Noise from Imperfectly Expanded Supersonic Coaxial Jets

Experiments have characterized the acoustics of axisymmetric high-speed jets at a variety of Mach numbers and velocities and at pressure-matched, overexpanded, and underexpanded conditions. The effect of an annular secondary e ow on noise emission was also investigated. The fully expanded jet velocity ranged from 630 to 920 m/s and thefully expanded jet Mach number ranged from 1.27 to 1.75. The secondary e owwassupplied at400 m/s and was designed for Mach wave elimination conditions. Imperfect expansion creates screech and broadband shock noise. Screech is dominant in the near e eld whereas broadband shock noise affects mainly the lateral direction of the far e eld. The secondary e ow practically eliminates the screech tones, but has little impact on broadband shock noise. With exception of localized and weak screech tones, the far-e eld spectra in the direction of peak noise emission (aft quadrant ) are insensitive on nozzle exit pressure and depend solely on the fully expanded Mach number and velocity. Addition of the secondary e ow produces substantial noise reduction in the aft quadrant, a consequence of Mach wave elimination, and modest noise reduction in the lateral direction, an effect attributed to mean shear reduction. Lowering the velocity and/or Mach number of the jet enhances the benee t of the secondary e ow by shortening the region of the principal noise sources, thus improving the coverage of that region by the secondary e ow. Far-e eld noise reductions of up to 17 dB were recorded at frequencies most relevant to aircraft noise.