Noise Measurements in Supersonic Jets Treated with the Mach Wave Elimination Method

We report noise measurements for perfectly expanded coaxial jets composed of a supersonic primary stream at velocity of 920 m/s and a coflow stream at conditions designed to prevent formation of Mach waves. Both the primary and secondary streams consisted of helium-air mixtures to simulate approximately the conditions of hot flows, The resulting sound field was compared to that emitted by a single jet at the conditions of the primary stream. Overall sound pressure levels (OASPL) and noise spectra were obtained at many radial and azimuthal positions around the jet exit. Equal-thrust comparisons were made by using geometric scaling. At equal thrust, Mach wave elimination reduced the near-field OASPL by 11 dB and the far-field OASPL by 5 dB. The mid-to-high-frequency region of the spectrum, which is most pertinent to aircraft noise, was reduced by 20 dB in the near field and by 9 dB in the far field. It is shown that Mach waves account for at least 85 % of the sound field most relevant to aircraft noise.