Experimental investigations of a swirling jet in both stationary and rotating surroundings |
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Authors: | Hanzhuang Liang T Maxworthy |
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Institution: | (1) Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089, USA;(2) Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA |
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Abstract: | The ‘plug’ flow emerging from a long rotating tube into a large stationary reservoir was used in the experimental investigation
of swirling jets with Reynolds numbers, Re = 600, 1,000 and 2,000, and swirl numbers, S = ΩR/U, in the range 0–1.1, to cover flow regimes from the non-rotating jet to vortex breakdown. Here Ω is the nozzle rotation rate,
R is the radius of the nozzle exit, and U is the mean mass axial velocity. The jet was more turbulent and eddies shed faster at larger Re. However the flow criticality and shear layer morphology remained unchanged with Re. After the introduction of sufficient rotation, co-rotating and counter-winding helical waves replaced vortex rings to become
the dominant vortex structure. The winding direction of the vortex lines suggests that Kelvin–Helmholtz and generalized centrifugal
instability dominated the shear layer. A quantitative visualization study has been carried out for cases where the reservoir
was rotating independently with S
a
= Ω
a
R/U = ±0.35, ±0.51 and ±0.70 at Re = 1,000 and 2000, where Ω
a
is the rotation rate of the reservoir. The criterion for breakdown was found to be mainly dependent on the absolute swirl
number of the jet, S. This critical swirl number was slightly different in stationary and counter-swirl surroundings but obviously smaller when
the reservoir co-rotated, i.e. S
c
= 0.88, 0.85 and 0.70, respectively. These results suggest that the flow criticality depends mainly on the velocity distributions
of the vortex core, while instabilities resulting from the swirl difference between the jet and its ambient seem to have only
a secondary effect. |
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Keywords: | |
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