蝴蝶身體俯仰動態之飛行動力機制與飛行操控研究

Flying with Body Rotation: the Unique Flight Dynamics Revealed from Butterflies in Free Flight

費約翰(Yueh-Han John Fei)

指導教授 ： 楊鏡堂

國立臺灣大學/工學院/機械工程學研究所/博士(2017年)

https://doi.org/10.6342/NTU201701085

摘要

相較於其他飛行昆蟲，蝴蝶飛行伴隨明顯身體俯仰動態與飛行軌跡變化，本研建立蝴蝶飛行之數值模型，探討暫態飛行下蝴蝶特殊身體俯仰動態之氣動力學效果與特色，以及飛行時如運用身體俯仰動態進行飛行模式之調控。
蝴蝶飛行時，翅膀前後翅交疊大幅地限制翅膀旋轉之自由度，翅膀僅相對於身體軸進行往復之拍撲，且飛行時伴隨明顯之身體旋轉動作。透過數值模擬與流場可視化分析，結果顯示蝴蝶飛行時翅膀操縱在較高之攻角，其產生之翼後緣與翼前緣渦漩皆貼附在翅膀表面，分別於翅膀表面形成局部低壓區，提升單次拍撲所產生之空氣之作用力，進一步透過身體俯仰動態改變此作用力方向；此外，蝴蝶低展弦比翼型亦提升空氣作用力之產生及拍翅產所需之耗能，進一步提升飛行表現。由於此特殊空氣動力學產生方式，實驗中可觀察蝴蝶到飛行時明顯之軌跡變化與速度變化，因此本文進一步探討蝴蝶於暫態飛行下之空氣動力學效果。
為準確了解於暫態下之飛行，本研究透過計算空氣作用力與重力所造成之瞬間加速度建立蝴蝶於自由飛行之數值模擬。模擬結果顯示，蝴蝶於前飛之水平速度變化劇烈；水平速度於上拍結束達最大值(1.2 m s-1)，此時翅膀交疊，使水平方向投影面積減小，以避免高速氣流帶來之飛行阻力；當下拍轉換上拍階段水平速度達最小值(0.2 m s-1)，此時翅膀往返捕捉下拍所產生之誘導氣流，使產生之推進力提升，透過暫態氣流與翅膀動態之巧妙交互作用；相較於等速飛行條件，蝴蝶於暫態下之平均飛行速度提升47%。在過去昆蟲飛行研究中大多假設飛行速度為等速，然而，透過本研究發現此簡化可能導致低估昆蟲之飛行表現，甚至對其飛行給予錯誤之詮釋。
蝴蝶飛行時被觀察到伴隨明顯之俯仰動作，且身體之俯仰轉動隨飛行模式改變而有所不同，本文最後透過改變身體俯仰起始角度與擺動振幅，以觀察飛行時軌跡與流場隨俯仰動態改變之變化。研究結果顯示，水平位移量與起始身體角度之大小呈負相關，而垂直位移量與俯仰振幅大小呈正相關；此外，在身體角度垂直下，俯仰振幅對垂直位移之增益效果更加明顯，蝴蝶透過身體俯仰動態可有效達到飛行速度與飛行模式之調控。在飛行上，蝴蝶具備低拍撲頻率與優良操控等特色，為微飛行器設計上理想之參考對象，本文解釋蝴蝶如何透過特殊之身體俯仰姿態來強化空氣作用力之產生以及進行飛行調控，其可提供未來微飛行器操控策略設計上不同思維。

關鍵字

蝴蝶飛行 ； 自由飛行 ； 軌跡變化 ； 翅膀結構 ； 身體俯仰 ； 飛行操控

並列摘要

Flying with low wing beats, erratic trajectory and broad wings, the flight of butterfles is unique and different from most of flying insect. In our observation, the dancing-liked flight motion of butterflies can be attributed to two reasons -- unsteady flight speed and significant body pitching motion. These aerodynamic effects, however, were scarcely examined before. In this study, we create a numerical model of butterflies to investigate the aerodynamic and performance of their unique flight in transient conditions; the flight control strategy with the body pitching motion is also revealed.
Nature butterflies are observed flying with obvious body pitching motion but limited wing rotation, which is unlike the flight motion that observed in most of flying insect. Using numerical models with simplified geometry to compare the aerodynamic forces, power requirement and flow structure generated by the motions of typical insect and butterflies, we indicate that the difference of flight kinematics of butterflies leads to larger force generation. A butterfly operates its wings in a high angle of attack, and both of the leading edge and trailing edge vortices are observed attaching on the wing surface during flight. They provide two regions of low pressure on the wing surface, which enhance the net force generation. The low aspect ratio wings of butterflies is also found to enhance the flight performance of butterflies by increasing the force generation and reducing the aerodynamic power in each stroke.
The large aerodynamic force generation of butterflies increases their flight maneuverability, but also leads to the unstable flight trajectories and large variations of their flight speed. A three-dimensional model of a butterfly in transient flight is then created based on the experimental data. The fluid domain is solved with the commercial software (FLUENT), and the flight speed is solved with the equation of motion in each step by integrating the aerodynamic force and the body force acting on butterflies. The model translates freely both in vertical and horizontal directions.
The numerical results indicate that flight speed (1.2 m s-1 to 0.2 m s-1) largely variates in a stroke and nicely match with the values recorded from experiments. The butterfly wings clap to reduce the drag when the flight speed is high, and capture the induced wakes to increase thrust when the flight speed is low. The wing motion of a butterfly skillfully interacts with its transient flight speed and enables an increase of averaged speed by 47% compared with the model in the same motion but in a constant flight speed. The results indicate that a butterfly flies faster in transient conditions. Considering a butterfly flying in a constant inflow leads to an underestimation of its real speed, which might yield an inaccurate interpretation on the insect's flight behavior.
Finally, we investigate how body postures affect butterflies’ flight. Body motion in a simulation is prescribed and parametrically tested by changing the initial body angle and rotational amplitude. The results indicate that vertical translation increases when the rotation amplitude increasing, and the horizontal translation decreases when the initial body angle increasing. The body motion of butterflies changes the direction of jet-flow, and further affects their flight modes. Butterflies’ body motion can effectively control the flight modes. In engineering perspective, low flapping frequency and highly maneuverable flight made the butterflies an ideal model for the small flight vehicle designing; therefore, the inspiration of the unique flight of a butterfly might yield an alternative way to control future small flight vehicles.

並列關鍵字

butterfly flight ； rotation of body ； computational fluid dynamics ； irregular flight trajectory ； flight maneuver

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