Numerical And Experimental Study On Structure Water Entry

Water entry problem has a wide range of applications in the military industry,shipbuilding industry and bionics.In the numerical solution of the water entry problem,the large deformation of the free surface becomes the difficulty of dealing with the water entry problem by the traditional method based on grid.Because the SPH method is easy to deal with the large deformation problem,the two-phase SPH method is used to solve the water entry problem in this thesis.The experimental device which can control the water entry speed accurately is developed to study the water entry of a body by the use of linear motor.The experimental data provide the basis for validating the SPH results.In this thesis,experimental and numerical studies on water entry are carried out to analyze the slamming force,the pressure distribution on the structure,the fine flow field and the evolution of the cavity during the water entry process.The two-phase SPH method based on air and water is developed.Two-dimensional and three-dimensional cases are simulated respectively.The method of measuring the flow field of the structure water entry based on the PIV system is established.The benchmark experiment of the evolution of the cavity and fine flow field is carried out.The experimental data are compared with the numerical results.The main work of this thesis is as follows:Firstly the three research directions into the water entry problem are introduced: slamming force,evolution of the cavity and ballistic trajectory.A literature survey on the research progress of water entry problems covering theoretical,experimental and numerical methods.In the numerical simulation,the advantages and disadvantages of grid method and meshless method in dealing with water entry problem are compared.This thesis introduces the development course of SPH method in detail,and provides reference and guidance for the work of this thesis.Two-phase SPH model of three-dimensional water entry problem is deduced.The mathematic model of the three-dimensional water entry problem is given and the SPH formulation of the governing equations are deduced.The boundary treatment is improved and verified by the classical dam break model.Finally,through several different conditions of the cylinder water entry to verify the reliability of the SPH model to deal with the water entry problem.The experimental device which can control the speed accurately is developed by the use of linear motor.Experimental study on two-dimensional wedge water entry and axisymmetric body is implemented.During the experiment,the slamming force is obtained directly through the 3-component force balance.Through the high speed PIV system,the fine flow field in the water entry process is obtained,and the basis is provided for the numerical simulation of SPH.The performance of the linear motor in the experiment is described in detail,and the measurement result at the typical experimental condition is presented.A two-dimensional wedge water entry at constant speed is carried out.The two-phase SPH method is utilized for numerical simulation.The SPH data is compared with the experimental results.The pressure field inside the cavity is provided in the SPH method.The numerical simulation of the two-dimensional wedge and the ship section structure water entry freely was carried out and the results are compared with the experimental results of the WILS JIP-III team.The results of this thesis are included in the 26 th International Ocean and Polar Engineering Conference(ISOPE)on the water entry problem.From the results of each unit,the two-phase SPH results are of competitive advantage.Based on the experimental device and the two-phase SPH method,the disk water entry at constant speed is investigated experimentally and numerically.The slamming force curve against time is obtained.The dimensionless maximum slamming force curve against Froude number is summarized.The pinch-off time and pinch-off depth are recorded using the high-speed photography.The alternative relation of the dimensionless pinch-off time and pinch-off depth with the Froude number is summarized.Experiments on the disk-cylinder water entry at constant speed are carried out.The typical cavity closure is summarized.The two-phase SPH model is utilized to simulate three-dimensional structure water entry problem.A three-dimensional sphere vertical water entry is developed.The water entry of a sphere is simulated to understand the evolution of the cavity and the changes of pressure inside the cavity.The simulation of the sphere oblique water entry is carried out.