Experimental Study of Air Bubbles Entrained by Breaking Waves near a Submerged Breakwater

• Main Authors: Tsung-mo Tien, 田宗謨
• Other Authors: Twi-wen Hsu
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/07040463575786623596

博士 === 國立成功大學 === 水利及海洋工程學系碩博士班 === 97 === In this study, wave gauges, the underwater acoustic instruments and an array of fiber optic sensor were used to explore the characteristics of the air bubbles entrained by breaking waves near a submerged breakwater. The characteristics of air bubbles are discussed in terms of bubble size, numbers, probability density and air void fraction. In the experiments of wave reflection and transmission, wave gauges were mounted 1 m away from the front and rear sides of the submerged breakwater, to measure the water surface elevation. The experimental data revealed that when the relative height of the breakwater is more than 0.6, an obvious decreasing trend is observed at the transmission coefficient. In order to excavate more about the characteristics of entrained bubbles, eight different wave conditions were selected and carried out in the acoustical and fiber optical experiments. The hydrophone was utilized to detect the noise radiated from the air bubbles. The time-series data of the underwater sound were transformed into the time-frequency domain for capturing the information of the bubbles, such as the bubble size and numbers. The probability density function of the bubbles was also determined. The results of this study reveal that the radius of the bubbles entrained by the breaking waves near a submerged breakwater ranges between 0.1 mm and 7.6 mm. The probability density function of the bubble shows a different slope with a Hinze scale of 0.8 mm. The number of air bubble increases along with the incident wave height and the width of submerged breakwater. Array of fiber optic probe is deployed in the surf zone to monitor the optic signals. The time fraction ratio of signals affected by air bubbles was then obtained. A calibration experiment was carried out in a cylindrical tank to establish the rating curve between the time fraction ratio and the void fraction. The rating curve enables the transfer of the time fraction ratio into the void fraction. In the void fraction measurement experiment, distributions of void fraction along the lee side of the breakwater are also presented in the text. There is a nice agreement after comparing the results of energy loss with entrained air bubbles.