Abstract
The noise radiated by a subsonic, axial-flow fan at its rotational frequency and harmonics is related to the nonsteady force field created at the rotor blade/fluid interface. This force field is highly dependent on the time-invariant flow distortions that enter the fan. In this basic study, a typical cooling fan used in the electronic and computer industry was instrumented with an unsteady axial force sensor. Its output is proportional to the total unsteady axial force created by the rotor. The inflow field of the fan was systematically distorted by placing a small cylinder at various positions in the inlet plane. The nonuniform, three-dimensional flow field entering the rotor was measured by traversing a miniature five-hole pressure probe. The total pressure outputs from this probe can be related to the axial, tangential, and radial velocity vectors. Fourier decomposition of the inflow velocity data is coupled with analysis to give information on the unsteady rotor force harmonic content. The on-axis sound pressure levels were measured and compared to coherent output power spectra involving the unsteady force sensor and the microphone. Very good coherence at the discrete tones is observed. [Work supported by IBM Corp.]
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