An investigation of anisotropy in the bubbly turbulent flow via direct numerical simulations

TL;DR

This paper investigates how bubbles influence turbulence anisotropy in channel flows using direct numerical simulations, revealing flow direction-dependent bubble clustering and turbulence modulation effects.

Contribution

It provides detailed insights into bubble-induced turbulence anisotropy and energy transfer mechanisms in bubbly turbulent flows through high-fidelity DNS.

Findings

Bubbles tend to accumulate near the wall in upward flows, increasing turbulence.

In downward flows, bubbles cluster in the channel center, inducing pseudo-turbulence.

Bubble motion near walls significantly affects velocity gradients and turbulence structure.

Abstract

This study explores the dynamics of dispersed bubbly turbulent flow in a channel using interface-resolved direct numerical simulation (DNS) with an efficient Coupled Level-Set Volume-of-Fluid (CLSVOF) solver. The influence of number of bubbles (96 and 192), flow direction, and Eotvos number was examined across eight distinct cases. The results indicate that in upward flows, bubbles tend to accumulate near the wall, with smaller Eotvos numbers bringing them closer to the wall and enhancing energy dissipation through increased turbulence and vorticity. This proximity causes the liquid phase velocity to attenuate, and the bubbles, being more spherical, induce more isotropic turbulence. Conversely, in downward flows, bubbles cluster in the middle of the channel and induce additional pseudo-turbulence in the channel center, which induce additional turbulent kinetic energy in the channel center. The study further examines budget of Turbulent Kinetic Energy (TKE) and the exact balance equation for the Reynolds stresses, revealing that near-wall bubble motion generates substantial velocity gradients, particularly in the wall-normal direction, significantly impacting the turbulence structure.