Analytical Prediction of the Pylon-Wake Effect on the Tonal Noise radiated by the Front-Rotor of CROR Propulsion Systems

Abstract

The present paper aims at predicting the tonal noise radiated from the first propeller of Counter-Rotating Open Rotors (CROR) in a pusher configuration, considering both the pylon-wake and the uniform flow effects. The computations are carried out in the frequency domain according to Ffowcs Williams and Hawkings′ analogy. In order to point out the role of the steady-state source mechanisms in the blade reference frame, three noise sources are identified in the process. First the unsteady loading is computed using a similar procedure to the one used for the rotor-rotor wake interaction noise prediction. The velocity deficit in the wake is locally expanded in two-dimensional Fourier gusts in a reference frame attached to the front rotor. The unsteady lift induced by each gust on a blade segment is calculated using a linearized analytical response function that accounts for a realistic geometry. The steady loading constitutes the second source contribution and is computed using a software based on the lifting-line theory. To ensure an accurate interference of sources, the resulting compact loads are distributed on the same blade geometry that the one used for the unsteady loading computation. Finally the thickness noise due to the blade volume displacement is included in the analysis using Isom’s approach. From the linear acoustic assumptions all these sources are modelled as equivalent rotating dipoles and then summed to calculate the far-field noise. The whole methodology is then assessed against wind-tunnel test data. A parametric study considering several pylon positionings and pylon-wake configurations with blowing is finally performed in order to emphasise the relative contribution of the three noise sources.