Suppression of Fan Noise of High Circumferential Mode Using Optimized Duct Wall Properties

C1b - Noise Control by Passive Means

Sound absorption by duct lining is most effective in the medium frequency range. The attenuation spectrum tails off at both low- and high-frequency extremes. The reduction of absorption rate towards the high frequency end is more difficult to deal with, especially when the source has complex circumferential and radial modes.

This work investigates, theoretically, the absorption of high frequency, high circumferential order rotor noise by a wall lining of finite axial length. The wave equation is solved in the frequency domain with the Chebyshev collocation method. The duct has a segment of lining with a typical length equal to the casing diameter. The scattered, out-going waves are propagated out perfectly using a non-local, Dirichlet-to-Neumann condition that covers all the modes resolved by the Gauss Lobatto grid. The incident wave simulates a typical concentrated source located near the circumference of the duct, which may be expanded into multiple radial modes. An axially distributed, locally reactive impedance is used for the duct-lining interface. An optimization scheme is used to maximize the transmission loss.

For a given frequency, it is shown that such optimization may yield significant extra noise reduction when compared with the baseline configuration of a uniform lining which is itself optimized for a frequency. Details of the acoustic field are analyzed to reveal the mechanism of the high absorption rate derived from the optimized liner.