On broadband jet-ring interaction noise and aerofoil turbulence-interaction noise predictions

The aerodynamic noise of a thin rigid annulus (referred to as the ring here) placed in the mixing layer of a subsonic circular jet is investigated in the paper, both theoretically and experimentally. From the experimental point of view, the jet–ring configuration is understood as an axisymmetric alternative to more usual ones involving a rectangular aerofoil held between parallel side plates, dedicated to the study of the noise due to the impingement of upstream turbulence. The main advantages of the circular geometry are a minimum background noise, the absence of tip effects and more specifically the account for all radiation angles from the surface in the far-field acoustic signature. The circular set-up is well suited for the study of pure broadband interaction noise only if the flow remains free of self-sustained oscillations. This is ensured by keeping a sufficient interaction distance between the nozzle and the ring, and by shaping serrations on the nozzle lip. From the theoretical point of view, an analytical model is derived as a straightforward extension of existing formulations. The induced unsteady lift forces on the ring are first inferred from a linearized unsteady aerodynamic theory and the far field is calculated in a second step by a radiation integral. This relates the far-field acoustic pressure power spectral density (PSD) to the two-wavenumber spectrum of the radial turbulent velocity at the ring location, by means of an aeroacoustic transfer function. The latter is shown asymptotically identical to the one detailed in the Appendix for a rectangular aerofoil, in the limit of relatively high frequencies. The analytical acoustic predictions are found to agree well with the measurements over an extended frequency range, provided that the model is fed with turbulent velocity input data measured by a hot-wire probe. Indirectly, this agreement validates the transfer function for a rectangular aerofoil at oblique radiation angles, which is not achievable in a set-up involving side plates and a rectangular nozzle.