A parametric study of jet-noise reduction by uidic injection on co-axial jets

In this work we consider the noise-reduction produced by fluidic injection in the secondary flow of an isothermal co-axial jet. The control device, which we call a fluidevron, has been shown to produce reductions in jet noise which are comparable with those achieved using conventional microjets, but the underlying physics have been shown to be very different (see Laurendeau et al. for details). A negative effect produced by the control comprises a high-frequency noise increase. Having already optimised for convergence and penetration angle, we here present the impact of injection velocity on the low-frequency noise-reduction, the high-frequency noise-increase and the cross-over frequency. We furthermore assess the performance of the controller after transposition to full scale has been effected. This transposition includes both standard scaling based on the Strouhal number, a weighting to account for the sensitivity of the human ear, and a further weighting which incorporates the integration of all other noise sources on the aircraft. An important result is the very high sensitivity of the ‘useful’ noise-reduction to the cross-over frequency (where we pass from noise-reduction to noise-increase). Configurations which look good at laboratory-scale are often found to produce negligible benefit at full-scale, while laboratory configurations which are more modest, in terms of overall or low-frequency reduction, are sometimes found to be beneficial at full-scale on account of high cross-over frequencies.