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Interactions non-linéaires d’ondes internes de gravité et mélange en milieu stratifié

Abstract : To be able to explain the oceanic mixing, the understanding of phenomena at small scales is crucial, and especially the energy transfers to those scales. Internal waves are studied in this context: by breaking, destabilizing and generating subharmonic triadic resonant instability (TRI), they contribute to this transfer. Our experimental study is focusing on wave propagation during long times. Previous experiments conducted under these conditions within the team had led, after several hours, to a profile showing clear mixing layers. The physical explanation given until then, purely turbulent, was incorrect. Our experimental setup actually places us in a singular situation where the stratification is modified jointly by wave propagation and by double diffusion: mixing layers can thus be present and the fluid behaves like a waveguide. In addition, stratification strongly modified by the waves has a feedback effect on their propagation: the daughter waves resulting from the TRI gradually disappear from the fluid, which limits the transfer to the smallest scales and therefore the mixing. Two internal waves can also interact with each other to generate, this time, a super-harmonic wave whose structure is determined by the structure of the waves that gave birth to it. Our work led to the first experimental demonstration of the generation of such a super-harmonic wave. The results are in excellent agreement with the theory developed by Thorpe (1966). In particular, the super-harmonic wave has the predicted pattern, is observed over a finite range of frequency around the resonant value and the spatial growth rates are very close to those expected theoretically
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Pauline Husseini. Interactions non-linéaires d’ondes internes de gravité et mélange en milieu stratifié. Mécanique des fluides [physics.class-ph]. Université de Lyon, 2019. Français. ⟨NNT : 2019LYSE1285⟩. ⟨tel-02507942⟩

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