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研究生:余詩敏
研究生(外文):Shih-Min Yu
論文名稱:孤立波底板邊界層之流場特性探討
論文名稱(外文):The Characteristic of Bottom Boundary Layer Flow Induced by Solitary Wave
指導教授:林呈林呈引用關係
學位類別:碩士
校院名稱:國立中興大學
系所名稱:土木工程學系所
學門:工程學門
學類:土木工程學類
論文種類:學術論文
論文出版年:2008
畢業學年度:96
語文別:中文
論文頁數:71
中文關鍵詞:孤立波
外文關鍵詞:solitary wave
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本研究應用具備高時間解析之PIV速度量測系統,於孤立波通過水平底床之層流邊界層流場進行實驗量測。於量測進行時,使用高通濾波,讓追蹤粒子的亮度更為明顯,並且使用邊界層影像處理,解決邊界層底部泛光的問題。在速度向量分析方面,利用動態網格PIV速度計算方法,增加空間解析度,以解析具有較大之速度梯度之邊界層流場。
由實驗量測結果可知,在孤立波通過後,由於波形的變化,造成水位升降,產生壓力梯度的改變,使得水分子受到影響,而有加速或減速的現象。在孤立波通過後,底部邊界層受到逆壓力梯度的影響,有逆向流發生。而本研究針對此現象作進一步的探討,發現隨著波高水深比越來越大,逆壓力梯度的變化也隨之變大,而發生逆向速度的時間跟著提早,且發生的位置也有越來越靠近拍攝區域的中心。
至於底部邊界層之速度分佈,則可利用孤立波波峰通過的時間作為一區分,依照波峰通過前與通過後之速度剖面分佈的特性,將分別進行相似性分析。孤立波之波峰通過前之底部速度剖面可用兩個雙曲線正切函數,進行相似性分析。而在特徵長度尺度的選取上除了使用邊界層厚度,另外還嘗試使用位移厚度、動量厚度與能量厚度作為特徵長度尺度,以勢能區速度作為特徵速度尺度。分析結果發現,位移厚度、動量厚度與能量厚度都為邊界層厚度的倍數。因此不論選用何種厚度當作特徵長度尺度,最後都為邊界層厚度的倍數,故找到相似曲線。
於波峰通過後,則使用雙曲線正切函數與餘弦函數的組合,進行相似性分析。在分析過程中,曾嘗試使用最大負速度所對應之厚度為長度尺度,但結果不佳。因此採用最大逆向流厚度與半寬度為長度尺度,以勢能區速度與最大負速度之差值當作特徵速度尺度。由於這兩個長度尺度所分析出的結果都相當良好,最後採用較具物理意義的最大逆向流厚度為特徵長度尺度。在剪應力部分,將上述所找到的相似曲線,對Y方向上之水平底部作微分,可得到底部黏滯剪應力。由結果知底部黏滯剪應力隨著波高水深比有逐漸增加之趨勢,且最大剪應力所發生的時間約在T =-2~0之間。
The objective of the study is to investigate the characteristic of bottom boundary layer flow induced by a solitary wave using high-speed particle image velocimetry (PIV). Image processing techniques and recursive supper reduction were utilized in the study to improve the time and space resolutions of PIV system. Particle trajectory photography was also used for flow visualization to observe flow fields near the bottom boundary.
The characteristics of bottom boundary layer flow induced by a solitary wave are discussed in the following items: (1)The pressure gradient decreases after the passing of solitary wave and also results in the reverse velocity of the boundary layer flow. The occurring time of reverse flow becomes earlier and the location of the separation point gets close to the center of the area with respect to the ratio of wave height to water depth. (2)The velocity distribution of boundary layer can be classified into two parts for similarity analysis. Before the passing of wave crest of the solitary wave, a regression curve which consists of two hyperbolic tangent functions is used to fit the velocity profiles. Corresponding characteristic values were calculated by this fitting curve. These characteristic values were used to conduct similarity analysis. Boundary layer, displacement layer, momentum layer and energy layer were tried to be various length scales. Finally a similarity curve could be obtained and it is also found that displacement layer thickness, momentum layer thickness and energy layer thickness are the multiple of boundary layer thickness. A regression curve combining both hyperbolic tangent function and cosine function was used to fit the velocity profiles after the passing of wave crest of the solitary wave. The height of corresponding of minimum velocity was tried to be used as a length scale. The result is not good. Besides, the thickness of maximum reverse flow could also be used as the length scale for each velocity profiles. This study tried to selecting the subtraction of the potential velocity and the minimum velocity as velocity scale, and to use the maximum reverse flow layer thickness and half-velocity-defectas different length scales, using the maximum reverse flow layer thickness of physical significance as length scale, a similarity profile can be obtained. (3)Based on the similarity profile, the viscous bottom shear stress can be calculated. It is found that the bottom shear stress increases with the ratio of wave height to water depth. The maximum viscous bottom shear stress occurs for T = - 2 ~ 0.
中文摘要 I
英文摘要 III
目錄 V
圖目錄 VII
相片目錄 XI
表目錄 XII
符號說明 XIII
第一章 緒論
1-1研究動機與目的 1
1-2相關文獻回顧 4
1-3本文組織架構 6
第二章 實驗設備與量測系統
2-1實驗水槽、模型與座標系統 8
2-2流場可視化實驗佈置 9
2-3量測系統 12
2-3-1 高時間解析度之PIV量測系統 12
2-3-2 雙波高計與高速PIV同步量測 14
2-3-3 FLDV量測系統 15
2-4量測系統 16
第三章 初步實驗
3-1孤立波波形檢測 21
3-2孤立波波速檢測 25
3-3高時間解析度之PIV檢測 27
第四章 孤立波邊界層流場量測結果與討論
4-1流場特性分析 32
4-2水平底床邊界層特性分析 39
4-3邊界層相似性分析與剪應力分析 44
第五章 結論與建議
5-1結論 65
5-2建議 67
參考文獻 68
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