### ASA 124th Meeting New Orleans 1992 October

## 5aPAb7. Noise radiation from ducted fans with realistic duct-rotor
coupling.

**F. Farassat
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*NASA---Langley Res. Ctr. M.S. 460, Hampton, VA 23665
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**P. L. Spence
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*Lockheed Eng. and Sci. Co.
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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.