Effect of riblets on turbulence in the wake of an airfoil

Measurements were made in the near and intermediate wake regions of a NACA 0012 airfoil with three different riblet sizes at a freestream Reynolds number of 2.5 x 105. The riblets tested were a symmetric v-groove type of heights 0.0229, 0.076, and 0.152 mm, respectively. To ensure turbulent flow on the airfoil, a transition strip was applied at 10% of the chord. Wake velocity profiles and turbulence parameters were measured with a temperature compensated hot-film x-probe. Riblet effectiveness was indicated by marked decrease in turbulence levels in the wake. Integration of the wake mean velocity profiles indicated a net reduction in drag of up to 13.3%, which was possibly caused by the combined effect of decreased turbulent shear stress and lower momentum exchange near the surface due to a viscosity-dominated region in the riblet valleys.