Coaxial Rotor Broadband Noise Prediction in Hover

This paper presents an efficient prediction of coaxial rotor broadband noise, particularly trailing-edge noise. The method combines a newly developed iterative coaxial rotor BEMT, a viscous panel method, an empirical wall pressure spectrum, and Amiet's trailing-edge noise model. Aerodynamic data including the induced velocity and angle of attack on each rotor are calculated by the iterative BEMT. Then, turbulent boundary layer flows, such as the boundary layer thickness, skin friction coefficient, pressure gradient, etc., are computed by a viscous panel code, XFOIL. Based on these boundary layer parameters, the wall pressure spectrum near the trailing edge is computed by Lee's semi-empirical model. Finally, trailing-edge noise is predicted by Amiet’s model from the wall pressure spectrum. This method provides fast computations for aerodynamics and acoustics for coaxial rotors. Acoustic predictions can be performed for various design and operating conditions including the effect of rotor-to-rotor separation distance. In addition to the overall noise of the combined rotor system, each rotor's contributions to noise can be analyzed. A small-scaled untwisted rotor is selected to analyze aerodynamics and aeroacoustics. It is found that the noise contribution from each rotor is about the same at small separation distances. At large separation distances, the lower rotor generates higher noise levels than the upper rotor mainly due to the change in rotor distance with respect to the observer. The detailed boundary-layer flow properties are investigated on both rotors.