力元理論的紊流效應：大尺度渦漩模擬

Abstract

本論文以力元理論檢視均勻入流流經翼型NACA0012，在雷諾數20,700，攻 角5度及10度兩種情形下之無限翼的紊流效應。流場部份是以大渦旋模擬法來做預測，並在正交網格的建構下，以貼體座標法來執行固體邊界條件。 研究中，將力元理論(Chang(1992))推廣至紊流層面，以環境渦度的擾動量來表現出物體於平均流場所受的紊流效應，並將其應用於大渦旋模擬法，得到總體紊流貢獻被分為計算尺度擾動量與次網格模型兩部份。我們以此來檢視紊流對無限翼的總體力貢獻以及其於局部空間中的力效應，得到無限翼受紊流影響不顯著的結果。接著針對攻角10度的情形，於紊流發展前期所形成的暫態紊流極値現象做討論。我們透過截面渦渡分析的方式，找出此極值源自於近翼尾下游的尾流區，並量化出此區域因紊流擾動量的瞬間提升，導致局部紊流提供高升、阻力的貢獻比例。

In this study, turbulence effects on a NACA-0012 airfoil at Reynolds number of 20,700 and the angles of attack (AOA) = 5^° and 〖10〗^° are investigated through the force element theory. Flow fields are computationally obtained through the large-eddy simulation (LES). The computation is carried out on the body-fitted grids with the H-type orthogonal-grid construction. The study extends the force element theory (Chang (1992)) to the analysis for the turbulent flow filed. Turbulence effects can be divided into the computationally resolved field and the sub-grid-scale (SGS) model on the framework of LES. The total force contribution and the local effect of turbulence to the infinite wing are examined, and the results show that both give insignificant effect. Moreover, in the case of AOA = 〖10〗^°, the temporary extreme value of turbulent kinetic energy is investigated. It is found that the extreme is resulting from the interaction between turbulent fluctuation and the trailing-edge wakes. We also measure the resulting increase of turbulent fluctuation, which in turn causes the high ratio of lift and drag contribution due to the local fluctuation.