Improving aerodynamic control strategies for low Reynolds number airfoils

标准翼型是根据雷诺数、流动惯性比和流体粘度设计的高速气动性能最优的翼型。例如，一架波音747飞机的雷诺数约为8000万，而昆虫的雷诺数可能约为1000。低雷诺数(低于100万)的翼型在包括低速无人机、风力涡轮机和低速/高空飞行器在内的许多工程应用中都具有重要意义；然而，翼型的性能会显著降低。对于安大略省大部分地区的风力涡轮机安装来说，这是一个令人担忧的问题，因为只有有限的地点有足够高的风速来发电。翼型上的流动分离，即边界层从表面到尾迹的分离，在低雷诺数时特别普遍，这是由于吸力面上的层流边界层的相互作用，其中流动压力沿流动方向增加(逆压梯度)。包含在边界层中的动量往往无法承受不利压力梯度所施加的力，从而导致流动分离。使用周期性激振，即主动流动控制，在表面局部应用，以减轻流动分离和恢复失速翼型的气动性能，这是一种已应用了几年的技术，取得了不同程度的成功。因为这些装置与翼型表面是平的，所以不会引入几何阻力。自2005年以来，我们实验室一直在研究合成射流致动器(SJA)作为一种候选的控制方法。SJA有一个安装在空腔中的振动膜片，其中有一个通向需要控制的表面的孔口/槽。隔膜的变形导致工作流体被空腔交替地吸入和排出，从而增加了流动的动量(但不是质量)。这项技术面临的主要困难，特别是这里提出的方法是，尽管

Standard airfoil profiles are designed for optimal aerodynamic performance at high speed based on the Reynolds number, a ratio of flow inertia and fluid viscosity. For example, a Boeing 747 operates at a Reynolds number of approximately 80 million while an insect might have a Reynolds number of roughly 1000. Operating airfoils at low Reynolds number (below one million) is of interest in many engineering applications including low-speed unmanned aerial vehicles, wind turbines, and low-speed/high-altitude aircraft; however, airfoil performance is significantly reduced. For wind turbine installation in most of Ontario, this is a concern as there are limited locations with high enough wind speeds to produce power. Flow separation on airfoils, the detachment of the boundary layer from a surface into a wake, is particularly prevalent at low Reynolds numbers due to the interaction of the laminar boundary layer on the suction surface where flow pressure increases in the flow direction (an adverse pressure gradient). The momentum contained in the boundary layer is often unable to withstand the forces imposed by the adverse pressure gradient, which causes the flow to separate. The use of periodic excitation, i.e., active flow control, applied locally at the surface to mitigate flow separation and restore the aerodynamic performance of stalled airfoils is a technique that has been applied with varying degrees of success for several years. Because these devices are flat to the airfoil surface, no geometric drag is introduced. Since 2005, our lab has been studying the synthetic jet actuator (SJA) as a candidate control method. The SJA has a vibrating diaphragm mounted in a cavity with an orifice/slot leading to the surface where control is desired. Deformation of the diaphragm causes the working fluid to be alternately ingested and expelled by the cavity, thereby adding momentum (but not mass) to the flow. The main difficulties faced with this technology and, particularly the approach proposed here, is that despite the relat