Influence of Karman Vortex Street on Broadband Frequency Noise Generated from a Multiblade Fan

Abstract

In the prediction theory for a broadband frequency noise generated from a multiblade fan, the vortices in Karman vortex street were divided into n pieces. The frequency distribution of the noise was estimated so that the Strouhal number could become constant even if the wake is spread by the diffusion. From the results of the measurement of the internal flow of the fan, it was found that the noise was related to the wake characteristics of the specific location in the scroll casing where the relative flow velocity was high. The noise operating in the vicinity of the maximum efficiency point of the fan was distributed over the domain from 500Hz to 1250Hz. It was experimentally proved that when the distribution of the estimated sound pressure level corresponded to the measured broadband frequency noise, no influence of the vortices on the noise in the domains of high and low frequencies existed.