Biomimetic Aerodynamics

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Flapping motion has been considered as a fascinating aerodynamic subject to many scientists and engineers because of its potential applicability to various propulsive devices or next-generation micro-aerial-vehicle. In order to uncover its curious unsteady behavior, many researchers have conducted experimental or computational studies on the unsteady aerodynamics of various flapping motions, such as periodic oscillating airfoils, insect's and/or bird's wing motions, and so on.



         Fig. 1 Hovering flight posture                Fig. 2 Flying insects            Fig. 3 Numerical study on insect flight


It is the pattern of vortical flows that plays an important role in generating unsteady aerodynamic forces and determining the efficiency of flapping motion. Thus, detailed analysis of vertical structure should be carried out to physically understand unsteady flapping aerodynamics. Moreover, it has been reported that conventional aerodynamics based on quasi-steady assumption could not be applied to insect's flapping motion.


Vortex in insect's flapping motion (or the "figure-of-eight" motion) yields favorable aerodynamic consequences. It can produce a sufficiently large amount of lift using dynamic stall phenomenon. Ellington et al. observed the LEV (Leading Edge Vortex) and wake capture using smoke visualization around both a real moth and a 3-D model called "flapper". They observed a strong vortex attached at leading edge of wing in downstroke motion. As another unsteady lift enhancement mechanism, a vortical pattern produced by the "clap-fling" motion was observed.


Although previous works could explain many interesting aspects of flapping motions, further study is necessary to understand unsteady flow fields of flapping motion under forward flight condition. Airfoil under sinusoidal flapping (or combined pitching and heaving) motion can not produce lift for hovering. On the other hand, unsteady mechanisms on insect's hovering need to be verified for more general flight condition, such as forward flight and rapid maneuvering. Rapid maneuvering, which is one of the most vivid characteristics in insect flight, has to be carefully examined to design highly maneuverable flapping MAVs.


Keeping these in mind, our laboratory is focused on the unsteady aerodynamic force (both lift and thrust) generation mechanism of two-dimensional insect's flapping motion under forward flight condition. Based on previous researches, two kinds of research strategies are followed. The first one is to extend to three-dimensional rigid wing simulation which could capture three-dimensional complex vortex structures. The second one is to consider the flexibility of the wing. In order to reflect the flexibility effect, two-dimensional FSI (Fluid-Structure Interaction) simulations are conducted.


Fig. 4 Numerical simulation of 2- and 3-D insects' flapping flight