Climate & Environment



Mixing of deep ocean water masses induced by gravity waves

In an attempt to explain and develop strategy for control of aircraft wake vortex in a turbulent atmosphere, the evolution of a vortex column dipole (a pair of counter-rotating vortices) in the presence of one-scale (homogeneous and isotropic) freestream turbulence is studied via direct numerical simulation of the Navier-Stokes equations. The free-stream turbulence is found to signifcantly accelerate the vortex decay via a complex vortex-turbulence coupling scenario. External one-scale turbulence is ?rst stretched into azimuthal ?laments which merge into threads through successive pairings and advect along the column dipole by self-induction.
Oppositely-directed advection of opposite-signed threads forms thread dipoles which then move outward by mutual-induction and also eject column ?uid. This has the e?ect of enhancing both mixing with the ambient ?uid and the nominally planar reconnection (cross-di?usion) between the column vortex pair. In addition, the interaction of the column vortex dipole and turbulence and the vortex decay are explored in terms of their dependence on parameters like the column vortex Reynolds number, separation of the vortices, and the intensity and scale of free-stream turbulence.

The above-mentioned study provided the motivation for the study of the stability mechanisms of a single (Oseen) vortex. Since, a vortex embedded in fine-scale turbulence will encounter perturbations comprising a vast range of magnitudes and wavenumbers, a study of the evolution of such perturbations, i.e. a study of the instability mechanisms of the vortex, was undertaken. Attempts have been made to explain the physical mechanisms responsible for growth or decay of perturbations in an Oseen Vortex. In addition, attempts have been made to explain the physical mechanisms responsible for the transient energy growth in a stochastically-forced Oseen vortex.
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Ecole Polytechnique