Numerical Investigation Of Scour Around Cylinder Based On MP-PIC Method

Local scour around piles has always been a hot topic in river and coastal engineering research.Traditional research methods mainly describe sediment transport and bed scour evolution through various empirical and semi-empirical sediment transport formulas or equations.With the rapid development of computer technology,there are more and more ways to study the local scour around structures from the mesoscale,but most of the current research methods have the shortcomings of large amount of calculation,low calculation efficiency and limited number of simulated particles.In recent years,MP-PIC method has attracted wide attention.It uses a new particle collision stress model and can use a parcel to represent multiple particles,which can greatly reduce the calculation cost and improve the efficiency of calculation.In this paper,a three-dimensional numerical model based on MP-PIC method is established to simulate the local scour around a cylinder under unidirectional flow.The main contents and results of this thesis are as follows.(1)In order to verify MP-PIC model in simulating sediment motion,the sediment particle settling was simulated.Firstly,the sensitivity of grids is analyzed.It is recommended to simulate the sediment motion on the grid scale of 2-5 times the sediment particle size.The sedimentation of single sediment particle with different sizes is simulated,and the simulated settling velocities agree well with the experimental results and empirical formulas.The hindered settling of group particles with different volume concentration are simulated.The simulation results are consistent with the experimental results of Peysson et al,which has a tendency that the settling velocity of particles in the middle region is larger than that near the side wall.At the same time,the simulated settling velocity is close to the theoretical formula.(2)The open channel flow and sediment motion under different velocity conditions were simulated.It is found that there are turbulent phenomena in the flow field,such as the upward injection of low-speed fluid and the sweeping of high-speed fluid along the flow direction.And the simulated time-averaged velocity profile is in good agreement with the logarithmic law velocity formula and results of CFDEM.At the same time,the components of turbulence intensity and the vertical distribution of Reynolds stress are analyzed.The simulation results of single width sediment transport rate and bed load transport intensity of each example are in good agreement with the empirical formula and experimental values,which verifies the rationality of the model for bed load transport simulation.(3)The local scour around a cylinder under unidirectional flow is simulated.Through the analysis of the change process of scouring pit shape and scouring depth with time,it is found that the whole scouring process can be divided into three stages: rapid scouring,steady scouring and balanced scouring.The simulation results show that the maximum scour depth is 1.25 D,which is close to the CFD-DEM simulation results.The velocity distributions at different positions around the cylinder are analyzed,and it is concluded that the maximum influence range of the cylinder on the flow field around the cylinder is 3D.By comparing the computational time,it is found that the computational efficiency of MP-PIC is 8.8 times that of CFD-DEM,which verifies the rationality and efficiency of the model for local scour simulation.