Primary and secondary vortical structures contribution in the entrainment of low Reynolds number jet flows (original) (raw)
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Journal of Visualization, 2008
Classical planar 2D-PIV measurements and time-resolved visualizations enriched by low-level processing are used for the reconstruction of the Kelvin-Helmholtz vortex passing in the near field of a circular and a 6-lobed orifice jet flow. In the circular jet, the entrainment is produced in the braid region, being interrupted in the presence of the Kelvin-Helmholtz ring. The latter compresses the streamwise vortices and alters their self-induction role. Conversely, the 6-lobed orifice geometry allows the cutting of the Kelvin-Helmholtz structures into discontinuous ring segments. Consequently, into these discontinuity regions streamwise large scale structures are developing. These streamwise structures are permanent thus controlling and enhancing the jet entrainment which is not altered by the Kelvin-Helmholtz structures passing.
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Journal of Fluid Mechanics, 1992
The role of streamwise vortex structures in the near-field (xjd < 10) evolution of a round jet is examined. In free shear layers the streamwise vorticity develops into Bernal-Roshko structures which are streamwise vortex pairs. Similar structures are shown to exist in round jets. These structures, which evolve and amplify in the braid region between primary vortical structures, are shown to drastically alter the entrainment process in the near field and to increase the rate at which fluid is entrained into the jet. As the flow evolves downstream, the efficiency of the streamwise vorticity in entraining fluid increases relative to that of the azimuthal vorticity. Beyond the end of the potential core regime, the entrainment process is mainly controlled by streamwise vorticity. These processes are identified via flow visualization and confirmed by detailed global entrainment measurements.
Vortical structures in a laminar V-notched indeterminate-origin jet
Physics of Fluids, 2005
A flow visualization investigation using dye-injection and laser-induced fluorescence techniques has been carried out to understand the vortex dynamics resulting from a Vnotched indeterminate-origin jet with two peaks and two troughs. The laminar jet was studied under forcing and non-forcing conditions to investigate the resultant dynamics of coherent large-and small-scale flow structures. Present experimental observations indicated that the effects of the nozzle peaks and troughs differ from those reported previously. Instead of the peaks producing streamwise vortex-pairs which spread outwards into the ambient fluid and the troughs generating similar vortex-pairs but entrain ambient fluid into the jet flows as indicated by earlier studies, the present experimental observations showed that both peaks and troughs produce outward-spreading streamwise vortex-pairs. Laser cross-sections further showed that the subsequent formation of azimuthal ring-vortices causes these streamwise vortex-pairs to be entrained. This entrainment causes the streamwise vortex-pairs to "rollup" together with the ring vortices, leading to intense flow interactions between them.
Sequence behavior of vortical development and evolution of a low-speed rectangular jet
— The main objective of present study is to understand the vortical structure development and evolution of a low-speed rectangular jet via smoke low visualization. The side-and front-view near the jet exit are utilized simultaneously to know the development relationship in detail. Results not only connirm the development of vortex ring but also indicate this process has strong relationship with the vortical structure development in a plane jet.
Flow visualization of the non-parallel jet-vortex interaction
Journal of Visualization, 2018
The jet-vortex interaction is observed in settings ranging from aeronautics to physiology. In aeronautics, it presents as a parallel interaction of the jet exhaust and aircraft wing-tip vortex, and in the diseased state of the heart called aortic regurgitation, the interaction between blood flows is characterized by a non-parallel interaction. While there is substantial research into the mechanisms of the parallel interaction, there is comparatively limited scientific material focused on the non-parallel interaction. The objective of this study was to characterize three distinct orientations (30°, 60°and 90°) of the non-parallel jet-vortex interaction in a simplified flow loop. The ratio of the jet Reynolds number to the vortex ring Reynolds number was used to define four levels of jet strength. Flow visualization and particle image velocimetry were used to qualitatively and quantitatively describe how the flow structures interacted, and the energy dissipation rate of each condition was calculated. It was determined that as the relative jet strength increases, the vortex ring dissipates more rapidly and the energy dissipation rate increases. This information provides a basis for the understanding of a vortex ring's interaction with an impinging jet. When the angle between the jet and vortex ring flows is perpendicular, the energy dissipation rate decreased from 6.1 W at the highest jet strength to 0.3 W at the lowest jet strength, while at an angle of 30°the energy dissipation rate decreased from 51.8 to 10.3 W. This finding contradicts the expected result, which potentiates further studies of various non-parallel arrangements.
Measurement of Entrainment Rate in the Initial Region of Swirling Jets
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The entrainment rate in the initial region of axisymmetric turbulent swirling air jets has been measured by Particle Image Velocimetry (PIV) for a number of swirl strengths from S = 0 up to S = 1.5 and two Reynolds numbers. From the complete PIV maps of the jet and its surroundings, entrainment has been evaluated by using two methods: a) integration of the radial profiles of time-mean axial velocity of the jet; b) direct measurements of the radial velocity component of ambient air entrained into the jet. The entrainment rate increases with increasing axial distance from the nozzle in a non linear way and shows a rapid enhancement with the swirl number, particularly after inception of vortex breakdown and when the processing vortex core (PVC) is observed. The results allow to better understand the entrainment process in swirling jets and related effects on mixture formation and flame structure in industrial burners. Introduction Entrainment is an essential feature of shear flows and ...
On the formation of the counter-rotating vortex pair in transverse jets
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Among the important physical phenomena associated with the jet in crossflow is the formation and evolution of vortical structures in the flow field, in particular the counter-rotating vortex pair (CVP) associated with the jet cross-section. The present computational study focuses on the mechanisms for the dynamical generation and evolution of these vortical structures. Transient numerical simulations of the flow field are performed using three-dimensional vortex elements. Vortex ring rollup, interactions, tilting, and folding are observed in the near field, consistent with the ideas described in the experimental work of , for example. The time-averaged effect of these jet shear layer vortices, even over a single period of their evolution, is seen to result in initiation of the CVP. Further insight into the topology of the flow field, the formation of wake vortices, the entrainment of crossflow, and the effect of upstream boundary layer thickness is also provided in this study.
Characteristics of Small Vortices in a Turbulent Axisymmetric Jet
Journal of Fluids Engineering-transactions of The Asme, 2005
Characteristics of Small Vortices in a Turbulent Axisymmetric Jet Characteristics of small vortices were studied in axisymmetric jets wherein the Kolmogorov scale was approached by progressively decreasing the Reynolds number while still maintaining turbulent flow. A periodic forcing introduced far upstream of the jet nozzle ensured that the jet was turbulent. A vortex eduction tool was developed and applied to the high-pass filtered 2D velocity field in the axial plane of a turbulent jet while varying Re between 140 and 2600. Vortex population, energy, vorticity, and rms (root-meansquare velocity fluctuations) of the high-pass filtered field were measured to elucidate vortex characteristics. The observed population of vortices decreases dramatically at the Kolmogorov scale. The observed increase in vortex population with decreasing vortex size appears to be in accord with the space-filling argument, in that the vortex population in a two-dimensional domain should grow as R −2. The energy density curve obtained from vortex statistics reproduces the −5/3 slope for the inertial subrange, and the highpass filtered field accounts for approximately two-thirds of the total rms.
International Journal of Heat and Fluid Flow, 2000
Vortex generating jets (VGJs) are jets that pass through a wall and into a cross¯ow to create a dominant streamwise vortex that remains embedded in the boundary layer over the wall. The VGJ is characterized by its pitch and skew angles (U and H) and the velocity ratio (VR) between the jet and the cross¯ow. For VR 1.0, the VGJ con®guration of U 30°Y H 60°has been identi®ed as that which produces the vortex with the highest peak mean vorticity. Three-component laser Doppler velocimetry (LDV) data for this particular con®guration demonstrate many interesting features of the¯ow. Mean velocity data show a de®cit of streamwise momentum in the core of the vortex, thinning of the boundary layer on the downwash side of the vortex, and thickening of the boundary layer on the upwash side. Plots of the turbulent kinetic energy and the turbulent shear stress huvi show that the turbulent structure of the boundary layer is grossly disturbed by the presence of the vortex. The turbulent transport of the turbulent kinetic energy shows the possibility for a gradient diusion model in most regions, but not the vortex core. Ó