蝴蝶飛行之翅膀旋轉動力機制分析與翅膀撓性效應

Abstract

本研究以蝴蝶飛行為研究標的，探討其翅膀旋轉之飛行動力機制與翅膀撓性效應，提出仿昆蟲微飛行器翅膀動作與設計之創新見解。蝴蝶飛行機動性極高，舉凡後飛、俯衝、倒飛或急轉皆易如反掌，較低的拍撲頻率在工程應用上也更具有可行性，是拍撲微飛行器設計很好的參考對象。 相較於其他昆蟲，蝴蝶使用較小幅度的翅膀旋轉動作飛行，通過對真實飛行蝴蝶動力學的分析，蝴蝶之翅膀旋轉振幅大約為45度，且旋轉的振幅與飛行方向呈現正相關。根據量測的蝴蝶動作，建立數值模擬重現蝴蝶飛行，發現具有旋轉角的蝴蝶相比無旋轉角的蝴蝶能產生更大升力，蝴蝶控制自身的翅膀旋轉幅度則可增強尾流捕獲效應，從而增加向前推進力。對於翅膀旋轉幅度較小的動作，下拍中產生的脫離渦漩會誘導尾流區域之空氣流向翅膀，使蝴蝶能夠捕獲誘導氣流獲得額外的向前推進力，這股額外推力佔所有推進力的47%以上。而當翅膀旋轉振幅超過45度時，脫離渦漩誘導的氣流流向則遠離翅膀，並減少了尾流捕獲效應造成蝴蝶失去部分向前推進力。 脫離渦漩誘導的氣流影響蝴蝶的空氣作用力，當左右翅之脫離渦漩融合成渦漩環時，將形成強烈的誘導射流，對蝴蝶的飛行方向十分很重要，過去的研究指出，此射流可以通過自身的身體角度來操控。本研究中發現除了身體角度之外，翅膀的撓性變形也能改變射流方向。真實蝴蝶飛行實驗中，身體角度隨著前飛速度的降低而逐漸增大，蝴蝶懸停飛行時(飛行速度極低)，身體幾乎垂直於地面；從流場可視化實驗中得知，當翅膀前後拍動時，誘導射流卻不斷向下方流動。數值模擬結果顯示，翅膀在具有撓性的情況下，翼後緣渦漩會在拍撲的中段之後分離。逆時針旋轉的脫離翼後緣渦漩與順時針旋轉的翼前緣渦漩，在尾流區域同時誘導空氣向下流動。此外，隨著翅膀變形幅度增加，射流的方向將更為垂直，形成強烈的向下射流，而這種情況沒有出現在剛性翅膀模型中。受真實蝴蝶啟發，研究發現翅膀撓性可改變射流方向，在蝴蝶懸停飛行時得以支撐其身體重量。 大多數昆蟲在飛行過程中都會經歷某種程度的翅膀變形，翅膀撓性的增加改變昆蟲的空氣動力學表現，本研究最後藉由可視化流場技術與仿蝴蝶拍撲機構，結合翅膀旋轉及翅膀撓性，研究翅膀在不同旋轉時機之流場變化，研究發現在領先、對稱及延遲三個不同旋轉時機下，撓性翅膀結合對稱旋轉，相比於領先及延遲旋轉有更強的翼前緣渦漩，且三種旋轉時機之撓性翅膀上的翼前緣渦漩皆比剛性翅膀更貼附於翅膀表面上，較強與較為貼附的翼前緣渦漩表示有更大的空氣作用力產生。從變形量量測實驗得知，撓性翅膀通過對稱旋轉能產生更大的彎曲位移，從而增加翅膀額外的拍撲速度，增強翼前緣渦漩的強度。 本研究的觀察與分析得到蝴蝶飛行之機制或策略，可運用在仿昆蟲拍撲微飛行器開發上，不論翅膀設計、翅膀動作或是研究概念、實驗架設方法，皆能提供未來設計上參考之不同思維。

This dissertation conducts the biological experiment on the butterfly flight. By exploring the flight dynamics of the wing-pitch rotation and the wing flexibility effect, we propose innovative insights into the wing motion and the design of insect-like micro-aerial vehicles. Butterflies have an extraordinary flying behavior. They can easily fly backward, upside down, dive or even sharp turn in few seconds. Lower flapping frequency of a butterfly is more feasible in engineering applications, which is a good reference for a flapping micro-aerial aircraft design. Compared with other insects, butterflies fly with a smaller wing-pitch amplitude during flight. Through the dynamic analysis of a real flying butterflies, the wing-pitch amplitude of a butterfly is about 45 degrees, and the amplitude is positively correlated with the flight direction. Based on an established numerical simulation refined by the experimental data, the flight of the butterfly model with wing-pitch rotation is found to be generated more aerodynamic forces than the butterfly model without wing-pitch rotation. A butterfly can control its wing-pitch angle to increases the forward propulsion by the wake-capture effect. For a small wing-pitch amplitude, the detached vortex generated in the downstroke induce the air in the wake to flow to the wings, so that the butterfly captured the induced flow to obtain additional forward propulsion, which accounts for no less than 47% of all the propulsion. When the wing-pitch amplitude exceeds 45 degrees, the direction of the induced airflow by the detached vortex is move away from the wing, reducing the wake-capture effect, and causing the butterfly to lose part of its forward propulsion. The induced flow of the detached vortex affects the aerodynamic forces of the butterfly. When the detached vortices of the left and the right wings merges into a vortex ring in the wake, a strong induced jet-flow, which is very important for the flight direction of the butterfly will be formed behind the wing. The jet-flow directed the propulsion of a butterfly, which can be manipulated by its body angle. In this study, we found that, besides the body angle, the chordwise deformation of the wings can also alter the direction of the jet-flow. In the experiment of a real flying butterfly, the body angle gradually increased with the decrease of the forward flight speed. When the butterfly hovers (lower limit of flight speed), the body is almost perpendicular to the ground; the induced jet continuously flows downward when the wing flaps forward and backward in the flow visualization experiment. The results of the numerical simulation show that the trailing-edge vortex detached after the mid-downstroke with the flexible wing model. The counterclockwise detached trailing-edge vortex and the clockwise leading-edge vortex induce air to flow downward in the wake region. In addition, the direction of the jet-flow become more vertical as the chordwise deformation of the wing increases, forming a strong downward jet-flow; this condition did not appear in the rigid wing model. Inspired by real butterflies, this study found that the chordwise deformation of the wing can alter the direction of the jet-flow to support its weight during hovering flight. Most insects experience some degree of wing deformation during flight. The increase in wing flexibility changes the aerodynamic performance of insects. In this study, the visualization of flow field technology and the imitation butterfly flapping mechanism combined with wing rotation and wing Flexibility, study the change of the flow field of the wing at different rotation timings. The study found that under the three different rotation timings of lead, symmetry and delay, the flexible wing combined with symmetrical rotation has a stronger leading edge than the leading and delayed rotation. vortices, and the leading edge vortices on the flexible wings of the three rotation timings are more attached to the wing surface than the rigid wings. According to past research, the stronger and more attached leading edge vortices both show Greater air force is generated. It is known from the deformation measurement experiment that the flexible wing can generate a larger bending displacement through symmetrical rotation, thereby increasing the additional flapping speed of the wing and enhancing the strength of the leading-edge vortex of the wing. Most of the insects experience some degree of wing flexibility during flight, which changes the aerodynamic performance. In this study, the butterfly-like flapping mechanism combined with wing-pitch angle and wing flexibility was established to study the flow field of different timing of wing-pitch angle by the flow field visualization experiment. The study found that under three different timing of wing-pitch angle, including advanced, symmetric and delayed rotation, the flexible wing combined with symmetric rotation has a stronger leading-edge vortex than advanced and delayed rotation. In addition, the leading-edge vortices on the flexible wings are more attached to the wing surface than the rigid wings; it indicates the aerodynamic forces on the flexible wing is greater than on the rigid wing. From the experimental measurement of wing deformation we found the flexible wing with symmetric rotation induced more bending displacement, which increased an additional flapping speed of the wing and enhanced the strength of the leading edge vortex of the wing. The mechanism or strategy of the butterfly flight obtained in this study through the observation and the analysis of a real flying butterfly can be used in the development of insect-like micro-aerial vehicles. The novelty of the wing design, the wing motion, research concepts, or even the methods of experimental procedures all provide different ideas for the design of micro-aerial vehicles in the future.