A parametric study of the effect of leading edge spherical tubercle amplitudes on the aerodynamic performance of a 2D wind turbine airfoil at low Reynolds numbers using computational fluid dynamics

Abstract

The majority of wind power is currently produced on high wind speed sites by large wind turbine, whereas small wind turbines often operate in light wind conditions. Small capacity wind turbines have not received the same engineering attention as their larger counterparts. This is partially due to a number of unique problems that small wind turbines experience. The most relevant are: low operating Reynolds number (Re<500,000), and poor performance at high angles of attack. Low and medium wind speed sites (Class II–IV) are more common than high wind speed sites, meaning there is a large source of energy not being taken advantage of. Several studies have suggested that flow control devices such as the spherical tubercle could be used to increase lift before stall and generate more power in such situations. The aim of this study is to determine the effect of tubercle amplitude on aerodynamic performance of an airfoil at low-Re numbers (Re=300,000&Re=400,000). Three amplitudes were considered in this study: A1=0.005c, A2=0.01c, and A3=0.03c. A detailed 2D simulation study is carried out, using FLUENT (a commercial CFD software) and the TransitionSSTk−ω turbulence model, to obtain aerodynamic coefficients and flow characteristics. Results indicate that small tubercles perform better overall than larger tubercles. The airfoil with the smallest tubercle outperforms the unmodified airfoil at both studied Reynolds numbers at angles of attack 0° – 4 °. Moreover, the airfoil with the largest tubercle outperformed all of the airfoils at an angle of attack of 0° and Re=300,000. The analysis of the aerodynamic coefficients indicates that the improvement of the aerodynamic performance of airfoils with tubercles is due to the reduction of the drag coefficient. Pressure, intermittency and wall shear stress contours suggest that the overall drag reduction is achieved through the decrease of friction drag. The decrease in friction drag is attributed to the thickening of the laminar boundary layer, caused by a more favorable pressure distribution around the airfoils with the aerodynamic improvements. Moreover, the drastic deterioration in aerodynamic performance at higher angles of attack is attributed to the turbulence generated by the tubercles. This study suggests that spherical tubercles could have a potential application in small wind turbines.