Jet preferred mode vs shear layer mode

Mair, Michael; Bacic, Marko; Chakravarthy, Kharthik; Williams, B. A. O.

doi:10.1063/5.0007934

Cited by 7 publications

(7 citation statements)

References 49 publications

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“…The momentum thickness was computed numerically using each side of the velocity profile in Fig. 4 to give a mean and difference of h ¼ 0:0340 6 0:0028 D. This value is consistent with the calculations of Mair et al 11 who used a nearly identical test section and experimental setup. The frequency corresponding to the shear layer mode can then be calculated from (3) to be f n ¼ 1080 6 89 Hz.…”

Section: Resultssupporting

confidence: 75%

“…A first approximation for the length along which vortices exist is the length of the potential core, although they do persist beyond this region for some distance before breaking down into isotropic turbulence. Mair et al 11 reported potential core lengths ranging from 3D when excited at the jet preferred mode to 7D when unexcited. To get a sense of the order of the average number of vortices present in the shear layer, the length of the region where vortices at f c exist is taken to be L ve % 5D h .…”

Section: Introductionmentioning

confidence: 99%

“…However, lock-on can be induced away from the preferred (or any other) mode if the excitation amplitude is large enough. 11 In this paper, a free, round jet is excited upstream of the nozzle orifice with a loudspeaker and studied with particle image velocimetry (PIV). It is shown that the lock-on effect occurs, and the vortices are produced at the excitation frequency in most cases.…”

Section: Introductionmentioning

confidence: 99%

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On acoustically modulated jet shear layers and the Nyquist–Shannon sampling theorem

et al. 2022

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The goal of this paper is to present the behaviour of a jet shear layer in response to acoustic excitation from a signal processing perspective. The main idea is that the vortices that roll-up in the jet shear layer are similar to the discrete samples of a digital control system, and hence that the Nyquist-Shannon sampling theorem should apply. We further hypothesize that the strength of a vortex is determined by the mean amplitude of the excitation waveform during its creation. We also argue that, at least in some cases, demodulation occurs as a result of the vorticity signal generated by the convection of discrete vortices past a point in the shear layer. This vorticity signal is related to the amplitude modulation (AM) excitation waveform by a half-wave rectification operation, a common implementation of an AM demodulator. To investigate these ideas, a free, round jet that is excited upstream of the nozzle is studied using particle image velocimetry (PIV). Experiments are conducted that confirm that the sampling theorem applies and an aliased response is observed when the Nyquist limit is exceeded. Previous authors have attributed demodulation to a vortex merging mechanism, but we demonstrate that merging is not always required for demodulation, and suggest that it is one of two mechanisms at play.

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Section: Resultssupporting

confidence: 75%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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On acoustically modulated jet shear layers and the Nyquist–Shannon sampling theorem

et al. 2022

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“…This transition theoretically explains the disagreement about the origin of the jet preferred mode and supports the conclusion of Petersen & Samet (1988). The question of the nature of the preferred mode was readdressed in a recent experimental study by Mair et al (2020), but no definite conclusion was drawn by the authors.…”

Section: Introductionmentioning

confidence: 64%

“…Thus, currently, there is no consensus on the nature of the jet preferred mode, in both theoretical and experimental studies. As was argued by Mair et al (2020), this is partially a result of attempts to apply linear stability theory, which assumes laminar flow, to turbulent jet flows. Moreover, a numerical study by Boguslawski, Wawrzak & Tyliszczak (2019) clearly demonstrates that the turbulent jet's response to forced excitation depends not only on the jet's velocity profile, as linear stability theory predicts, but also on the combination of incoming turbulence intensity and excitation amplitude and frequency.…”

Section: Introductionmentioning

confidence: 99%

Experimental validation of inviscid linear stability theory applied to an axisymmetric jet

et al. 2022

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We study the development of perturbations in a submerged air jet with a round cross-section and a long laminar region (five jet diameters) at a Reynolds number of 5400 by both inviscid linear stability theory and experiments. The theoretical analysis shows that there are two modes of growing axisymmetric perturbations, which are generated by three generalized inflection points of the jet's velocity profile. To validate the results of linear stability theory, we conduct experiments with controlled axisymmetric perturbations to the jet. The characteristics of growing waves are obtained by visualization, thermoanemometer measurements and correlation analysis. Experimentally measured wavelengths, growth rates and spatial distributions of velocity fluctuations for both growing modes are in good agreement with theoretical calculations. Therefore, it is demonstrated that small perturbations to the laminar jet closely follow the predictions of inviscid linear stability theory.

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Nonlinear dynamics and acoustic radiation of coherent structures consisting of multiple ring–helical modes in the near-nozzle region of a subsonic turbulent circular jet

Zhang,

2023

J. Fluid Mech.

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This paper investigates the nonlinear evolution and acoustic radiation of coherent structures (CS) in the near-nozzle region of a subsonic turbulent circular jet. A CS is taken to be a wavepacket consisting of multiple ring/helical modes, which are considered to be inviscid instability waves supported by the mean-flow profile. As the three-dimensionality of helical modes is weak in the near-nozzle region, the ring and helical modes with the same frequency have nearly the same growth rates and critical levels. They coexist and interact with each other in their common critical layer at high Reynolds numbers. The self and mutual quadratic interactions generate a mean-flow distortion and streaks, which act back on the fundamental components through the cubic interaction. The amplitude of the CS is governed by an integro-partial-differential equation, a significant feature of which is that differentiations with respect to the azimuthal coordinate appear in the history-dependent nonlinear terms. The non-parallelism of the mean flow as well as the impact of fine-scale turbulence on CS are taken into account and found to affect the nonlinear terms. By solving the amplitude equation, the development of the constituting modes, streamwise vortices and streaks are described. For CS consisting of frequency sideband, low-frequency components are excited nonlinearly and amplify to reach a considerable level. By analysing the large-distance asymptote of the perturbation, the low-frequency acoustic waves are found to be emitted by the temporally–spatially varying mean-flow distortion and streaks generated by the nonlinear interactions of the CS, and are thereby determined on the basis of first principles. Interestingly, the energetic part of the streaky structure that contributes to the nonlinear dynamics does not radiate directly, and instead the Reynolds stresses driving the subdominant radiating components represent the true physical sources.

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Jet preferred mode vs shear layer mode

Cited by 7 publications

References 49 publications

On acoustically modulated jet shear layers and the Nyquist–Shannon sampling theorem

On acoustically modulated jet shear layers and the Nyquist–Shannon sampling theorem

Experimental validation of inviscid linear stability theory applied to an axisymmetric jet

Nonlinear dynamics and acoustic radiation of coherent structures consisting of multiple ring–helical modes in the near-nozzle region of a subsonic turbulent circular jet

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