Research On Wake Field Characteristics Of Solid Rocket Motor After Underwater Ignition

Solid rocket motors that generate thrust by the thermodynamic expansion of gases,have gradually become the preferred underwater jet propulsion devices due to their simple structure and high reliability.The combustion products of solid rocket motors after underwater ignition are injected into water at supersonic speed.As a result,highly turbulent and strongly transient flow are generated,which may lead to serious ignition safety and thrust stability problems.Therefore,it is of great significance to deepen the understanding of flow mechanism.The methods of mechanism experiments,numerical calculations and theoretical analysis are used in this paper.And the wake field characteristics of solid rocket motors after ignition in still water,flow around underwater vehicles and tail cavity of vertically launched underwater vehicles are studied.The main contents are as follows:(1)Based on the method for capturing multiphase flow field structure with solving unsteady Reynolds-averaged Navier-Stokes equation,a mathematical model of underwater supersonic gas jet considering evaporation and condensation is established by utilizing Lee model.Furthermore,combined with dynamic mesh technology,the calculation method is extended for flow around moving boundary.Compared with the experimental data,the effectiveness of proposed method in predicting gas bubble evolution,pressure oscillation and thrust are verified.(2)Combining the static ignition experiment in pressure tank and numerical simulation,the flow field characteristics of underwater under-expanded gas jets are studied.The results show that,the gas bubble initially has a cap-like shape and gradually evolves into an ellipsoid,and after full development,a conical jet channel is formed near the nozzle.There are shear vortex rings and repeated shock cells in the gas bubble,and the gas-liquid interaction accelerates the flow direction attenuation of turbulent jets.The downstream flow oscillates in the process of momentum transfer,and the amplitude and frequency of near-field pressure pulsation increase with the increase of water depth.The main characteristics of thrust are peak thrust,negative thrust and thrust oscillation,and the average thrust decreases with the increase of water depth.(3)Taking the Laval nozzle with area expansion ratio of 14 and design Mach number of 3.5as the research object,the flow field characteristics of underwater over-expanded gas jets are studied.The results show that,after the over-expanded gas jet forms a jet channel,dramatic periodic oscillation changes occur in the jet core,which is specifically manifested as the expansion and contraction process of 100-200 Hz.Shock-induced separation flow exacerbates the complex changes of wave structure,and the flow separation point periodically moves back and forth.Under the modulation of irregular shock movement,the flow parameters at nozzle outlet become particularly unstable.The pressure pulsation in the separation zone presents the characteristic of high oscillation,which is manly concentrated in the broadband of 100-600 Hz.The simple flow separation criterion for underwater conical nozzles is that the outlet pressure of the nozzle is not less than 0.44 times of the ambient pressure.(4)Through the cold jet experiment,hot jet experiment and numerical simulation of the underwater vertically moving vehicle,the cavity characteristics of supersonic gas jets in tail shear flow are studied.The results show that,asymmetric cavity may be formed in the early stage of ventilation in shear flow,which can induce cavity oscillation phenomenon.The supersonic gas jets continuously produce disturbances near the nozzle.And the pressure at underwater vehicle bottom successively presents the characteristics of transient impact peak,initial wide pulsation,and high-frequency pulsation.The shear flow formed by high-speed motion can suppress the high-frequency oscillation of tail jets.After the jets break through the initial recirculation zone,the vortex with different sizes and opposite directions are formed at the underwater vehicle bottom,accounting for complex changes of local multiphase flow.(5)Aiming at the near-tube ignition technology of underwater vertical launch vehicle,the flow field characteristics of solid rocket motors after ignition in tail cavity are studied.The evolution law of tail cavity,wave system and vortex structure are obtained when the underwater environmental parameters change in a wide range.The oscillation characteristics of tail cavity induced by thermal jet and the fluid dynamics of the vehicle are analyzed,so as to find out the mechanism of pressure pulsation in the coupling process of thermal jet and tail cavity.The influence of mass flow rate,nozzle area expansion ratio and launch water depth on the tail cavity oscillation are obtained,and schematic wake diagrams of the underwater vertical launch vehicle are established.In this paper,based on the study of the interaction process between high-speed gas and water,the flow field characteristics of underwater supersonic gas jets and their coupling with surrounding flow and cavitation flow are obtained,and the understanding of underwater ignition mechanism of solid rocket motor is deepened.The research results have important reference value for engineering application.