Aeroacoustic prediction method of open rotor based on unsteady vortex lattice and vortex particle method

Open rotor engines have the advantages of high efficiency and high fuel economy， but the huge noise radiation of the open rotor blades is an urgent problem to be solved. Based on Unsteady Vortex Lattice Method（UVLM）， Vortex Particle Method（VPM）， and Ffowcs-Williams and Hawkings equation， a fast aerodynamic-noise prediction method for open rotor has been developed， which is free from the space-volume mesh. The three-dimensional Prandtl-Glauert law has been introduced for compressibility correction. Taking NASA SR-7A single open rotor as the research object， the aerodynamic performance and far-field noise under take-off condition have been calculated and compared with NASA test results and numerical results from commercial software. The error of performance prediction is less than 1%， and the error of main tonal noise and overall sound pressure level directivity can be less than 1 dB. With the same research object and hardware， the computational cost of present method is lower than 1/100 that of common URANS simulation. Considering the circumferential flow distortion at flight angles of attack under take-off condition， the aerodynamic parameters and noise results of SR-7A open rotor at angles of attack from 0° to 20° have been studied systematically. It is found that with the increase of the incoming flow angle of attack， the time-averaged thrust， power， and efficiency show exponential growth trends， while the rotor in-plane force shows linear growth trend. The unsteady fluctuation amplitude of the rotor load increases with the increase of angle of attack. The peak-to-valley value of the load fluctuation of a single blade has a phase-lag phenomenon （about 17°~56°） compared with the blade motion phase， which is highly relevant to the deflection in the circumferential noise directivity results. As the angle of attack increases from 0° to 20°， the overall sound pressure level decreases from 107 dB to 99 dB at microphone upon the rotor， while increases from 107 dB to 114 dB at microphone beneath the rotor. In addition， the interaction between the incoming angle of attack and the rotation plane can lead to an additional tonal noise at shaft frequency up to 73 dB. The noise source decomposition analysis indicates that flight angle of attack has significant effects on the increase of unsteady loading noise. The method developed in the present work can obtain the unsteady aerodynamic and noise characteristics of open rotor with low cost， and has the ability of noise source decomposition， which provides a powerful tool for the design of low noise open rotor in the future.