Improving aerodynamic control strategies for low Reynolds number airfoils

标准翼型是根据雷诺数、流动惯性和流体粘度之比设计的，以实现高速下的最佳空气动力学性能。例如，波音747在大约8000万的雷诺数下运行，而昆虫可能具有大约1000的雷诺数。在低雷诺数（低于一百万）下操作翼型在许多工程应用中是令人感兴趣的，包括低速无人驾驶飞行器、风力涡轮机和低速/高空飞行器;然而，翼型性能显著降低。对于安大略大部分地区的风力涡轮机安装来说，这是一个问题，因为风速足够高的地方有限，无法发电。由于吸力面上层流边界层的相互作用，翼型上的流动分离，即边界层从表面分离成尾流，在低雷诺数下特别普遍，吸力面上的流动压力沿流动方向增加（逆压梯度）。边界层中包含的动量通常无法承受由逆压梯度施加的力，这导致流动分离。使用周期性激励，即，在表面局部应用以减轻流动分离并恢复失速翼型的空气动力学性能的主动流动控制是一种多年来已经应用并取得不同程度成功的技术。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，我们的实验室一直在研究合成射流激励器（SJA）作为一种候选的控制方法。SJA有一个安装在腔体中的振动膜，腔体中有一个孔/槽，通向需要控制的表面。隔膜的变形导致工作流体被腔体交替地吸入和排出，从而向流动增加动量（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，尽管关系，

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