本研究旨在釐清不同直徑筐網之流場特性以及其對橋墩流場之影響機制，透過在鋼構渠道中設計一定床實驗，並利用都普勒聲波剖面流速儀(ADV)實際量測流場，將所得之資料做流速場分布圖、紊流分析及頻譜分析以達到研究之目的。
所有實驗皆使用孔隙比0.5之筐網，並在相同水理條件(Q = 35710 cm3/s 、H=10cm)下進行，將實驗劃分為兩大部分，先探討不同直徑筐網之流場特性，再將各直徑之筐網分別設置於直徑8cm的橋墩前距離墩心1.5倍橋墩直徑處，以瞭解筐網直徑之變化對橋墩流場之影響效應。
實驗結果可知，筐網後方之紊流強度與能量頻譜皆明顯低於不透水圓柱後方，故其後方有一穩定低速區，隨著筐網直徑增加，此區域在長度與寬度上皆有所增長。而在受到筐網保護下之橋墩前方來流速度降低，向下射流因此減弱，橋墩後方之渦流強度則隨著直徑比(Db/DP)之增加而下降，最多可降低55.9%。

The change in flow characteristics around a cylinder sheltered by a porous cylinder was investigated experimentally. The models of these two cylinders were mounted on a flat-bed of a horizontal flume and velocities around them were measured by using an Acoustic Doppler Velocimeter (ADV). In order to examine the effects of diameter of the upstream porous cylinder on the studied flow, seven different diameters were tested. It was shown that porous cylinder’s diameter has substantial effects on the flow characteristics around the downstream cylinder. Time-average velocities were calculated to study the main flow feature. In addition, turbulence statistics in velocity field and the related power-spectrum analysis were also presented. The result clearly demonstrates that in comparison with bare cylinder, the turbulence intensities, turbulence kinetic energy and flow velocities around cylinder decrease remarkably when the porous cylinder’s diameter is larger than the downstream cylinder. Consequently, the flow structure around the cylinder has become less complex. It is thus suggested from this result that Db/Dp ≥1 would be preferable in flow characteristics.

1. Akilli, H. and Rockwell, D., Vortex formation from a cylinder in shallow wa-ter, Phys. Fluids, Vol.14(9), 2957-2967 (2002).
2. Baker, C. J., The laminar horseshoe vortex, J. Fluid Mech., Vol. 95(2), 347-367 (1979)
3. Bhattacharyya, S., Dhinakaran, S. and Khalili, A., Fluid motion around and through a porous cylinder, Chem. Eng. Sci., Vol. 61, 4451-4461 (2006).
4. Dargahi, B., The turbulent flow field around a circular cylinder, Exp. Fluids, Vol. 8, 1-12 (1989).
5. Ettema, R., Kirkil, G. and Muste, M., Similitude of large-scale turbulence in experiments on local scour at cylinders, J. Hydraul. Eng., Vol. 132, 33-40 (2006).
6. Fransson, J.H.M., Konieczny, P. and Alfredsson, P. H., Flow around a porous cylinder subject to continuous suction or blowing, J. Fluid Struct., Vol. 19, 1031-1048 (2004).
7. Lienhard, J. H., Synopsis of lift, drag and vortex frequency data for rigid cir-cular cylinders, Washington State University, College of Engineering, Re-search Division Bulletin, 300 (1966).
8. Ozkan, G.M., Oruc, V., Akilli, H. and Sahin, B., Flow around a cylinder sur-rounded by a permeable cylinder in shallow water, Exp.Fluids, Vol. 53, 1751-1763 (2012).
9. Ozkan, G.M., Akilli, H. and Sahin, B., Effect of high porosity screen on the near wake of a circular cylinder, EPJ Web conference, Vol. 45, 01071 (2013).
10. Sahin, B., Pressure losses in an isolated perforated plate and jets emerging from the perforated plate, Int. J. Mech. Sci., Vol. 31(1), 51-61 (1989).
11. Von Karman, T. Uber den Mechanismuss des windersstandes den ein bewegter korper in einen flussigkeit erfahart, Nachrichten der k. Gesellschaft der Wissenschaften zu Gottingen, 547-556 (1912).
12. Yu, P., Zeng, Y., Lee, T. S., Chen, X. B. and Low, H. T., Steady flow around and through a permeable circular cylinder, Comput. Fluids, Vol. 42, 1-12 (2011).
13. 石武融，「透水性筐網圓柱之流場試驗研究」，國立成功大學水利及海洋工程學系碩士論文 (2007)。
14. 朱宏翊，「筐網群結構物對橋墩沖刷保護效果之研究」，國立成功大學水利及海洋工程學系碩士論文 (2007)。
15. 吳虹邑，「筐網結構物對橋墩沖刷保護之研究」，國立成功大學水利及海洋工程學系碩士論文 (2005)。
16. 林芳宇，「不同孔隙比筐網圓柱之定床流場試驗研究」，國立成功大學水利及海洋工程學系碩士論文 (2013)。
17. 洪思維，「透水筐網圓柱距離底床不同高度之流場試驗研究」，國立成功大學水利及海洋工程學系碩士論文 (2008)。
18. 徐淑女，「三角形排列之筐網圓柱群對橋墩沖刷防護之研究」，國立嘉義大學土木與水資源工程學系碩士論文 (2010)。
19. 黃進坤，「橋墩保護新工法之研究」，臺灣公路工程，第32卷第8期，39-44 (2006)。
20. 傅家揚，「筐網結構物在不同水流攻角對橋墩沖刷保護之影響」，國立成功大學水利及海洋工程學系碩士論文 (2006)。
21. 曾玟義，「成功筐網對減少橋墩沖刷之現地試驗」，國立成功大學水利及海洋工程學系碩士論文 (2012)。
22. 鄭聰信，「橋墩沖刷保護機構之現地實驗與探討」，國立成功大學水利及海洋工程學系碩士論文 (2007)。