jeudi 24 novembre, à 14h00 – Thèse de Misa Ishimura

Publié le mar 8 Nov 2022

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



Gravity-driven falling liquid films are known to develop large-amplitude surface waves due to the long-wave Kapitza instability. In many applications, such as cryogenic air separation in rectification columns, a falling liquid film is put into contact with a turbulent counter-current gas flow within a strongly confined geometry. In this case, surface waves greatly enhance inter-phase mass transfer, but, through their interaction with the gas, they can also trigger critical events (obstruction of the channel, upward travelling waves, liquid arrest). To identify optimal wavy-film regimes that reconcile these two opposing effects, it is important to elucidate the linear and nonlinear hydrodynamic response of the wavy falling liquid film to the gas flow.
We have studied this problem based on a prototype configuration: a liquid film falling in a weakly-inclined channel of height H*=10 mm and sheared by a counter-current turbulent gas flow. On the one hand, we have performed experiments in a setup allowing to visualize and measure the spatio-temporal evolution of surface waves under the effect of an increasingly strong gas flow. On the other hand, we have developed a low-dimensional long-wave model based on the weighted residual integral boundary layer (WRIBL) approach and a Reynolds-averaged description of turbulence in the gas. Thirdly, we have performed linear stability calculations based on the full governing equations, allowing to identify the different long- and short-wave instability modes, as well as the transition from convective to absolute instability (AI).
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