Visualization of the wake behind a sliding bubble

Abstract

In this work, Schlieren measurements are presented for the wake of an air bubble sliding under a heated, inclined surface in quiescent water to provide new insights into the intricate sliding bubble wake structure and the associated convective cooling process. This is a two-phase flow configuration that is pertinent to thermal management solutions, where the fundamental flow physics have yet to be fully described. In this work, we present an experimental apparatus that enables high-quality Schlieren images for different bubble sizes and measurement planes. By combining these visualizations with an advanced bubble tracking technique, we can simultaneously quantify the symbiotic relationship that exists between the sliding bubble dynamics and its associated wake. An unstable, dynamic wake structure is revealed, consisting of multiple hairpin-shaped vortex structures interacting within the macroscopic area affected by the bubble. As vorticity is generated in the near wake, the bubble shape is observed to recoil and rebound. This also occurs normal to the surface and is particularly noticeable for larger bubble sizes, with a periodic ejection of material from the near wake corresponding to significant shape changes. These findings, along with their implications from a thermal management perspective, provide information on the rich dynamics of this natural flow that cannot be obtained using alternate experimental techniques.

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

“…Therefore, the vortex detaches prematurely in the bubble wake and vortex loops are no longer observed.It is noteworthy, that the vortex leg did not detach from the bubble beneath for angles α ≤30°, because of the in-plane confinement conditions that force a rectilinear behavior of bubble. It is well known that, vortex detachment needs a 'zig-zagging' bubble behavior, where vortex shedding is occurring at points where bubbles changes their trajectory directions, called, "the inversion point"[26,28,33]. This was not observed in this study, at least for the range of inclination tested from 10° to 60°.…”

“…1). Confined sliding bubbles are confronted to several forces: the buoyancy due to the upper inclined wall [9,28], and higher inertia due to the confinement [8], which slightly slows down their overall motion. Before the investigation of the interfacial area shown in figure 5, it is worthwhile to characterize the effect of the inclined wall on the bubble shape elongation.…”

“…Particle Image Velocimetry PIV under three different angles of inclination (α=20°, 30°, and 40°) [26][27][28]. It has been shown that an increase of α and bubble db increased the bubble velocities [9,30,32].…”

“…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]).…”

“…In addition, the spatial and temporal evolution of the flow structures consisted of two distinct regions: (i) a near wake moving in close association with the bubble, forming a recirculation region after which fluid separated from this high-velocity region and was drawn towards the inclined surface. (ii) A far wake, where fluid moved away from the surface in an asymmetrical shape, corresponding to oppositely oriented tails of hairpin vortices at a greater distance from the bubble [28,33].…”