- 學位論文
無風環境下機翼近地效應之實驗分析
Experiments of Ground Effect of an Airfoil under No Wind Environment
摘要
翼地效應(ground effect)是空氣動力學中相當重要的一個課題，一般認為:在近地或水面處於某個尺度範圍時，即出現翼體升阻力變化的現象。現實環境下各式飛行器、鳥類與昆蟲於起飛及降落過程中，皆會受此效應之影響。而對於趨於微型化的飛行器，低雷諾數環境的探討更顯重要。 本文以循環式水洞(circuit water tunnel)進行實驗，並配合自行設計之輸送帶(moving belt)以模擬真實情形下NACA4412機翼前行時無風狀態(no wind environment)之流場情形。搭配PIV顯影系統以確認近地流場之情況，觀察/量測的條件包括雷諾數Re < 10^4下，固定15度攻角(α), 各定常高度(h^*=高度/翼弦長)，及由高處(h^*=2.02)平飛後以不同的β(β=下降速度/平飛速度)下降至低處(h^*=0.02)平飛之各種近地情況。上述動態實驗結果將與固定高度之飛行實驗比對發現，結果無論升、阻力情形， Re越高受到β的影響越緩，而Re較低時β改變而產生的影響較劇烈。下降速度明顯地幫助升力係數的增長，而於較低的Re中，於β=0.13時，阻力係數幾近與定常高度 (h^*=2.02~0.02) 之值重合，且高的β也會助長阻力係數的增長。最後並由數值模擬對照與實驗情形之一致性。
並列摘要
Ground effects are an important issue in aerodynamics. It is generally considered that the lift and drag (coefficients) may alter drastically when a flying object is approaching to the ground. Examples in reality include such cases of landing and takeoffs of aircrafts, birds and insects flights under similar conditions. With advances in the technology of aircraft miniaturization today, it becomes important to explore these effects at low Reynolds numbers To achieve the no wind environment, the experiments are conducted in a circulating water tunnel with a self-designed moving belt. The particle image velocimetry (PIV) visualization system is employed to record the interaction between an airfoil and the ground when an airfoil of NACA4412 is landing. In this study, the Reynolds number is moderately low (Re <10^4). Measurements/visualizations were made for the airfoil held at various heights (h^*= height/chord length) as well as for the airfoil landing from a higher altitude h^*=2.02 to the lower h^*=0.02 at the fixed angle of attack α = 15° under various speed ratios β (downward speed/horizontal speed). In particular, we compare the results at the fixed height and at landing. It is shown that the lift and drag coefficients are less dependent on β at higher Re, but change significantly with varying β at higher Re. The lift coefficient increasing with increase in β at the same Re. On the other hand, at lower Re, the drag coefficient for landing at β = 0.13 remains almost the same as that for the height fixed at h^*=2.02~0.02, and, increasing β will also enhance the drag coefficient. Moreover, the simulation results are shown to be in good consistency with the experiments.
參考文獻
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