Multipolar vortex structures have been observed frequently in both laboratory experiments and geophysical fows during the last few decades. These coherent structures are characterized by multiple satellite vortices that surround a core of opposite-signed vorticity and can emerge from the barotropic instability of a shielded, monopolar vortex. Analytical instability criteria for a quasi-geostrophic vortex subject to a constant background rotation relate the azimuthal mode of the instability to the characteristics of the initial vorticity profile. In this study this relation has been investigated experimentally for isolated vortices with a vorticity distribution that is close to piecewise uniform. Vortices have been created with a rotating inner cylinder and a coaxial, non- rotating outer cylinder. Experiments have proved that the desired initial vorticity distribution is obtained by using an inner cylinder with a co-rotating bottom plate. Disturbances can be reduced by using thin-walled cylinders. By varying the diameter ratios of the cylinders the criteria for the emergence of a tripole and quadrupole have been verified. Also signatures of a pentapole and higher mode instabilities have been observed. Finally it is shown that the experimentally found multipolar structures match with semi-analytical solutions found in previous studies.
|Date of Award||31 Oct 2008|
|Supervisor||Ruben R. Trieling (Supervisor 1), Z. Kizner (External coach) & G.J.F. van Heijst (Supervisor 2)|