Two-stage dispersion mechanism of clean spherical bubbles rising in a chain

TL;DR

This paper reveals a two-stage dispersion mechanism for bubbles in a chain, involving wake interactions and flow modifications from bubble-induced mean flows, enhancing understanding of bubble dynamics.

Contribution

It introduces a model that accounts for self-induced flow effects, explaining large-scale dispersion beyond wake interactions alone.

Findings

Large-scale lateral dispersion occurs even at large inter-bubble spacings.

A modified model incorporating bubble-induced flow accurately predicts dispersion.

Flow modification from bubbles enhances lateral migration, explaining observed dispersion.

Abstract

Wake-induced lift is a key mechanism governing the initial destabilization of bubbles rising in a chain (Atasi et al., 2023). Moore's wake model predicts limited interfacial vorticity and a relatively slender, spatially confined wake for clean spherical bubbles, suggesting that wake-mediated interactions weaken as the inter-bubble spacing increases. However, we observed pronounced large-scale lateral dispersion and strong bubble frequency dependence in controlled experiments where bubble diameter and generation frequency were independently varied, even when the inter-bubble separation exceed the characteristic wake length. A reduced-order model incorporating pairwise wake-induced interactions captured the onset of bubble chain destabilization but systematically underpredicted the subsequent emergence of large-scale dispersion. We demonstrate that bubbles rising in a chain collectively generate a mean upward liquid flow that modifies the local shear field, enhancing the lateral migration through shear-induced lift. Incorporating this self-induced weak flow into the model quantitatively reproduced both the dispersion magnitude and its frequency dependence. These results suggest that the dispersion of bubbles rising in a chain involves a two-stage mechanism, with initial chain destabilization mediated by wake interactions, followed by flow modification arising from two-way coupling between bubbles and the liquid. This collective mechanism highlights the importance of self-induced mean flow effects in continuum descriptions of bubble flows.