Experimental study on a zigzagging bubble using tomographic particle image velocimetry with shadow image reconstruction

Abstract

For decades, it has been proven by numerous experiments and simulations that a single bubble freely rises in an unstable path and shape in a surface tension force dominant regime. Using time-resolved tomographic particle image velocimetry combined with three-dimensional shadow image reconstruction, the present study experimentally provides a full three-dimensional diagnosis of the shape and wake structures of a zigzagging bubble. An ellipsoidal bubble with an equivalent diameter of [Formula: see text] = 5.47 mm freely rising in stagnant water is investigated at a terminal Reynolds number of 1390 with a zigzag path. The results show a typical double-threaded vortex structure generated during the initial ascending stage. In the regular zigzagging stage, a four-ring mode of vortex generation is observed, which is composed of alternatively discharged and induced hairpin vortices. Thanks to the volumetric measurement, the shedding or inducing mechanism of complicated wake structures is clearly achieved. We speculate that the secondary shape oscillation of the bubble is excited by the shedding of the primary hairpin vortex. Frequencies of the bubble trajectory, variation of velocity, and bubble shape oscillation are analyzed in detail. Their associated harmonics are classified to indicate the interactions between the bubble and the wakes.

Advancements in the Understanding of Single‐Bubble Dynamics: Integrated Experimental and Simulation Studies

“…It is evident that the calculated shedding frequency of the wake is approximately two times the path oscillation frequency, which is consistent with previous research conclusions. [ 24 ]…”

“…This result is consistent with previous research reports. [24] de Vries et al [54] and Mougin and Magnaudet [35] demonstrated through experiments and numerical simulations that zigzag bubbles have a two-threaded wake with an axial vortex component that converts signs twice in one zigzag period. They believe that this is the reason for the vertical velocity frequency of the bubble being twice that of its horizontal velocity frequency.…”

“…The previous studies have generally ignored the instability of bubble rise paths. [22][23][24][25][26][27] As shown by Zhang et al [25,28] bubbles rising in liquid metal undergo zigzag, helical path due to bubble deformation and vortex shedding in the wake region. Therefore, a more accurate bubble motion model is necessary to accurately predict the subsequent behavior of the bubbles, improve the cleanliness of liquid steel, and improve the internal and surface quality of the casting billets.…”

“…Sharf et al [ 31 ] experimentally investigated the dynamics of bubble rise in aqueous glycerol solution and plotted Ga – Eo planes based on the shape of the bubbles. She et al [ 24 ] used time‐resolved chromatographic particle image velocimetry combined with 3D shadowing image reconstruction to experimentally provide full 3D diagnosis of zigzag bubbles and wake structures. The previous studies advance a qualitative understanding of bubble behavior without quantitative force measurements.…”

An experimental study on near-field tip vortex of an elliptical hydrofoil using tomographic particle image velocimetry

“…In recent years, with the development of a novel multiview PIV system: tomographic particle image velocimetry (TPIV) (Elsinga et al, 2006), the volumetric measurement of the three-component (3C) velocity field has become feasible. As a mature technique for three-dimensional flow measurement, TPIV has been successfully applied in various flow problems (Scarano, 2012;Gao et al, 2013), such as vortex breakdown over a non-slender delta wing (Wang et al, 2016a), unsteady vortices on a flapping wing (Ehlers et al, 2016) and wake structures of a zigzagging bubble (She et al, 2021). The TPIV technique is also rather practicable for researches in the field of hydrodynamics.…”