Acoustic Optimization of Rotor-Stator Interaction Noise by Trailing-Edge Blowing

Abstract

The paper deals with a novel approach for minimizing tonal rotor-stator interaction noise. A stage of a low-pressure axial compressor with a axial rotor and a downstream stator was designed and investigated experimentally. Seven discrete orifices at the rear end of each rotor blade are fed with pressurized air via separate internal passages and pressure control valves and allow the generation of arbitrary spanwise trailing edge blowing profiles from hub to tip. 3D hot wire measurement revealed that the upwash velocity fluctuations downstream of the rotor are caused by (i) the wakes from the rotor blades, (ii) vortex structures at hub and tip, (iii) the potential flow field of the rotor. Former studies proposed that perfect blade wake filling will lead to maximum reduction of rotor/stator interaction noise. In this study some reduction could be achieved by this strategy, but the utillization of an evolutionary optimiziation algorithm in the experiments proved to be much more effective: Targeting directly the far field sound pressure level we identified a spanwise trailing edge flowing distribution that eliminates the wake related tone completely. Spectral analysis proved that the blade wakes cause the acoustic rotor/stator interaction tone at twice the blade passing frequency whereas the fundamental tone is predominantely due to the vortex structures at hub and tip and the potential flow field interacting with the rotor. This explains, why with trailing edge blowing only the second harmonic of the blade passing frequency could be eliminated but not the fundamental tone. 2 w 2 = relative velocity z = number of blades ε r = relative combined standard uncertainty  = dimensionless flow rate coefficient  = circumferential angle * = dimensionless

Reduction of airfoil trailing edge noise by trailing edge blowing

“…1.5 dB. More recent studies were carried out by Winkler et al [13] and Kohlhaas et al [14]. Winkler was focusing on broad band noise reduction from an isolated airfoil and a tandem airfoil assembly.…”