Ondes internes divergentes et convergentes : étude expérimentale de la marée interne

Abstract : The Earth's oceans are stratified in density by temperature and salinity gradients.The interaction of tidal currents with ocean bottom topography results therefore in the radiation of internal gravity waves into the ocean interior. These waves are called internal tides and their dissipation owing to nonlinear wave breaking plays an important role in the mixing of the abyssal ocean, and hence in the large-scale ocean circulation.In this context we investigate the generation of internal waves by oscillating objects of different idealized geometries as a model of barotropic flow over ocean topography, and consider linear as well as nonlinear effects on these waves resulting from interactions with the object and from wave--wave interactions.The relatively novel contribution of this thesis is the investigation of three-dimensional flow aspects that were accessible with our experimental approach, and are generally difficult to investigate by numerical and analytical modelling.First we investigate the wave structure of the first and higher harmonics for an oscillating spheroid, emitting diverging waves. Higher harmonics are generated by nonlinear instability at the surface of the object together with nonlinear effects in the zone of intersection of the primary beams. They may intersect and focus, therefore increase in energy, and become dominant over the first harmonic. The horizontal structures of both, first and higher harmonics are determined.We then consider waves generated by an oscillating torus, that are converging to a focal point. Outside this focal region experimental results and theoretical predictions are in good agreement, but in the focal region the wave amplitude is twice as large as it is close to the torus, leading to local nonlinear wave amplification and incipient wave breaking for large oscillation amplitudes. As a result, the propagation of the first harmonic waves is found to be hindered in the focal region. A standing pattern forms, while new waves are generated and emitted away from this focal region.A larger torus has been tested at the Coriolis platform to compare the focusing of internal gravity, inertia--gravity and inertial waves in a low viscous regime. Owing to the complexity of the focal region, a second harmonic is observed even at low oscillation amplitude. The vertical vorticity field of internal gravity waves exhibits a dipolar structure in the focal zone, which transforms in the rotating case into a Yin--Yang-shaped'' monopolar vortex structure. The overall structure of the inertial wave beams is close to that for internal gravity waves, though relatively more intense.
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Natalia Shmakova. Ondes internes divergentes et convergentes : étude expérimentale de la marée interne. Sciences de la Terre. Université Grenoble Alpes, 2016. Français. ⟨NNT : 2016GREAU040⟩. ⟨tel-01562044⟩

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