Measuring Jet Noise Source Locations with Acoustic Beamforming

Abstract

Numerous methods have been used by the aeroacoustics community to locate noise sources of different frequencies in subsonic jets. These include: acoustic mirrors, microphone arrays, two-microphone methods, causality correlation and coherence techniques, acoustic beamforming, and automated source breakdown. Except for the acoustic beamforming, most of these methods are quite time consuming. No one seems to have ever compared the performance of these techniques. This paper compares the performance of various existing techniques. As the beamforming technique is the least time consuming, emphasis has been given to examining the performance of this technique in measuring source location in a subsonic jet for a range of Reynolds numbers. A 48-microphone acoustic beamformer was used to locate the noise sources for several subsonic Mach numbers from a one-inch and a half-inch nozzle. The effects of tabs on source location are also examined for the one-inch nozzle. Once set up, this method allows for faster and thus more numerous tests. It is shown that jet noise source locations may be a strong function of either Reynolds number or nozzle exit boundary layer. In general, there is a shift of all the noise sources away from the nozzle exit as Reynolds number decreases. Nomenclature BW = bandwidth, in Hz CF = center frequency, in Hz D = diameter of the nozzle, in inches f = source center frequency, in Hz M = Mach number U = nozzle velocity, in feet per second X = distance from the nozzle exit, in inches

Study of the Position of Sound Sources in a Turbulent Jet Using Nozzles of Different Configurations

“…As can be seen in Fig. 6, the results obtained by the beamforming method are in good agreement with the results of [22], in which a similar method was used. The positions of the noise sources under similar outflow conditions practically coincide with each other for different jet installations.…”

“…The results obtained for the jet outflowing from the conical nozzle were compared with the results of other authors and the expression for the Strouhal number from [22]. It was found that the position of the noise sources in the jet is close enough to the calculated one.…”

“…As can be seen in the figure, the center of the source in the area of the maximum jet noise for a given speed (500-2000 Hz) is located at a distance of 4-9 calibers from the nozzle output, which corresponds to the end of the initial section and the beginning of the main section in the buoyant jet [32]. Subsequently, a more detailed analysis of the position of the source of a clean jet was carried out in accordance with [22]. In this work, the position of the center of the noise source on the localization map for turbulent jets was considered.…”

“…The differences in the position of the source at close frequencies are no more than 1 caliber. It should be noted that the most reliable is the comparison of the results of determining the location of the source obtained using different methods on the same setup under the same outflow conditions [22]. Fig.…”

“…Using this method, it is possible to obtain information on the distribution of acoustic sources in space, to determine the sound pressure level of these sources at frequencies of interest. Beamforming methods are widely used in aeroacoustic research [19][20][21][22][23][24][25].…”