豆娘穩定前飛與急停迴旋之力學機制探討

Abstract

本文選用台灣常見的豆娘中華珈璁以及細胸珈璁作為實驗物種，將其放入特製之狹長型透明壓克力觀測箱中，觀測豆娘在其中試圖迴避四周壁面而由原本向前飛行轉換至向後飛行的急遽變化。實驗分析使用高速攝影機捕捉豆娘迴旋時的拍翅動作以及身體俯仰動作，並利用二維PIV技術將流場可視化，找出影響轉彎的關鍵運動學參數及限制，並藉由比較拍翅運動學和流場渦旋互動歸納出豆娘特殊的轉彎策略。 結果顯示豆娘在不同雷諾數(Reynolds number)區間下會採用兩種不同的迴旋模式，分別是低雷諾數(50~300)的往復式急迴旋以及中高雷諾數(200~700)的旋轉式急迴旋。頭部加速度與飛行雷諾數成正相關。低雷諾數下(Re < 300)豆娘以增加拍翅頻率(+26%)獲得頭部加速度的大幅提升(+100~120%)，高雷諾數(Re > 200)下豆娘則普遍藉由降低拍翅平面角度改變阻力作用點，使身體旋轉以大幅增加頭部加速度(+70~75%)。並且隨著雷諾數提高，豆娘之史卓荷數(Strouhal number)會趨近約0.2，以Taylor (2003) 之觀點可視為迴旋飛行操控之最佳能量效率操作點。 流場分析顯示在穩定前飛期間，豆娘前翅翼前緣渦旋(leading edge vortex, LEV)逸散緩慢，上下拍轉換期間會停留在前翅後方與後翅上方之狹窄區間，並與後翅翼前緣渦旋融合，強度提升至最高160 1/s並持續維持約半周期；此現象說明豆娘能夠以低於蜻蜓一半的拍翅頻率飛行，並在急迴旋期間之懸停階段藉由相似機制提供足夠升力之原因。 研究中之豆娘運動學參數可被使用於更具有操控潛能之微飛行器利用，並期望能改善能源使用效率之目的。

The turning mechanics of damselfly species Psolodesmus mandarinus and Mnais tenuis is investigated. When free-flying in highly-confined acrylic chambers, damselflies are observed to decelerate from forward fly status and perform special turning maneuver in order to avoid impact. High speed camera (fps>1000) is used to capture transient flapping trajectories and body posture while two dimensional PIV technique reveals the transient flow field and vortices patterns. The study focuses on finding the key kinematic parameters influencing the turning ability and deducing the turning strategies by combining flapping kinematics and vortices interaction. Results show that damselflies mainly use two different kinds of turning modes according to their forward fly Reynolds number and the head acceleration increases with increasing Reynolds number. When the Reynolds number is lower than 300, damselflies perform “reciprocating turn” while they shift to “rotational turn” when the Reynolds number is greater than 200. During reciprocating turn, damselflies increase their flapping frequency to increase their head acceleration up to 120% compared to counterparts with same Reynolds number. In rotational turn, damselflies adjust their stroke plane angle to change the location of the drag force. A net moment is thus created to rotate the body, enhancing the head acceleration up to 75% compared to counterparts with same Reynolds number. Additionally, as Reynolds number increases, the operating Strouhal number for damselflies is shown to approach a constant of 0.2, which agrees with Taylor (2003) that flapping insects operate at certain Strouhal numbers in order to achieve high energy efficiency. The study also discovers that during both forward fly and turning maneuver, the slowly-shed fore wing LEVs interact with hind wing LEVs between the gaps of two wings, forming sustaining vortices with vorticity up to 160 1/s. The vortices interaction mechanism strengthens the facts that under same flying speed a damselfly can operate with half the flapping frequency compared to a dragonfly and, during hovering stage in turning maneuver, the lift is sufficient to provide body support. Flapping kinematics obtained in the study can be used to design and fabric more agile and energy-saving micro air vehicles capable of challenging tasks.