蜻蜓撲翅運動之仿生模擬及力元理論分析

本論文主要探討蜻蜓在向前飛行時，非定常撲翅運動的撲翅受力情形，蜻蜓藉由雙對翅膀，能應付各種不同的飛行需求，其飛行模式一直是仿生空氣動力學中的課題之一。分析蜻蜓撲翅運動的受力情形，能有助於仿生微飛行器的設計。本論文使用商用軟體Ansys Fluent進行蜻蜓撲翅運動的模擬。以兩組旋轉網格模擬三維蜻蜓於飛行時前後翅的拍撲情形，並利用DES紊流模型進行數值運算，分析並比較蜻蜓在同相及完全異相飛行模式下的受力情形，並利用力學的角度解釋蜻蜓為何在起飛及爬升之時會採用前後翅同相的飛行模式，而穩定平飛時則會採用前後翅完全異相的飛行模式。本論文並透過探討不同的參數條件，包括撲翅角度(±20°, ±30°, ±40°)、飛行仰角(2°, 9°, 16°)、及雷諾數(Re=1125, 2250, 4500, 6750)，配合前後翅相位差(0T, 0.5T)來分析更多飛行型態的受力情形及飛行效率的比較。本論文特別引入力元理論(Chang, 1992)分析方法，透過勢流場的角度觀察受力，將蜻蜓非定常撲翅運動之受力與渦度作連結。力元理論藉由力的分解，將物體之受力分解為體渦度項、表面渦度項、勢流力項、摩擦力項、及雷諾應力項，藉此計算出各個流元(fluid element)對蜻蜓撲翅流場造成各項力的貢獻。一般計算升阻力時會採用表面壓力積分法進行整體受力的計算，而力元理論分析可探討此非定常流場各項受力的逐時變化，如此便能深入分析翅膀的受力來源。經過力元理論分析後得知蜻蜓撲翅流場之受力係由體渦度項所主導，故透過繪製力元理論中的體渦度分布圖，清楚表達蜻蜓於撲翅時，周圍流場之渦漩結構變化，以及體渦度對翅膀的力貢獻分布，經過繪製分析後得知，前緣渦的消長與結構變化，主導著撲翅運動的受力情形，其為昆蟲及鳥類等撲翅生物得以飛行的重要因素。經過計算分析後得知，前後翅同相飛行，即前後翅相位差零個週期時，與一般鳥類撲翅之飛行效果類似，能帶來大的升力有助於滿足飛行時的爬升需求；而前後翅完全異相飛行，即前後翅相位差1/2個週期時，因前後翅相反方向運動，會形成一正一負的體渦度力而互相抵消，使得在各種飛行參數下，平均升阻力及週期內的升阻力大小皆相較同相飛行時來的小，不過又能保有足以支撐身體重量的升力，以及飛行所需的推力，此有助於蜻蜓穩定平飛之需求。

關鍵字

蜻蜓；撲翅；相位差；力元理論；非定常流動；微飛行器

並列摘要

The study mainly discusses the unsteady flow field of dragonfly flapping wings when flying forward. Dragonflies can cope with various flight needs by means of two pairs of wings, and their flight patterns have always been the subject of bionic aerodynamics. Analyzing the force of dragonfly flapping motion can be helpful for the design of bionic micro-aircraft. This study uses the commercial software Ansys Fluent to simulate the flapping motion of dragonfly. Two sets of rotating grids were used to simulate the flapping of the fore and hind wings of a three-dimensional dragonfly during flight, and the DES turbulence model was used for numerical calculation to analyze and compare the force of the dragonfly in the in-phase stroking and counter-stroking flight modes. Explains why dragonflies use a flight pattern with their fore and hind wings in-phase stroking during takeoff and climb, and a counter-stroking flight pattern for stable flight. This study also discusses different parameters, including flapping angles (±20°, ±30°, ±40°), body angles (2°, 9°, 16°), and Reynolds number (Re=1125, 2250 , 4500, 6750), combined with the phase difference between the fore and hind wings (0T, 0.5T) to analyze the drag and lift of more flight patterns and the comparison of flight efficiency. The study especially introduces the force element theory (Chang, 1992) analysis method, observes the force from the perspective of the potential flow field, and connects the force and vorticity of the unsteady flapping motion of dragonfly. The force element theory decomposes the force into a volume-vorticity term, a surface vorticity term, a potential flow force term, a friction term, and a Reynolds stress term, thereby calculating the fluid element to the contribution of various forces to the flow field of dragonfly flapping wings. Generally, the surface pressure integration method is used to calculate the overall force when calculating the lift and drag. The force element theory analysis can explore the time-to-time changes of the forces in this unsteady flow field, so that the force source of wings can be deeply analyzed. After the analysis of force element theory, it is known that the force of the flow field of the dragonfly flapping wings is dominated by the volume-vorticity term. Therefore, by drawing the volume-vorticity distribution diagram in the force element theory, the change of the vortex structure and the distribution of the force contribution of the volume-vorticity can be clearly expressed when it flaps its wings. After drawing and analysis, it is known that the structural change of the leading edge vortex dominates the force of the flapping motion, which is an important factor for winged creatures to fly. After calculation and analysis, it is known that the fore and hind wings fly in-phase stroking, that is, when the fore and hind wings have a phase difference of zero period, it is similar to the flying effect of general birds flapping their wings, which can bring large lift and help reach the climbing needs during flight; The dragonflies fly as counter stroking, that is, when the fore and hind wings have a phase difference of 1/2 period, due to the movement of the fore and hind wings in opposite directions, a positive and a negative volume-vorticity force will be formed to cancel each other out, so that under various flight parameters, the average lift and drag, and the magnitude of the lift and drag during the period is smaller than in-phase stroking flight, but it can retain enough lift to support the weight of the body and the thrust required for flight, which helps the dragonfly fly stably.

並列關鍵字

dragonfly ； flapping wings ； phase difference ； force element theory ； unsteady flow field ； micro air vehicles

參考文獻

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