Rotor-stator interaction noise of axial turbo machinery: Minimization by experimental optimization of trailing-edge blowing

The work deals with tonal noise of an axial turbo machinery stage due to rotor-stator interaction. The objective of this study is the reduction of this noise by ejection of a secondary mass flow at the rotor blades' trailing edge (trailing edge blowing, TEB). A widely investigated strategy is a perfect filling of the wake downstream of the blade trailing edge. Recent studies, however, indicated that this state of the art trailing edge blowing strategy is problematic since it may stimulate undesired secondary flows which overcompensate for the benefits of trailing edge wake filling. Hence, in this paper a novel strategy of trailing edge blowing is described. The key idea is the application of an evolutionary optimization algorithm to trailing edge blowing experiments with the measured far field sound pressure level as target function. As a result spanwise trailing edge blowing distributions that reduce the fundamental tone at blade passing frequency (BPF) by 2.4 dB, and at its first harmonic by 21.4 dB (the former representing a small benefit, but the latter corresponding to a complete elimination of the tone), have been found. The required blowing mass flow rate is 2% of the mass flow rate through the stage and hence relatively moderate. 3D hot wire measurements proved that the acoustically relevant upwash velocity fluctuations as seen by the stator are not only caused by the wakes from the rotor blades but also by vortex structures at the rotor's hub and tip. TEB, which is obtained by optimization as proposed in this contribution, either does not enhance these secondary flow structures or puts them out of action.