Mean Stator Loading Effect on the Acoustic Response of a Rotating Cascade

Abstract

Discrete-frequency tones generated by unsteady blade row interactions are of particular concern in the design of advanced turbine engines. With a rotor ‐ stator mounted in a duct, only certain specie c spatial modes are generated by the rotor ‐ stator interaction, where the generated modes are a function of the number of rotor blades and stator vanes. In addition, only some of these modes propagate to the far e eld, with the rest decaying before reaching the far e eld. Thus, it is only those spatial modes that propagate to the far e eld that represent the discrete-frequency noise received by an observer. This paper’ s aim is to determine the ine uence of steady stator loading on the acoustic response of an annular cascade. To accomplish this, the existence of the propagating modes generated by a rotor ‐ stator interaction must e rst be verie ed. Microphones placed in an axial plane in the outer annulus of the inlet of the Purdue Annular Cascade Research Facility are sampled simultaneously over one rotor revolution, and an ensemble-averaged data set is acquired. With the microphone signals treated as a function of time and space, dual Fourier transforms are utilized to determine the magnitude of the spatial modes at multiples of blade pass frequency. The wave equation is used to predict the propagation characteristics of these modes in the inlet duct. The two predicted propagating modes were found to have signie cantly higher amplitudes than modes that were predicted to decay, or were not to be generated by the rotor ‐ stator interaction, and steady stator loading had a profound ine uence on acoustic response of the cascade. The acoustic response at blade pass and twice blade pass frequency increased by more than 20 dB for angles of attack ranging from 220 to 25 deg.