Experimental investigation of the flow past passive vortex generators on an airfoil experiencing three-dimensional separation

The use of passive vortex generators (VGs) as a simple and effective way to delay or suppress separation on an airfoil optimized for wind turbine blades is examined experimentally. The profile experiences three-dimensional separation of the Stall Cell type. Pressure, Flow Visualization and Stereo Particle Image Velocimetry experiments are discussed for the case of triangular counter rotating VGs with common flow up. For a Reynolds number of 0.87×106, Stall Cell formation is delayed for 5°, and lift increases up to α=15°. In total, maximum lift increases by 44%, while drag increases by 0.002 at pre-stall angles of attack. At α=16° the flow bifurcates between separated and attached flow conditions. At α=10° the flow is examined in detail and an investigation on the turbulence characteristics is carried out by correlating Reynolds stresses production to time averaged flow gradients. Strong turbulent interaction is observed between the two vortices and the underlying flow up to 37.2 VG heights downstream of the VGs, while further downstream (up to 47.2 VG heights) diffusion governs the flow. A wandering motion of the VG vortices leads to increased v`v`¯ normal stress values between the two vortices.