Behavior of microbubbles in homogeneous stratified turbulence

Dynamic behaviors of spherical microbubbles, within a one-way coupling frame, in stratified turbulence are investigated by direct numerical simulation. To simulate stratified turbulence, the Navier-Stokes equations and heat equation with a background linear temperature variation are solved in a periodic cube domain. Stratification creates a predominantly horizontal motion in a fluid, thereby inducing the well-defined "quasi" mean oscillatory horizontal flow. By solving the equation of motion for microbubbles, we found that the clustering of bubbles with a Stokes number below 0.1 becomes weaker as the stratification is increased. Consequently, the reduction of rise velocity of bubbles in turbulence compared to that in still fluid decreases from 7% to 2%. Owing to the alternating mean motion of the fluid, bubbles rise in a zigzag pattern, thereby resulting in an oscillatory Lagrangian correlation of the horizontal velocity component of bubbles. Despite this oscillating rising motion of the bubbles, the horizontal dispersion of a single bubble is suppressed. From the statistics of the horizontal separation of pair bubbles, we observe a power-law growth in the Batchelor and Richardson regimes for the separation.