微型飞行器低雷诺数空气动力学

微型飞行器(MAVs)设计绝不是常规飞行器在尺度上的简单缩小,面临许多技术难题.其中微型飞行器低雷诺数空气动力学是其最为根本的技术瓶颈之一,也是当前受到广泛关注的热点之一.本文紧密结合微型飞行器技术,对这一领域中所面临的低雷诺数空气动力学问题和近两年来该方向国内一些新的进展进行了较为详细的介绍.按照MAVs飞行方式和结构特性进行分类,简单介绍微型飞行器研究中的低$Re$数空气动力学问题.首先介绍了二维和三维固定翼低雷诺数空气动力学问题:包括层流分离泡,翼型升力系数小攻角非线性效应,静态迟滞效应,以及低$Re$数小展弦比机翼气动特性.第2,介绍了拍动翼低雷诺数空气动力学方面的研究工作.包括前人提出的昆虫低$Re$数下获得高升力的多种非定常拍动翼飞行机制:Wagner效应、Weis-Fogh效应(clap-and-fling)、延迟失速效应(delayedstall)、Kramer效应(rotational forces)、尾迹捕获效应(wakecapture)、附加质量效应(addedmass)等.以及国内学者近几年在拍动翼方面取得的一些研究成果.第3,介绍了柔性翼低雷诺数气动问题.研究表明柔性翼对于固定翼微型飞行器提高抗阵风能力,拍动翼微型飞行器产生足够的升力和推力.最后简单介绍了可变形翼(morphingwing)微型飞行器方面的一些研究工作,指出微型飞行器技术可以通过采用可变形翼设计,突破众多的技术瓶颈.另一方面,可变形翼概念可以通过在低成本,低速的MAVs上进行飞行试验,获得非常好的验证平台. 

LOW REYNOLDS NUMBER AERODYNAMICS OF MICRO AIR VEHICLES

The micro air vehicles(MAVs)are not simple scale-downs of common aircraft,but face many technical challenges.One of the most serious bottlenecks is low Reynolds number aerodynamics,which is also one of the most attractive research fields.This paper discusses the aerodynamic problems realted with the MAVs at low Reynolds numbers,and the latest progresses in China in this field.These problems are classified according to the flight style and the structure characteristics of MAVs in this paper.Firstly,the low Reynolds number aerodynamic problems of 2-D and 3-D rigid wing MAVs are described,including the laminar separation bubbles,nonlinear effects of lift coefficient at small angle of attack,the effects of static hystereses at middle angle of attack,and the aerodynamic characteristics of the low-aspect-ratio wing.Secondly,the flapping wing low Reynolds number aerodynamic problems,which involve many unsteady mechanisms of low Reynolds number flapping flight for the insects,such as the Weis-Fogh (clap-and-fling),the delayed stall,the Kramer(rotational forces),the wake capture,the added mass,where some studies in China in recent years are also included.Thirdly,the low Reynolds number aerodynamics of the flexible wing.It is shown that the flexible wing could effectively improve the ability of resisting gust for the rigid wing MAVs,and have significant effects for producing enough lift and thrust for the flapping wing MAVs.Finally,some studies on the morphing wing MAVs are briefly discussed.It is indicated that many bottlenecks of MAVs design could be broken through the morphing wing.On the other hand,the concept of morphing wing technology could be tested through flight experiments on this low speed and cheap air vehicles.