Elucidation of surfactant effect on macroscopic structure of bubbly flows

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To conduct 3-dimensional measurement of upward bubbly flows in a vertical channel, Scanning Stereo PIV system was developed. By scanning the laser sheet in span-direction using galvano-mirror in the measurement area of 30mm in the streamwise(x-), 40mm in the wallnormal(y-) and 20mm in the spanwise(z-) directions, we succeeded to measure the 3-dimensional time-dependent velocity fields. Approximately 10, 000 velocity vectors was obtained at each time step with the time resolution of 30Hz and spatial resolution of 0. 5mm, 0. 5mm, and 1. 0mm in x, y, z directions, respectively. Using the developed PIV system, single phase turbulent flow was measured and the quantitatively good agreement with previous results(both experimental and numerical) was achieved. Then, using this system, the liquid velocity was measured under the situation that the clusters of a few tens of bubbles were sliding up along the vicinity of the wall. We succeeded to measure the drastic reduction of Reynolds stress in this type of bubbly flow which was previously obtained only through the LDV measurement. Furthermore, we succeeded to visualize the change of turbulent structures in the center region of channel which is caused by bubble clusters.