F. Farassat
NASA---Langley Res. Ctr. M.S. 460, Hampton, VA 23665
P. L. Spence
Lockheed Eng. and Sci. Co.
The problem of noise radiation from inlet and exhaust of ducted fans is important because of anticipated stringent noise standards. The aerodynamic and duct propagation problems of these engines have been solved. What is needed is the coupling of rotor aerodynamics and duct propagation using realistic physical and geometrical models on today's high-speed computers. In this paper, a method is presented for prediction of noise from the inlet of ducted fans correctly accounting for fan blade aerodynamics. An Euler code is used giving the fluid dynamic parameters such as pressure and velocity at a duct section approximately one chord length from the fan face. Here, all of the pressure disturbance is propagating. This pressure disturbance is decomposed into spinning modes using Fourier and Hankel transforms in circumferential and radial directions, respectively. The amplitude of each mode is produced from interaction of upstream and downstream (due to reflections from the inlet) moving waves. The relative amplitudes of these waves are computed using a duct propagation code based on FEM by initializing the upstream moving wave of a given mode to unit amplitude and finding the reflection coefficients of all modes. A system of linear simultaneous equations for amplitudes of upstream and downstream moving waves is constructed and solved. From these amplitudes, the radiation from the duct inlet can be calculated. Computed examples of radiation from ducted fans in forward flight are presented.