Oscillating Taylor bubble during the emptying of a partially filled water bottle.

The oscillatory behavior observed during the emptying of a vertical cylinder partially filled with water has been studied for large neck-to-bottle diameter ratios, d^{*}. For large apertures (d^{*}>0.8), a Taylor bubble invades the cylinder from the bottom, and its rising speed exhibits periodic oscillations coupled to periodic motions of the free surface limiting the top air buffer initially present in the bottle. We introduce an elementary model where the vertical oscillation of the free surface is represented by a variable mass oscillator exciting the oscillatory dynamics of the Taylor bubble. In this system, the top-air buffer acts as a spring, whose stiffness is related to its compressibility. The variable mass is the mass of the liquid in the cylinder that decreases as the Taylor bubble progresses during the emptying. The motion of the bubble is solved assuming that the unsteady flow generated by the free-surface motion is potential in the vicinity of the apex of the bubble. A comparison with experimental results obtained at the laboratory shows that the model agrees well with the data if it takes into account dissipation. This study shows that a viscous damping, proportional to the velocity, with a constant damping coefficient is able to accurately represent the dissipative processes such as the effect of viscous Stokes boundary layers at the walls.

Interaction and coalescence of large bubbles rising in a thin gap.

We present accurate measurements of the relative motion and deformation of two large bubbles released consecutively in a quiescent liquid confined in a thin-gap cell. Although the second injected bubble was smaller, we observed that, in all cases, it accelerated and caught up with the leading bubble. This acceleration is related to the wake of the leading bubble, which also induces significant changes in the width and curvature of the trailing bubble. On the contrary, the velocity of the leading bubble is unaltered during the whole interaction and coalescence process. Shape adaptation of the two bubbles is observed just prior to coalescence. After pinch-off, the liquid film is drained at a constant velocity.

Wake attenuation in large Reynolds number dispersed two-phase flows.

The dynamics of high Reynolds number-dispersed two-phase flow strongly depends on the wakes generated behind the moving bodies that constitute the dispersed phase. The length of these wakes is considerably reduced compared with those developing behind isolated bodies. In this paper, this wake attenuation is studied from several complementary experimental investigations with the aim of determining how it depends on the body Reynolds number and the volume fraction alpha. It is first shown that the wakes inside a homogeneous swarm of rising bubbles decay exponentially with a characteristic length that scales as the ratio of the bubble diameter d to the drag coefficient Cd, and surprisingly does not depend on alpha for 10(-2)