Impact of inlet distortion on fan tonal noise

Abstract

Fan tonal noise is currently supposed to be mainly caused by the interaction of fan-blade wakes with Outlet Guide Vanes and by the shocks in transonic regimes. Evolution of aircraft engines towards Ultra High Bypass Ratio architectures may affect this noise with the additional distortion coming from the shortened and asymmetric air inlets. The influence of this distortion is studied here by means of simulations based on the Unsteady Reynolds-Averaged Navier-Stokes equations. The results show that the asymmetric air inlet is responsible for a multiplication of the distortion level by four to six and for a deformation of fan-blade wakes. A filtering consisting in eliminating the flow patterns convected with the flow is achieved to extract only the acoustic fluctuations from the simulations. Acoustic power estimations are made and allow for evaluating the acoustic penalty induced by air inlet distortion.