| Water-entry of the small moving body at subsonic and transonic speeds is a nonlinear and instantaneous process involving medium change,phase transition strong turbulence,and shake wave.The phenomena that the interaction between multi-degree-of-freedom motion and multiphase flow field during oblique water-entry and the impact of compressibility on cavitation during water-entry at transonic speed are the difficulties in the research of water-entry,which are affected by the parameters of entry and body shape.These complex multiphase flow characteristics involved in the subsonic water-entry are the theoretical basis for the development of weapons such as supercavitating projectiles and trans-media aircraft.Therefore,it is of great theoretical and practical value to carry out research on the multiphase flow characteristics of water-entry of the small moving body at subsonic and transonic speeds.In this study,the combination of experiment and numerical simulation is used to investigate the multiphase flow characteristics of subsonic oblique water-entry and transonic vertical water-entry.The main research contents and results are as follows:Based on the high-speed camera experimental method,the evolution of cavity and drag characteristic of subsonic oblique water-entry are investigated.Through the subsonic oblique water-entry experiments with different entry parameters and shape parameters,the evolution of cavity and splash,and the influence of tail slap on cavity and trajectory are qualitatively investigated.The change that the dimensionless time of surface closure is linear to cavitation number and the dimensionless time of surface closure is proportional to entty angle is obtained by the quantitative analysis of experimental data.The predicting formulas for the cavity profile and the maximum cavity size underwater of the subsonic oblique water-entry are established based on the principle of conservation of energy and experimental data.Based on the theory of homogeneous equilibrium flow,a three-dimensional and three degree-of-freedom numerical simulation method of subsonic oblique water-entry with self-compiled programs is improved,and it is validated by comparing the simulation results with the experimental results.Numerical calculations of subsonic oblique water-entry of the small moving body with typical entry parameters are carried out.The distribution of pressure and velocity of cavitation flow field,and the variation of hydrodynamic parameters are achieved.The interaction mechanism between multi-degree-of-freedom motion and multiphase flow field during oblique water-entry is revealed.The multi-degree-of-freedom motion changes the flow field,and the multiphase flow field provides the moving body with restoring force to make the body back to the cavity.Numerical calculations of subsonic oblique water-entry of the small moving body with different entry angles and entry velocities are carried out.The variation of the cavity shape,hydrodynamic coefficients,and trajectory with entry angle and entry velocity are achieved.The cavity size increases with an increase in the entry velocity and a decrease in the entry angle.During the sobsonic oblique water-entry,the drag coefficient increases with an increase in the entry angle in the impact phase.However,entry velocity and entry angle have minimal influence on the drag coefficient in the cavity-running phase.The moving body with the greater entry velocity and entry angle is not prone to tumbling.Based on the theory of homogeneous equilibrium flow,a numerical simulation method of transonic vertical water-entry is improved,in which the compressibility of air,vapor and water is taken into account.Air and vapor are treated as ideal gases,and a temperature-adjusted Tait equation is used to describe the compressibility effects in water.The numerical simulation method is validated by comparing the simulation results with the experimental and theoretical results.Based on the established numerical simulation method for transonic vertical water-entry,the numerical calculation of vertical water-entry considering the fluid compressibility is carried out,and the calculation results are compared with that ignoring the fluid compressibility.By comparing the calculation results of the changing process of pressure,density and volume fraction of each phase,and cavity shape,the influence of fluid compressibility on the multiphase flow characteristics in the impact phase and the cavity-running phase is achieved.The mechanism of the influence of fluid compressibility on cavitation effect is revealed.The influence of entry velocity on the multiphase flow characteristics of transonic vertical water-entry is analyzed.With different entry velocities,the pressure peak in the impact phase,the drag coefficient when the body runs with cavity,and the volume of vapor inside the cavity are studied.The predicting formulas for impact pressure and drag coefficient are established based on the calculation results. |
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