Prediction of discrete-frequency noise generated by a propfan at cruise

Abstract This study employs the specialized prediction program FODNOPPb, based on Farassat’s time-domain collapsing sphere method (Formulation 1-Sph), to calculate the discrete-frequency noise generated by the propfan model SRPF with a diameter of 0.622 m. Designed by using compressible lifting surface theory, SRPF operates at a cruising altitude of 11 km and a flight Mach number of 0.8, featuring a transonic helical-tip Mach number of 1.13. The observer point, located in the far field, moves synchronously with the airframe. The results indicate that the maximum harmonic sound pressure level of the overall noise is 107.6 dB, and the acoustic pressure waveform of the overall noise is primarily determined by the acoustic pressure waveform of the thickness noise. By the collapsing sphere method, quite a smooth shape is obtained in the sharply varying segment of the blade acoustic pressure curve, which corresponds to significant changes in the blade acoustic planforms at various observer times. This paper provides valuable insights into the low-noise design of propfans during the aerodynamic inverse design phase.