PhD defense : Path instability of spheres, spheroids and bubbles

PhD defense : Wei ZHOU

Team : MécaFlu

Title : Path instability of spheres, spheroids and bubbles

Abstract : The thesis presents a numerical study of path instabilities for spheres, oblate spheroids and bubbles moving freely under the effect of the gravity, buoyancy and hydrodynamic forces.
For spheres, the parametric study of Jenny et al. (2004) is revisited, improved end extended with a special focus on the chaotic states. The results reveal that the effect of density ratio responsible for different oblique oscillating states of low and high frequencies has a significant impact both on the onset of chaos and on the behavior of fully chaotic states. Several quantitative statistical quantities are proposed and shown to be relevant for establishing the relation between chaotic and ordered states and for an objective comparison of random data of numerical or experimental origin.
The extensive study on freely moving spheroids establishes the link between disks and spheres by varying the aspect ratio of spheroids from infinitely flat to almost spherical. The state diagrams provided for eight different aspect ratios of spheroid show in detail how the transition scenario varies depending of the body shape. The investigation of almost spherical spheroids reveals the specificities of the dynamics light imperfect spheres.
For the deformable gas bubble in the limit of zero gas/liquid density and viscosity ratio, a marginal stability curve is given in the two-parameter plane of the Galileo and the Bond number indicating the critical Galileo numbers for the loss of stability of vertical trajectories. The numerical investigation covers more than two decades of Bond number going from 0.1 to 20. The results clearly show the crucial role of the surface deformation.

This thesis was supervised by Jan Dusek, professor of the University of Strasbourg.

The presentation will be held in English on Thursday, September 29th, at 10:30 am in the seminar room on the 2nd floor of the 4, rue Boussingault.