Rotor-strut interaction force is identified as the main tonal noise source of a typical computer cooling fan for which obvious design faults have been corrected. It is hypothesized that the interaction force arising from one encounter between rotor blade and strut can be approximated by that in a special design in which the number of rotor blades is equal to the number of struts. in such a design, the on-axis radiation of noise gives the exact signature of the time derivative of the interaction force which can be experimentally quantified and further used to predict noise radiation by a practical design in which the number of struts is different from the number of blades. This hypothesis is applied to a sample fan of 9 cm in diameter with 7 rotor blades and 4 struts. Analysis of the 4-strut fan noise agrees very well with the prediction based on the force signature experimentally obtained from the corresponding 7-strut design. Measurement also shows that a potential of around 10 dB sound power reduction can be realized when the usual design of cooling fans are corrected for its inlet flow conditions and unequal strut sizes. (C) Institute of Noise Control Engineering.
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