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Experimental Flow Analysis on Autorotation of Maple Samaras

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Many samaras or winged-seeds such as maple seeds make autorotational flight during their fall. Autorotation is the result of the delicate equilibrium between gravity, inertia and aerodynamic forces. The very low descent speed of the autorotating maple seed has been known to be attributed to the high lift generated by leading-edge vortex (LEV). The autorotation of maple seeds was observed to be very stable during its flight. Maple seeds were found to start a stable autorotation within 1-m distance after detaching from their parent plants, regardless of the initial attitudes. In this study, several prominent features of the autorotation flight of maple samaras were investigated experimentally. At first, synchrotron X-ray computed tomography was employed to obtain detailed information about the three-dimensional geometry of maple seeds such as morphology and surface roughness. Acer palmatum was used as test samples a prevailing maple species in Korea. These information can be valuably used to analyze the functions of the morphological features and to provide geometrical data for numerical simulation. Through free-fall tests, the effect of initial attitude of maple seeds on the autorotational flight was investigated. Flight distance, transient attitude and autorotation parameters at the terminal state were measured. The downward facing of the seed’s leading edge exhibits the fastest stabilization in free-fall. The stable autorotation of maple seeds were validated by a vertical wind tunnel experiment and the results were compared with the results of the free-fall test. In addition, a new wind turbine rotor model bio-inspired from maple seeds was fabricated. In order to analyze the interrelation between the autorotation parameters, the spinning rate was measured at various oncoming wind speeds as changing the pitch and coning angles of rotors. Furthermore, the wind turbine model bio-mimicking natural maple seeds has a strong potential to be used as a rotor of a small wind turbine.

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