On the Kinematic Trends of Micro-Bubbles Rising in Shear-Layer Flows

• Main Authors: Yu-Chi Chang, 張育齊
• Other Authors: Yi-Chih Chow
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/89942896036391038537

碩士 === 國立臺灣海洋大學 === 系統工程暨造船學系 === 97 === Energy shortage has become a world-wide problem, causing the increase of research efforts on energy saving. In the field of naval architecture,energy saving can be effectively achieved by reducing ship drag. Among several drag reduction methodologies, bubble drag reduction(BDR) has attracted much attention probably due to the fact that previous “water-tunnel” tests of BDR have shown as high as 80% drag reduction efficiency. However, the tests of “towing-tank” type have consistently shown a much less drag reduction efficiency than the cases of water-tunnel type. To gain more insights into this performance difference and the kinematic trends of bubbles rising in these two different shear-layer flows, we construct a shear-layer water tunnel consisting of upper and lower channels through which flow velocities can be independently controlled. By merging these two flows of different velocities we are able to simulate the specific type of shear-layer flow that corresponds to either water-tunnel or towing-tank tests of BDR. We adopt the method of electrolysis to generate small bubbles in the water flow, and use PIV (Particle Image Velocimetry) system to acquire image data of the bubbles and the background shear-layer flow. The experimental results indicate that the overall rising velocity of bubble in the flow of towing-tank type is less than that in the flow of water-tunnel type. We also use a simplified bubble-dynamics theory to clarify the key mechanism that causes this trend of bubble kinematics.