Rainbow-Color Imaging Of Microbubbles Strongly Clustering In A Turbulent Boundary Layer

Rainbow-color volumetric illumination for visualizing 3D microbubble distributions was examined in order to find microbubble clustering in a spatially developing turbulent boundary layer. The color coding was defined according to the wall unit from 0 to 200, straddling viscous-sub layer, buffer layer and log-law region. This allowed understanding where the microbubbles were accumulated or diffused within the boundary layer. The results showed microbubble clouding in the low-speed streak regions along the viscous sublayer, clustering into cores of streamwise vortices ejected into the buffer layer, and intermittent exhaust of the clusters toward the log-law region. Furthermore, correlation of these behaviors with local Reynolds shear stress event (ejection, sweep) was found with quadrant analysis of the liquid velocity vector data obtained by PIV.