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Title: Instabilities of a shear layer in a barotropic rotating fluid
Author: Aguiar, Ana Claudia Barbosa
ISNI:       0000 0001 3399 4804
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2008
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Above a critical value of horizontal stress, the flow within a bounded system in rotation is driven to an unstable limit, beyond which it develops chains of vortices. The number of these vortices depends not only upon the value of the stress imposed but also on the sense of the shear in some configurations, highlighting discrepancies between earlier experiments. Quasi-geostrophic theory, however, predicts that there should be no qualitative differences with respect to the sign of the forcing. We have studied barotropic instability in laboratory experiments with flat cylindrical geometry, where a detached shear layer occurs tangent to differentially rotating sections. These sections can either be two discs placed at the top and bottom of the tank or a single thick disc immersed in the fluid. \Vhen a single thick disc is used, we observe that the azimuthal wavenumber depends on the sign of differential rotation. \Vith axisymmetric numerical simulations, we were able to study the differences in the meridional circulation for different configurations and sign of forcing. \Vhen two discs are used, the circulation occurs in pairs of counterrotating cells of similar size, if the forcing is weak. For strong and positive forcing only, centrifugal instability sets in. \Vhen a single disc is used, one of the circulation cells is typically much stronger than the other and the flow is strongly asymmetric in radius. The influence of a topographic ,a-effect was also investigated in laboratory experiments, using four distinct sloping bottom combinations with the setup of the two discs. In the configurations studied, unstable modes of shear instability can be stabilised by topography, depending on the combination of sgn(,B) and sign of forcing. Finally, we studied a possible example of barotropic instability in planetary atmospheres and propose that long-lived polygonal jets such as Saturn's north polar hexagon should be interpreted as a finite-amplitude, nonlinear equilibration of a barotropic instability of Saturn's zonal jet.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available