Comparison of measured and calculated mode redistribution associated with spinning mode transmission through circumferentially segmented lined ducts

Abstract

An experiment was conducted to investigate mode redistribution in lined circular ducts with circumferential impedance nonuniformities. Mode insertion losses were measured for six-segment, two-segment, and uniform liners in a spinning mode synthesizer (SMS) in the quiet flow facility in the Langley Aircraft Noise Reduction Laboratory and correlated with predicted transmission losses. Both experiment and theory show that circumferentially segmented duct liners do scatter energy into circumferential modes of different orders. Good agreement measured and calculated attenuations was obtained for circumferential mode numbers paired with zero order radial mode numbers. For the higher order radials, the agreement was not as good