低雷諾數下低展弦比翼板之起動流場實驗分析

Abstract

微飛行器有許多研究學者投入心力去研究，較有前瞻性的是模仿鳥類或昆蟲 的撲翼運動，因其具有很高的機動性。而微飛行器主要有兩個特點：(1)在低雷諾數(Re=100、300)下運作；(2)低展弦比。在低雷諾數下，許多複雜的現象可能在邊界層(Boundary Layer)發生，例如：流場分離 (Separation)、過渡流(Transition Flow)、流體再附著(Reattachment)；且低展弦比的情況下，翼尖渦的影響及其與前緣渦的交互作用變得重要得多。為簡化問題，本實驗以低展弦比的平板作為研究對象，使用流場顯影配合升、阻力量測，探討有限平板渦漩的發展以及前緣渦與翼尖渦的結構。本實驗在甘油和水的混合溶液下以雷諾數100、300進行低展弦比翼板高攻角瞬間起動且作平移運動，使用的平板展弦比有1、2、3，攻角15、30、45、60度，用六軸量力感測器記錄其升、阻力係數隨時間的變化，並分析無因次化時間T=2與T=7升阻力係數的差異以及展弦比對升阻力係數的影響。流場觀測方面，藉著釋放微塑膠顆粒，並使用雷射光切頁與CCD擷取圖像，在不同剖面觀察前緣渦與翼尖渦隨時間的發展，配合PIV方法計算流場的速度場及渦度值。由量力結果發現在翼板剛起動時有著較大的升阻力係數以及較高的升阻力係數比值，符合延遲失速(Delayed Stall)效應；且在觀察前緣渦的發展中，發現在較低雷諾數與較高的展弦比情況下，前緣渦聚集成巨大的渦漩；反之，前緣渦則容易受到翼尖渦的影響，使前緣渦消散變成不規則的運動，甚至是滯流的狀態。在翼尖渦可自由發展的情況下，本實驗並無發現馮卡門渦漩(Von Kármán Vortex)的產生。

Micro air vehicles (MAV) is a popular research topic. Because of the high maneuverability, simulation of flapping wings of birds and insects may have good potential in the MAV development. Micro air vehicles operate in a relatively low Reynolds number regime. In this regime, many complex flow phenomena take place within the boundary layers: separation, transition, and reattachment can all occur within a short distance along the chord of a wing. Under the condition of low-aspect-ratio, the effect of wing-tip vortex and it’s interaction with leading-edge vortex become more important. To simplify the problem, low-aspect-ratio flat plates are used to investigate the formation of leading-edge vortex, wing-tip vortices by means of quantitative flow visualization. The experiment is performed in a tank filled with a Glycerine/water mixer. The low-aspect-ratio rectangular plate is impulsively started and translated at high angles of attack at low Reynolds numbers(Re=100、300). The considered aspect-ratios are 1, 2 and 3, respectively. The angles of attack are 15, 30, 45 and 60 degrees. The plate is rigidly mounted to a six-axis force sensor recording lift and drag force with time. The variations of lift and drag coefficients between nondimensional time T=2 and T=7, and the effect of aspect-ratio to lift and drag coefficients are analyzed. For flow visualization, small particles are released in the fluids illuminated by laser light sheet. The trajectories of the particles are captured by CCD camera. The velocity and vorticity fields can be calculated by PIV method, through which, the formation of leading-edge vortex and wing-tip vortices can be observed. Results show that at the beginning of translation the plate has larger lift and drag coefficients and larger lift to drag ratios, which conform to the effect of delayed stall. In the observation, we find that under the conditions of lower Reynolds number and higher aspect-ratio, the leading-edge vortex can be gathered to form a bigger vortex; otherwise, the leading-edge vortex will be influenced by the wing-tip vortices and dissipate to an irregular and slow motion. In the situation of free development of wing-tip, the Von Kármán Vortex was not observed in the present study.