Experimental Study and Simulation of Rocket Engine Freejet Noise

On behalf of CNES and in the framework of studies concerning the acoustic environment of Ariane 5 launcher at lift-off, ONERA has adapted to the near sound field simulation a NASA's jet noise model of the 1970's. In order to validate this model, tests have been carried out with static rockets of various sizes. Several improvements appear to be necessary for a precise simulation of the acoustic measurements: in particular, the characteristic length of the model must be reduced and the fully expanded jet data must be used instead of the jet exhaust data. It is shown that the acoustic efficiency, for a given Mach number, is not dependent on the rocket size, and may be estimated from a semi-empirical formula found in the literature. The model changes are checked with other jets of various characteristics. On the basis of experiments and following an approach deduced from an earlier study, the spatial characteristics of the sound field and more particularly the sound power peak location are related to the supersonic length of the flow, which appears to be the adequate reference length for a future jet noise model.