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By adopting a proper slip length boundary condition in Large Eddy Simulations of airfoil flows at high Reynolds numbers it can be shown that riblets reduce both friction and form drag. We show that riblets induce small but significant modifications of the pressure distribution which tends towards inviscid with improved pressure recovery in the airfoil aft. A quantitative analysis using a classical matched asymptotic expansion reveals that riblets reduce the boundary layer displacement thickness. Reduced thickening of the equivalent body introduced by the boundary layer reduces form drag leading to the unexpected improvement in pressure gradient flows measured in various experiments.
Mele et al. (Mon,) studied this question.