Optimization by CFD analyses of riblet distribution over a transonic civil aircraft configuration

The effectiveness of the riblets, one of the most interesting drag-reduction device, is discussed in this paper. Numerical simulations by the Reynolds Averaged Navier-Stokes equations with the riblets properly taken into account are presented. It is proposed to model riblets as a singular roughness problem by modifying the classical Wilcox boundary condition for rough walls. The boundary condition is able to predict the flow features in the low roughness range (transitional roughness) where riblets operate. First, the results for the incompressible flow around a flat plate and for airfoils at subsonic and transonic speed are compared with the available experimental data. Then, a complex configuration is analyzed and the overall effect of riblets on the aerodynamic coefficients evaluated. Calculations of a complete aircraft configuration at transonic conditions show how a proper optimized choice of the riblet height can significantly improve the drag reduction.