鳥類拍撲翼飛行對翼地效應的影響

Abstract

本研究以三維數值模擬方法研究小型鳥類在近地航行時因為翼地效應(ground effect)的影響造成升力增加、阻力減少的現象。以定量與定性的方式探討物理模型在不同距地高度下，地面阻斷翼尖渦流(wingtip-vortices)結構現象的差異性，並分析翅膀翼面下方與地面間的流體因翼尖渦流以及氣墊效應(ram effect)的干擾所產生流動特徵的變化加以討論。前人之研究大多以雙翅物理模型以及靜態滑翔模擬為研究主軸，忽略了身體以及動態拍撲對於流場帶來的變化，本研究探討這些被忽略的因素對於翼地效應所產生的影響。靜態滑翔之模擬結果顯示：翼面下方的流體會因為翼尖渦流導致下洗速度(downwash)產生，靠近雙翼中心的流體較不易因翼尖渦流而改變流動方向。在僅有雙翼的情況下，當翅膀距地低於一倍的半翼展長(semi-span)時，越靠近地面翼地效應現象將會越顯著，特別是在距地0.37倍半翼展長時，阻力減少12%。然而對於完整鳥類模型翼地效應現象卻不明顯，原因是身體與地面間吸引效應(suction effect)的產生。 在鳥類動態拍撲模擬之子題，耦合三種拍撲模式，分別為拍動(flapping)、摺疊(folding)以及扭轉(twisting)。研究結果顯示:鳥類拍撲一週期僅有下拍過程產生升力，上拍過程對於升力並無貢獻。對於滑翔飛行型態，翼尖渦流與氣墊效應皆為翼地效應的主導者。然而對於動態拍撲飛行，由於拍撲對於流體速度的影響遠大於翼尖渦流所造成的效應，致使翼尖渦流之影響減弱而氣墊效應取得流場流動特徵之主導權。本研究亦發現由於氣墊效應主導整個流場的變化，導致鳥類近地拍撲飛行相對於遠地拍撲飛行，其平均升力約增加47%且平均阻力約減少20%。本研究所揭示之鳥類翅膀與身體間的影響及動態拍撲的分析，將可啟發鳥類相關的模擬與實驗研究者不同的思維。

An investigation with computational fluid dynamics of the ground effect on a small bird revealed quantitatively the obstruction of the vortex expansion resulting from the presence of the ground at varied distance. Preceding authors focused mainly on the simulation of static bird’s wings, generally neglecting the bird’s body and maneuverable wings; we discuss specifically the distinction of the aerodynamic effect between cases with and without the presence of the bird’s body. The results of static simulation show that, considering only two wings, for a distance between the wing model and the ground smaller than a semi-span, the smaller is the ground clearance, the more significant is the ground effect. At clearance 0.37 times a semi-span, the drag is decreased 12%. The ground effect for an intact bird model composed of both wings and body is less effective than that for a simplified model with body omitted, because a suction was observed on the lower surface of the intact bird’s trunk at clearance 0.37 times a semi-span; for this reason the intact bird model benefits less from the ground effect than the model with body excluded, but increased lift and decreased drag remain observable. In our research we treat the dynamic simulation with maneuverable wings coupling of the flapping, folding, and twisting motion. The results of dynamic simulation show that: There is almost no lift generation in upstroke process; lift is generated in down stroke process. We unexpectedly found that in the model with flapping wings the ram effect dominate the fluid field rather than the wingtip vortex. It is because that the acceleration of the fluid caused by flapping is much bigger than the wingtip vortex does, resulting the effect of the obstruction of the vortex to become weaker. We also found at β = 1.026 the average of lift is increased 47%, and the drag is decreased 20%. The present study provides an insight into experimental and computational research dealing with ground effect of a flapping bird and also reveals the importance of the presence of bird’s body in computational or experimental models.