Acoustic mode detection in an engine nacelle with a scaled rig fan

Abstract Acquiring acoustic modes with high quality data in large-scale nacelles is quite challenging in the engine industry because of the complex configuration, high flow speed, tremendous number of acoustic modes, and some other extraordinary interference. A complete procedure for mode detection in the engine industry that is applicable to full-size situations is proposed. Two different array patterns are adopted: a circular array for azimuthal modes in both the intake and bypass ducts, and a rotating linear array for radial modes only in the bypass duct. The azimuthal locations of sensors in the circumferential array are non-uniformly distributed to get more modes than the Nyquist limit. For each individual channel signal, an adaptive resampling method is adopted to reduce the components incoherent with source rotation and frequency shifts caused by shaft speed variation. At high flow speeds, boundary turbulence contaminates acoustic signals of wall-flush mounted sensors. A wavenumber decomposition method is used to separate the acoustic part and the dynamic pressure part in the bypass duct during radial mode detection. Finally, both the azimuthal and radial acoustic modes in bypass and intake ducts are acquired successfully.