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Three-dimensional vortex dynamics in oscillatory flow separation

Published online by Cambridge University Press:  23 March 2011

MIGUEL CANALS  and
GENO PAWLAK
MIGUEL CANALS*
Affiliation:
Department of Engineering Science and Materials, University of Puerto Rico, Mayagüez, PR 00681, USA
GENO PAWLAK
Affiliation:
Department of Ocean and Resources Engineering, University of Hawaii, Honolulu, HI 96822, USA
*
†Email address for correspondence: miguelf.canals@upr.edu
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Abstract

The dynamics of coherent columnar vortices and their interactions in an oscillatory flow past an obstacle are examined experimentally. The main focus is on the low Keulegan–Carpenter number range (0.2 < KC < 2), where KC is the ratio between the fluid particle excursion during half an oscillation cycle and the obstacle size, and for moderate Reynolds numbers (700 < Rev < 7500). For this parameter range, a periodic unidirectional vortex pair ejection regime is observed, in which the direction of vortex propagation is set by the initial conditions of the oscillations. These vortex pairs provide a direct mechanism for the transfer of momentum and enstrophy to the outer region of rough oscillating boundary layers. Vortices are observed to be short-lived relative to the oscillation time scale, which limits their propagation distance from the boundary. The instability mechanisms leading to vortex decay are elucidated via flow visualizations and digital particle image velocimetry (DPIV). Dye visualizations reveal complex three-dimensional vortex interactions resulting in rapid vortex destruction. These visualizations suggest that one of the instabilities affecting the spanwise vortices is an elliptical instability of the strained vortex cores. This is supported by DPIV measurements which identify the spatial structure of the perturbations associated with the elliptical instability in the divergence field. We also identify regions in the periphery of the vortex cores which are unstable to the centrifugal instability. Vortex longevity is quantified via a vortex decay time scale, and the results indicate that vortex pair lifetimes are of the order of an oscillation period T.

JFM classification

Geophysical and Geological Flows: Coastal engineering Wakes/Jets: Separated flows Vortex Flows: Vortex breakdown
Type
Papers
Information
Journal of Fluid Mechanics , Volume 674 , 10 May 2011 , pp. 408 - 432
Copyright
Copyright © Cambridge University Press 2011

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