Far-field radiation of aerodynamic sound sources in axial fans fitted with passive noise control features

This study investigates the causal relationship between flow aerodynamics and radiated noise in low-speed axial fans. Using blades with three distinctive tip configurations, including two that were developed with a view to reducing noise emissions, the pressure fluctuations of the exhaust flow in the near field are correlated with the noise measured in the far field in an anechoic chamber. By varying the far-field microphone's azimuthal position, the study investigates the source signatures and directivity of noise sources distributed along the blade span. Several distinctive features in the noise directivity pattern are identified and correlated with the noise sources of aerodynamic origin dissected along the blade span. Utilising the directional far-field auto-spectra of the three blade configurations in combination with the near-field/far-field cross-spectra, the emission characteristics of the aerodynamic sources are analysed and their roles with respect to the overall acoustic signature of the fan are discussed. It is apparent that the ability to decompose the output of the aerodynamic noise sources in the near field is a useful tool in designing fans to achieve desirable low noise targets. Copyright © 2009 by ASME.