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標題: 孤立波於陡坡底床溯降時之流場及加速度特性探討
Acceleration and Velocity Characteristics of Retreated Flow during Run-down Motion of Solitary Wave over Sloping Beach
作者: Wei-Ying Wong
關鍵字: 孤立波
渦流結 構
solitary wave
acceleration and pressure gradient
incipient flow separation
vortex structure
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摘要: 本研究係利用流場可視化法與具高時間解析度之PIV(Particle Image Velocimetry)量測系統,針對孤立波於1:3斜坡上溯升及溯降過程中之流場相關特性進行探討。本實驗孤立波之入射條件為波高水深比(H0/h0) 0.171、0.263 及0.363,並使用數個不同大小及位置之觀測區域(Field of View, FOV)進行流場可視化觀測及速度場PIV量測。此外,本文中所呈現之速度向量場係經過多次實驗重複拍攝後相位平均取得,且亦針對高時間解析度PIV量測系統之速度量測結果適用性、高速攝影機影像獲取水面之時空間變化結果適用性等進行檢測。再者,流場之渦度、加速度及對應之壓力梯度係透過速度向量場之速度剖面取得。本研究亦針對加速度計算之時空間間距選取進行敏感度測試。 本研究根據流場可視化法與具高時間解析度之PIV量測系統呈現及探討之流場現象係過去利用基本或簡單之儀器設備所無法呈現。首先,本研究指出於孤立波溯升過程中之流場分枝情形及其當中之細節;本研究說明了於孤立波溯降過程中之流場加速及減速之特性,包括局部加速度項、對流加速度項及對應之壓力梯度等;本文亦探討孤立波溯降過程中溯降流之福祿數相等於1.0之臨界流斷面之空間變化。 此外,於溯降運動初期及前半段中期之溯降流因受逆壓力梯度變化之影響而發生邊界層流場分離之發生機制、及其隨後所發生之分離剪力層下伴隨產生之明顯的渦流結構,包括主渦流渦心之空間變化、於後半段中期及末期主渦流結構上緣外之高速蜿蜒流流場特性等,亦在本文中進行相關細部探討。
The evolution and detailed characteristics of the retreated flow during the run-down phase of a solitary wave propagating over 1:3 sloping bottom were investigated experimentally, using particle trajectory method of flow visualization technique and high-speed particle image velocimetry (HSPIV). A series of experiments were performed with several different fields of view, and with several incident solitary waves having wave height (H0) to water depth (h0) ratio of 0.171, 0.263 and 0.363. The velocity fields were obtained by ensemble averaging the repeated measurements. Several preliminary tests were carried out to validate and ensure the quality of the measured data. In addition, the vorticity, acceleration, and pressure gradient were obtained by the velocity profiles. The sensitivity tests for acceleration calculation were performed in order to reduce the bias of the results. Furthermore, an interesting phenomenon of the vortex structure generated from a separated boundary layer in the retreated flow during run-down motion is explored. According to the results obtained qualitatively and quantitatively by utilizing flow visualization technique and HSPIV measurements, related features of flow field and vortex structure, which were hardly discovered and depicted in the past due to the rudimentary equipment, are presented and discussed. Firstly, a striking phenomenon of flow bifurcation during run-up phase is investigated. Second, the characteristics of flow deceleration or acceleration, including the local and convective accelerations, being accompanied by the adverse or favorable pressure gradient in the retreated flow are illustrated. Furthermore, the spatial variation of the critical section where the critical flow with Froude number being equal to unity takes place is investigated. Moreover, the mechanism for the incipient flow separation, accompanied by the formation of the separated shear layer from the sloping bottom during early and first-half middle stages of run-down motion, and the variation of the pressure gradient before, during and after the incipient flow separation are explored. Besides, the evolution of the vortex structure beneath the separated shear layer, including the position and vorticity of the primary vortex core moving offshore under the high-speed meandering stream during second-half middle and late stages of run-down motion are discussed.
文章公開時間: 2020-08-16
Appears in Collections:土木工程學系所



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