Analysis of flow characteristics in turbulent bubbly channel flow through the velocity decomposition

The instantaneous velocity in a turbulent bubbly flow has been so far decomposed by the mean and fluctuation velocities. In our study, we decompose the velocity into the mean velocity without bubbles (U$_{\mathrm{0}})$, mean velocity deviated from U$_{\mathrm{0}}$ by bubbles ($\Delta $U), and the fluctuating velocity. In this decomposition, we use an ensemble averaging on the velocity fields containing bubbles with respect to their wall-normal positions. We apply this decomposition to turbulent bubbly channel flow, using direct numerical simulation. $\Delta $U contains a horseshoe vortex behind each bubble located close to the wall, which significantly affects the Reynolds stress distribution there. From turbulent fluctuations, counter-rotating vortices behind bubbles are mainly observed irrespective of their wall-normal locations. Quasi-streamwise vortices occurring in a single-phase flow is suppressed by the dynamic motion of near-wall bubbles.