Primary wave of an air-water mixing layer : a numerical study
This thesis was prepared within the LEGI MOST team, as well as during an 18 months stay at Cornell University (Ithaca, NY, USA).
Abstract :
The shear instability occuring at the interface between a slow water layer and a fast air stream is a complex phenomenon driven by momentum and viscosity differences across the interface, velocity gradients, as well as by injector geometries. Simulating such an instability in the conditions of experiments is numerically challenging and few studies exist in the literature. This work aims at filling a part of this gap by presenting a study of the convergence between two-dimensional simulations, linear theory, and experiments, in regimes where the instability is triggered by confinement, i.e., the finite thicknesses of the gas and liquid streams. Very good agreement between the three approaches is obtained. Moreover, using simulations and linear theory, we explore in details the effects of confinement on the stability of the flow and on the transition between absolute and convective instability regimes, which is shown to depend on the lengthscale of confinement as well as on dynamic pressure ratio. In the absolute regime under study, interfacial wave frequency is found to be inversely proportional to the smallest injector size (liquid or gas). We then study the transition between primary and secondary instability through wave acceleration. In additional, we explore the impact of three-dimensional effects on the flow. Finally, we present the development of an open boundary condition for turbulent multiphase flows and surface waves simulations. Initially thought as a way to improve accuracy and lower needed computational ressources of air-water mixing layer simulations, this work leads to improvements in the use of traction boundary conditions. Particularly, this novel boundary treatment couples Lagrangian traction estimation to backflow stabilization which provides stability, accuracy and non-reflectivity of artificial boundaries.
Directeur de thèse (PhD director) : Guillaume Balarac (LEGI/GINP)
Co-directeur de thèse (PhD co-director) : Olivier Desjardins (Cornell University)
Jury (committee) :
Pr Stéphane Zaleski, rapporteur
Pr Stéphane Vincent, rapporteur
Pr Jean-Philippe Matas, examinateur
Pr Eric Blayo, examinateur