Active Control of Discrete-Frequency Noise Generated by Rotor-Stator Interactions

Abstract

Discrete-frequency tones generated by unsteady blade row interactions are of particular concern in the design of advanced turbine engines. In the annular fan inlet and exit duct, the acoustic waves generated at the multiples of the rotor blade pass frequency must have circumferential periodicity. The rotor-stator generated discrete-frequency noise is characterized as a summation of these circumferentially periodic acoustic waves or spatial modes over the multiples of the rotor blade pass frequency. Only the spatial modes that propagate to the far field represent the discrete-frequency noise received by an observer. A series of fundamental discrete-frequency noise source control experiments is described. Active airfoil source control is utilized to generate propagating spatial modes to interact with, and simultaneously cancel, the upstream and downstream propagating spatial modes generated by the rotor-stator interaction. The active airfoil source control is optimized for the control of the propagating spatial modes. The active noise control system incorporates the active airfoil source control with in-duct spatial mode measurements. Data are acquired and analyzed that demonstrate the viability of this unique active noise control technique. Significant simultaneous noise reductions are achieved for the upstream and downstream propagating spatial modes. The control system is successfully demonstrated over a range of rotor blade pass frequencies.