Experimental Investigation Of Ventilated Cavity Flow Characteristics Of Water Entry With Gas Jet Cavitator

The water entry of vehicles from air to underwater is a typical penetration media problem.Due to the difference in density between water and air,a strong nonlinear impact will be exerting on the vehicle when encounters a sudden change in the media,which will have an important impact on its structure,internal device,and water entry ballistics.To solve such problems,two obvious advantages are using the gas jet assisting vehicle water entry.On the one hand,the gas jet impacts the water surface to generate induced pressure which can significantly reduce the water entry impact momentum of the vehicle,avoiding strong nonlinear impact effectively.On the other hand,the surrounding air entrained by the gas jet and the open ventilated cavity which is formed by the gas jet can reduce the viscous resistance during the water entry.In this paper,to investigate the flow and impact load characteristics of vehicle water entry with gas jet cavitator,the experiments of vehicle water entry with gas jet cavitator under different gas ventilation rates and water entry angles were performed.Firstly,the experimental images and data processing are acquired and analyzed based on the self-programmed program,and the limitations of the data processing method which is mentioned in the EPPS doctoral thesis are pointed out.Then,the characteristics of multiphase flow in the early stage of water entry with gas jet cavitator were analyzed,and the results indicate that the water surface appears to fluctuate due to gas-liquid shear which caused by viscosity,and the splash formed by gas jet significantly different from that of the water entry without the gas jet.At the same time,there are transitions in different regions(i.e.,disturbance region,transition region,and the cavity develop region)during the formation of the open cavity,and it is related to the position of the vehicle.As the vehicle vertical height decreased,the open cavity depth increases nonlinearly,and the experimental data are in good line with the theoretical results.The diameter of the open cavity shows an increasing trend firstly and then decreases after increasing to a certain value.The open cavity depth and diameter increase linearly with increasing ventilation coefficient.The diameter and depth of the open cavity increase with the increase of the water entry angle,however,when the water entry angle increases to a certain value,the depth will increase and weaken,and the penetration depth will decrease with the increase of the water entry angle.After the vehicle penetrates the air-water interface,the cavity surface(gas-liquid interface)exhibits fluctuation phenomenon significantly due to the K-H instability,and the cavity collapse phenomenon was observed under the low ventilation coefficient.As the vehicle penetrates the water surface,the cavity diameter and gas jet length decreased gradually but decays insignificantly at the later stage of water entry.These changes are not only affected by the ambient pressure but also be related to the turbulent jet fluctuation.The cavity diameter and gas jet length increase linearly with increasing gas ventilation coefficient with limited effect by the water entry angles.Finally,the impact load characteristics of vehicle water entry with gas jet cavitator under different gas ventilation rates and water entry angles were discussed.The results indicate that the velocity of the vehicle water entry decreases with increasing gas ventilation coefficient,and the velocity decay significantly after penetrating the water surface,and the impact force and force coefficient become greater at the later stage of water entry.As the water entry angle decreased,the velocity of the vehicle at the early stage of the water entry process becomes greater.Also,the impact force and force coefficient become greater due to the great wetted area of the vehicle after penetrating the water surface.