Bubble deformation and flow structure measured by double shadow images and PIV/LIF

An experiment on bubble motion in a simple shear layer was performed in order to obtain fundamental knowledge of the force on the bubble and its lateral motion induced by the surrounding flow field. We explored the flow structure in the vicinity of the bubble in one plane and its deformation in two planes by particle image velocimetry (PIV) - Laser-induced fluorescence (LIF) and a projection technique for two perpendicular planes, respectively. For our experiment, we chose a single air bubble with an equivalent bubble diameter D_eq of 2-6 mm in a vertical shear flow. Velocity measurements were made using a digital high-speed CCD camera for PIV with fluorescent tracer particles. The second and third CCD cameras were used to detect the bubble's shape and motion via backlighting from an array of infrared LEDs. We quantitatively studied the three-dimensional wake structure from measurements of the two-dimensional vortex structure and approximated three-dimensional shape deformation arranged from two perpendicular bubble images.