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Acceleration and Velocity Characteristics of Retreated Flow during Run-down Motion of Solitary Wave over Sloping Beach
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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。
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.
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