Coherent Structures in Jet Turbulence and Noise

Armstrong, R. R.; Michalke, A.; Fuchs, Helmut V.

doi:10.2514/3.60743

Cited by 77 publications

(37 citation statements)

References 9 publications

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“…This can be attributed to the lower growth rate predicted by stability theory for higher Mach numbers, since the differences in the mean velocity profiles are slight (see figure 1) and could not cause such a significant effect. The reduction of the normalized amplitude for higher Mach numbers was also observed by Armstrong, Fuchs & Michalke (1977) and Suzuki & Colonius (2006) in experimental results for the near-field pressure. On the other hand, for the velocity spectra on the jet lipline, shown in figure 19(c), the results for the three Mach numbers collapse.…”

Section: Velocity Dependence Of the Sound Radiation For Each Azimuthasupporting

confidence: 50%

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: Velocity Dependence Of the Sound Radiation For Each Azimuthasupporting

confidence: 50%

Axisymmetric superdirectivity in subsonic jets

Cavalieri

Jordan

Colonius

et al. 2011

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

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“…Several important findings were nevertheless made, which are relevant to our present work. Azimuthally coherent wavepackets were found to be most clearly observed in the near pressure field of natural round jets, which is dominated only by a few low-order azimuthal modes (Michalke & Fuchs 1975;Armstrong, Michalke & Fuchs 1977;Fuchs & Michel 1978). Michalke & Fuchs (1975) also demonstrated that the two-point cross-spectral density (CSD) for any azimuthal mode depends only on the constituent of the pressure field for that azimuthal mode, which motivates azimuthally resolved analysis of wavepackets and source mechanisms.…”

Section: Introductionmentioning

confidence: 93%

A coherence-matched linear source mechanism for subsonic jet noise

et al. 2015

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We investigate source mechanisms for subsonic jet noise using experimentally obtained datasets of high-Reynolds-number Mach 0.4 and 0.6 turbulent jets. The focus is on the axisymmetric mode which dominates downstream sound radiation for low polar angles and the frequency range at which peak noise occurs. A linearized Euler equation (LEE) solver with an inflow boundary condition is used to generate single-frequency hydrodynamic instability waves, and the resulting near-field fluctuations and far-field acoustics are compared with those from experiments and linear parabolized stability equation (LPSE) computations. It is found that the near-field velocity fluctuations closely agree with experiments and LPSE computations up to the end of the potential core, downstream of which deviations occur, but the LEE results match experiments better than the LPSE results. Both the near-field wavepackets and the sound field are observed directly from LEE computations, but the far-field sound pressure levels (SPLs) obtained are more than an order of magnitude lower than experimental values despite close statistical agreement of the near hydrodynamic field up to the potential core region. We explore the possibility that this discrepancy is due to the mismatch between the decay of two-point coherence with increasing distance in experimental flow fluctuations and the perfect coherence in linear models. To match the near-field coherence, experimentally obtained coherence profiles are imposed on the two-point cross-spectral density (CSD) at cylindrical and conical surfaces that enclose near-field structures generated with LEEs. The surface pressure is propagated to the far field using boundary value formulations based on the linear wave equation. Coherence matching yields far-field SPLs which show improved agreement with experimental results, indicating that coherence decay is the main missing component in linear models. The CSD on the enclosing surfaces reveals that the application of a decaying coherence profile spreads the hydrodynamic component of the linear wavepacket source on to acoustic wavenumbers, resulting in a more efficient acoustic source.

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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%

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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Coherent Structures in Jet Turbulence and Noise

Cited by 77 publications

References 9 publications

Axisymmetric superdirectivity in subsonic jets

Axisymmetric superdirectivity in subsonic jets

A coherence-matched linear source mechanism for subsonic jet noise

Wavepackets in the velocity field of turbulent jets

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