Investigation on supersonic jet noise reduction by water injection: Effects of water injection parameters

In order to protect the payload and the rocket structure from severe vibrations, water injection is commonly used on launch pads to reduce the high noise level radiated by supersonic plumes at lift-off. Even if this technique is widely used, the mechanisms behind sound reduction remain unclear. Sound reduction occurs due to multiple phenomena, each of them having different effects on the jet noise. Experiments based on acoustic measurements have been carried out on a hot supersonic jet on MARTEL facility (Mach number of 3 and a total temperature of 1700 K) to deeper identify the effect of evaporation and water injection parameters. Water flow rate, water injection speed and injection distance to the nozzle have been tested independently to identify the most important water injection parameters. It appears that water flow rate is the most important parameter to decrease noise level and that injecting water at high speed results in noise increase in the downstream direction. Results also highlight the importance of having liquid droplets at the end of the potential core to reduce noise.