背階圓柱尾流特性之試驗研究

Abstract

水流於橋墩基礎造成局部沖刷之現象，為台灣多數橋梁面臨之安全問題，在工程上多以固床工、蛇籠等結構體穩固橋墩基礎，以確保橋梁安全。無論是固床工或蛇籠，大部分皆以階梯型式存在於流場中，可視為圓柱與階梯之組合，過去對於水流流經圓柱或階梯之相關研究甚多，但鮮少探討兩者組合之流場特性。因此，本研究於圓柱底部設置一階梯，應用質點影像測速法沿水深方向分層量測圓柱後方尾流之水平面流場，藉此探討圓柱與階梯組合之流場特性。 量測結果顯示，圓柱正後方階梯流場中存在一逆向水流，往階梯方向流動，因其流速較兩側階梯迴流區高，故形成以逆向水流為主之迴流運動；此逆向水流向上發展影響圓柱尾流流場，使尾流中心軸上形成一低速區，導致圓柱渦流向下游移動時，有往兩側偏移之現象。本研究亦分析圓柱渦流脫離頻率與階梯高度、福祿數之關係，作為往後研究橋墩震動、局部沖刷深度評估之參考。

In Taiwan, one of the safety problems of many bridges is the local scour caused by flow passing around the bridge piers. Regarding engineering methods, groundsill works, gabions and other structures are usually used to protect the foundations of piers to ensure bridges safety. Because groundsill work or gabion is constructed on step, the structure can be regarded as the combination of circular cylinder and step. In the past, there were many researches about flow passing either circular cylinder or step, but few studies on the flow field of the combination of circular cylinder and step. Therefore, this experiment is conducted to study the flow field passing a circular cylinder located on a backward-facing step by using the particle image velocimetry to measure the horizontal flow fields at different layers of the wake generated by the circular cylinder. The experimental result reveals an adverse flow zone occurs in the recirculation region of step behind the cylinder. In the adverse flow zone, fluids flow toward step and faster than those on both sides of the recirculation region. Therefore, the adverse flow will become the main flow of the recirculation region. The adverse flow zone affects the wake of the circular cylinder causing a field of low velocity existing in the middle of the wake. It also results in that vortices displace laterally when flowing downstream. Furthermore, the relationship among vortex shedding frequency, height of step, and Froude number is analyzed to provide the information for the future study about the vibration of the pier and the estimation of the local scour depth.