Base Drag Reduction Caused by Riblets on a GAW(2) Airfoil

Abstract

AMONG various methods explored for turbulent drag reduction on aerodynamic surfaces, riblets have beer the most promising.' As much as 4-8% of viscous drag reduction has been reported for simple two-dimensional configurations Plastic sheets with symmetric v-grooves (manufactured by the 3M Co.) have been employed widely in research- Assessment of viscous drag reduction on two-dimensional airfoils. both at low and transonic speeds, has been reported as well.- Excellent reviews on the subject covering aspects of drag reduction and flow structure are contained in Refs. 1 and 7. 13; There have been very few attempts exploring the fuse of giblets in separated flows, either from the point of view of drag reduction or separation control-r'xB0; Recently. Krishnan et al.' showed that riblets actually increase the base drag (about 8.7 on a long axisymmetric body with a blunt base at low speeds: the base diameter was about four times the boundary -layer thickness ahead of the base corner-They used 3M riblet sheers and systematically studied the effect of h+` on base pressure. They also speculated that, while riblets caused an increase in the base drag for a large-scale separated flow (like on the axisymmetric blunt base'), the effect could be favorable on an airfoil with a blunt trading edge, which is a case of a small-scale separated flow. 13; The present investigation was undertaken specifically to assess the effect of 3M riblets on the base pressure of an airfoil with a blunt trailing edge. Experiments were made at low speeds on a 13.6% thick GAW(2) airfoil model, which has a trailing-edge thickness ratio of 0.5%. The results show very clearly that the base drag reduction of an engineering value can be achieved for the optimized riblet geometry.