Study On Multiphase Flow During Water Entry Of Semi-closed Cylinder

The semi-closed cylinder is a kind of barrel structure with an opening hollow cell.As the opening end of the semi-closed cylinder impact the freesurface firstly and entry water from air,the air in the cell is compressed and water flow into the cell.The cell forms a internal space where gas and liquid two phase flow coupling and connects with the exterior space through the opening end.In the process of water entry for semi-closed cylinder,involved solid,liquid and gas three phases coupling movement and internal and external two fields coupling flowing.It is distinct for the flow phenomenons which created by the semi-closed cylinder water entry and the solid of revolution water entry.In general,the complex flowing generate the peculiar air cavity characteristic,flow structure characteristic,hydrodynamic force characteristic and submarine ballistics characteristic.Therefore,it is very important to study the unsteady multiphase flow in water entry of semi-closed cylinder.In present study,the unsteady cavity flows,flow structure,hydrodynamic force and kinetic stability in water entry of semi-closed cylinder are investigated by combining potential theory,numerical method and experiment.The main research contents are as follows:Based on the theory of potential theory,the basic mathematical model for cavitating flows in water entry of semi-closed cylinder is established.According to aseries of basic assumptions,the equation of gas moving in the opening cell,the equations of cylinder movement and the equation of cavity flowing are established.We adopt the asymptotic theory and the three-dimensional potential theory to simplify and solve the equatons of motion of semi-closed cylinder in water entry.Based on the experimental method by high-speed video cameras and visualization technique to study the cavitating flows patterns and regimes in vertical water-entry cavity of semi-closed cylinder.Following the speed of impacting freesurface increased,the cavitating flows present two flow pattern forms of fluctuation cavitation and cloud cavitation.The formation and development of fluctuation cavitation is investigated and the effect of Froude number is studied.The formation mechanism of cavitation fluctuations and cavitation clouds are studied based on the basic theory of fluid mechanics.Comparisons of the experiment results of models with different structure parameter,we analyse the influence to cavity flow for volume of the opening cell and area of the opening end.Based on the computational fluid mechanics method,a numerical model of unsteady multipase flows is adopted to simulation the water entry of semi-closed cylinder.The application of numerical model is evaluated and validated by comparing the numerical time evolution of cavitating flow with the experimental results.Numerical simulation is conducted over three dimensional semi-closed cylinder water entry to obtain the hydraulic resistance and pressure on the surface of cylinder.Furthermore,the movement of floating and sinking periodically after the first sinking is obtained by numerical simulation for positive-buoyancy semi-closed cylinder model.Based on the result of experiments and numerical simulations,the perturbation of the periodic jet is investigated to the flow structure near the opening end.Pressure pulsation characteristics are analyzed in the water impacting process and the pressure distribution and velocity distribution are obtained in the process of air compressing and expanding.Furthermore,a series of cavity flow instability created by the perturbation are discovered based on experimental results and the perturbation mechanism is analyzed by the boundary layer theory.Based on the inclined water entry experimental method,the trajectory and kinetic stability of semi-closed cylinder underwater are conducted by the inclined experiments to obtain the typical motion characteristics including trajectory,velocity,posture and rotation angular velocity.The effect of the initial motion parameters to the trajectory and kinetic stability,such as velocity and angle of pitch,are also executed.