Beschreibung:
<jats:p>This paper shows that a long vertical columnar vortex pair created by a double flap
apparatus in a strongly stratified fluid is subjected to an instability distinct from the
Crow and short-wavelength instabilities known to occur in homogeneous fluid. This
new instability, which we name zigzag instability, is antisymmetric with respect to the
plane separating the vortices. It is characterized by a vertically modulated twisting
and bending of the whole vortex pair with almost no change of the dipole's cross-
sectional structure. No saturation is observed and, ultimately, the vortex pair is sliced
into thin horizontal layers of independent pancake dipoles. For the largest Brunt–Väisälä
frequency <jats:italic>N</jats:italic> = 1.75 rad s<jats:sup>−1</jats:sup> that may be achieved in the experiments, the zigzag
instability is observed only in the range of Froude numbers:
0.13 < <jats:italic>F</jats:italic><jats:sub><jats:italic>h</jats:italic>0</jats:sub> < 0.21 (<jats:italic>F</jats:italic><jats:sub><jats:italic>h</jats:italic>0</jats:sub>
= <jats:italic>U</jats:italic><jats:sub>0</jats:sub>/<jats:italic>NR</jats:italic>, where <jats:italic>U</jats:italic><jats:sub>0</jats:sub> and <jats:italic>R</jats:italic>
are the initial dipole travelling velocity and radius). When
<jats:italic>F</jats:italic><jats:sub><jats:italic>h</jats:italic>0</jats:sub> > 0.21, the elliptic instability develops resulting in three-dimensional motions
which eventually collapse into a relaminarized vortex pair. Irregular zigzags are then
also observed to grow. The threshold for the inhibition of the elliptic instability
<jats:italic>F</jats:italic><jats:sub><jats:italic>h</jats:italic>0</jats:sub> = 0.2±0.01 is independent of <jats:italic>N</jats:italic> and in good agreement with the theoretical study
of Miyazaki & Fukumoto (1992). Complete stabilization for <jats:italic>F</jats:italic><jats:sub><jats:italic>h</jats:italic>0</jats:sub> < 0.13 is probably
due to viscous effects since the associated Reynolds number is low, <jats:italic>Re</jats:italic><jats:sub>0</jats:sub> < 260. In
geophysical flows characterized by low Froude numbers and large Reynolds numbers,
we conjecture that this viscous stabilization will occur at much lower Froude number.</jats:p><jats:p>It is tentatively argued that this new type of instability may explain the layering
widely observed in stratified turbulent flows.</jats:p>