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Obstacle-induced spiral vortex breakdown

Pasche, Simon ; Gallaire, François ; Dreyer, Matthieu ; Farhat, Mohamed

In: Experiments in Fluids, 2014, vol. 55, no. 8, p. 1-11

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    Titre
    • Obstacle-induced spiral vortex breakdown
    Auteur
    • Pasche, Simon. Laboratory of Hydraulic Machines, Ecole Polytechnique Fédérale de Lausanne, 1015, Lausanne, Switzerland
    • Gallaire, François. Laboratory of Fluid Mechanics and Instabilities, Ecole Polytechnique Fédérale de Lausanne, 1015, Lausanne, Switzerland
    • Dreyer, Matthieu. Laboratory of Hydraulic Machines, Ecole Polytechnique Fédérale de Lausanne, 1015, Lausanne, Switzerland
    • Farhat, Mohamed. Laboratory of Hydraulic Machines, Ecole Polytechnique Fédérale de Lausanne, 1015, Lausanne, Switzerland
    Type de document
    • Postprint
    Langue
    • Anglais
    Publié dans
    • Experiments in Fluids, 2014, vol. 55, no. 8, p. 1-11. Springer Berlin Heidelberg
    Autre version électronique
    Classification
    • Technologie, Sciences de l'ingénieur
    Identifiant OAI-PMH
    • oai:doc.rero.ch:326258
    Summary
    An experimental investigation on vortex breakdown dynamics is performed. An adverse pressure gradient is created along the axis of a wing-tip vortex by introducing a sphere downstream of an elliptical hydrofoil. The instrumentation involves high-speed visualizations with air bubbles used as tracers and 2D Laser Doppler Velocimeter (LDV). Two key parameters are identified and varied to control the onset of vortex breakdown: the swirl number, defined as the maximum azimuthal velocity divided by the free-stream velocity, and the adverse pressure gradient. They were controlled through the incidence angle of the elliptical hydrofoil, the free-stream velocity and the sphere diameter. A single helical breakdown of the vortex was systematically observed over a wide range of experimental parameters. The helical breakdown coiled around the sphere in the direction opposite to the vortex but rotated along the vortex direction. We have observed that the location of vortex breakdown moved upstream as the swirl number or the sphere diameter was increased. LDV measurements were corrected using a reconstruction procedure taking into account the so-called vortex wandering and the size of the LDV measurement volume. This allows us to investigate the spatio-temporal linear stability properties of the flow and demonstrate that the flow transition from columnar to single helical shape is due to a transition from convective to absolute instability.