ISPH Simulation And Experimental Research Of Local Scour Around Structures Under Currents

Local scour around isolated structures has always been a very important research topic in the field of port,coastal and offshore engineering.Excessive sediment erosion will directly result in instability and even destruction of breakwaters,piers,revetments and other marine structures.Hence,prediction of the local scour process and the equilibrium scour depth around these structures is important basis for the design of isolated structures.On the other hand,research on incompressible smoothed particle hydrodynamic(ISPH)method has been paid more and more attention in recent years by the researchers because of its advantages in saving computation of free surface tracking when solving the governing equation of fluid movement and without the necessity of solving the sediment transport equation additionally in treatment of sediment movement.The main research achievements of this paper are summarized as follows.(1)Theoretical achievements of the ISPH method are studied in detail,including the improvements of N-S equation discretization based on ISPH,the simulation method of bed erosion and the treatment method of two-phase flow.After comparing the advantages and disadvantages of various methods,the local scour numerical model is to be developed on the basis of the ISPH multiphase flow model with high accuracy,high stability and CISPH-HS-HL-ECS modified scheme.(2)For the situation of scoured sediment transported mainly in the form of suspended load,concepts of numerical turbid water particle(TWP)and clear water particle(CWP)are put forward to deal with the sediment-entrained flow.A method is proposed to determine the initial density of TWP at the instant of sediment pick-up by following the studies of sheet flow.An erosion model is proposed for initiating the movement of the TWPs by fixing the theoretical datum of zero-velocity at the center of the first dummy particle(DMP)below the interface wall particle(IWP),and calculating the shear stress on the bed surface by using the weighted average velocity at the center of IWP based on the logarithmic velocity law,which are proved to have the advantages of simple and good stability.(3)The process of sand bed erosion caused by dam break is simulated.It is found that in the bedload dominated situation,basically consistent computational results can be obtained for the sand-water interface form and its time development process as compared with the experiment data,as long as the initial density of TWP in the numerical model is evaluated the same with the actual value of sand and the diameter of numerical particle is directly set as the actual diameter of sand.(4)The erosion process behind the seawall under continuous tsunami overflow is simulated using ISPH scour model.The numerical results are compared with the data of flume experiment.The predicted results of the maximum depth as well as the shape of the scour pit reasonably agree with the experimental data,it is found that the maximum scour depth is proportional to the height of overflow and inversely proportional to the falling height,and also the size of scour pit is closely related with the scale of the vortex.However,the numerical simulation of the present model has much faster erosion pace than the physical model,the reason is that the diameter of the numerical particle is much larger than the actual size of sediment.(5)The local scour around a cylinder under unidirectional currents is studied by physical model test.In the experiment,the high-speed video camera device is used for real-time monitoring of the sediment erosion process in front,back and lateral side of the cylinder.The experiment focuses on the comparison of the final equilibrium scour depth and equilibrium scour time under the conditions of different depth and velocity,which provides validation data for the three-dimensional numerical model.(6)The flow field and the local scour process around an isolated cylinder are simulated by using the ISPH.In the erosion model,the effect of three-dimensional slope on sediment incipient motion conditions is considered.The modeled results of velocity and water surface elevation are consistent with the measured data,and the horseshoe vortex and the lee-wake flow around cylinder are successfully reproduced.The 3D numerical results are compared with the data of the physical test,both the scour shape and maximum scour depth show basically consistent with the experimental values.