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非常复杂的，这是典型的非定常流动，因为存在表征这种流动的许多参数和旋涡的动力学行为。本文采用数值计算和实验方法，对翼型绕流的非定常流动进行了研究。然而，大多数研究都集中在Re=10^6以上的高雷诺数区域。近年来，随着微机电系统技术的发展和微型飞行器、微型飞行机器人的发展，低雷诺数区非定常流动的研究越来越受到人们的重视。由于对水生动物、鸟类和昆虫的推进机理的研究具有很高的需求，因此该流场也引起了生物流体动力学的极大关注。然而，运动翼型背后的详细涡流结构与动力特性之间的关系， ...更多信息 本文利用PIV技术对低雷诺数区俯仰翼型和升沉翼型后的旋涡流场进行了详细的测量。并在水洞中用六轴传感器对它们进行了动态推力测量。研究结果不仅揭示了涡的流态、涡量分布和射流特性等细节涡结构，而且揭示了动推力特性与细节涡结构的关系，回流区由少量离散涡组成。从前缘脱落的离散涡尺度约为弦长的四分之一，与翼型外形无关。回流区长度与弦长之比决定了回流区中离散涡的数量。翼型外形对离散涡的动力学行为有一定的影响，而翼型外形对离散涡的脱落频率没有明显的影响。另外，离散涡的动力学行为对动升力的影响很大，在高无量纲尾缘速度和无量纲升沉速度下，产生推力的涡街明显。随着无量纲尾缘速度和无量纲升沉速度的增大，涡量增大，涡间距变窄。结果表明，强涡诱导的射流速度较高，作用在俯仰翼型和升沉翼型上的平均动推力随无量纲后缘速度和无量纲升沉速度的增大而增大。俯仰翼型和升沉翼型的动推力迟滞回线具有内凹和外凸的形状特征。升力翼型的动态推力特性与俯仰翼型的动态推力特性不同，俯仰翼型的推力效率迅速提高，最高可达Vp =0.7，最大推力效率为0.34。在V_p=0.5时，升力翼型的推力效率迅速提高，最大推力效率为0.20。在这两种翼型中，推力效率随着无量纲速度的增加而降低，因为不仅推力而且作用在俯仰翼型上的力矩和作用在起伏翼型上的升力迅速增加。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