Aeroacoustic source identification using frequency dependent velocity scaling

Abstract

This paper discusses a method for identifying broadband aeroacoustic sources. The goal of the method is to experimentally derive the dependence of sound power level upon a characteristic velocity and to use that dependence as an indicator of primary aeroacoustic processes. The analysis provides an identification of distinct frequency bands within which the variation in velocity dependence is relatively small. As such, each band is presumed to be controlled by different processes, and the set of likely processes is fixed according to the average velocity ex ponent value obtained. In principal, the method is applicable to any aeroacoustic source identification problem where a characteristic flow speed can be measured and systematically varied. Tests using synthesized acoustic data and measured jet impingement data were used to evaluate the performance of the method. The procedure was then applied to a small axial fan typical of those used to cool electronic systems. The results added support and insight into earlier experiments related to the radiation of high frequency (greater than 2.5 kHz) noise. More specifically, fluid-dynamic and acoustic measurements had shown that flow unsteadiness related to tip gap flows was the primary source of high frequency noise. The frequency-dependent velocity scaling demonstrated that shifts in dominant aeroacoustic processes occur near 2.5 and 5-6 kHz. Additionally, the velocity scaling analysis indicated that the low frequency regime may be controlled by sources which are not compact. (Author)