abstract
The dynamics of a vortex subject to a localised stretching is numerically
investigated. The structure of the flow is analysed in the case of
an initially bidimensional vortex submitted to a periodic localised stretching-compression.
It shows amplified oscillations of the axial vorticity and stretching in
strong contrast with Burgers-like vortices. The resulting dynamics
is the appearance, around the vortex, of axisymmetrical successive vortical
structures of smaller and smaller radius and alternated sign embedded inside
the previous ones. The frequency scaling of the oscillations is recovered
by Kelvin modes analysis but not the amplification nor the shape of successive
tori. An inviscid model based on structures is presented, which compares
better with the numerical computations. These results suggest that
the formalism of Kelvin waves is not sufficient to describe the full dynamics,
which is instead related to feedback of rotation on stretching and more
conveniently described in terms of localised structures. We finally
discuss the relative timescales of vortex stretching and of vortex retroaction.
The Burgers-like vortices, where there is no such retroaction, turn out
to correspond to nearly pure strain field, slightly disturbed by rotation.