低雷諾數下有限翼近地效應之實驗與數值模擬分析

Abstract

實際上，飛行器在著陸的過程中，其與地面之距離將不斷的變化。因此，在降落過程應將受到為動態地面效應(Dynamic Ground Effect)的影響。本研究將利用循環式水洞搭配輸送帶系統，來針對的定翼型微型飛行器進行近地效應分析。本論文，首先對於不同攻角、雷諾數與展弦比之結果做分析，再探討靜態(Static Effect)分析與動態分析結果之差異，最後由數值計算之結果與張建成教授所提出的力元理論做比對與驗證。本實驗將針對NACA4412的翼型做探討，其規格為弦長5cm而展弦比為2、3與4的模型翼。文中將探討6~25之攻角以及雷諾數5000、6000、7500、9000與10000等條件下進行近地效應之模擬。由結果分析可知，不論動態或靜態分析，其近地效應約從1.00c處開始影響，而在0.5c以下影響更劇烈。在相同雷諾數的環境下，靜態分析存在臨界攻角而動態則無明顯的臨界攻角(以下)，此差異來自於靜態與動態在高攻角條件下兩者在翼面分離渦旋的情況不同。在適當的攻角條件下，升力大小隨雷諾數提升而增加，而高攻角條件下則無此趨勢。在不同展弦比的結果中顯示，展弦比長的力的能夠在低雷諾數下得到更多升力來源。在較低攻角的條件下，可由靜態分析結果來分析真實情況；而在高攻角的條件下，應由動態分析來探討，否則將存在30%~40%之間的誤差。

This study is aimed to investigate the influence of ground effect during the landing process of the airfoil NACA 4412 under static and dynamic experimental force measurements in a water tunnel that has moving ground simulation with numerical validations. Airfoils of 5‑cm chord length (C) and 3° pitch angle (θ) at various aspect ratios (AR=2, 3, & 4) are vertically immersed in the water with a narrowed‑neck support linked between the load cell above water surface and the airfoil beside the conveyor belt. The conveyor belt is designed to synchronize with the inflow velocity (U) in order to remove the boundary effects generated by the velocity difference at the side wall, and is capable of matching Reynolds numbers (Re) from 5,000 to 10,000 as utilize in the present study. The forces exerted on the airfoil are measured by a 2‑D load cell under the two prescribed situations, namely static ground effect (SGE) and dynamics ground effect (DGE).In SGE, the forces exerted on the airfoil are measured statically at various clearance and angle of attacks α=θ+γ, where γ is chosen to match with those in the DGE. As in DGE, the forces are measured at a continuous descending motion of normalized constant velocity vf=vf*∕U=tan(γ) towards the ground (moving conveyor), so‑called dynamic. It is emphasized that γ is the inflow angle relative to the descending airfoil determined by vf, and α in DGE is varied from 6° to 25° (with θ=3°).The results show that the ground effect in either SGE or DGE is essential to the airfoil only when the clearance decreases below a unit chord length (C), and the influence becomes rather tremendous as the clearance deceases below 0.5C. Next, it is found that the lift coefficient CL increases with increasing α in SEG and DGE cases under the same Re. However, the increment of CL ceases when a critical α is achieved in SGE. When α is low‑to‑moderate, CL is shown to increase with increasing Re in both SGE and DGE cases. In addition, the lift curve with respect to the clearance in SGE and DGE are almost the same when α is low‑to‑moderate, and thus the results in SGE can be reasonably applied to that in DGE. However, when dealing with α beyond a certain angle, the analysis of DGE must be taken into account in order to prevent an error that could reach about 30 to 40%.