Numerical Simulation Study On The Flow-Field Around And Local Scour At Bridge Piers Under Unsteady Flow

Flood in natural rivers is unsteady in essense.The excessive scour of bridge foundation by unsteady flood is the major reason for bridge collapse.However,existing theory and method for predicting local scour depth are mainly based on research on steady flows.In order to effectively improve the prediction accuracy of local scour around the bridge pier,this paper used both the measured natural-flood data and the three-dimensional hydrodynamic numerical simulation method to study the process and mechanism of local scour around the bridge pier under steady flow and unsteady flow conditions.The major contents and conclusions are as follows.(1)Based on the natural-flood data of typical rivers in the Yangtze River and Yellow River basins,the unsteady characteristics of the flood in natural rivers are analyzed statistically.It is found that the discharge variation amplitude and duration of flood increases and decreases,respectively,with the lowering of the stream order in the same basin,indicating that the flood is more significantly unsteady in low-order streams.Meanwhile,the phenomenon of swift rise and slow fall of flood discharge becomes more significant in low-order streams.The increasing of the unsteadiness of the flood with the lowering of the stream order is more significant in the Yellow River Basin than that in the Yangtze River Basin.(2)A prediction formula for the development length of open channel turbulence was proposed based on two-dimensional hydrodynamic numerical simulation of uniform open channel flows.The feasibility of unsteady flow simulation was valiated with two-dimensional hydrodynamic model as well.It is also found that the bed shear stress peak appears earlier than the flow discharge peak in unsteady flows.The asynchrony between the bed shear stress and the discharge becomes more significant in flows with shorter flood duration and larger discharge variation amplitude.(3)A three-dimensional numerical model for local scour of bridge piers including both flow model and scour model was constucted by using Ansys-Fluent.The flow model can accurately determine the initial and boundary conditions of unsteady flows.The scour model takes into account both the correction of the critical shear stress of sediment over a non-flat bed and the gravity sliding of bed material.The bed deformation was realized by using the dynamic mesh technology.The model can be used to simulate the process of local scour at the bridge pier under steady flow and unsteady flow.(4)The local scour around the bridge pier under both steady and unsteady flows starts beside the pier with a radual angle of 75°～105° where the maximum bed shear stress appears,and the scour hole develops gradually to encircle the bridge pier.The downflow and horseshoe vortices are the main driving forces of local scour in front of the bridge pier.The horseshoe vortices and lateral accelerated flow are the main driving forces of local scour at the bridge pier side,but the contribution of horseshoe vortices decreases faster from the front to the side of the bridge pier.The local scour at the bridge pier under steady flow condition develops quickly at first and then gradually.On the other hand,the local scour process within the rise stage of unsteady flow develops slowly at first and then quickly.(5)Under the condition of steady flow,the strength of downflow in front of the bridge pier increases gradually during the scour process,while the strength of horseshoe vortices in front of the bridge pier first increases slightly and then decreases gradually,resulting bed shear stress in front of the bridge pier increases firstly and then decreases.At the bridge pier side,the strength of horseshoe vortices and lateral accelerated flow decrease gradually with the scour process,making the bed shear stress decreases continuously.Within the rise stage of the unsteady flow,the strength of downflow in front of the bridge pier,horseshoe vortices around the pier,and the lateral accelerated flow at the pier side increase gradually with the local scour process,making the bed shear stress around the pier increases gradually as well.