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Axisymmetric superdirectivity in subsonic jets

Cavalieri, André V. G. and Jordan, Peter and Gervais, Yves and Colonius, Tim (2011) Axisymmetric superdirectivity in subsonic jets. In: 17th AIAA/CEAS Aeroacoustics Conference (32nd AIAA Aeroacoustics Conference), 5-8 June 2011, Portland, OR.

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We present experimental results for the acoustic field of jets in the Mach number range 0.35 ≤ M ≤ 0.6. Data acquired by means of an azimuthal ring of six microphones, whose polar angle, θ, was progressively varied, is decomposed into azimuthal Fourier modes. In agreement with past observations, the sound field for low polar angles (measured with respect to the jet axis) is found to be dominated by the axisymmetric mode, particularly at the peak Strouhal number. As θ is increased, modes 1 and 2 become increasingly important and dominate at angles greater than θ ≈ 30°. A number of features of the axisymmetric mode of the acoustic field suggest that it can be associated with an axially non-compact source, in the form of a convected wave comprising amplification, saturation and decay, and whose axial extension is of the order of several jet diameters: (a) the sound pressure level for peak frequencies is shown be superdirective for all Mach numbers considered, with exponential decay as a function of (1 − M_c cos θ)^2, in agreement with wavepacket models for an axially non-compact axisymmetric source; (b) while the mode m = 1 spectrum scales with Strouhal number, suggesting that its energy content is associated with turbulence scales, the axisymmetric mode scales with Helmholtz number—the ratio between source length scale and acoustic wavelength; (c) the axisymmetric radiation has a stronger velocity dependence than the higher order azimuthal modes, again in agreement with predictions of the said wave-packet models. We use such a wave-packet model to estimate that the axial extension of the source structure underpinning the axisymmetric component of the sound field is of the order of 6–8 jet diameters, and that the source comprises a convected wave with three spatial oscillations, weighted by a Gaussian envelope; such a source structure is in good agreement with past observations based on coherent structure eduction techniques. The present results show that the narrow-band spectrum of the axisymmetric mode contributes to the appearance of the characteristic jet-noise spectrum at low angles, an effect that becomes more marked as the Mach number is increased.

Item Type:Conference or Workshop Item (Paper)
Related URLs:
URLURL TypeDescription Paper Paper ItemArticle
Cavalieri, André V. G.0000-0003-4283-0232
Jordan, Peter0000-0001-8576-5587
Colonius, Tim0000-0003-0326-3909
Additional Information:© 2011 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Published Online: 4 Nov 2012. The present work is partially supported by CNPq, National Council of Scientific and Technological Development – Brazil, and by the European-Russian program ORINOCO (FP7-AAT-2010-RTD-Russia; project number 266103).
Funding AgencyGrant Number
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)UNSPECIFIED
European Research Council (ERC)266103
Subject Keywords:Aeroacoustics
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AIAA Paper2011-2743
Record Number:CaltechAUTHORS:20190717-102319217
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:97203
Deposited By: Melissa Ray
Deposited On:18 Jul 2019 16:37
Last Modified:16 Nov 2021 17:29

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