Rainbow Imaging Of 3-D Microbubble Distribution Clustered Inside A Turbulent Boundary Layer

Abstract

Microbubbles dispersed in a turbulent boundary layer spatially developing along a flat plate is investigated experimentally. The aim of study is to find out visually how microbubbles interact with turbulent eddies to achieve frictional drag reduction on a wall. For stream Reynolds number up to 3×104 , microbubble suspended at Stokes number smaller than 10 − 3 is measured. Stream-wise vortices in the boundary layer were visualized by flake optics, of which spacing expands with injection of microbubbles. 3-D microbubble distributions were measured by a color-coded volumetric illumination applied to the boundary layer. The result showed preferentially accumulated microbubble clouds to low-speed streaks close to viscous sublayer and to hair-pin vortices in the buffer layer.