Graduate School in Nonlinear Science

Sponsored by the Danish Research Academy

MIDIT                               OFD                           CATS
Modelling, Nonlinear Dynamics       Optics and Fluid Dynamics     Chaos and Turbulence Studies
and Irreversible Thermodynamics     Risø National Laboratory      Niels Bohr Institute and 
Technical University of Denmark     Building 128                  Department of Chemistry
Building 321                        P.O. Box 49                   University of Copenhagen 
DK-2800 Lyngby                      DK-4000 Roskilde              DK-2100 Copenhagen Ø
Denmark                             Denmark                       Denmark


by Andreas Spohn
Ecole Nationale Superieure de Mécanique et d'Aérotechnique
Poitiers, France

MIDIT-seminar 425

Wednesday December 2, 1998, 15.15 h
at MIDIT, MAT Building 303, room 026

Abstract: The flow entrained by the rotating bottom inside a closed cylinder gives rise to a wide variety of flow structures, including the formation of recirculation zones very similar to vortex breakdown. Here we focus on recent experimental results that show the formation and evolution of these bubbles in detail. Special attention is paid to the interpretation of flow visualisations obtained by the electrolytic precipitation technique and the flourescent dye technique. The experiments confirm the overall behaviour for the bubble formation previously found by Vogel (1968) and Escudier (1984). In particular they show the bubble interaction in the three-bubble region predicted by Brøns, Voigt, Sørensen (1998). Although the global flow structure appears to be highly axisymmetric, local asymmetries appear along the container axis and inside the boundary layers along the container walls. The bubbles are found to be open with in- and outflow. Close to the bifurcation curves spiral-like structures similar to the spiral-type vortex breakdown are found. Both PIV measurements and visualisations show that the bubble first appear inside these spirals. All results highlight on the one hand the great difficulty to obtain valuable results but suggest on the other hand that asymmetric modes should be taken in account for a more complete analysis of the flow.