Vortex dynamics and entrainment mechanisms in lobed jets (original) (raw)
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Experiments in Fluids, 2008
Particle image velocimetry measurements and time-resolved visualization are used for the reconstruction of the Kelvin–Helmholtz vortex passing in the near field of a round jet and of a lobed jet. For the round jet, the entrainment is produced in the braid region, where streamwise structures develop. In the Kelvin–Helmholtz ring, entrainment is dramatically affected by the attenuation of the streamwise structures. As for the lobed jet, the special geometry introduces a transverse shear leading to a breakdown of the Kelvin–Helmholtz structures into “ring segments.” Streamwise structures continuously develop at the resulting discontinuity regions and control the lobed jet self-induction. In this case, the entrainment rate is less affected by the primary structures dynamics.
Vortex dynamics and entrainment mechanisms in low Reynolds orifice jets
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.
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.
International Journal of Heat and Fluid Flow, 2012
This paper describes an experimental investigation at Reynolds number equal to 5000 on circular and chevron impinging jets by means of time-resolved tomographic particle image velocimetry (TR-TOMO PIV) and infrared (IR) thermography. TR-TOMO PIV experiments are performed at kilo-hertz repetition rate in a tailored water jet facility where a plate is placed at a distance of 4 diameters from the nozzle exit. Using air as working fluid, time-averaged convective heat transfer is measured on the impinged plate by means of IR thermography with the heated-thin-foil heat transfer sensor for nozzle-to-plate distances ranging from 2 to 10 diameters. The circular impingement shows the shedding and pairing of axisymmetric toroidal vortices with the later growth of azimuthal instabilities and counter-rotating streamwise vortices. In the chevron case, instead, the azimuthal coherence is replaced by counter-rotating pairs of streamwise vortices that develop from the chevron notches. The heat transfer performances of the chevron impingement are compared with those of the circular one, analyzing the influence of the nozzle-to-plate distance on the distribution of Nusselt number. The chevron configuration leads to enhanced heat transfer performances for all the nozzle-to-plate distances hereby investigated with improvements up to 44% at the center of the impinged area for nozzle-to-plate distance of 4. Such enhancements are discussed in relation to the streamwise structures that, compared with the toroidal vortices, are associated with an earlier penetration of turbulence towards the jet axis and a higher arrival speed.
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.
Hydrodynamics During the Transient Evolution of Open Jet Flows from/to Wall Attached Jets
Flow, Turbulence and Combustion, 2016
Swirl stabilized flows are the most widely deployed technology used to stabilize gas turbine combustion systems. However, there are some coherent structures that appear in these flows close to the nozzle whose occurrence and stability are still poorly understood during transition. The external recirculation zone and the Precessing Vortex Core to/from the Coanda effect are some of them. Thus, in this paper the transition of an Open Jet Flow-Medium Swirl flow pattern to/from a Coanda jet flow is studied using various geometries at a fixed Swirl number. Phase Locked Stereo Particle Image Velocimetry and High Speed Photography experiments were conducted to determine fundamental characteristics of the phenomenon. It was observed that the coherent structures in the field experience a complete annihilation during transition, with no dependency between the structures formed in each of the flow states. Moreover, transition occurs at a particular normalized step size whilst some acoustic shifts in the frequencies of the system were noticed, a phenomenon related to the strength of the vortical structures and vortices convection. It is concluded that a transient, precessing, Coanda Vortex Breakdown is formed, changing flow dynamics. The structure progresses to a less coherent Trapped Vortex between the two states. During the phenomenon there are different interactions between structures such as the Central Recirculation Zone, the High Momentum Flow Region and the Precessing Vortex Core that were also documented.
Numerical Simulation of the Vortical Structures in a Lobed Jet Mixing Flow
43rd AIAA Aerospace Sciences Meeting and Exhibit, 2005
Numerical simulations of an incompressible jet mixing flow exhausted from a circular lobed mixer/nozzle were presented and validated against whole-field quantitative Dual-Plane Stereoscopic PIV (DP-SPIV) measurement results of the same flow. The numerical simulations were conducted using a Reynolds Averaged Navier-Stokes approach with a modest number of unstructured tetrahedral cells and four widely used turbulence models to predict the vortex structures in a lobed jet mixing flow. The predictability of the turbulence models to the lobed jet mixing flow were assessed and quantified based on quantitative comparisons of the numerical predictions with the DP-SPIV measurement results. It is found that the numerical simulations agree with the experimental measurements reasonably well in terms of streamwise vorticity and azimuthal (spanwise) vorticity. Although all the four turbulent models investigated over predict the magnitude of the turbulent kinetic energy significantly (about 50% to 130% over predicted), the numerical simulations are found to agree with the experimental results in predicting the locations of the regions with high turbulent kinetic energy, and the trends associated turbulent kinetic energy production and decay with downstream distance. It is found that the k-ε Realizable turbulence model provides the most accurate prediction of the lobed jet mixing flow among the turbulence models compared. Nomenclature
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.
On the formation of the counter-rotating vortex pair in transverse jets
2001
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.