The steady propagation of an air finger into a rectangular tube

摘要

The steady propagation of an air finger into a fluid-filled tube of uniform rectangular cross-section is investigated. This paper is primarily focused on the influence of the aspect ratio, alpha. on the flow properties, but the effects of a transverse gravitational field are also considered. The three-dimensional interfacial problem is solved numerically using the object-oriented multi-physics finite-element library oomph-lib and the results agree with our previous experimental results (de Lozar et al. Phys. Rev. Lett. vol. 99, 2007, article 234501) to within the +/- 1 % experimental error. At a fixed capillary number Ca (ratio of viscous to surface-tension forces) the pressure drops across the finger tip and relative finger widths decrease with increasing alpha. The dependence of the wet fraction in (the relative quantity of liquid that remains on the tube walls after the propagation of the finger) is more complicated: in decreases with increasing alpha for low Ca but it increases with alpha at high Ca. Our results also indicate that the system is approximately quasi-two-dimensional for alpha >= 8, when we obtain quantitative agreement with McLean & Saffman's two-dimensional model for the relative finger width as a function of the governing parameter 1/B = 12 alpha Ca-2. The action of gravity causes an increase in the pressure drops, finger widths and wet fractions at fixed capillary number. In particular, when the Bond number (ratio of gravitational to surface-tension forces) is greater than one the finger lifts off the bottom wall of the tube leading to dramatic increases in the finger width and wet fraction at a given Ca. For alpha >= 3 a previously unobserved flow regime has been identified in which a small recirculation flow is situated in front of the finger tip, shielding it from any contaminants in the flow. In addition, for greater than or similar to 2 the capillary number, Ca-c, above which global recirculation flows disappear has been observed to follow the simple empirical law: Ca-c(2/3) alpha = 1.21.