Rotating instability waves as a noise source in a ducted axial fan

Experimental evidence for efficient sound generation by a rotating instability in a small, low tip speed, ducted axial cooling fan is described. The radiated noise is spectrally decomposed to obtain the acoustic source spectra. A notable feature of the acoustic spectra is a broad peak that becomes dominant close to the best efficiency point of the fan. The existence of a rotating modal pattern in the fan flowfield is demonstrated using both hot-wire and blade-mounted hot-film sensor measurements. The autospectra of both velocity and hotfilm voltage show a broad hump composed of harmonically related narrow-band tones in the frequency range at which the large peaks appear in the acoustic source spectra. Correlation analysis of hot-film sensor signals demonstrates that this spectral pattern is highly coherent in the tip region of the blades and that it is rotating relative to the fan. Furthermore, the fluctuations on adjacent blades are shown to be in phase at the frequency of the acoustic spectral peaks, indicating that the acoustic peaks arise from constructive interference of pressure fluctuations on all blades.