The noise from the large-scale structure of a jet

Williams, J. E. Ffowcs; Kempton, A. J.

doi:10.1007/bfb0012629

Cited by 18 publications

(16 citation statements)

References 4 publications

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“…Furthermore, the educed field comprises the saturation and decay phases characteristic of instability waves, and which are so important for sound radiation (Crow & Champagne 1971;Michalke & Fuchs 1975;Ffowcs Williams & Kempton 1978;Tam & Burton 1984a,b;Crighton & Huerre 1990); but no a priori assumption is made that the waves should behave in this way. It is true that the source Ansatz used, subsequently, to extract time-varying amplitudes and length scales does contain this growth-to-decay cycle, implicitly, but this behaviour is clearly present in the educed field prior to any consideration of the said Ansatz.…”

Section: Discussionmentioning

confidence: 98%

Educing the source mechanism associated with downstream radiation in subsonic jets

et al. 2012

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This work belongs to the ongoing debate surrounding the mechanism responsible for low-angle sound emission from subsonic jets. The flow, simulated by large eddy simulation (Bogey & Bailly, Comput. Fluids, vol. 35 (10), 2006a, pp. 1344-1358, is a Mach 0.9 jet with Reynolds number, based on the exit diameter, of 4 × 10 5 . A methodology is implemented to educe, explore and model the flow motions associated with low-angle sound radiation. The eduction procedure, which is based on frequency-wavenumber filtering of the sound field and subsequent conditional analysis of the turbulent jet, provides access to space-and time-dependent (hydrodynamic) pressure and velocity fields. Analysis of these shows the low-angle sound emission to be underpinned by dynamics comprising space and time modulation of axially coherent wavepackets: temporally localized energization of wavepackets is observed to be correlated with the generation of high-amplitude acoustic bursts. Quantitative validation is provided by means of a simplified line-source Ansatz (Cavalieri et al. J. Sound Vib., vol. 330, 2011b, pp. 4474-4492). The dynamic nature of the educed field is then assessed using linear stability theory (LST). The educed pressure and velocity fields are found to compare well with LST: the radial structures of these match the corresponding LST eigenfunctions; the axial evolutions of their fluctuation energy are consistent with the LST amplification rates; and the relative amplitudes of the pressure and velocity fluctuations, which are educed independently of one another, are consistent with LST.

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Section: Discussionmentioning

confidence: 98%

Educing the source mechanism associated with downstream radiation in subsonic jets

et al. 2012

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“…Although Ffowcs Williams & Kempton (1978) found that, in general, jitter can increase the amplitude of radiated sound, Cavalieri et al (2011) showed that this has a significant effect in subsonic jets and only a minor influence in supersonic ones. This explains why the LPSE implementation used by Sinha et al (2014) did not encounter this problem.…”

Section: Introductionmentioning

confidence: 95%

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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“…Following the observations of Mollo-Christensen (1963, 1967, a large body of experimental, theoretical and more recently numerical work has been dedicated to the eduction of wavepackets from turbulence (Crow & Champagne 1971;Lau, Fisher & Fuchs 1972;Hussain & Zaman 1981;Kerhervé et al 2012), to their study as a sound-generation mechanism (Michalke 1970;Crow 1972;Crighton 1975;Michalke & Fuchs 1975;Ffowcs-Williams & Kempton 1978;Crighton & Huerre 1990;Sandham, Morfey & Hu 2006;Cavalieri et al 2011aCavalieri et al , 2012bKerhervé et al 2012) and to their dynamic modelling (Michalke 1971;Liu 1974;Crighton & Gaster 1976;Tam & Morris 1980;Mankbadi & Liu 1984). Eduction methodologies (e.g.…”

Section: Introductionmentioning

confidence: 99%

Jet-noise control by fluidic injection from a rotating plug: linear and nonlinear sound-source mechanisms

et al. 2016

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We present a study of the turbulent and acoustic fields of subsonic jets, controlled by means of a novel actuator that introduces perturbations via steady-fluidic actuation from a rotating centrebody. The actuation can produce louder or quieter jets, and these are analysed using time-resolved stereoscopic particle image velocimetry and a hot-wire anemometer. We place the analysis in the framework of wavepackets and linear stability theory, whence we show, using solutions of the linear parabolised stability equations, that the quieter flows can be understood to result from a mean-flow deformation that modifies wavepacket dynamics, and in particular their phase velocities, which are significantly reduced. The mean-flow deformation is shown, by a triple decomposition, to be due to the generation of Reynolds stresses associated with incoherent turbulence (rather than coherent structures) which arises when the actuation energises the flow with a frequency-azimuthal wavenumber (ω-m) combination to which the mean flow is stable. When the actuation excites the flow with an ω-m combination to which the mean flow is unstable, the response is dominated by coherent structures, whose rapid growth takes them beyond the linear limit, where they undergo quadratic wave interactions and lead, consequently, to a louder flow.

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The noise from the large-scale structure of a jet

Cited by 18 publications

References 4 publications

Educing the source mechanism associated with downstream radiation in subsonic jets

Educing the source mechanism associated with downstream radiation in subsonic jets

A coherence-matched linear source mechanism for subsonic jet noise

Jet-noise control by fluidic injection from a rotating plug: linear and nonlinear sound-source mechanisms

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