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

众所周知，运动翼型周围的流场是一种典型的非定常流动，由于存在许多表征这种流动的参数和涡的动态行为，因此流场非常复杂。采用数值和实验方法对运动翼型的非定常流场进行了研究。然而，他们中的大多数集中在Re=10^6以上的高雷诺数区域。近年来，随着微机电系统以流动控制为目的的改进和微型飞行器和微型飞行机器人的发展，低雷诺数区域非定常流场的研究受到了人们的关注。由于对水生动物、鸟类和昆虫的推进机制有很高的了解需求，该流场在生物流体动力学中也引起了极大的关注。然而，对运动翼型后涡结构的详细研究以及作用在其上的动力特性与低雷诺数区域涡结构的关系还没有得到充分的阐明。在本研究中，作者用PIV测量方法对低雷诺数区域俯仰翼型和起伏翼型后的涡流进行了详细的测量。此外，作者还利用六轴传感器在水洞中进行了作用于它们的动态推力测量。研究结果不仅揭示了涡旋流型、涡度分布和射流特性等细部涡结构，还揭示了动力推力特性与细部涡流结构之间的关系。再环流区是由几个离散涡形成的。前缘离散涡落的尺度约为弦长的四分之一，且与翼型结构无关。再循环区与弦长之间的长度决定了离散涡的数量。离散涡的动力学特性与翼型构型有关，但离散涡的脱落频率与翼型构型无关。此外，离散涡的动力特性对动力升力有很大影响。在高无量纲尾缘速度和无量纲升沉速度下，产推力涡街明显形成。此外，随着无量纲尾缘速度和无量纲起伏速度的增大，不仅涡距变窄，而且涡量增大。结果表明，强涡度引起的射流速度较高。作用于俯仰翼型和升沉翼型的平均动态推力随无量纲尾缘速度和无量纲升沉速度的增大而增大。作用于俯仰翼型和升沉翼型的动态推力滞回线表现出折回和凸形特征。动推力作用于升沉型和俯仰型的动态特性是不同的。俯仰翼型的推力效率迅速提高到V_p=0.7，最大推力效率为0.34。升沉翼型的推力效率迅速提高到V_p=0.5，最大推力效率为0.20。在两种翼型中，推力效率都随着无量纲速度的增加而降低，这是因为作用在俯仰翼型上的力矩和作用在起伏翼型上的升力都迅速增加。少

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