Attractors in stratified fluids

Continuously stratified fluids support internal waves. The energy of these waves propagates under an angle with the vertical direction. In enclosed domains multiple reflections of the waves lead to the formation of patterns. These patterns arise because the domain focuses all energy onto an attractor, the path onto which all energy converges. These attractors manifest themselves as regions of intense wave motion. This thesis describes experiments performed in the laboratory on internal wave attractors in stratified fluids. First the attractors in a simple narrow trapezoidal tank are studied. The attractors are generated by a gentle sloshing of the stratified fluid. The formation and decay of the attractor are explained in terms of the wave number spectrum. The robustness of the attractor under change of forcing frequency, domain shape and stratification is investigated. Also the internal wave patterns in a fully three-dimensional tank are measured, using a tomographic reconstruction technique. In all experiments the formation of internal wave patterns dominates the internal dynamics, even when the attractor is of complicated shape. This finding suggests that the possibility of attractor formation has to be considered, whenever internal waves are found in stratified fluids. The second finding of the thesis deals with particle transport by internal wave beams, found both from theory and in experiments. This transport is in a direction opposite to that into which wave energy propagates and has a weak vertical component. This vertical component results in displacements that are working against the stable stratification and could lead to mixing. The horizontal component of the transport is into the wave beam and leads to convergence. This finding suggests that near internal wave beams, strong and spatially varying transport occurs

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