Research On Some Problems In Structure Impact With Water Using SPH Method

Water entry phenomenon exists widely in aerospace and navigation,such as aircraft crashing into water,space capsule ending its trip with water landing,and the impact of ship.The force on structures is large during these processes,which strongly influence the safety of these structures and the personnels in them.Therefore,the research of water entry is vital.Smoothed particle hydrodynamics(SPH)method is one of the mesh-free methods.Mesh distortion could be avoided in SPH.It is also easy to track the motion of particles and treat interface.SPH method has wide application in hydrodynamics.During the structure entering water,the liquids and the solids interacts with each other.The key issue during this process is the interface treatment between different objects.In this thesis,elastic body impact with water,the bounce phenomenon of structure during water entry and the effect of air in different structures impact with water are studied using SPH method,which involve the interface treatment between different objects.Firstly,the interface between elastic body and water is studied.This thesis finds that the big difference of velocities near interface causes the interface problems in SPH.First,the model of two aluminium rectangles collision shows the errors from discontinuous velocities near interface.Then,in the model of elastic cylinder impact with water,the splash of water is restrained and the pressure on elastic body shows the alternation of negative and positive values from SPH simulation.This thesis proposes a new strategy that combines DSPH method and interface force to deal the interface,which improves the simulation.Finally,elastic ring impact with water is simulated.The result shows that the strategy is able to characterize the big deformation during this process,which verifies our new simulation strategy further.Secondly,the accuracy improvement of DSPH method is studied.Multidimensional RDSPH method that includes first order and second order is proposed based on Taylor series expansion.The accuracies of RDSPH method and DSPH method near the interface,boundary and in un-uniform interior region are tested in one-dimensional and two-dimensional spaces by numerical models.It turns out that the second order RDSPH shows the highest accuracy,then the first order RDSPH,and the DSPH shows the lowest accuracy.Thirdly,the bounce phenomenon during structure impact with water is studied.The research in this chapter verifies the contact algorithm of rigid structure and water.The bounce of a given plate depends on initial velocity,attack angle and incidence angle,which result in different types of motion including skipping,surfing and rotating.These three motion types are simulated by SPH method.The motion equations of plate in 2D are worked out,so SPH simulation results can be compared with theory.This thesis finds that plates in different size can have same motion in certain condition.The best attack angle is about 10° because it has minimal initial velocity for bounce and loses less energy.Moreover,this thesis simulates the skipping of curved structure that is compared with flat plate,which aims at confirming better shape on skipping.Finally,continuous skips is studied.Multi-small tanks model that makes the simulation efficiently are set up.The incident angle changes to a certain value during continuous skips.The certain value is influenced by the attack angle.From the linear relationship between dissipative energy of each bounce and the square of velocity,we can predict the number of skips under different velocities by theory.Fourthly,the air/water interface and gas/solid boundary are tested.First,two-phase problem that air bubble raises in water is simulated,which tests the air/water interface.The adiabatic state equation is worked out based on thermodynamics theory.This equation of state is compared with weakly compressible equation of state,which is used to verify the latter one.The bubble shape and velocity are investigated during rise.Second,the treatment of gas/solid boundary is tested.The model that a piston is driven by the pressure of air in a tube is set up for the first time.Three boundary conditions(no slip,free-slip and periodic boundaries)are compared.No-slip result has rapid damping,which is faster with the increase of viscosity.The tube size has influence on the amplitude and period,which shows that the higher tube has bigger amplitude and period.Fifthly,the effect of air on different structures(wedge,flat plate and multihull structure)impact with water is studied,which involves air,water and rigid body.First structure is wedge.It produces vortex during falling in the air.The accelerations of wedge from simulations are compared with theory,in which the effects of three-dimensional experiment and two-dimensional simulation on acceleration are researched.The effect of air in the motion of wedge is small because the air can escape easily as the wedge enters the water.Second structure is flat plate.It traps air during impact water,which produce "air cushion" and make the increase of acceleration slowly and decrease the peak of acceleration.The pressure on plate is more uniform when air exists.The influence of plate width,velocity and mass on acceleration and pressure are studied.Last structure is multihull structure that shows the SPH algorithm in this thesis works well on complicate structures.This thesis studies several problems on structure impact with water,which could be a reference for engineering and also provide a SPH strategy for future research on the similar problems.All the simulations are done by SPH codes that are written in C language.Post-processing software is Paraview.