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Formation of Turbulent Vortex Breakdown: Intermittency, Criticality, and Global Instability

Oberleithner, K. and Paschereit, C. O. and Seele, R. and Wygnanski, I. (2012) Formation of Turbulent Vortex Breakdown: Intermittency, Criticality, and Global Instability. AIAA Journal, 50 (7). pp. 1437-1452. ISSN 0001-1452. doi:10.2514/1.J050642.

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This study provides quantitative insight into the formation of vortex breakdown and the onset of global instability in a turbulent swirling jet. A water jet is guided through a rotating honeycomb that imparts the rotational motion, passed through a contraction, and discharged into a large water tank. The flow states evolving at increasing swirl are mapped out via time-resolved particle image velocimetry. The experimental results scale properly with the swirl number based on the axial momentum flux when the commonly used boundary-layer approximations are omitted. The instantaneous velocity field reveals that vortex breakdown occurs intermittently at a wide range of swirl numbers before it appears in the mean flow. At this intermittent state, the evolving breakdown bubble oscillates heavily between two streamwise locations where the vortex core is subcritical. Upon further increasing the swirl, the breakdown oscillations decay and a region of reversed flow appears in the mean flowfield. The formation of this socalled axisymmetric breakdown state is accompanied by a supercritical-to-subcritical transition of the inflowing vortex core. The reversed flow region is found to grow linearly with increasing swirl until the flow undergoes a supercritical Hopf bifurcation to a global single-helical mode, and vortex breakdown adopts a spiral shape. The global mode shape is extracted from the particle image velocimetry snapshots by means of proper orthogonal decomposition and Fourier analysis. The present experiment reveals that, at gradually increasing swirl, the jet first transitions to an axisymmetric breakdown state that remains globally stable until a critical swirl number is exceeded. This sequence of flow states agrees well with the transient formation of vortex breakdown observed in laminar flows.

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Additional Information:© 2012 Kilian Oberleithner. Published by the American Institute of Aeronautics and Astronautics, Inc. Received 3 May 2010; revision received 15 December 2011; accepted for publication 3 January 2012. The authors kindly acknowledge financial support from the German Science Foundation (DFG). The authors would also like to thank the referees for their thorough review of the manuscript and for posing significant questions and concerns that significantly improved the quality of the manuscript.
Funding AgencyGrant Number
German Science Foundation (DFG)UNSPECIFIED
Issue or Number:7
Record Number:CaltechAUTHORS:20120806-093241053
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Official Citation:Formation of Turbulent Vortex Breakdown: Intermittency, Criticality, and Global Instability Kilian Oberleithner, Technical University of Berlin; C. Paschereit, Technical University of Berlin; R. Seele, California Institute of Technology; I. Wygnanski, University of Arizona AIAA Journal 2012 0001-1452 vol.50 no.7 (1437-1452) doi: 10.2514/1.59037
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:32935
Deposited By: Jason Perez
Deposited On:06 Aug 2012 20:44
Last Modified:09 Nov 2021 21:31

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