Microbubble Skin Friction Reduction under the Influence of Pressure Gradients and Curved Surface

The influence of streamwise pressure gradients and curved surfaces on microbubble skin friction reduction was examined in cavitation tunnel at free-stream speeds of 5, 7 and 10m/sec. A 1.5m long, 0.3m width flat plate and 2D convex model were used. In order to change streamwise pressure gradient, using temporary slopping bottom changed sectional area at the test section.The effect of skin friction reduction under favorable pressure gradients tended to rather reduce compared with the zero pressure gradient case. In opposition, the adverse pressure gradients tended to rather increase. Moreover, there was a condition that the skin friction reduction effect increased extremely under the adverse pressure gradient. Air bubbles under that condition looked like cloud and the void fraction of inner layer was high. There seems to be possible to find an effective microbubble method by the optimization with air bubble diameter and distribution.