空氣阻力在競技自行車中是很重要的影響因素。為了獲得更低空氣阻力的優勢並兼顧車架本身的結構強度，水滴翼型廣泛被自行車製造商使用。它是由圓柱體和對稱NACA翼型的組合。水滴翼型確實有降低空氣阻力的效果，但這樣的設計在0度角攻角，低雷諾數範圍（30000-50000）時還是有流場分離和渦流產生的現象。透過計算流體動力學（CFD）我們能充分了解翼型表面的壓力分佈和流場的發展，藉由這些研究我們能夠得到一個新的翼型設計稱為截尾橢圓(Truncated Ellipse，簡稱TE)。與水滴翼型相比，TE翼型之空氣阻力係數較低且能使表面流場分離延遲。接著我們使用視覺流體力學實驗比較兩種翼型的流場發展現象。在水槽中使用注入染料及粒子圖像觀察兩種相同弦長與寬度的翼型之流場發展現象。在風洞中進行油流實驗表面油膜的可視化，觀察其流場分離位置。實驗結果發現，在攻角為0度時TE的流場分離發生在其後緣。在低攻角時流場分離位置皆有延遲（<15度），這是因為分離泡出現在其前緣所造成的現象。這表明TE翼型具有比水滴翼型更好的空氣動力學性能。我們的研究對未來空氣動力自行車設計有很大的幫助。

關鍵字：截尾橢圓翼型，水滴翼型，空氣阻力係數，流場分離，分離泡。

Aerodynamic resistance is always an issue in competitive cycling. In order to gain aerodynamic advantages and structure stiffness of the bike, the teardrop shape is widely used by bicycle manufacturers today. It is a combination of a circular cylinder and symmetric NACA airfoil. Teardrop shape airfoil does provide aerodynamic benefit, this design still suffers from early flow separation and vortex generation at yaw angle <0 degrees in the low Reynolds number range (30000-50000). Studying the pressure distribution and flow development on a number of airfoils with Computational Fluid Dynamics (CFD), we were able to obtain a new airfoil design called Truncated Ellipse (TE). The TE airfoil shows a much lower drag coefficient and flows separation delay compared with a teardrop shape. In our flow visualization experiment, we compared a TE airfoil and a teardrop shape with same chord length and width using dye-injection and Particle Image Velocimetry (PIV) visualization method in a water channel and surface oil-film method in a wind tunnel, respectively. The experimental result shows that the TE has flow separation occurred at the base of the airfoil at a 0-degree yaw angle and delayed flow separation at low yaw angles (<15 degrees) due to the development of separation bubble near the leading edge. This indicates that the TE airfoil has much better aerodynamic performance than the teardrop shape. Our research has implications for the future prospects of aerodynamic bicycle design.

Keywords: Truncated ellipse airfoil, Teardrop shape airfoil, drag coefficient, flow separation, separation bubble.

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