運動學參數對鳥類拍撲翼之升力影響

Effects of motion kinematics on lift production of a flapping bird-wing

章聿珩(Yu-Hung Chang)

指導教授 ： 楊鏡堂

國立臺灣大學/工學院/機械工程學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.00261

摘要

本文研究運動學參數對鳥類拍撲翼升力之影響，以綠繡眼和胡錦為實驗目標，探討小型鳥類在非對稱懸停與爬升飛行時的運動學原理與飛行機制，可運用至仿生拍撲飛行機構之設計，以提升飛行操控特性。
本文紀錄歸納綠繡眼與胡錦在懸停飛行與爬升時的翼拍撲特徵運動軌跡，進行三維運動學參數分析，並利用粒子影像測速儀進行流場定性量測與分析。本文進一步將特徵運動學參數分析歸納結果，應用於仿鳥類拍撲機構之設計與動作模擬，包含鳥類的上下拍翅（flap）與翼面攻角改變（twist）兩個重要撲翅模式，套用生物實驗的分析數據，重建生物模型，深入探討各運動學參數對鳥類拍撲飛行之影響與重要性。
由於前人研究大都著重於生物的定常態平飛（steady level flight），鮮少探究操控性飛行（maneuvering flight）的力學特性，因此本文特別著眼於小型鳥類操控式飛行的運動學參數與空氣動力學分析。
綠繡眼與胡錦在懸停與爬升飛行時，皆可粗略地將其動作分成三個階段：第一個階段為翅膀從鳥身體後方高舉延伸的狀態開始往下拍，直到翅膀在身體兩側完全延展為止；第二個階段從翅膀在鳥身體兩側完全延展至翅膀拍至身體前方，直到下拍飛行階段結束；第三階段為完整的上拍飛行，翅膀從鳥身體前方開始往身體內側蜷縮，在幾乎完全收起後再將翅膀往鳥身體後上方伸展拕舉，回到第一階段的起始動作為止。
儘管非對稱懸停與爬升動作相似，但於第二個階段末期與第三階段初期皆更顯著動作差異。懸停時在第二階段末期、下拍階段快要結束前，在鳥腹部前方的翅膀會用力地碰觸拍擊（ventral clap）。爬升時在第三階段初期，會更一小段時間翅膀緊貼身體不動，形成減低阻力的垂直上升拕體（vertical bound）。
本文亦揭示此二種鳥類的生物流場與升力機制在懸停與爬升時的特色。綠繡眼與胡錦拍撲飛行時皆僅由下拍階段提供升力，此時翼前緣渦流（leading edge vortex）、翼尖渦流（wingtip vortex） 、以及翼後緣渦流（trailing edge vortex）會在翼拍撲動作完成時形成渦流環。本文利用渦環動量理論計算此渦流環的動量，以估算鳥拍撲飛行的瞬時升力。
上述研究顯示非對稱懸停時利用翅膀拍擊產生的高升力，可補足上拍無升力的缺失。為了維持空中懸停，綠繡眼與胡錦在下拍階段末期，雙翼會產生拍擊（ventral clap）動作。此動作瞬間在鳥的雙翼中間產生一股強烈的向下噴流，並提供鳥身體約2至2.5倍鳥體重的升力，可彌補先前翅膀攻角過大產生的1至1.5倍負向升力，並剩餘約1倍的鳥體重升力補償上拍無升力的缺失。
懸停時，綠繡眼翅膀出力幅度約為3倍的鳥體重，胡錦約為4.5倍鳥體重；爬升時，綠繡眼翅膀出力幅度約為3.2倍的鳥體重，胡錦約為3.5倍鳥體重。表示綠繡眼較適合機動性的懸停；胡錦較適合直線爬升飛行。綠繡眼的展弦比為1.73，胡錦的展弦比為1.94，完全符合展弦比較大者適合定常態平飛的理論。
綠繡眼與胡錦爬升飛行時雖未進行拍擊以產生瞬間強烈噴流供給升力，但翼面攻角變化範圍變小使下拍時幾乎無負向升力（negative lift）。加上拍翅速度增快，爬升時正向升力（positive lift）增加，故可使鳥軀幹上升。下拍結束後，鳥會更一陣子呈現翅膀緊貼身體、減低阻力僅靠慣性上升的垂直拕體動作（vertical bound），減少鳥在爬升飛行時的阻力並節省體力。以上結果解釋二種鳥類於懸停與爬升時的升力機制與空氣動力學性質之差異。
本研究利用仿鳥類拍翅飛行的機構重建綠繡眼飛行模型，探討翼拍撲（flap）和翼扭轉（twist）動作對懸停飛行的影響。實驗證明使用渦漩環理論估算鳥飛行時的瞬時升力方法準確性高達85.6%，且加入twist的拍撲飛行比只更flap高出7至21%的效率。twist是攻角的改變，可對應到昆蟲的wing rotation，過去的文獻多認為wing rotation適用於雷諾數在101至103的昆蟲飛行，本機構實驗證實對於雷諾數在105至106的小型鳥類飛行運動，wing rotation機制依然相當重要。
綜合以上，本文對鳥類拍翅飛行機制與運動學參數之研究成果，除深入探討不同展弦比的小型鳥類之操控性飛行因素，研究結果亦可使用於仿鳥類拍撲機構之設計，於控制機構在懸停、直線上飛的設計和流場控制層面更著卓越的幫助，並可應用於多自由度仿生拍撲飛行器之概念設計與研發方向。

關鍵字

操控式飛行 ； 翼前緣渦流 ； 翼後緣渦流 ； 翼拍擊 ； 垂直上升拋體 ； 仿鳥類拍撲機構 ； 渦漩環理論

並列摘要

This thesis investigates the influence of motion kinematics on lift production of a flapping bird-wing. Biomechanical and aerodynamic mechanisms underlying asymmetrical-hovering and ascending flights in Zosterops japonicas and Erythrura gouldiae were experimentally explored. The flight mechanisms and bio-wisdom revealed from our research on live birds were applicable to the design of biomimetic flapping aerial-vehicles, beneficially enhancing the flight maneuverability.
I recorded and analyzed the characteristic three-dimensional locomotor trajectory of flapping wings of Zosterops japonicas and Erythrura gouldiae during both hovering and ascending flights. The wake flow fields of flying birds were quantitatively visualized employing the digital particle image velocimetry (DPIV). A mechanical flapper mimicking bird wings was devised and constructed according to the biomechanical principles extracted from the experiments. This mechanical flapper primarily emulates two important wing motions - flapping and twisting, enabling a detailed examination of the significance as well as impacts of wing kinematics on lift production.
The steady level flight has been the subject of a great deal of studies on bird flight. There is, however, remarkably little research focusing on the maneuvering flight of birds. This is why this thesis has particularly attempted to clarify the role of wing kinematics in aerodynamics of maneuvering flight of birds.
The motion of Zosterops japonicas and Erythrura gouldiae during hovering and ascending flight can be roughly divided into three stages. At the first stage, the wings initially situated on the dorsal side fling until the wings are fully extended. For the second stage, the fully extended wings sweep forward and downward, completing the downstroke phase. At the third stage corresponding to the upstroke phase, the bird wings are initially retracted and subsequently extended dorsally, resuming a posture in preparation for the succeeding downstroke.
Although motion of hovering and ascending are similar, the bird wings execute a downstroke ventral-clap only in the hovering flight. It should be noted that clapping wings of Zosterops japonicas act like two plates hitting each other. Erythrura gouldiae otherwise executes a bow with two wings. A remarkable difference between hovering and ascending flights is that a ventral clap is not observed during the ascending flight.
Lift forces produced by the birds were evaluated employing a vortex-ring model. Results manifest that merely the downstroke produces the required lift. Moreover, during asymmetrical hovering, Zosterops japonicas has a better flight performance than Erythrura gouldiae. Erythrura gouldiae expends less energy during ascending flight than hovering flight.
During asymmetrical hovering, ventral-clap produces a strong downward jet compensating for the zero lift-production during the upstroke. Prior to the ventral-clap, the aerodynamic drag is approximately 1~1.5 folds of the bird weight due to a large angle of attack of the wing. Then ventral-clap subsequently produces a lift that is 2~2.5 folds of the bird weight, counteracting the drag and also providing a lift sufficiently large to maintain the hovering flight.
During hovering, the peak-to-peak lift force amplitude of Erythrura gouldiae is around 4.5 times of the bird weight, while the peak-to-peak lift force amplitude of Zosterops japonicas is 3 times of the bird weight. For ascending flight, the peak-to-peak lift force amplitude of Erythrura gouldiae is 3.5 times of the bird weight, while Zosterops japonicas is 3.2 times of the bird weight. Additionally, the aspect ratios associated with a Zosterops japonicas and a Erythrura gouldiae were approximately 1.73 and 1.94 respectively. These findings suggest that a bird wing of a high aspect ratio is suited to the steady level flight, whereas a bird wing of a small aspect ratio is suited to flight maneuvers.
Although there is no ventral-clap in the ascending flight for both bird species, the changes in angle of attack of a downstroking wing during ascending flight are typically smaller than that of the hovering flight. Accordingly, there is nearly zero production of negative lift. With a higher flapping frequency and less production of the negative lift, more net positive lift is produced during the downstroke for ascending birds. Furthermore, at the beginning of an upstroke, a vertical-bound is observed for both the Zosterops japonicas and the Erythrura gouldiae.
Experiments with a biomimetic mechanical flapper indicate that evaluation of the lift force employing the vortex-ring model renders a result of 85.6% accuracy. The aerodynamic influences of both flapping and twisting motions on the flight performance are also clearly addressed. It was found that the ‘wing-rotation’ mechanism is also effective for flapping flight at Reynolds numbers ranging from 105 to106.
To summarize, the impacts of motion kinematics on lift production of a flapping bird-wing are clarified. Finding of this thesis can be beneficially applied to the design of biomimetic flapping aerial vehicles with multiple degrees of freedom.

並列關鍵字

maneuvering flight ； vortex-ring model ； ventral clap ； vertical bound ； asymmetrical flight ； leading-edge vortex ； trailing-edge vortex

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