Numerical Study Of Underwater Supersonic Gas Jets Of Solid Rocket Engine

The solid rocket motor’s working process under water is essentially a submerged supersonic gas jet.The gas accelerates to supersonic through the nozzle,jets into the water environment,blended with water vigorously.The process involving the shock wave structure,the motion of gas-liquid boundary,the instability of gas-liquid interface.Since the underwater propulsion process is under ultra-high temperature and high pressure,involving several physical processes such as heat exchange and phase change,it is a very complicated gasliquid two-phase flow.With the increase of thrust demand,the rocket motors with large expansion is more and more widely applied,which will greatly increase the complexity of the flow field structure.Due to the increasing depth of the rocket motor working,the environmental pressure changes have seriously influence on the motor operating characteristics.Therefore,it is of great significance to study the effect of expansion ratio and water depth on the underwater jet propulsion of the motor.In this paper,a two-dimensional axisymmetric model of numerical calculation based on ANSYS-FLUENT is established.According to the combustor temperature and pressure empirical curve,UDF method is used to define the inlet boundary conditions of the motor nozzle,and the VOF method is used to track the phase interface migration of gas-liquid twophase flow,two supersonic gas jets with different diameters were calculated at different depths in water.The development process of the jet development stage and flow field structure is described,and the variation law of flow field characteristics and thrust characteristics is obtained.According to the results,the jet consists of the expansion period and the periodic-change period.The period of periodic-change consists of three typical stages:necking,bulging and back-attacking.It is characterized by cyclical changes in morphology,pressure field,velocity field,etc.In this paper,the shock wave motion is used to explain the periodic-change of flow field structure and thrust characteristics.The formation of each stage is related to the interaction of water environment,such as extrusion,eddy current shearing,and jet gas expansion.By studying the mechanism of the flow field structure change on the shock wave motion,the periodic-change law of the thrust force and its component force is given.By comparing the flow field structure and thrust characteristics of the two expansion ratios,the influence of the increase of expansion ratio on the periodicity of the flow field is analyzed.The effect of thrust oscillation frequency and amplitude on the results shows that the reduction of expansion ratio can reduce the thrust oscillation frequency and reduce the negative thrust,which helps to improve the thrust characteristics.By comparing the processes in different water depths,the influence of ambient pressure on the penetration and periodic characteristics of the jet is analyzed.The results show that in the shallower depth of water,the smaller the axial penetration and radial expansion of the jet,and the shorter cycle time.During the expansion period,the back pressure peak phenomenon occurs,and the thrust does not basically change with the cycle but only with the inlet temperature pressure.