Study On Water Entry And Underwater Drag Reduction Of High Temperature Spheres

The water entry problem is a complex multiphase hydrodynamic problem involving solid,liquid,gas interaction in a very short time.This problem is closely related to navigational,aeronautic and astronautic fields,such as: aerial torpedo dropping,spacecraft recovery at sea and ship slamming and so on.Reducing the underwater drag is an important problem in hydrodynamics.Lower drag means faster speed and less energy consumption.Research on drag reduction technology is of great significance to energy saving and improving the performance of ships and underwater vehicles.Recent studies have shown that the boiling effect has a great influence on the flow characteristics of the moving body in the process of water entry and underwater motion.In this paper,numerical simulation and experiment are combined to study the multiphase flow characteristics during water entry and drag reduction characteristics during underwater motion of a high temperature sphere.The main research contents and results are as follows:Based on the experimental method of high-speed camera,the multiphase flow characteristics of high-temperature sphere during water entry in the whole boiling stage are systematically studied.Experiments on water entry of spheres at different impact velocities,initial temperatures of sphere and water temperatures are carried out.The flow pattern and evolution characteristics of the cavity during water entry of the spheres in the nucleate and film boiling stages are obtained,and the cavity morphology is divided.In the nucleate and film boiling stages the water-entry cavities can be formed at low impact velocities.The cavity surface is rough in the nucleate boiling stage and smooth in the film boiling stage.The influence of initial temperature and impact velocity of sphere on the stability of vapor film during film boiling is analyzed.The lower the temperature of the sphere and the higher the impact velocity,the more easily the vapor film is disturbed,thus a disturbed water-entry cavity is formed.The water-entry experiments of superhydrophobic high temperature spheres are carried out,and the coupling influence of boiling and wettability of the sphere surface on the evolution characteristics of water-entry cavity are analyzed.Based on VOF multiphase flow model,coupled with evaporation-condensation phase transition model,a numerical simulation method for water-entry process of a sphere in film boiling stage is established.The validity of the numerical method is verified by comparing with the experimental results.The differences of flow characteristics of film boiling spheres,hydrophilic and superhydrophobic spheres during water entry are analyzed through numerical simulation.There is no contact line between the sphere and the surface of cavity in the film boiling stage.The vaporized vapor film separates the bottom of the sphere from water.In the film boiling stage,the radial expansion velocity of the cavity surface is much faster than that of the superhydrophobic sphere,and the pinch off time of the cavity is longer.A numerical study on the water entry of spheres in the membrane boiling stage with different relative densities is carried out.The influence of relative density on the evolution of cavity and hydrodynamic characteristics is analyzed.The drag reduction characteristics of the high-temperature sphere moving in water are experimentally studied.The average velocities of spheres falling in different temperature water during nucleate boiling and film boiling stages are obtained.The influence of initial temperature of sphere and water temperature on underwater drag reduction is studied.The variations of velocity with time in three typical states,i.e.room temperature sphere,nucleate boiling and stable film boiling,are analyzed.The trajectory stability of the sphere falling in water under three conditions is also studied.The vapor formed in the nucleate boiling stage is not easy to accumulate,and the drag reduction effect is better than that of the sphere in the film boiling stage.Compared with the sphere at room temperature,the stability of the falling trajectory of the sphere is better in the film boiling stage and worse in the nucleate boiling stage.Mixture multiphase flow model is used to study the drag reduction characteristics of sphere moving underwater in film boiling stage.The influence of Reynolds number and surface heating position on underwater drag reduction is analyzed.The mechanism of drag reduction is also studied.In the film boiling stage,the low-speed vortex region at the tail of the sphere is narrow,and the vapor cloud formed is streamlined.The flow velocity of the fluid outside the vapor film is high.The slip effect of the vapor film reduces the viscous force of the wall surface on the fluid,makes the separation point of the flow move to the tail,and reduces the resistance of the sphere.With the increase of Reynolds number,the position of steam accumulation moves backwards,delaying the flow separation and decreasing the drag coefficient.The closer the surface heating position is to the inflow direction,the weaker the accumulation effect of steam is,the closer the separation position is to the tail of the sphere and the stronger the drag reduction ability is.