Numerical Investigation On Local Scour Based On CFD-DEM

The study of local scour around piles has practical significance in many engineering applications,such as piers of river Bridges,foundations of Marine generator fans,etc.Local scour has an important effect on the safety performance of structures.This problem has been studied for more than a hundred years,yet the difficulty in obtaining accurate information about the motion of particles in the flow field has limited the understanding of the mechanism of this process.This paper establishes a model using the discrete element method(DEM)dealing with particles and computational fluid dynamics method(CFD)dealing with flow.The interaction between water and sediment in scour is simulated through information transformation between two modules.Detailed information of the particles and flow are obtained.The mechanisms of local scour are analyzed and summarized.The main conclusions from the study are as follows:(1)Based on the open source CFD software Open FOAM and the discrete element library Liggghts,a CFD-DEM coupling model is established.Two benchmark experiments are used to study the selection of key parameters in the calculation model.The performance of the model under different computational Settings are evaluated.Suggestions on the settings of the coupling model are given.(2)The coupled model is used to study the movement of bed-load particles under shear stress.The accuracy of the coupling model for sediment flow simulation was verified by comparing the simulation results of bed bed mass transfer rate with the experimental data.The motion mode of particles in bed load movement was summarized,the relationship between the height of particles jumping distance and the shear stress on the bed surface was analyzed.The characteristics of particle velocity distribution at different depths were discussed.(3)The flow around the cylinder was simulated by the CFD module in the coupling model.The flow field velocity distribution around the cylinder was studied by setting the conditions of rough bottom bed and smooth bottom bed.The morphological characteristics of horseshoe vortices and wake vortices,which play an important role in local scouring,were analyzed.(4)The local scour process around the cylinder was simulated by the established coupling model,the simulation results of the equilibrium scour depth were compared with the experimental data.The phenomenon that the time needed to reach the equilibrium state in the simulation was shorter was analyzed.Based on the simulationresults,the development process of scour pits with time was analyzed.The development rate of scour pits was divided into three stages.The flow field and particle motion characteristics in different stages were studied.The different roles of horseshoe vortices and trailing vortices in the scour process were explained.The development mechanism of local scour was summarized.The development process of scour pit is a coupling process in which the change of flow field and the change of bed surface shape influence each other.The limitations of the previous numerical simulation method of predicting the height of bed surface solely based on the shear stress of bed surface are pointed out.