Noise from imperfectly expanded supersonic coaxial jets

Experiments have characterized the acoustics of axisymmetric high-speed jet s at a variety of Mach numbers and velocities and at pressure-matched, over expanded, and underexpanded conditions. The effect of an annular secondary flow on noise emission was also investigated. The fully expanded jet veloci ty 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 desi gned for Mach wave elimination conditions. Imperfect expansion creates scre ech and broadband shock noise. Screech is dominant in the near field wherea s broadband shuck noise affects mainly the lateral direction of the far fie ld. The secondary flow practically eliminates the screech tones, but has li ttle impact on broadband shock noise. With exception of localized and weak screech tones, the far-field spectra in the direction of peak noise emissio n (aft quadrant) are insensitive on nozzle exit pressure and depend solely on the fully expanded Mach number and velocity. Addition of the secondary R ow produces substantial noise reduction in the aft quadrant, a consequence of Mach wave elimination, and modest noise reduction in the lateral directi on, 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 sh ortening the region of the principal noise sources, thus improving the cove rage 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.