Soutenance de thèse : Tim LEISTER

Soutenance d'une thèse de doctorat de l’Université de Lyon en cotutelle internationale entre Karlsruher Institut für Technologie (Karlsruhe, Allemagne) et l’INSA Lyon (Villeurbanne, France)

Doctorante : Tim LEISTER

Laboratoire INSA : LaMCoS

Ecole doctorale :  ED162 : Mécanique, Énergétique, Génie Civil, Acoustique (MEGA)

The gas foil bearing (GFB) technology is considered one of the key factors for the intended transition to oil-free rotating machinery in future transportation systems. Besides numerous advantages in terms of size, weight, efficiency, and cleanliness, GFBs offer the unique ability to be lubricated with working fluids such as refrigerants. However, the computational analysis of refrigerant-lubricated GFB–rotor systems represents an interdisciplinary problem of enormous complexity and with conflicting interests between all-encompassing but efficient modeling and solution approaches. This thesis succeeds in exploring and pushing forward existing limits of feasibility and thereby establishes a new strategy that enables stability and bifurcation analyses. Owing to the precisely identifiable fluid–structure–rotor interaction mechanisms, three submodels that are of reasonable complexity but nevertheless take into account all relevant nonlinearities can finally be transformed into a single dynamical system. As an essential model feature, the non-ideal characteristics of a typical refrigerant that may undergo vapor–liquid phase transitions are described by thermodynamic equations of state to be included in modified Reynolds and temperature equations. Also, it becomes feasible with the proposed lumped-element foil structure model to include dry friction in various ways reaching from highly efficient regularized Coulomb friction models to bristle models that can reproduce stick–slip transitions. Altogether, this makes the entire problem accessible to rigorous mathematical theory and allows for developing a monolithic research code with interchangeable modules. Introduced by a brief overview of the mathematical theory of dynamical systems, the description and discussion of relevant findings is divided into three sections and guides through a selection of the most important nonlinear effects and phenomena. Firstly, the particular features of refrigerant lubrication with phase transitions are focused on with regard to both steady-state operation and vibrational self-excitation. Secondly, the influence of some foil structure design parameters and the importance of dry friction are thoroughly investigated using stability and bifurcation analyses. Finally, the addition of rotor unbalance gives an outlook to quasi-periodic behavior with even more complex scenarios resulting from combined excitation mechanisms.