Instability waves in a subsonic round jet detected using a near-field phased microphone array

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摘要：

We propose a diagnostic technique to detect instability waves in a subsonic round jet using a phased microphone array. The detection algorithm is analogous to the beam-forming technique, which is typically used with a far-field microphone array to localize noise sources. By replacing the reference solutions used in the conventional beam-forming with eigenfunctions from linear stability analysis, the amplitudes of instability waves in the axisymmetric and first two azimuthal modes are inferred. Experimental measurements with particle image velocimetry and a database from direct numerical simulation are incorporated to design a conical array that is placed just outside the mixing layer near the nozzle exit. The proposed diagnostic technique is tested in experiments by checking for consistency of the radial decay, streamwise evolution and phase correlation of hydrodynamic pressure. The results demonstrate that in a statistical sense, the pressure field is consistent with instability waves evolving in the turbulent mean flow from the nozzle exit to the end of the potential core, particularly near the most amplified frequency of each azimuthal mode. We apply this technique to study the effects of jet Mach number and temperature ratio on the azimuthal mode balance and evolution of instability waves. We also compare the results from the beam-forming algorithm with the proper orthogonal decomposition and discuss some implications for jet noise.

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Experimental/ aeroacoustics flow instability flow measurement hydrodynamics jets laser velocimetry Mach number nozzles subsonic flow turbulence waves/ subsonic round jet near-field phased microphone array diagnostic technique beam-forming technique analogy far-field microphone array noise sources linear stability analysis instability wave amplitudes axisymmetric mode azimuthal modes particle image velocimetry direct numerical simulation conical array mixing layer nozzle exit radial decay streamwise evolution phase correlation hydrodynamic pressure turbulent mean flow jet Mach number proper orthogonal decomposition jet noise/ A4720 Hydrodynamic stability and instability A4755C Jets in fluid dynamics A4740D General subsonic flows A4735 Waves in fluid dynamics A4780A Flow visualization and imaging A4755E Nozzles A4725 Turbulent flows, convection, and heat transfer A4328 Aeroacoustics and atmospheric sound A4725M Noise (turbulence generated)