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 flow on noise emission was also investigated. The fully expanded jet velocity ranged from 630 to 920 m/s and the fully expanded jet Mach number ranged from 1.27 to 1.75. The secondary flow was supplied at 400 m/s and was designed for Mach wave elimination conditions. Imperfect expansion creates screech and broadband shock noise. Screech is dominant in the near field whereas broadband shuck noise affects mainly the lateral direction of the far field. The secondary flow practically eliminates the screech tones, but has little impact on broadband shock noise. With exception of localized and weak screech tones, the far-field 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 Row 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 benefit of the secondary flow by shortening the region of the principal noise sources, thus improving the coverage of that region by the secondary flow. Far-field noise reductions of up to 17 dB were recorded at frequencies most relevant to aircraft noise.