Jean-Marc Chomaz & Paul Billant
LADHYX, Ecole Polytechnique, Palaiseau, France

Stability of columnar dipole in a stratified rotating fluid

We present new experiments on the dynamic of a vertical columnar
vortex pair in a rotating and strongly stratified fluid. The
different three-dimensional instabilities developing on the vortex
pair, created by a double flap apparatus, are analyzed as a function
of the horizontal Froude number Fh and Rossby number Ro. The zigzag
instability, which slices the vortex pair into thin horizontal layers
of pancake dipoles (Billant & Chomaz, JFM, 418, 419, 2000), prevails
for low Froude number regardless of the Rossby number investigated
and is even enhanced by rotation effects. For Rossby number of order
one and above a critical Froude number, a centrifugal instability
develops only on the anticyclone as recently observed in the
non-stratified case (Afanasyev, 2000). For lower Rossby number, a new
oscillatory instability also appears solely on the anticyclone. For
high Rossby and Froude numbers, the elliptic instability arises on
both cyclonic and anticyclonic vortices. Experimental observations
are compared to theoretical and numerical stability analyses.