Abstract
The effect of upstream obstructions on the discrete-frequency noise generation of small, axial-flow fans is investigated. These cooling fans, which operate at very low Mach numbers, generate discrete tones through fluctuating blade-lift forces. These dipolelike acoustic sources are a direct consequence of nonuniformities in the velocity inflow field interacting with the rotor blades. In typical applications, several types of protuberances act to disrupt the inflow. Three basic types of obstructions are modeled. These include cylindrical, rectangular, and periodic structures. The fans operate in an otherwise free, anechoic environment. On-axis, farfield sound pressure level data are obtained through narrow-band spectral analysis. Levels at the blades passage frequency and several prominent harmonics are presented as functions of the nondimensionalized distance of the various obstructions from the rotor hub. Regions critical to noise reduction are determined as aids to the designer. [Work supported by IBM Corporation.]
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