Organized motions in a jet in crossflow (original) (raw)
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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.
The Vortex Dominated Flow Field Associated with a Confined Jet in a Crossflow
2006
This study focuses on the near field mixing of a confined circular jet that is being injected normally from a wall into a subsonic crossflow in a confined area. The streamwise, transverse and lateral mean velocities were obtained using a two-component Laser Doppler Anemometer. From these quantities the time average vorticity in three dimensions were estimated for locations between 1.5 and 4.5 jet diameters downstream of the jet exit. The jet to crossflow velocity ratio was kept constant at four. The results highlight a complex system of vortices within this flowfield that is different to that of a free jet in a crossflow. In addition the present study was able to quantify changes in the vortical structures as a function of downstream distance as well as provides evidence of what effect freestream turbulence has on the evolving vorticity.
Experiments in Fluids, 2013
Circular flush Jets In Cross-Flow were experimentally studied in a water tunnel using Volumetric Particle Tracking Velocimetry, for a range of jet to cross-flow velocity ratios, r, from 0.5 to 3, jet exit diameters d from 0.8 cm to 1 cm and cross-flow boundary layer thickness δ from 1 to 2.5 cm. The analysis of the 3D mean velocity fields allows for the definition, computation and study of Counter-rotating Vortex Pair trajectories. The influences of r, d and δ were investigated. A new scaling based on momentum ratio r m taking into account jet and cross-flow momentum distributions is introduced based on the analysis of jet trajectories published in the literature. Using a rigorous scaling quality factor Q to quantify how well a given scaling successfully collapses trajectories, we show that the proposed scaling also improves the collapse of CVP trajectories, leading to a final scaling law for these trajectories.
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 vorticity dynamics of a turbulent jet in a crossflow
Journal of Fluid Mechanics, 1986
We present numerical solutions of the fully three-dimensional flow of a round, turbulent jet emitted normal to a uniform free stream. Comparisons with available laboratory data and comparison between different numerical grid resolutions are used to demonstrate the quality of the simulation. Examination of the detailed flow pattern within a computational domain, which extends 15 jet diameters from the source allows us to follow the vorticity dynamics in the transition from an initially vertical jet to a wake with a vortex pair essentially aligned with the free stream. The transition is presented as a function of the ratio of the jet exit velocity to free stream velocity. For large velocity ratios, the source of the streamwise vorticity in the vortex pair can be readily traced back to the original streamwise vorticity in the sides of the vertical jet.
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.
Streamwise vortices originating from synthetic jet–turbulent boundary layer interaction
Fluid Dynamics Research, 2014
The interaction between a flat plate turbulent boundary layer and a synthetic jet issuing from a rectangular slot slanted with respect to the free stream was studied experimentally using Digital Particle Image Velocimetry (DPIV). Instantaneous flow fields were sampled in a cross-plane downstream of the slot. Results concerning the effects of varying the synthetic jet velocity ratio at fixed stroke length L 0 and yaw angle, and the effects of varying the orifice yaw angle β at a fixed frequency are presented. The formation of a pair of counter-rotating vortical structures, completely embedded in the boundary layer, was observed in the mean flow field when the slot was aligned with the cross-flow. As the slot yaw angle was increased the leeward vortex intensified while the other became weaker. These vortical structures are the traces of streamwise vortices forming upstream, at the slot exit, during the blowing phases. As the jet velocity ratio and the slot yaw angle were increased the vortices grew in size and intensity. The vortex identification technique showed that these vortical structures are intermittently present in the instantaneous flow fields with a percentage growing with the frequency but not influenced by the yaw angle. Conditional averages showed that while the rotational core of the identified vortices is nearly unaffected, their outer region is greatly modified and grows in size and intensity as the jet velocity ratio and the yaw angle increased.
Characteristics of air jets discharging normally into a swirling crossflow
AIAA Journal, 1987
Jets discharging normally into a swirling crossflow are investigated experimentally. Total jet to total swirling flow axial momentum ratios of 0.43 and 0.96 are examined. A vane swirler with an overall swirl number of 2.25 is used to generate the crossflow in a 125-mm i.d. tube. The jet nozzle with an exit diameter of 8.73 mm is mounted at one tube diameter downstream of the swirler. A one-color, one-component laser Doppler anemometer, operating in the forward scatter mode, is used to measure the flow field downstream of the jet. Results show that the jet does not penetrate deeply into the tube. Instead, it moves downstream along a spiral created by the swirling flow in the absence of the jet. Consequently, the disturbances created by the jet are limited to a small region around the jet, and the reversed flow region along the tube core created by the swirling motion still remains intact. The turbulence field of the swirling flow is affected more by the jet than the mean flowfield. However, the disturbances created by the jet die off quickly. At two tube diameters away from the jet and farther downstream, the mean flow and turbulence fields exhibit behavior characteristic of the swirling flow alone. This demonstrates that the decay of the jet is fairly complete within this region. Finally, as the jet momentum is increased, the jet's effect is also noted in regions other than along the jet trajectory. This indicates jetbifurcation behavior at high jet momentum.
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.