Effect of boundary conditions on axial flow in a concentrated vortex core

Cohn, Richard K.; Koochesfahani, Manoochehr

doi:10.1063/1.858784

Cited by 22 publications

(15 citation statements)

References 3 publications

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“…Axial flow from a wall boundary condition through a vortex core has been established experimentally by Cohn & Koochesfahani (1993). It is suggested here that the bursting event in the r = 5 jet is caused when the CVP pulls the wake vortex into the jet, stretching it, and increasing its rotational velocity.…”

Section: Jet Fluid In the Wake Structuresmentioning

confidence: 79%

Mixing, structure and scaling of the jet in crossflow

1998

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The mixing of the round jet normal to a uniform crossflow is studied for a range of jet-to-crossflow velocity ratios, r, from 5 to 25. Planar laser-induced fluorescence (PLIF) of acetone vapour seeded into the jet is used to acquire quantitative twodimensional images of the scalar concentration field. Emphasis is placed on r = 10 and r = 20 and a few select images are acquired up to r = 200. The Reynolds number based on the jet exit diameter, d, and the exit velocity varies from 8400 to 41 500. Images are acquired for conditions in which the product rd is held constant, requiring decreasing d for increasing r.Results from this experimental study concern structural events of the vortex interaction region, and mixing and mean centreline concentration decay in the near and far fields. The results cover all three regions of the transverse jet, and suggest that the jet scales with three length scales: d, rd and r 2 d.Events within the vortex interaction region display d-scaling, including the crossflow boundary layer separation and roll-up. Over the range of velocity ratios studied, the vortex interaction region shows r-dependent variations in the flow field, including the emergence of jet fluid in the wake structures for r > 10 and a slower development of the counter-rotating vortex pair (CVP) in higher-r jets.The trajectory and physical dimension of the jet in both the near and far field display rd-scaling. The near field is characterized by a centreline concentration decay along the centreline coordinate s of s −1.3 , different from the decay rate (s −1 ) of the free jet. When normalized by rd, the decay of each velocity-ratio jet branches away from the s −1.3 decay, approaching a decay of s −2/3 , a rate predicted by modelling efforts. The branch points represent a transition in the flow field from enhanced mixing to reduced mixing compared to the free jet. When normalized by r 2 d, the branch points occur at a uniform jet position, s/r 2 d = 0.3, which is viewed to be the division between the near and far fields. Self-similarity is not seen in the near field, but may be present in the far field.The view of the branch points as a place of transition in the flow is supported by the probability density function (p.d.f.) of concentration along the upper edge of the jet. Before the branch points, the p.d.f.s are non-marching in character, and after the branch points, they are tilted in character.Instantaneously, the CVP is asymmetric in shape and concentration. End views reveal extensive motion of the CVP and plan views show this motion can occur in both axisymmetric and sinusoidal motion. Ensemble-averaged images show the jet concentration is asymmetric about the centreline plane.

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Section: Jet Fluid In the Wake Structuresmentioning

confidence: 79%

Mixing, structure and scaling of the jet in crossflow

1998

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“…This Reynolds number was chosen to closely match the conditions reported in Koochesfahani 6 and Cohn and Koochesfahani. 7 The airfoil oscillated sinusoidally about its 1 / 4-C axis with an amplitude of 2°and a frequency f in the range 1. 18-3.21 Hz.…”

Section: Methodsmentioning

confidence: 99%

Molecular tagging velocimetry measurements of axial flow in a concentrated vortex core

Bohl

Koochesfahani

2004

Physics of Fluids

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The characteristics of the axial flow within the core of a concentrated line vortex are investigated using molecular tagging velocimetry (MTV). A well-defined array of isolated vortices of alternating sign is generated in the wake of a NACA-0012 airfoil pitching sinusoidally at small amplitude and high reduced frequencies. The circulation and peak vorticity of the vortices are varied by the choice of oscillation frequency. Interaction of these two-dimensional vortices with the walls of the test section generates an axial flow within the vortex cores. The magnitude of the axial flow and its spatial/temporal characteristics are quantified using the MTV technique. Results show that the peak axial flow speeds can be very high, of the order maximum swirl speed of the vortices. The maximum axial speed ratio (maximum axial speed normalized by maximum swirl speed) is found to vary in the range 0.6-1.0 for the parameters investigated here. Initially, the axial flow is spatially confined in isolated structures corresponding to the core of vortices. As the vortex convects downstream, however, the spatial structure of axial flow changes from isolated regions to a continuous region for the highest reduced frequency investigated here. This change in structure is correlated with a significant decrease in the peak axial flow speed.

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“…End-wall effects may be an important factor in governing this phenomenon. For example, strong core-wise flow in vortices due to interaction with the end-wall boundary layer has been observed by Cohn and Koochesfahani (1993) and Bohl and Koochesfahani (2009), and Visbal (2011) observed spanwise propagating vortex breakdown due to the interaction of the vortex with an end wall. .…”

Section: Case Withmentioning

confidence: 96%