Shock Wave Behavior in Transonic Compressor Noise Generation

Abstract

Compressor noise at transonic tip speeds contains strong tones at multiples of shaft rotation frequency as well as harmonics of blade passage frequency. These multiple pure tones or combination tones are caused by rotor blade nonuniformities which result in pronounced irregularities in the shock pattern attached to the rotor. Nonlinear first-order theory, similar to that used in analysis of sonic boom strength, is utilized to determine shock wave decay with upstream distance. In the extreme near field of the rotor, shock strength varies inversely as the square root of upstream distance from the blade leading edge, as with an isolated airfoil. Somewhat further upstream, the expansion region from the neighboring blade in the cascade interacts with the shock so that shock strength varies as the inverse first power of distance. These aerodynamic results are used to infer some characteristics of transonic compressor noise which in turn are compared with experimental results.