Calculation On The Reduction Of Rocket Motor Jet Noise By Water Injection

Rocket motor gas jet produces powerful aerodynamic noise, which causes hugeharm to the launch site staff and the equipment and makes the weapon system easier tobe detected. Using the CFD method to simulate the jet flow field and the noise fielddistribution, this paper researches the characteristics of the flow field and theaerodynamic noise field generated by the rocket motor gas jet, simulates the affectcaused by the water injected to the gas jet shear layer, and studies the influence on thenoise reduction of different water spray angle and different MFR.We can draw a conclusion based on the analysis of the calculation results that theflow field formed by the high speed gas sprayed from the rocket motor nozzle is filledwith complicated expansion wave and compression wave. The potential core, in whichthe pressure, temperature, speed and the Mach number of the flow field changesdramatically, is the main source area of jet noise. The sound pressure caused by rocketmotor gas jet changes rapidly and large over time, however2～3orders of magnitudesmaller than the flow fluctuating pressure. The jet noise peak value is mainly distributedin a narrow frequency range, and which is adequately low while the frequency is morethan3000Hz. The directivity of the jet noise can be found in this work. The maximumoverall sound pressure level is in the60°direction. The overall sound pressure level isdecreases with increasing distance.Water injection changes the jet flow field structure, but mainly affects the fielddownstream the water jet point, slightest impacts the field upstream. the water jetvelocity component perpendicular to the axis of the gas jet is an important factorinfluencing the effect of noise reduction, too small, the water hardly injected into thepotential core and hence least curb the noise, whereas too big, the water jet possiblypenetrate the potential core and which is ineffective in suppressing the noise.The pressure, temperature, velocity and the Mach number decline significantly inthe water jet flow field and the sound pressure level is significantly lower. Analyzing theeffect of different water spray angles shows that an obvious noise reduction could beachieved with the water spray angle in the scope of30°～85°, while water sprayed at60°, the noise reduction effect is ideal in this paper. Analyzing the effect of different MFR shows that the noise reduction is increases with the rate of water and gas increasesin the range of2.4～5.5.