Study On The Multiphase Fluid Hydrodynamics Characteristics Of Water Entry And Water Exit For Trans-medium Structures

The problem of multiphase fluid hydrodynamic characteristics of water entry is an important technical problem faced by the development of cross-media equipment.The medium crossing process has a significant impact on the structure’s motion characteristics and the flow field evolution characteristics,while the load changes caused by the flow field have a close relationship with the stability of the structure’s motion.Therefore,research on the multiphase hydrodynamic characteristics of structures entering water is of great significance for clarifying the mechanism of ingress load generation,the safety assessment of trans-medium structures,and the design and application of trans-medium flying submersibles.Firstly,the experimental research is carried out for the cylinder entering the water under the initial conditions of different angles of attack.The results show that the evolution of cavity shows obvious unsteady characteristics,and the evolution process is closely related to the angle of attack.The influence of the angle of attack on the flow field is mainly reflected in parameters such as the area of the cavity,the time when the cavity is pinch-off,and the time when the splash surface seal.At the same time,we have carried out research on the change law of the impact load and surface load,it can be seen that the peak value of the impact load has a gradient decline with the increase of the attack angle.And in the early stage of the development of the cavity,the relative pressure on the surface of the cylinder inside the cavity is negative.When the cavity separation line passes through the surface,the surface flow field changes drastically,and then pressure pulsation occurs.The part in contact with water under the action of water pressure shows a significant positive relative pressure.Subsequently,experiments and numerical calculations are carried out on the problem of the truncated cone-cylinder entering water at different angles of attack.The results show that the characteristics of the non-dimensional parameters of the revolving body with different head shapes are roughly the same.The peak value of the impact load decreases with the increase of the angle of attack,and the difference between the peak values of two adjacent cases is also reduced.The change trend of the surface load is different from that of the cylinder,but the load change mechanism is the same.Through numerical calculation,it can be found that the change of the surface pressure of the revolving body is mainly concentrated near the cavity separation line.The bottom surface pressure is distributed asymmetrically,and it periodically propagates from the upstream surface to the downstream surface in the form of ripples.Moreover,the movement characteristics of the flow field around the body are closely related to the surface pressure distribution.When the velocity forms a vortex on the surface,the surface pressure is usually less than the ambient pressure.In addition,a trans-media flying submerged vehicle based on the principle of bionics is proposed,and the water entry test and numerical calculation research are carried out for this model.The results show that the flying submersible model can automatically adjust the direction of movement after entering the water,and has a good attitude adjustment ability.At the same time,it is found through numerical prediction that the flying submersible has a greater horizontal velocity and a smaller vertical velocity when entering the water in a wave environment,and the lift and drag coefficients also change in response.In addition,different wave heights will also have a certain impact on the water entry motion characteristics of the flying submersible.In the calculated conditions,the higher wave height case flying submersible has greater horizontal velocity and acceleration peak,drag coefficient is always smaller than the case of small wave height.Finally,we designed a trans-media flying submerged vehicle propel by propeller based on the submersible-launched unmanned vehicle,and carried out a numerical prediction study of the vehicle exit the water.The results show that the thrust generated by the propeller can make the vehicle complete the process of media crossing from underwater navigation to air flight.The higher the rotational speed of the propeller,the larger the volume of the cavity produced after exiting the water,and the more drastic the flow field changes.The number of small-scale vortices entrained by the wing is also proportional to the rotational speed.Finally,it is found that the vehicle always has a greater horizontal and vertical velocity at high rotational speed cases,the force on the vehicle increases instantaneously after exiting the water,and the peak of the force is also proportional to the rotational speed.When navigating underwater,the head of the aircraft tends to sink and rotates clockwise.And the deflection angle is maximum when the vehicle contacts the water surface.