The shape of subsonic jet noise

We investigate the scaling laws of the sound radiated by isothermal subsonic jets, in the Mach-number range, 0.4 ≤ M ≤ 0.9. Pressure measurements are decomposed to give the directivity of the most acoustic relevant azimuthal components of the acoustic field, resolved in frequency. We then explore systematically the trends with Mach number and frequency, in order to obtain a parametric function, fitted to the azimuthally decomposed database that describes downstream radiation. The trends of sound pressure level (SPL) with respect to radiation angle, frequency and Mach number are analyzed to inform a Lighthill line-source model. The identified source model is based on an acoustically matched wavepacket characterized by the acoustic (rather than hydrodynamic) wavenumber and that captures the OASPL at low polar angles to within 2.5dB for Mach numbers ranging from 0.4 to 0.9.