SEPARATION STRUCTURE OF BUBBLY FLOW AROUND A CIRCULAR CYLINDER WITH BUBBLE - BOUNDARY INTERACTION

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In the present research, the bubble behavior in the boundary layer along the cylinder surface is investigated experimentally. The bubble motion is observed by a high speed video from the inside of the cylinder through the transparent wall. The flow behavior in the boundary layer is visualized by an injected dye. In the present experimental conditions, the Reynolds number is corresponding to a sub-critical flow condition for single-phase flow and a super-critical one in the bubbly flow. The laminar separation is disturbed by the passing bubble, and the separation is attached on the surface. This causes that the separation point shifts backward and the form drag is reduced. The following results are also obtained from the photograph and the measurement of pressure distribution around the cylinder. The bubbles break up into small bubbles near the circular cylinder surface due to the strong shear stress in the boundary layer in the larger Reynolds number cases. The pressure distribution around the cylinder approaches to that of the super critical condition with the increase of void fraction.