在有限水深下高密度底床射流引致之回流現象：速度場及密度場量測分析

Back flow due to dense bottom jets in ambients of finite depth: velocimetry and densimetry

梁昕茹(Sin-Ru Liang)

指導教授 ： 卡艾瑋

國立臺灣大學/工學院/土木工程學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201902197

摘要

本論文主要研究高密度底床射流於有限水深下之發展特性。使用鹽水模擬高密度流體，藉由入流裝置提供高密度射流於單寬長渠道底床，進行一系列小尺度水工模型試驗。使用高速攝影機拍攝被雷射切葉照亮之流場，因鹽水中含有染劑，利用其對濃度變化有亮度上的差異量測濃度場；在速度場方面，則利用粒子追蹤測速法分析流速。透過實驗求得之流速以及濃度，可進一步計算流線方程式，並分析回流層之特性。最後將不同實驗條件下之分析結果進行比較。結果顯示，當水深下降時，回流速度增加並往上游移動；下降到一定水深範圍時，高密度流會形成內部水躍，並在下游發展出一小段順流。另外，當渠道坡度增加時回流流量增加，並往下游移動；當高密度射流流速增加時，上游回流流量些微增加且下游清水之回流流量減少，但位置沒有明顯改變。在交界面上會產生捲入現象，當下降到一定水深時，回流層捲入順流層後隨即在下游捲出到回流層，而當水深過淺時，捲入現象將由高密度射流主導。

關鍵字

高密度射流 ； 高速攝影機 ； 粒子追蹤測速法 (PTV) ； 流線方程式 ； 回流 ； 雙層流

並列摘要

The propose of this thesis is to analyze the flow field of dense bottom jets in ambients of finite depth. We would conduct a series of small-scale laboratory experiments by using salt water to simulate the high-density fluid. Through the inflow equipment, it would flow as the dense jets in the bottom of a long narrow channel. The high-speed camera is used to capture the flow field illuminated by the laser sheet. Since the salt water contains dye, the density field could be measured by the difference of luminance. The velocity field is analyzed by particle tracking velocimetry. Through the analysis of the velocity field and density field, the streamline function can be estimated and the characteristics of the backflow layer can be observed. Finally, compare the analysis results under different experimental conditions.
The results show that while the ambient depth declines, the surface backflow velocity increases and moves upstream. When the depth decrease to a specified range, the internal hydraulic jump would evolve and a small forward flow will develop in the downstream. In addition, while the channel slope increases, the back flow rate increases and moves downstream. While the discharge of dense jet increases, the back flow rate increases slightly and the back flow rate of ambient fluid decreases in the downstream, but the position does not change significantly. The entrainment would occur on the interface of two-layer. When the depth decrease to a certain value, the upper layer would entrain into the under layer and then detrain to the upper layer downstream. When the depth is too shallow, the entrainment would be dominated by the dense jet.

並列關鍵字

dense bottom jets ； high speed camera ； particle tracking velocimetry ； stream function ； backflow ； two-layer flow

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