Suppression of High Mach Number Rocket Jet Noise by Water Injection

摘要

The present work experimentally investigates suppression of the sound level from an underexpanded jet of Machnnumber 2.8 by water injection. The jet is produced by a solid rocket motor being static test fired. Water is injectednfrom a radial distance of 5.2 jet diameters, at different axial locations from the exit of the nozzle, at two differentnangles of injection relative to the downstream jet axis. The ratio of mass flow rates of water to the nozzle exhaust gasn(referred to as the mass flow rate ratio) and the injection pressure are varied independently. Acoustic measurementsnare performed at a radius of 30 jet diameters, over angles in the range of 30–130 deg, relative to the downstream jetnaxis. Sound levels continuously decrease by 10 dB with the increase in the angle of observation. With water injection,nhigher levels of reduction in sound are observed in the upstream quadrant. Injection closer to the nozzle exit leads tonbetter reduction, mainly due to suppression in the high-frequency range when observed from downstream, but it isnalmost in the entire frequency range as observed at the upstream locations. At intermediate mass flow rate ratios, annoptimum injection pressure exists for maximum noise suppression, due to the penetration of water to the potentialncore and its evaporation there at high injection pressures. The results affirm that the validity of many past studiesnobtained on water injection to suppress noise levels on simulated jets can be extended to an actual rocket situation.