Experiments on the interaction of rising bubbles with solid boundaries and vortex rings
The dynamics of bubble motion are of great interest since the distribution and movement of bubbles govern mass and energy transfers in numerous industrial and natural processes, such as CO₂ absorption at the ocean surface and salt aerosol formation in the atmosphere. The problem of a bubble rising freely into an unbounded liquid is a classic and extremely rich problem in fluid mechanics which has attracted attention for decades, generating a vast body of literature (Duineveld 1995 ; Maxworthy et al. 1996 ; Magnaudet and Eames 2000 ; Mougin and Magnaudet 2001 ; Tripathi et al. 2015 ; Cano-Lozano et al. 2016). However, in most natural and industrial situations, bubbles inevitably move near a solid boundary, which introduces complex changes to the dynamics of the bubbles whose governing mechanisms have not yet been fully unravelled. Thus, we approach the simplified problem of a single bubble rising in a still liquid bounded laterally by a vertical wall, and aim to measure experimentally the kinematics of the bubble motion and the forces acting on it.
Experiments are performed in a 1.2 m tall tank using different liquids and air injectors to vary the value of the Galileo, Ga = ρg1/2D3/2/μ, and the Bond, Bo = ρgD2/σ, numbers, as well as the dimensionless distance between the vertical surface and the air injector, L. The bubble rising process is recorded using two synchronized high-speed cameras placed perpendicularly. Using an in-house code to analyze the images taken from both cameras, we determine not only the path of the bubble, but also the time evolution of their shape, major and minor diameters, and orientation. Three distinct ascending regimes are observed for different combinations of Ga and Bo : rectilinear, zigzagging, or spiralling ascent. The results for three values of L for each case are compared with those for the unbounded scenario (L = 1, 2, 4, ∞). It is observed that, in the three regimes, the surface induced a migration effect on the bubble that is stronger the closer they are to the wall, being the bubbles in the zigzag regime the ones that exhibited the greatest deviation from the unbounded path. We also focus on determining the forces that lead to the changes induced by the surface in the bubble path. We analyze the drag and lift coefficients (CD, CL) using the generalized Kirchhoff equations applied to the motion of a bubble in a viscous liquid, using the bubble aspect ratio, orientation, and mass coefficient obtained from the experiments.
In addition to the above study, the talk will cover some experimental results on the interaction between bubbles and vortex rings. We use a time-resolved 3D particle tracking velocimetry (PTV) technique, coupled with shadowgraphy, to obtain simultaneous three-dimensional measurements of bubble motion and fluid behavior during bubble-vortex ring interaction. We experimentally characterize the dynamics of the interacting pair by simultaneously capturing the bubble dynamics, deformation, and eventual breakup, as well as the fluid motion, for a range of situations in which we vary the vortex circulation, the vortex-to-bubble size ratio, and the Weber number. We present the results of the evolution of vortex velocity, kinetic energy and enstrophy, alongside the time evolution of bubble dynamics under various conditions. Our analysis of these results also provides insight into the bubble breakup process and the mechanisms that lead to vortex ring destabilization.
Contact Nathanaël Machicoane for more information or to schedule a discussion with the seminar speaker.