潛堤附近碎波氣泡特性之實驗研究

Abstract

本文分別使用水中聲學儀器及光纖感測器，探討波浪通過潛堤碎波時氣泡的特性。水中聲學實驗主要是利用水中麥克風擷取碎波氣泡之水中聲音訊號，在濾除環境噪音後，利用Gabor Transform將時域訊號轉換爲時間－頻率域訊號，並由此分析出氣泡之大小及數量，並進一步得出碎波氣泡之機率密度譜。光纖感測實驗則是利用陣列式光纖探針量測碎波帶內的光訊號，並求出訊號受氣泡影響之時間分率比(time fraction ratio)。經由圓柱管氣泡率定實驗，得到訊號時間分率比與局部氣泡體積分率之率定曲線之後，可藉由時間分率比判識出氣泡體積分率。本研究結果顯示當波浪通過潛堤產生碎波時，氣泡半徑範圍在0.1 mm到7.6 mm之間，且當氣泡半徑大於或小於0.7 mm時，氣泡機率密度的斜率不太一樣。隨著波高與堤寬增加，碎波氣泡數量也會增加。此外，本文所探討的實驗案例中，碎波氣泡主要出現在自由液面附近，並平均分佈在堤的後方。In this study both the underwater acoustic instruments and the fiber optic sensors were used to explore the characteristics of the air bubbles entrained by breaking waves near a submerged breakwater. 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. Array of fiber optic probe was 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. 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 0.7 mm. The number of air bubble increases along with the incident wave height and the width of submerged breakwater. Furthermore, the entrained air bubbles appear mainly near the free surface and distribute uniformly at the lee side of the breakwater.