Aerodynamic Characteristics of Ground Effect in Flapping Flight of Small Birds
Date Issued
2012
Date
2012
Author(s)
Tang, Jhen-Han
Abstract
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.
Subjects
small bird
ground effect
ram effect
wing-tip vortex
flapping
gliding
Type
thesis
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