Experimental Investigation of Mode-Frequency Scattering at Fan Stages

Abstract For the development of low-noise fan designs the sound generation must be reduced at the source, but also sound transmission effects through the rotor can be exploited to reduce the noise emitted to the far-field. Rotor shielding has been identified as a means of noise abatement, where rotor counter-rotating modes are blocked by the rotor and transmit poorly to the inlet. An effect that is rarely considered in this context is mode-frequency scattering at the rotor, which can circumvent the blockage effect by shifting the sound energy in frequency and mode order, typically to a rotor co-rotating mode order that transmits efficiently to the inlet. An experimental investigation is presented, where the transmission, reflection and scattering of individually generated modes through a fan stage is measured by means of a loudspeaker array and microphone arrays, upstream and downstream. The highly resolved measured data comprise the variation of mode order, frequency, rotor speed and aerodynamic operating conditions. In this study the focus is on mode-frequency scattering, for which results are presented for high and low rotor speed.