Suppression of High Mach Number Rocket Jet Noise by Water Injection

TLDR: In this paper, the authors experimentally investigated suppression of the sound level from an underexpanded jet of Mach number 2.8 by water injection and found that water injection closer to the nozzle exit leads to better reduction, mainly due to suppression in the highfrequency range when observed from downstream, but it is almost in the entire frequency range as observed at the upstream locations.

Abstract: DOI: 10.2514/1.43421 The present work experimentally investigates suppression of the sound level from an underexpanded jet of Mach number 2.8 by water injection. The jet is produced by a solid rocket motor being static test fired. Water is injected from a radial distance of 5.2 jet diameters, at different axial locations from the exit of the nozzle, at two different angles of injection relative to the downstream jet axis. The ratio of mass flow rates of water to the nozzle exhaust gas (referred toas themass flow rateratio)andthe injection pressure arevariedindependently. Acoustic measurements are performed at a radius of 30 jet diameters, over angles in the range of 30–130 deg, relative to the downstream jet axis.Soundlevelscontinuouslydecreaseby10dBwiththeincreaseintheangleofobservation.Withwaterinjection, higher levels of reduction in sound are observedin the upstream quadrant. Injection closer to the nozzle exit leads to better reduction, mainly due to suppression in the high-frequency range when observed from downstream, but it is almost in the entire frequency rangeas observed at the upstream locations. At intermediate mass flow rateratios, an optimum injection pressure exists for maximum noise suppression, due to the penetration of water to the potential core and its evaporation there at high injection pressures. The results affirm that the validity of many past studies obtained on water injection to suppress noise levels on simulated jets can be extended to an actual rocket situation.