The use of frequency-dependent velocity scaling as a diagnostic tool

Abstract

This paper will discuss a method for investigating broadband aeroacoustic noise sources. The method is based upon the use of frequency-dependent velocity scaling. The dependence of sound power level upon a characteristic velocity is determined experimentally, and that dependence is used as an indicator of primary aeroacoustic processes. The procedure yields an identification of distinct frequency bands within which source trends are observable. Each band is presumed to be controlled by different processes, and the set of likely processes is fixed according to the average velocity exponent value obtained. In principal, the method is applicable to low-speed aeroacoustic source identification problems where a characteristic flow speed can be measured and systematically varied. Measurements of sound radiated by flow over a flat plate 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. For the fan, the frequency-dependent velocity scaling demonstrated that shifts in dominant aeroacoustic processes occur near 2 kHz and above 4 kHz. The results added insight to a related study which demonstrated that tip gap flows were the primary source of broadband noise above 2 kHz.