Numerical investigation into rotational augmentation with passive vortex generators on the NREL Phase VI blade

Abstract

Passive vortex generators (VGs) have been widely used on wind turbines to improve the aerodynamic power. Nevertheless, most studies of VGs are conducted on flat plates and airfoils rather than rotating blades. The effect of VGs on rotational blade flow therefore remains unclear. This paper presents a comparative study between the NREL S809 airfoil flow and NREL Phase VI blade flow, to highlight rotational augmentation with VGs. There are 36 pairs of rectangular VGs installed at about 20% c between 25% and 50% spans of the blade. Fully-resolved RANS simulations are used to identify the airfoil and blade flow characteristics with VGs. Although VGs greatly increase the maximum lift coefficient of airfoil by almost 60%, they cause unexpected decreases in aerodynamic forces on the inboard blade. This is mainly because VGs can reduce the radial flow and thus undermine rotational augmentation. Furthermore, increasing VG size amplifies the VG effect on radial flow and enlarges the separation bubble at 30% span from 43% c to 57% c in height. Positioning VGs in a skewed layout, however, effectively diminishes this negative VG effect and reduces the separation bubble height to 28% c. These findings indicate a huge difference between the airfoil flow and blade flow with VGs due to rotational effects. This work might deepen the understanding of rotational augmentation with VGs on a rotating blade.