The Role of Source Interference in Jet Noise

Michel, U.

doi:10.2514/6.2009-3377

Cited by 21 publications

(25 citation statements)

References 11 publications

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“…Furthermore, the results suggest that the axisymmetric radiation can be appropriately modelled if, instead of considering the turbulent field to be formed by stochastic eddies with random phase (Lee & Ribner 1972;Crighton 1975), the axial interference over a non-compact source region is taken into account (see for instance Michalke 1970;Michel 2009). Some of the shortcomings in acoustic analogies may be overcome if we use appropriate source models for the large-scale structures in jets, accounting for axially extended wavepackets such as we have studied here.…”

Section: Resultsmentioning

confidence: 99%

Axisymmetric superdirectivity in subsonic jets

Cavalieri

Jordan

Colonius

et al. 2011

17th AIAA/CEAS Aeroacoustics Conference (32nd AIAA Aeroacoustics Conference)

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We present experimental results for the acoustic field of jets with Mach numbers between 0.35 and 0.6. An azimuthal ring array of six microphones, whose polar angle, θ , was progressively varied, allows the decomposition of the acoustic pressure 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. The axisymmetric mode of the acoustic field can be clearly associated with an axially non-compact source, in the form of a wavepacket: the sound pressure level for peak frequencies is found be superdirective for all Mach numbers considered, with exponential decay as a function of (1 − M c cos θ ) 2 , where M c is the Mach number based on the phase velocity U c of the convected wave. 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. The axisymmetric radiation has a stronger velocity dependence than the higher-order azimuthal modes, again in agreement with predictions of wavepacket models. We estimate the axial extent of the source of the axisymmetric component of the sound field to be of the order of six to eight jet diameters. This estimate is obtained in two different ways, using, respectively, the directivity shape and the velocity exponent of the sound radiation. The analysis furthermore shows that compressibility plays a significant role in the wavepacket dynamics, even at this low Mach number. Velocity fluctuations on the jet centreline are reduced as the Mach number is increased, an effect that must be accounted for in order to obtain a correct estimation of the velocity dependence of sound radiation. Finally, the higher-order azimuthal modes of the sound field are considered, and a model for the low-angle sound radiation by helical wavepackets is developed. The measured sound for azimuthal modes 1 and 2 at low Strouhal numbers is seen to correspond closely to the predicted directivity shapes.

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Section: Resultsmentioning

confidence: 99%

Axisymmetric superdirectivity in subsonic jets

Cavalieri

Jordan

Colonius

et al. 2011

17th AIAA/CEAS Aeroacoustics Conference (32nd AIAA Aeroacoustics Conference)

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“…The strong directivity and low-angle velocity component (n > 8) can be mimicked using an axially coherent, noncompact, wavy line-source model (Crow 1972;Michalke 1970Michalke , 1972Michel 2009). Michalke & Fuchs (1975) provided an explanatory analysis for the azimuthal structure by invoking the azimuthal dependence of the integral solution to an inhomogeneous wave equation, the Bessel function, J m (2π St M sin φ).…”

Section: Jet-noise Sensitivity To Upstream Conditionsmentioning

confidence: 99%

Wave Packets and Turbulent Jet Noise

Jordan

Colonius

2013

Annu. Rev. Fluid Mech.

485

298

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Turbulent jet noise is a controversial fluid mechanical puzzle that has amused and bewildered researchers for more than half a century. Whereas numerical simulations are now capable of simultaneously predicting turbulence and its radiated sound, the theoretical framework that would guide noise-control efforts is incomplete. Wave packets are intermittent, advecting disturbances that are correlated over distances far exceeding the integral scales of turbulence. Their signatures are readily distinguished in vortical turbulence; the irrotational, evanescent near field; and the propagating far field. We review evidence of the existence, energetics, dynamics, and acoustic efficiency of wave packets. We highlight how extensive data available from simulations and modern measurement techniques can be used to distill acoustically relevant turbulent motions. The evidence supports theories that seek to represent wave packets as instability waves, or more general modal solutions of the governing equations, and confirms the acoustic importance of these structures in the aft-angle radiation of high subsonic and supersonic jets. The resulting unified view of wave packets provides insights that can help guide control strategies.

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“…It is worth noting that the acoustic analogy model of Michalke & Fuchs (1975) and Michel (2009) shares the simplicity of the two-source model of Tam and co-workers and, yet, it does not need to make any assumptions about the existence of two separate mechanisms of jet noise to predict many integral characteristics of jet noise. Moreover, the Michel results indicate that the best agreement with the experiment corresponds to fL x ∼ v p ∼ c 0 .…”

Section: The Role Of Interference Within the Distributed Noise Sourcementioning

confidence: 99%

Understanding jet noise

Karabasov

2010

Phil. Trans. R. Soc. A.

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Jets are one of the most fascinating topics in fluid mechanics. For aeronautics, turbulent jet-noise modelling is particularly challenging, not only because of the poor understanding of high Reynolds number turbulence, but also because of the extremely low acoustic efficiency of high-speed jets.Turbulent jet-noise models starting from the classical Lighthill acoustic analogy to state-of-the art models were considered. No attempt was made to present any complete overview of jet-noise theories. Instead, the aim was to emphasize the importance of sound generation and mean-flow propagation effects, as well as their interference, for the understanding and prediction of jet noise.

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The Role of Source Interference in Jet Noise

Cited by 21 publications

References 11 publications

Axisymmetric superdirectivity in subsonic jets

Axisymmetric superdirectivity in subsonic jets

Wave Packets and Turbulent Jet Noise

Understanding jet noise

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