Experiments on internal waves: propagation and reflection

Kurtakoti, Prajvala Kishore (2014) Experiments on internal waves: propagation and reflection. Masters thesis, Memorial University of Newfoundland.

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Abstract

Internal waves are present both in the atmosphere and the oceans. Internal waves transport momentum across long distances and their breaking provides energy for mixing. The mechanisms of internal wave dissipation and breaking is not completely understood. We are interested in understanding how internal waves propagate and reflect off sloping topography such as the continental shelf and dissipation. We conducted a series of laboratory experiments to study the energy flux of propagating and reflecting mode-1 vertically trapped internal waves in a linearly stratified salt water fluid. The internal waves are generated by a wave generator that is capable of producing monochromatic vertical mode-1 internal waves. In our experiments, we record a video in a rectangular section of the tank where these waves propagate along the length of the tank (5m) and reflect back. We repeated this for 3 different types of end wall boundary conditions : no slope, supercritical slope and subcritical slope. Their quantitative measurements of the structure and amplitude of the internal waves are measured using an optical technique called synthetic schlieren from which we can extract the energy flux of the wave. We use a complex demodulation function called the Hilbert transform to filter out internal waves on the basis of the sign of their wave vector. We employ this technique to filter out the generated waves that are travelling left to right from their reflection that are travelling right to left. From the filtered data we can understand how much energy from incoming waves is present in reflected internal waves and how internal waves dissipate across experiments from all the cases and across different wave generator amplitudes. The analysis of energy flux of internal waves during propagation and reflection using Hilbert transform is helpful as it brings insight into phenomena that are difficult to observe during field studies. Comparison between the reflected energy efficiency across the 3 types of experiments improves our understanding of how waves reflect from different end boundaries, and also sheds light on the how dissipation changes when we change wave generator amplitude.

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