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

• Published in: Journal of fluid mechanics Vol. 565; pp. 197 - 226
• Main Authors: SUZUKI, TAKAO, COLONIUS, TIM
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 25.10.2006
• Subjects: Acoustics; Aeroacoustics and atmospheric sound; Aeroacoustics, atmospheric sound; Algorithms; Exact sciences and technology; Fluid dynamics; Fundamental areas of phenomenology (including applications); Instrumentation for fluid dynamics; Noise (turbulence generated); Physics; Stability analysis; Turbulent flows, convection, and heat transfer; Waves; Array; Pressure fluctuation; Microphone; Test facility; Subsonic flow; Circular jet; Hydrodynamic instability; Turbulence noise; Experimental study; Aeroacoustics
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/S0022112006001613

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.