Near-wall rising behaviour of a deformable bubble at high Reynolds number

Abstract

The dynamics of a large deformable bubble (Re ∼ O(10 3 )) rising near a vertical wall in quiescent water is experimentally investigated. The reference (without the wall) rising path of the considered bubble is a two-dimensional zigzag. For a range of wall configurations (i.e. initial wall distance and boundary condition), using high-speed shadowgraphy, various rising behaviours such as periodic bouncing, sliding, migrating away, and non-periodic oscillation without collisions are measured and analysed. Unlike low-Reynolds-and Weber-number bubbles, the contribution of the surface deformation to the transport between the energy components becomes significant during the bubble's rise. In particular, across the bubble-wall collision, the excess surface energy compensates the deficit of kinetic energy. This enables a large deformable bubble to maintain a relatively constant bouncing kinematics, despite the obvious wall-induced energy dissipation. The wall effect, predominantly appearing as energy loss, is found to decrease as the initial distance from the bubble centre to the wall increases. Compared to the regular (no-slip) wall, a hydrophobic surface enhances or reduces the wall effect depending on the wall distance, whereas a porous surface reduces the energy loss due to the wall, regardless of the initial distance from the wall. Furthermore, the bubble-wall collision behaviour is assessed in terms of a restitution coefficient and modified impact Stokes number (ratio of the inertia to viscous forces), which shows a good correlation, the trends of which agree well with the variations in the energy components. The dependence of near-wall bubble motion on the wall distance and boundary condition may suggest a way of predicting or controlling the near-wall gas void-fraction distribution in gas-liquid flow systems.

Superhydrophobic drag reduction in turbulent flows: a critical review

“…Although it was claimed that the gas bubbles introduced into the flow would enhance the sustainability and amount of plastron and hence reduce the drag on the random-roughness SHPo surface (Fukuda et al 2000;Du et al 2017), the exact mechanism of interaction was not found, although one can speculate the bubbles in the flow replenish the air pockets. Jeong and Park (2015) experimentally showed that rising air bubbles attach and slide on a SHPo vertical wall without merging with the plastron unless the thin water layer present between the surface and bubble ( d l ) becomes thinner than O(1) μm. Following the theoretical relation of d l ∼ ( l l v l v g ) 0.5 d 2 g −1 Δt −0.5 for the critical thickness in their study ( l : viscosity of water, l : density of water, v l : velocity of water, v g : velocity of bubble, d g : bubble diameter, : surface tension of water, and Δt : time duration at which the bubble resides on the surface), the product of v l v g ∕Δt should be smaller than O(1) for the bubble of d g ∼ O(100) μm, for example, to have a chance to merge into the air layer on the SHPo surface.…”

Hydrodynamics and gas-liquid mass transfer around a confined sliding bubble

“…By changing from rising bubbles to sliding ones, Maxworthy [9] performed one of the first studies of the dynamics of a bubble sliding under an inclined surface, explaining that sliding bubbles differed from free rising bubbles in that they only experienced a predominant buoyancy force [9,[26][27][28][29]. Depending on the angle of inclination of the upper wall and the bubble size, several behaviors of bubbles can be found: (i) bouncing for low angles of α < 5° [29], (ii) sliding for intermediate angles 5°< α < 80° [26,27,30], or (iii) steady bouncing of constant amplitude for high angles [18,31]).…”

Hydrodynamic interaction of bubbles rising side-by-side in viscous liquids

“…The gravitational potential energy is neglected as well. The kinetic energy of lateral motion ( E kx ) reads (Jeong and Park 2015;Newman 1977) where is the density of the surrounding liquid. R 1 and R 2 are the semi-major and minor axes of bubbles, respectively.…”