Study on High Lift and Thrust Mechanisms of Unsteady Airfoil and Its Effective Application

点击翻译按钮获取中文摘要

It is well known that a flow field around a moving airfoil, which is a typical unsteady flow, is extremely complicated since there are a number of parameters and dynamic behaviors of vortices that characterize such a flow. A number of studies on unsteady flow around a moving airfoil have been carried out with numerical and experimental approaches. Most of them, however, focused high Reynolds number regions over Re=10^6. Recently, a few studies on unsteady flows in low Reynolds number regions have been attracting attentions since the Micro-Electro-Mechanical-Systems has been improved with the aim of flow control and development of Micro-Air-Vehicle and micro flight robot. This flow field has also attracted significant attentions in biohydrodynamics as there is a high need to understand the propulsion mechanisms of aquatic animals, birds and insects. However, the detailed vortex flow structure behind moving airfoils and the relationship between the characteristics of dynamic forces actin … More g on them and the vortex flow structure at low Reynolds number region have not been clarified sufficiently.In this study, the authors have measured the detailed vortex flow behind a pitching airfoil and a heaving airfoil, at low Reynolds number region by PIV measurement. Moreover, the authors have performed dynamic thrust measurement acting on them by a six-axes sensor in a water tunnel. The result clarified not only the detailed vortex structure, such as vortex flow pattern, vorticity distribution and jet characteristics, but also the relationship between the characteristics of dynamic thrust and detailed vortex flow structure.The re-circulation region was formed by a few discrete vortices. The scale of discrete vortex shed from the leading edge was about one fourth of the chord length and it did not depend on the airfoil configuration. The length of the re-circulation region to the chord length determined the number of discrete vortex consisting there. The dynamic behavior of discrete vortex depended on the airfoil configuration, however the vortex shedding frequency of the discrete vortices did not depend on the airfoil configuration. Moreover, the dynamic behavior of discrete vortex influenced much on the dynamic lift.At the high non-dimensional trailing edge velocity and the non-dimensional heaving velocity, the thrust producing vortex street is formed clearly. Moreover, it has been founded that not only the distance between vortices becomes narrow but also vorticity increases as the non-dimensional trailing edge velocity and the non-dimensional heaving velocity increase. As a result, the jet velocity induced by the strong vorticity turns out to be high.The averaged dynamic thrust acting on a pitching airfoil and a heaving airfoil increases as the non-dimensional trailing edge velocity and the non-dimensional heaving velocity increase. The hysteresis loops of dynamic thrust acting on a pitching airfoil and a heaving airfoil show reentrant and convexity shapes characteristics. The dynamic behavior of dynamic thrust acting on a heaving airfoil is different from that on a pitching airfoil.The thrust efficiency of a pitching airfoil increased up to V_p=0.7 rapidly and maximum thrust efficiency was 0.34. The thrust efficiency of a heaving airfoil increased up to V_p=0.5 rapidly and the maximum thrust efficiency was 0.20. In both airfoils, the thrust efficiency decreases with increase of the non-dimensional velocity because not only thrust but also moment acting on a pitching airfoil and lift acting on a heaving airfoil increases rapidly. Less