The instability of a forced, circular shear layer in a rotating fluid has been studied
experimentally and numerically. The experiments were performed with a shallow layer
of water in a parabolic tank, in which it is possible to apply radial pumping and to
model a geophysical beta-effect. A shear layer was produced by a secondary rotation
of the central part of the parabolic vessel. In most experiments, the shear layer
takes on the appearance of a sequence of vortices, the number of which decreases
with increasing strength of the shear. A beta-effect may prevent the formation of a
steady vortex chain. Continuous pumping of fluid from the periphery to the centre
or vice versa leads to an azimuthal velocity field corresponding to a point vortex.
This azimuthal flow appears to stabilize the shear flow if it is opposite to the inner
rotation, and to be destabilizing otherwise.
The numerical investigations consist of the solution of the quasi-geostrophic equation
in a geometry similar to the experimental situation and with a term modelling
the experimental forcing. Though the numerical computations are based on a two-dimensional model, they capture the essential features of the instability and the
resulting vortex structures.