Examination of Sound Suppression by Water Injection at Lift-off of Launch Vehicles

Abstract

Analytical investigation of noise suppression effect of water injection to exhaust plume from rocket motors was carried out. The results showed acoustic absorption by water droplets, acoustic scatter by water droplets, absorption through air, and water curtain effect increase as frequency becomes high. It was also confirmed that acoustic absorption by water droplets has the most significant effects among the four effects. Furthermore, steady 2D-axisymmetric Reynolds-Averaged Navier-Stokes (RANS) simulations of a supersonic free jet were carried out in order to evaluate reduction of jet energy due to water injection. The reduction of the acoustic source strength along the jet axis was evaluated considering the difference of ρk value between with and without water injection. The far field sound power level (SPL) was analyzed using an empirical prediction method, NASA SP-8072 and compared to sub-scale motor test data. The strength of the acoustic source power along the jet axis was set based on the reduction rate of the ρk value due to water injection and the propagation to the far filed points was analyzed based on the NASA SP-8072. The results showed that the water injection effect can be reasonably evaluated by using both the analytical prediction methodology and the evaluation methodology of reduction of the jet energy based on the change of ρk .