Experimental study of path instability of rising bubbles

Abstract:

The main objective is to study the path instability of a freely ascending air bubble in water and silicon oil at varied Bond numbers (ratio of body forces to surface tension) and Galileo numbers (ratio of gravitational force to viscous force). Below a certain threshold of the Galileo number, the path of the bubble remains straight. But above a critical Galileo number (related to the size of the bubble) a new regime sets in, and the bubble changes its shape and follows a zig-zagging path.

Experimental investigation is needed because earlier existing numerical work suffers from simplifications likely to bias predictions, such as the assumption of fixed shape [1,2] and recent simulations accounting fully for shape deformation are either still too sparse to provide a reliably established marginal stability curve [3,4] or bring controversial results [5]. Our work aims to contribute reliable observations of the onset of instability using various silicon oils.

In our experimental set up, bubbles of variable size are released in a rectangular tank filled with silicon oil known to present no problem of free surface impurities. Images obtained by a fast camera and are post-processed to determine the size and shapes of the bubbles and their trajectory in the fluid. Bond and Galileo numbers are calculated based on the volume of bubble and on the physical properties of the used working fluid. 

Experiments will be conducted with silicon oils of varying viscosity to obtain as many points in the Galileo – Bond number parameter plane as possible.

Bibilography :

[1] Tchoufag J., Magnaudet J. and Fabre D., Linear stability and sensitivity of the flow past a fixed oblate spheroidal bubble, Physics of Fluids, 25 (2013) 052108

[2] Cano-Lozano J.C., Bohorquez P. and Martinez-Bazán C., Wake instability of a fixed axisymmetric bubble of realistic shape, Int. J. Multiphase Flow, 51 (2013) 11.

[3] Tripathi M.K., Saku K.C. and Govindarajan R., Nature Comm., 6:6268, (2015).

[4] Cano-Lozano J.C. , Martinez-Bazán, Magnaudet J. and Tchoufag J., Paths and wakes of deformable nearly spherical rising bubbles close to the transition to path instability, Phys. Rev. Fluids 1, 053604 (2016).

[5] Zhou W. and Dusek J., Marginal stability curve of a deformable bubble, Int. J. Multiphase Flow, 89 (2017) 218.