Experimental study of effects of circular-cross-section riblets on the aerodynamic performance of Risø airfoil at transient flow regime

New drag reduction methods have received much attention due to the importance of drag reduction in airplanes and wind turbines. One of the ways for drag reduction is the use of riblets. We investigated the effects of riblets on the aerodynamic performance of the Risø airfoil quantitatively. By installing a load cell and using the one-sided force measurement method, the drag and lift coefficients of the Risø airfoil were measured in two modes: With and without riblets at three different arrangements. The shape of riblets is a circularcross- section and the ratio of riblets’ diameter to the airfoil chord is equal to 0.005. The tests were carried out in transient flow regime (Two Reynolds numbers of 2.02×105 and 1.4×105), and at attack angles from 0 to 20 degrees. The results indicate that the extent of the riblets effect on the aerodynamic performance of the airfoil depends on the angle of attack, Reynolds number, and arrangement of the riblets on the airfoil. The maximum drag reduction at the Reynolds numbers of 2.02×105 and 1.4×105 is about 29.7 % and 54 %, respectively, that occurs at an attack angle of 7 degrees for both two Reynolds numbers.