低Reynolds数翼型绕流主动控制技术

在低Reynolds数条件下，翼型绕流的上表面边界层由于抗逆压梯度能力变差容易发生流动分离，从而形成长层流分离泡.分离泡通常是非定常的，会诱发边界层的转捩、再附并形成湍流边界层.这个过程会使翼型的气动性能急剧下降，并伴随着强非线性效应.转捩后形成的湍流边界层也会产生高摩擦阻力.针对这种现象，文章以NACA0012翼型为例，通过隐式大涡模拟研究了有效的主动控制方案.为了统一分离控制技术和湍流边界层减阻技术，研究了在平板或槽道湍流中取得较好控制效果的壁面垂向反向控制方案.首先利用隐式大涡模拟研究了低Reynolds数条件下NACA0012翼型绕流的流场特征.其次分析并验证了反向控制方案在分离区控制流场的可行性，发现反向控制在分离区的作用相当于基于流场信息的壁面抽吸控制，且控制具有实时性和高效性，控制抽吸了前缘的低能流体，使得翼型前缘附面层变薄，并增强了其抗逆压梯度的能力，较大程度提高了翼型的气动性能.最后在湍流边界层验证了其减阻控制效果，发现反向控制阻断了流向涡的法向输运，抑制了涡结构的发展，并减弱了猝发过程，使得湍流的高摩阻力得到了有效降低. 

Active Control of Low Reynolds Number Airfoil Flow by Implicit Large Eddy Simulation

At low Reynolds number, the boundary layer on the upper surface of the airfoil is easily separated from the leading edge due to its poor ability of adverse pressure gradient resistance. The separated shear layer may undergo transition to turbulence and flow reattachment and form a long separation bubble. The instability of this separation bubble affects the performance and the stability of the airfoil. Moreover, the detached turbulent layer may bring high skin friction. To cope with this problem, the flow characteristics of NACA0012 airfoil at low Reynolds number were studied by using implicit large eddy simulation. Based on the flow characteristics, the opposition control schemes were applied to the separation zone as well as the turbulent shear layer to improve the performance of the airfoil. Numerical results show that boundary layer becomes thinner and its ability to resistance the adverse pressure gradient is enhanced by using the opposition control scheme. Therefore, the separation zone is reduced effectively. At the turbulent zone, the normal transport of streamwise vortex is blocked and the development of the vortex structures is inhibited by the opposition control scheme, which effectively reduces the high friction of turbulence.