SUPPRESSION OF HORSESHOE VORTEX BY A LINEAR STRAKE

直線型導流片對馬蹄渦抑制的研究

In the present study, we conducted a parametric study by computations to investigate horseshoe vortex suppression due to a flat-plate boundary layer flow past a wing of finite span by a leading-edge strake. The cross section of the wing is a combination of an ellipse at the nose and a NACA0020 airfoil at the tail which join each other at the location of maximum thickness. The Reynolds number is 5x10^5, based on the chord length of the wing, c. The maximum thickness is 0.235c and the span of the wing is 0.705c. The cross-sectional shape of the strake on planes parallel to the flat plate is an ellipse connected to the wing body and its shape on symmetric plan is linear. The ratio of the strake length to the strake height in front of the leading edge varies from 1 to 4. And the absolute height also varies from 0.25T to 1.0T, where T is the maximum thickness of the wing. In total, there are 16 cases in this study. The Spalart-Allmaras model (1-equation model) was employed to account for the turbulence effect. The computational results show that different ratio leads to different flow development. However, in all cases, the horseshoe vortex can be effectively suppressed and a better wake is created due to a smoother transition in geometric setup. The detailed flow field near the leading edge of the wing and the wake development are presented for different cases. 在本論文中，我們透過計算流體力學，探討導流片對平板邊界層通過有限翼展機翼之馬蹄渦的抑制效應。在本研究中，我們所採用的機翼截面是由橢圓與NACA0020翼面組合而成，兩者最大厚度處相切銜接；流場的Reynolds數為5×10^5（以機翼的弦長為特徵長度），機翼的最大厚度為弦長的0.235倍，翼展為弦長的0.705倍。另外，在中線對稱面上，導流片係從機翼導緣某一高度以線性遞減的方式往上游延伸，直到與平板接觸，而同時在平行於平板的平面上，其截面呈橢圓形。我們變動導流片的長高比與高寬比，前者是導流片的長度和高度的比值，從1變到4；後者是導流片的高度與機翼最大寬度的比值，從0.25變到1.0。至於紊流上的模擬，我們則採用Spalart-Allmaras模型。相關結果顯示，導流片的引入使結構幾何變得相對平順，因此可有效抑制馬蹄渦，帶來較佳的尾流結構；我們特別討論導流片如何影響機翼導緣附近的流場、以及尾流的發展。