the prediction accuracy for the near-wall flow has been improved by the modified model.(2)The non-equilibrium sediment transport model was established based on the experiments of local scour induced by upstream bed level lowering.It shows that this model is suitable for the situation of local scour with significant bed deformation.The adaptation length in this model was also determined based on present experimeantal data.(3)The tuebulent horseshoe vortex(THV)in front of the pier results in a significant amplification of fluctuating bed shear stress.The sediment transport rate can be underestimated only based on mean bed shear stress.The quantatitive relationship between sediment transport and fluctuation was established,which extends its application.(4)Using the open source CFD code-OpenFOAM,a numerical model of wave-induced scour coupling flow,sediment and morphology was developed.In this numerical model,the hydrodynamic,sediment and morphological models are resolved in different computational meshes,and the coupling solutions were obtained by the method of variable at the same grid point translating among different meshes.The validation with experimental data shows that this numerical model and coupling method are reliable.(5)The mesh repairing technique was devoloped by combing MATLAB and further utilized to solve the issue of mesh distortion during the process of scour computation.It is the first time to realize the modeling of local scour at a bridge pier in coastal area.The validation shows that this mesh repairing technique is reliable,which also supplies a method to solve the issue of mesh distortion during dynamic mesh deformation.