Noise Characteristics of Combustion Augmented Jets at Midsubsonic Speeds

Abstract

Noise generation by a subsonic flow discharging from a combustion chamber is examined with regard to the relative importance of combustion as a source of noise in such a flow system. Measurements of pressure fluctuations inside the combustor are compared with far-field noise measurements by direct cross-correlations. The cross-correlations and derived cross-spectral densities verify that much of the noise originates inside the combustor. A fluid mechanic perturbation model is used to predict exit plane velocity fluctuations due to internal pressure fluctuations. This unsteadiness at the exit plane is assumed to behave as an acoustic monopole which radiates to the far field. Far-field noise levels estimated on this basis are in good agreement with measured values. The over-all noise level from the combustor/jet is found to be 10-20 db higher than for an equivalent clean, cold jet at the same exit velocity, in the range of 500-700 fps.