Trailing vortex instabilities

Advisor : François Gallaire, EPFL

Key words : swirling jets, vortex breakdown

Swirling jets, that combine an azimuthal velocity component to an axial one, are observed in numerous industrial configurations, among them wing tip vortices, apex vortices on delta wings or combustion chambers.

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Fig 1. Swirling jet meandering and bursting on an F18 visualized by smoke injection (NASA)

Depending on the swirl number, which quantifies the ratio of azimuthal to axial velocity, the dynamics of swirling jets are dominated by two main physical phenomena : helicoidal instabilities that grow along the vortex core and even more spectacular, the vortex breakdown phenomenon, associated to the appearance of a stagnation point and a recirculation region. This latter behaviour, discovered in the 50’s, is still only poorly understood and remains counter-intuitive : the flow seems to go round a virtual obstacle.

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Fig 2. Instantaneous streaklines associated to the three-dimensional flow obtained by DNS (Gallaire et al., 2005)

The main goal of the project is to conduct numerical experiments of forced vortices: little perturbations are introduced at the entrance of the flow at a prescribed frequency and one analyses their amplification as they travel along the vortex. This analysis will provide precious guidelines for control strategies favouring or inhibiting vortex bursting.