Numerical Investigation Of Scour Around Cylinder Based On CFD-DEM Coupling Simulation

The sediment incipience,transport and local scour in the case of complex flows are the hot research topic in river and coastal engineering all the time.The traditional theory in which only the mean bed shear stress or sediment carrying capacity formula is considered to study sediment transport and scour is not suitable in the case of complex flows containing turbulence and large-scale vortices.With the rapid development of discrete element method,it has developed fast to use numerical model to investigate this problem from the mesoscale view,which may reveal the mechnasim of sediment motion in the case of complex flows.Using computational fluid dynamics software Open FOAM and particle motion simulation software LIGGGHTS,a coupled fluid-particle model,in which the flow field is simulated by LES model,is presented and applied in the investigation of sediment incipience,transport and local scour.The main contents and results of this thesis are as follows.(1)The characteristics of the flow around the cylinder are studied through the dynamic one-equation LES model,the velocity profile,leeward of the cylinder and the pressure distribution are analyzed.The simulated results are verified by comparion between RNG?-? numerical results and experimental results.The simulation results show that the dynamic one-equation LES model can describe the complex flows around cylinder and the pressure distribution better.(2)The coupled fluid-particle model is applied in the investigation of three dimensional turbulence,sediment incipience and the sediment transport at the downstream of the backward-facing step flow.The model describes the mean velocity profile,the turbulence intensity and the distribution of Reynold stress well.The probabilitv of sediment incipience and the sediment flux obtained by the coupled model at the downstream of the backward-facing step agree well with the experiment.The results indicate that the turbulence and large-scale vortices induce the sediment incipience and the mean flow contributes to its transport in the case of complex flows.(3)The coupled fluid-particle model is used to verify the sediment motion and simulate the scour around the cylinder,and the scour process and depth are analyzed.The variation of flow velocity,turbulent kinetic energy and vorticity development during scouring are studied.The results reveal that the horseshoe vortex and the accelerating flow contribute dominantly to local scour in front of the cylinder while the turbulent fluctuation and wake vortex results in the scour developmentbehind the cylinder.