SPIV study of passive flow control on a WT airfoil

Stereoscopic Particle Image Velocimetry (SPIV) measurements investigating the effect of vortex generators (VGs) on the flow near stall have been carried out in the purpose built LM Glasfiber wind tunnel on a DU 91-W2-250 profile. Measurements have been conducted at Re=0.9*10^6, corresponding to free stream velocity U=15 m/s. The objective was to investigate the flow structures induced by and separation controlling behavior of vortex generators on the airfoil. The experimental results show strong separation of the uncontrolled flow whereas an intermittent behavior appears for the controlled flow, where the appearance of vortex structures alternate with a slightly separated turbulent boundary layer. However, the controlled case is yielding less backflow than the uncontrolled one: On average, the controlled flow leaves a nicely attached flow as opposed to the incident one. For the controlled flow, the observed vortex structures generated by the devices are similar to those found in studies at lower Reynolds numbers. Further, mixing close to the wall, transferring high momentum fluid into the near wall region, is seen. The hypothesis of intermittent fluctuations in the boundary layer is further supported by a Snapshot Proper Orthogonal Decomposition (POD) analysis. This analysis also reveals some of the dynamics of the induced vortices, such as pulsations of axial velocity across the vortex cores and oscillations of the vortices primarily in the spanwise direction.