Predicting the Tonal Noise of an Open Rotor in a Wind Tunnel with Acoustically Lined Walls

Abstract

An analytic method by which to predict the loading noise generated by a rotor in a rectangular cross-section wind tunnel is presented in this paper. An acoustic liner is modelled using an impedance boundary condition. For zero flow, a Green's function is derived which accounts for finite impedance at the walls. Green's functions with flow but with only zero and infinite impedance are also presented. These are used to obtain expressions for the noise generated by rotating dipole sources, which in turn can be used to predict rotor loading noise. Comparisons between each of the models are presented in the latter part of the paper. The objective is to gain insight into the acoustic field propagation in this situation and to replicate results encountered in model-scale open rotor tests in a high speed, closed test-section wind tunnel.