淺化碎波衝擊直立壁及傾斜壁之流場量測與分析

本研究採用坡度為1/10的堤前底床，使波浪經淺化後，形成完美碎波，分別拍擊在傾斜角度為0°、10°、20°、30°之傾斜壁，並使用粒子影像測速儀(P.I.V.)，觀測碎波撞擊前後堤壁附近的流場變化，其中包含速度向量場與渦度場之分析，以及撞擊瞬間堤壁所受波壓之估算。堤壁所受波壓分布之計算則是根據實驗所得的瞬時速度場，配合控制體積與Stokes二階波方程式而得。從結果可看出，隨著堤壁傾斜角度的增加，波浪撞擊堤壁後有較多向右的空間，速度變化較不劇烈，流體從堤壁上滑落的情況與波形的變化也漸趨緩和。碎波衝擊堤壁，下滑後會產生一個明顯的渦旋，此現象會隨著堤壁傾斜角度增加，變得較不顯著。而堤壁附近的紊流，也會隨著傾斜角度的增加而減少。本研究並針對波壓估算模式的準確性進行驗證，發現時間觀測尺度對估算模式之準確度有很大的影響。在撞擊瞬間，在靠近堤壁1 cm內，壓力急遽上升，在距離堤壁3~5 cm處，壓力在垂直堤壁方向變化非常小，類似以均勻流方式入射堤壁。以P/γH、y/d與PH/γhL、y/H兩種參數做無因次壓力分析時，發現四種堤壁波壓分布差不多，皆看不出傾斜角度對波壓的影響。

關鍵字

粒子影像測速儀；傾斜壁；壓力場估算；速度場；渦旋場；淺化碎波

並列摘要

In this study the slope of the approaching bed in front of the wall is 1/10. The impacts on the different sloping walls of 0°, 10°, 20°, and 30° by the perfect breaking waves after shoaling are studied, by using particle image velocimetry (P.I.V.) to observe the flow field variations near the walls right before and after the impact of the breaking waves on the walls, from which the velocity fields and vortex fields are analyzed and the wave pressures on the walls right on the instant of impact are evaluated. The distributions of the wave pressures on the walls are derived according to the instant velocity fields obtained from the experiments along with the control volume concept and the second-order Stokes wave equation. It is observed from the results that, with the increase of the oblique angles, there is more space toward the right hand side after waves impacting the wall; therefore, the variation of the velocity is not so severe, in addition, the fluid sliding down from the walls and the changes of the wave shapes are both mitigated. After impact of the breaking wave on the wall, there is a vortex induced by the sliding of the fluid. This phenomenon is less significant with the increase of the oblique angle of the wall. While the turbulence near the wall is mitigated with the increase of the oblique angle. In addition, the accuracy of the model for the wave pressure evaluation has been verified. It is found that the observation time scale is significant on the accuracy of the evaluation. Right on the instant of impact, within the proximity of 1 cm to the wall, the pressure increases rapidly; however, away from the wall 3 to 5 cm, the pressure normal to the wall has no much change, similar to the uniform flow impinging on the wall. From the non-dimensional pressure analyses with (P/γH,y/d) and (PH/γhL,y/H) two parameters, the wave pressure distributions for the four different oblique walls are no much difference; therefore, there is no influence on the wave pressure due to the oblique angles.

並列關鍵字

P.I.V. ； pressure field ； velocity filed ； vorticity field ； sloping wall ； shoaling breaking wave

參考文獻

15. 趙偉廷，2009。「深水碎波衝擊直立壁之流場分析研究」，國立臺灣大學工程科學與海洋工程學研究所碩士論文。

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國際替代計量

淺化碎波衝擊直立壁及傾斜壁之流場量測與分析

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