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Giulio Facchini

Soutenance de thèse

The present PhD work comes with the scope of characterizing, analysing and modelling some laboratory flows in the simultaneous presence of rotation, stratification and shear. To this aim we address three specific questions inspired by geophysics and astrophysics where these three ambient features are commonly relevant. In a first study we charac- terize and model the time evolution of a compact anticyclone in a rotating and stratified laboratory flow. We aim to understand the longevity of analogous vortices known as Meddies, which form at the Gibraltar Strait and populate the Atlantic Ocean at the exit of the Mediterranean Sea. We observe that viscous relaxation happens in an unusual way because of the balance between rotation and stratification. The results are confirmed by a quasi-geostrophic model and numerical simulations which show the crucial role played by secondary circulations. Secondly we consider the linear stability of one of the simplest parallel shear flow, namely the Plane Couette flow, and show that it becomes unstable when adding a vertical stratification. Interestingly the unstable pattern reminds of deep oceanic jets observed close to the equator. The signature of this instability is observed in a real flow performed in an ad-hoc experimental setup and interpreted with the support of direct numerical simulations. Finally we characterize the behaviour of a recently disclosed finite amplitude instability, namely the Zombie Vortex Instability or ZVI. This instability appears when rotation, stratification and shear are simultaneously present and may play a crucial role in the destabilization of proto-planetary disks, where these three features are of the same order of magnitude. We construct a stability dia- gram for ZVI in the space of the three ambient frequencies and additionally analyse the effect of viscous dissipation. Finally we discuss and compare our results to the most recent contributions on ZVI. 

Jury:

Michael Le Bars                               DR CNRS         IRPHE                                                              Directeur

Pascale BOURUET-AUBERTOT     Professeur       Univ. Pierre et Marie Curie UPMC Paris VI       Rapporteur

Paul BILLANT                                  DR CNRS        Ecole Polytechnique LadHyx                            Rapporteur 

Colm-cille CAULFIELD                    Professeur       DAMTP, Cambridge University                         Examinateur 

Thierry DAUXOIS                            CR CNRS        ENS de Lyon                                                     Examinateur 

Sebastien FROMANG                     Professeur       Service d’Astrophysique CEA/DSM/IRFU         Examinateur 

Philip MARCUS                               Professeur       University of California Berkeley                       Examinateur 

Patrice LE GAL                                DR CNRS        IRPHE                                                              Examinateur