The work deals with tonal noise of axial turbo fan stages by rotor-stator interaction. The objective of this study is the reduction of this noise by injection of a secondary mass flow at the rotor blades’ trailing edge (trailing edge blowing, TEB).
A literature survey suggested that at least the tonal rotor/stator interaction sound can be reduced considerably by perfect blade wake filling which usually is achieved by blowing air through slots in the blade trailing edge region. Our own studies proved that this traditional trailing edge blowing strategy is problematic. Hence, in this paper a novel strategy of trailing edge blowing is described. The key idea is the combination of experimental trailing edge blowing with an evolutionary optimization algorithm. Aiming directly at a minimum of far field sound pressure level we identified spanwise trailing edge flowing distributions that reduced the fundamental tone at blade passing frequency (BPF) by 1.4 dB, and at its first harmonic by 21.4 dB (the latter corresponding to a complete elimination). The required blowing mass flow rate is 2% of the mass flow rate through the stage and hence relatively moderate.
3D hot wire measurement revealed 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. Traditional trailing edge blowing primarily aims at eliminating the rotor blade wakes. By applying the novel optimization strategy even some (but not all) of those secondary flow structures could be put out of action which eventually led to the reduction of the spectral components observed.