Improving aerodynamic control strategies for low Reynolds number airfoils

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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