Vortex evolution in a round jet

Becker, H. A.; Massaro, Thomas A.

doi:10.1017/s0022112068000248

Cited by 220 publications

(104 citation statements)

References 14 publications

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“…More complex geometries such as lobed and chevron nozzles at high Reynolds numbers have been recently afforded with LES, 6,7 whereas investigations with DNS have mostly focused on the fundamental aspects of circular jets at low Reynolds number. 4 Experimental investigations on the organization of coherent flow structures require field measurement techniques such as flow visualization [8][9][10][11] and, in the last decades, particle image velocimetry (PIV). PIV measurements are typically conducted along streamwise or cross-sectional planes to investigate the unsteady behavior of the shear layer and the transition patterns.…”

Section: Introductionmentioning

confidence: 99%

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%

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

2011

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“…For laminar inlet conditions, the prominent flow feature in the region close to the nozzle of a round jet is the roll-up of the vortex sheet into axisymmetric vortex rings due to Kelvin-Helmholtz instability. 12,40,41 Azimuthal instability of the vortex rings lead to the formation of secondary streamwise vortices which distort the main rings further to the point of breakdown. 42 For our fully turbulent inlet conditions these vortex structures will not be as dominant as for a transitional flow but we do expect the flow features in our downstream experimental domain to be highly vortical.…”

Section: Analysis Of Coherent Structuresmentioning

confidence: 99%

Scanning tomographic particle image velocimetry applied to a turbulent jet

2013

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We introduce a modified tomographic PIV technique using four high-speed video cameras and a scanning pulsed laser-volume. By rapidly illuminating adjacent subvolumes onto separate video frames, we can resolve a larger total volume of velocity vectors, while retaining good spatial resolution. We demonstrate this technique by performing time-resolved measurements of the turbulent structure of a round jet, using up to 9 adjacent volume slices. In essence this technique resolves more velocity planes in the depth direction by maintaining optimal particle image density and limiting the number of ghost particles. The total measurement volumes contain between 1 ×106 and 3 ×106 velocity vectors calculated from up to 1500 reconstructed depthwise image planes, showing time-resolved evolution of the large-scale vortical structures for a turbulent jet of Re up to 10 000.

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“…In consideration of ambient room drafts, we decided not to run the jet below 20 ft/sec, corre&ponding to a Reynolds number Re of about 10,000, where Re = U eD/, and %-is the kinematic viscosity of air. We therefore could not explore the Reynol&-number range 1000-10,000, but fortunately excellent photographs have been taken in that range by Brown (1935) of a two-dimensional jet and by Becker & Massaro (1968) Using the lighting arrangenent shown in figure 5, we made motion pictures of the fog jet at frame rates ranging from 5000 to 11,003 per second.…”

Section: Flow-visualization Experimentsmentioning

confidence: 99%

“…The jet is to be regarded as a "black box", a nonlinear oscillator whose properties are to be understood in terms of the inputs ue /Ue, St and response u/UNow that the relevant parameters have been defined, it is worth noting exactly how this study fits with previous work involving loudspeaker excitation of jets. We deal with Reynolds numbers Re z 100,000, about ten times higher than those explored by Becker & Massaro (1968 Except for a large accessible amplitude, the mode driven at St = 0.30 is typical and serves as a useful introduction to the quantitativ-work.…”

Section: Flow-visualization Experimentsmentioning

confidence: 99%

Orderly structure in jet turbulence

1971

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Past evidence suggests that a large-scale orderly pattern may exist in the noiseproducing region of a jet. Using several methods to visualize the flow of round subsonic jets, we watched the evolution of orderly flow with advancing Reynolds number. As the Reynolds number increases from order 102 to 103, the instability of the jet evolves from a sinusoid to a helix, and finally to a train of axisymmetric waves. At a Reynolds number around 104, the boundary layer of the jet is thin, and two kinds of axisymmetric structure can be discerned: surface ripples on the jet column, thoroughly studied by previous workers, and a more tenuous train of large-scale vortex puffs. The surface ripples scale on the boundary-layer thickness and shorten as the Reynolds number increases toward 105. The structure of the puffs, by contrast, remains much the same: they form at an average Strouhal number of about 0·3 based on frequency, exit speed, and diameter.To isolate the large-scale pattern at Reynolds numbers around 105, we destroyed the surface ripples by tripping the boundary layer inside the nozzle. We imposed a periodic surging of controllable frequency and amplitude at the jet exit, and studied the response downstream by hot-wire anemometry and schlieren photography. The forcing generates a fundamental wave, whose phase velocity accords with the linear theory of temporally growing instabilities. The fundamental grows in amplitude downstream until non-linearity generates a harmonic. The harmonic retards the growth of the fundamental, and the two attain saturation intensities roughly independent of forcing amplitude. The saturation amplitude depends on the Strouhal number of the imposed surging and reaches a maximum at a Strouhal number of 0·30. A root-mean-square sinusoidal surging only 2% of the mean exit speed brings the preferred mode to saturation four diameters downstream from the nozzle, at which point the entrained volume flow has increased 32% over the unforced case. When forced at a Strouhal number of 0·60, the jet seems to act as a compound amplifier, forming a violent 0·30 subharmonic and suffering a large increase of spreading angle. We conclude with the conjecture that the preferred mode having a Strouhal number of 0·30 is in some sense the most dispersive wave on a jet column, the wave least capable of generating a harmonic, and therefore the wave most capable of reaching a large amplitude before saturating.

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Vortex evolution in a round jet

Cited by 220 publications

References 14 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

Scanning tomographic particle image velocimetry applied to a turbulent jet

Orderly structure in jet turbulence

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