Vortex Shedding of an Airfoil at Low Reynolds Numbers

Characteristics of airfoil wake vortices in low Reynolds number flows are investigated. The focus of this investigation is the effect of Reynolds number and airfoil thickness on the shedding frequency of these coherent structures. Experiments were conducted on a NACA 0018 airfoil at an angle of attack of 10 for a range of chord Reynolds numbers from 30,000 to 200,000. The experimental conditions were selected to investigate both laminar boundary-layer separation without reattachment and separation-bubble formation as well as to enable a comparison with data from previous studies. The results show that vortex shedding occurs in the airfoil wake for both flow regimes investigated. The dimensionless wake vortex shedding frequency is shown to vary linearly with Reynolds number; however, the slope of the linear fit depends on the shear-layer behavior. When an increase in Reynolds number results in shear-layer reattachment on the upper surface of the airfoil, the wake shedding frequency increases significantly, and the vortical structures become less coherent. The present results are compared with those available for NACA 0012 and NACA 0025 profiles as well as selected bluff bodies. The comparison demonstrates a similar dependence of the dimensionless frequency on the Reynolds number. In the absence of shear-layer reattachment on the upper surface, airfoil wake vortex shedding resembles that of a bluff body; however, the results show a more pronounced effect of the Reynolds number on the Strouhal number for airfoils. Using a frequency scaling based on a characteristic wake length, the data for two different airfoil profiles are shown to collapse on to a universal Strouhal number of approximately 0.18.