Beamformed Flow-Acoustic Correlations in a Supersonic Jet

Abstract

An experimental study of simultaneous multipoint measurements in the flowfield and acoustic field of a Mach 1.75 cold-air jet is presented. A series of four optical-deflectometer probes measured turbulent fluctuations in or near the jet flow, and eight microphones recorded the far-field pressure in the direction of peak emission. The correlation methodology involves the coherence between the delay-and-sum beamformer outputs of the optical-deflectometer probes and the microphones. This procedure yields results with greater fidelity and higher coherence levels than obtained with individual optical-deflectometer-to-microphone correlations. With the optical-deflectometer probes in the jet shear layer, there is a significant correlation, on the order of 0.1, between the turbulent fluctuations and farfield noise. As the optical-deflectometer probe moves transversely away from the jet, its correlation with the microphone beamformer first drops and then increases. This trend signifies the transition from hydrodynamic to acoustic pressure fields. In the vicinity of the nozzle exit, the peak coherence between the beamformed opticaldeflectometer and microphone signals coincides with the physical location of the optical-deflectometer probe. However, as the shear layer thickens downstream, the peak coherence generally lags the probe location, which is a probable result of acoustic refraction by the mean flow.

The separation of radiating and non-radiating near-field pressure fluctuations in supersonic jets

“…A simple, conventional beamforming method is used [15,37] in the present study to extract both the amplitude and phase of the wavepacket-like noise source. As shown in Fig.…”

“…6, W i ¼ e iωR=c0 = 4πR ð Þ. Eq. (A.3) is equivalent to the temporal Fourier transform of the simple delay-and-sum (DAS) in the time domain [15] q x…”

“…In the first method, a far-field virtual microphone array is used to record the acoustic pressure, and a simple beamforming method [15] is used in this study to extract the equivalent radiating noise sources. In the second method, the near-field noise sources are represented by the hydrodynamic pressure fluctuations.…”

Toward Active Control of Noise from Hot Supersonic Jets

“…The aforementioned work comprised spectral analysis of the far-field acoustic waveform produced by jets with supersonic exit speeds based on the gas dynamic Mach number (Mj = Uj/aj > 1). The work of Papamoschou et al (2010 and Baars et al (2011) comprised supersonic jets in terms of the acoustic Mach number (M a = Uj/aao > 1); this definition of the jet Mach number is most relevant to the formation of Mach waves. Aside from conventional spectral analysis, Laufer et al (1976), Gallagher (1982), Petitjean et al (2006) and Veitin et al (2011) studied the temporal characteristics of the acoustic pressure waveform in a laboratory (rangerestricted) environment while Gee et al (2008) focused on the nonlinear propagation effects of a full-scale static jet engine and compared the experimental data successfully to a numerical model based on the generalized Burgers equation.…”

Experimental Investigation of Jet Noise Sources in a Hypersonic Nozzle at Takeoff

“…Since the Rayleigh scattering method requires significant cost and complexity, the authors applied a simpler method, a variant of schlieren technique. Such optical deflectometry methods have also been employed by several researchers to investigate the behavior of the source [12][13][14]. In the present study, to gain a high sensitivity capable of detecting the acoustic wave propagation in the ambient, a laser schlieren optical system which realizes a small spot size is utilized.…”