Experiments on periodically forced flow over topography in a rotating fluid

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Abstract
Results from laboratory experiments on oscillatory flows over topograph in a rapidly rotating cylinder of homogeneous liquid are presented and compared with weakly nonlinear and low-order theories. With periodic forcing, the motion can be either periodic or chaotic. In the periodic regime, linear Rossby waves excited by the sloshing flow over shallow bottom topography become resonant at forcing frequencies that are integer multiples of the natural free Rossby wave frequency. As the topographic effect or the forcing amplitude is increased, the maximum response is shifted away from the linearly resonant frequency; to higher periods for azimuthal topographic wavenumbers of 1 and to lower periods for topographic zonal wavenumbers exceeding 1, in agreement with theory. The simple theories which use slippery sidewalk do not describe the observed chaotic flows. These complex states are associated with the development of small-scale vortices in the sidewall boundary layer that are shed into the interior. For both periodic and chaotic flows, long-time particle paths can contain significant chaotic components which are revealed in direct Poincaré sections constructed from observations of surface floats.