Soutenance de thèse – Misa Ishimura
jeudi 24 novembre, à 14h00, au Laboratoire FAST (Université Paris-Saclay) dans le Petit Amphithéâtre du Bâtiment Pascal (Bât. 530).
Films liquides tombants cisaillement par un écoulement turbulent de gaz à contre-courant confiné: stabilité, modélisation et expériences
The low-dimensional model has been validated with our experiments. It accurately predicts the effect of a turbulent counter-current gas flow on the frequency and growth rate of the linearly most-amplified waves,
including the onset of AI. And, it accurately captures the gas-effect on the height and shape of nonlinear surface waves.
Moreover, we have uncovered several physical phenomena. Firstly, depending on the inclination angle, we find that the turbulent counter-current gas flow can render the falling film unconditionally stable, unconditionally unstable or subject to stability islands linked to the laminar/turbulent transition. Secondly, we find that the gas flow disrupts the spatio-temporal coherence of nonlinear wave trains by precipitating
coalescence events and the emergence of dangerous large-amplitude tsunami waves (TW) that grow indefinitely by absorbing smaller waves in their path. This dynamics is much faster and violent than the
coarsening dynamics observed in a quiescent gas.
Thirdly, we find that transgressing the gas-induced AI limit of the Kapitza instability is not necessarily dangerous. On the contrary, the temporal growth associated with AI can lead to a narrow and effective linear wave selection near the liquid inlet. Once these waves enter the weakly-nonlinear regime, they propagate downstream in the form of a very regular train of limited-amplitude solitary waves, thus avoiding dangerous TW. This wave train is robust w.r.t. ambient noise. However, it can be disrupted by applying a strong monochromatic inlet forcing of competing frequency. In that case, coalescence-induced TW form once again and interact with almost-standing ripples emerging from AI on the thin residual film.
Fourthly, we have uncovered a new turbulence-induced short-wave interfacial instability mode associated with a negative wave velocity, which becomes dominant beyond the long-wave AI limit, but well below the threshold of the Tollmien-Schlichting instability. This finding allows to explain, at last, the occurrence of short-wave upward-travelling ripples observed in previous experiments. Our linear stability calculations accurately predict the wave speed and wave length of these ripples as compared to our own experiments. Also, we find that the short- and long-wave instability modes merge when the counter-current gas flow rate is large.
Composition du jury
Pierre-Yves LAGRÉE (Rapporteur), Directeur de recherche CNRS, Institut d’Alembert, Université Pierre et Marie Curie
Ranga NARAYANAN (Rapporteur), Professeur, Chemical Engineering dpt, University of Florida
Cathy CASTELAIN, Directrice de recherche CNRS, LTeN, Université de Nantes
Séverine MILLET, Maîtresse de conférences, LMFA, Université de Lyon 1
Gianluca LAVALLE, Maître assistant, Ecole des Mines de Saint-Etienne
Sophie MERGUI (Co-encadrant), Maîtresse de conferences, FAST, Universite Pierre et Marie Curie
Georg DIETZE (Co-directeur de thèse), Chargé de recherche CNRS, FAST, Université Paris-Saclay
Christian RUYER-QUIL (Directeur de thèse), Professeur, LOCIE, Université Savoie Mont Blanc