On turbulence and noise of an axisymmetric shear flow

Michalke, A.; Fuchs, Helmut V.

doi:10.1017/s0022112075001966

Cited by 205 publications

(115 citation statements)

References 11 publications

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“…The idea that coherent flow structures of the jet near region are involved in the sound generation mechanism 9,[32][33][34][35] suggests that a reduction of the flow azimuthal coherence, for instance by using chevrons, could promote local cancellation of large-scale acoustic fluctuations, thus leading to far-field acoustic suppression. 22 In a survey on chevron nozzles, Bridges and Brown 3 used PIV and microphone measurements to investigate the relationship between flow-field patterns, far field noise, and chevron geometric parameters.…”

Section: Introductionmentioning

confidence: 99%

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Three-dimensional evolution of flow structures in transitional circular and chevron jets

2011

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The three-dimensional behavior of flow transition in circular and 6-chevron jets at Re ¼ 5000 is investigated with experiments conducted on a free water jet by time-resolved tomographic particle image velocimetry. The emphasis is on the unsteady organization of coherent flow structures, which play a role in the generation of acoustic noise. Shedding and pairing of vortices are the most pronounced phenomena observed in the near field of the circular jet. The first and second pairing amplify the axial pulsatile motion in the jet column and lead to the growth of azimuthal waves culminating in the breakup of the vortex ring. Streamwise vortices of axial and radial vorticity are observed in the outer region and move inward and outward under the effect of the vortex rings. In the jet with chevrons, the axisymmetric ring-like coherence of the circular jet is not encountered. Instead, streamwise flow structures of azimuthal vorticity emanate from the chevron apices, and counter-rotating streamwise vortices of axial and radial vorticity develop from the chevron notches. The decay of streamwise vortices is accompanied by the formation of C-shaped structures. The three-dimensional analysis allows quantifying the vortex stretching and tilting activity, which, for the circular jet exit, is related to the azimuthal instabilities and the streamwise vortices connecting the vortex rings. In the chevron jet, stretching and tilting peak during the formation of C-structures. Following Powell's aeroacoustic analogy, the spatial distribution of the source term is mapped, evaluating the temporal derivative of the Lamb vector. The spatio-temporal evolution of such source term is visualized revealing that the events of highest activity are associated with the processes of vortex-ring pairing and vortex-ring disruption for the circular jet, and with the decay of streamwise instabilities and the formation of C-shaped structures for the chevron case.

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Section: Introductionmentioning

confidence: 99%

“…Nevertheless, as the acoustic production is strongly related to the large-scale flow motion, 32,33,40 the observation of the temporal evolution of large-scales structures is of major importance to understand the physical mechanism behind the production of acoustic noise.…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional evolution of flow structures in transitional circular and chevron jets

2011

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“…But the extension of the conclusions of such studies to their unforced counterparts is not straightforward. In natural jets the signature of azimuthally coherent wavepackets is clearest in the near pressure field, whose energy is concentrated in a few low-order azimuthal modes, in contrast to the velocity field (Michalke & Fuchs 1975;Armstrong, Fuchs & Michalke 1977;Fuchs & Michel 1978). Measurements using line arrays of microphones in the near field reveal a hydrodynamic wave extending several jet diameters downstream of the nozzle exit (Picard & Delville 2000;Tinney & Jordan 2008).…”

Section: Introductionmentioning

confidence: 99%

“…For low acoustic angles, consideration of azimuthal mode m and frequency ω of the T xx component of Lighthill's stress tensor leads to where k a is the acoustic wavenumber ω/c. The Bessel function of the first kind, J m , results from azimuthal integration, and leads to a lower efficiency of high azimuthal modes, as explored previously in the literature (Michalke 1970;Michalke & Fuchs 1975;Mankbadi & Liu 1984;Cavalieri et al 2011). This effect is illustrated in figure 9.…”

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confidence: 95%

Wavepackets in the velocity field of turbulent jets

et al. 2013

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We study the velocity fields of unforced, high Reynolds number, subsonic jets, issuing from round nozzles with turbulent boundary layers. The objective of the study is to educe wavepackets in such flows and to explore their relationship with the radiated sound. The velocity field is measured using a hot-wire anemometer and a stereoscopic, time-resolved PIV system. The field can be decomposed into frequency and azimuthal Fourier modes. The low-angle sound radiation is measured synchronously with a microphone ring array. Consistent with previous observations, the azimuthal wavenumber spectra of the velocity and acoustic pressure fields are distinct. The velocity spectrum of the initial mixing layer exhibits a peak at azimuthal wavenumbers m ranging from 4 to 11, and the peak is found to scale with the local momentum thickness of the mixing layer. The acoustic pressure field is, on the other hand, predominantly axisymmetric, suggesting an increased relative acoustic efficiency of the axisymmetric mode of the velocity field, a characteristic that can be shown theoretically to be caused by the radial compactness of the sound source. This is confirmed by significant correlations, as high as 10 %, between the axisymmetric modes of the velocity and acoustic pressure fields, these values being significantly higher than those reported for two-point flow-acoustic correlations in subsonic jets. The axisymmetric and first helical modes of the velocity field are then compared with solutions of linear parabolized stability equations (PSE) to ascertain if these modes correspond to linear wavepackets. For all but the lowest frequencies close agreement is obtained for the spatial amplification, up to the end of the potential core. The radial shapes of the linear PSE solutions also agree with the experimental results over the same region. The results suggests that, despite the broadband character of the turbulence, the evolution of Strouhal numbers 0.3 St 0.9 and azimuthal modes 0 and 1 can be modelled as linear wavepackets, and these are associated with the sound radiated to low polar angles.

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“…The special role of linear instability waves for jet noise was first noted by Michalke & Fuchs (1975), Laufer et al (1976), Michalke (1977), Moore (1977) and, later, by Tam & Auriault (1999), Tam & Burton (1984), Tam & Chen (1994) and Tam et al (2008). After an extensive analysis of experimental data, Tam et al (1996) empirically constructed two seemingly universal 'similarity sound spectra' that are able to give the best fits to all jet-noise data available to them for small (1982) and (b) the comparison of similarity spectra, FSS (G) and LSS (F ) with the measurements from the JEAN experiment (Power et al 2004).…”

Section: Introductionmentioning

confidence: 96%

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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On turbulence and noise of an axisymmetric shear flow

Cited by 205 publications

References 11 publications

Three-dimensional evolution of flow structures in transitional circular and chevron jets

Three-dimensional evolution of flow structures in transitional circular and chevron jets

Wavepackets in the velocity field of turbulent jets

Understanding jet noise

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