The two-dimensional erosion of a vortex subjected to an
external, adverse shear is
studied experimentally. The flow takes place in a thin stratified layer;
the vortex is
produced by electromagnetic forcing, whereas the shear is driven mechanically.
The
system thus allows the vortex strength and the external shear to be controlled
independently. We observe the so-called ‘erosion’
process, i.e. the progressive decrease
of the vortex area, leaving the vortex core unaffected. This process
is controlled by the
ratio γ=S/ωmax,
where ωmax and S are respectively the
maximum vorticity of the
vortex and the external shear. At small γ, the erosion is weak
and the vortex survives
over the duration of the experiment. At large γ, the vortex is
first eroded, and then,
after a critical time, becomes stretched and eventually breaks up into
filaments. During
the first period of time, the compensated maximum vorticity (with friction
decay
removed) is constant and the vortex area decreases, while beyond
the critical time, both
quantities decrease with time. A critical value for γ, defining
the transition between these two regimes, is determined experimentally:
γc=0.051±0.017. The breaking
process itself, during which the vorticity of the vortex core decreases,
is investigated.
All the qualitative aspects of the erosion process, the onset of breaking
and the
breaking process itself are found to be in excellent agreement
with the theoretical and
numerical description. The experimental value of γc
and the properties of the
filamentation process are consistent with the numerical estimates.