In the prediction theory for the broadband noise generated from a multiblade fan, the vortices in the Karman vortex street was 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 500 Hz to 1250 Hz. It was experimentally proved that the influence of the Karman vortex street on the noise in the domains of high and low frequencies did not exist when the distribution of the estimated sound pressure level corresponded to the measured broadband noise.

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In this study, the wake characteristics of an arc blade were measured by the wind tunnel experiment; the characteristics were defined as the width of the wake, diameter of the vortex, ratio of the vortex scale, and the local lift. The influence of the angle of attack on the aerodynamic noise of the blade was quantitatively predicted by using these characteristics. It was clarified experimentally that the sound pressure of the aerodynamic noise becomes small since the gradient of the differential of the lift fluctuation was reduced according to the increase of the angle of attack. The wake characteristics were applied to the prediction of the broadband noise generated from a multiblade fan; the fan noise level distribution was estimated with high accuracy to be in the range from 1000-3000 Hz and was used to analyze the broadband noise of the fan. From the analysis of the fan noise level, it was found that the difference in the relative velocity caused by the biased internal flow was related to the noise levels.

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 500 Hz to 1 250 Hz. 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.

No abstract