Since the observation of spontaneous generation of vorticity concentrations
from random 2D turbulence, a huge interest has been stimulated on
so-called coherent structures. The tripole is a much less common one
than the monopole and dipole in numerical and experimental observations.
It was first observed in both domains in the mid-late 1980s, and only
in 1991 was a tripolar structure detected in the oceans. Laboratory
tripoles occur as a result of the growth of a perturbation of azimuthal
wavenumber 2 in unstable axisymmetric shielded vortices (a vortex surrounded
by a ring of opposite vorticity). For this reason, shielded monopoles have
also been the subject of several numerical studies. In the present study,
in contrast, the tripole is seen to emerge from a nonshielded, Gaussian
monopole (which is stable), with a large nonlinear perturbation. In this
flow, the tripole was first observed by Rossi et al. (Phys. Fluids 9:2329,
1997). We extend the result by performing a parameter study, spanning
values of the amplitude of perturbation and Reynolds number, with numerous
simulations using a meshless vortex method. One of the goals is to
determine whether there is a threshold amplitude that separates two
asymptotic states (axisymmetric and tripolar). The possible relationship
between this threshold amplitude and Reynolds number is sought, and
several observations are made regarding the nonlinear, long-time evolution
of the structure.
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