Effect of body shape on riblets performance

The effect of partial slip flow on airfoil performance at high Reynolds numbers is analyzed in this paper. The link between the physical mechanism of drag reduction attained by many devices and the slip length concept has been well assessed in the literature. A slip length model is therefore here adopted in large eddy simulations to quantify the effect of slip flow on airfoil performance. The possibility to adopt a slip flow boundary condition to simulate riblets on airfoil is verified. Their effectiveness in reducing friction drag in turbulent flow has been well assessed since the end of the last century. Both theory and experiments proved that the effect of riblets only depends on the local Reynolds number. However, some experiments showed an increased effectiveness of riblets in the presence of pressure gradient. This secondary effect is still being debated and a physical explanation has not been found. This paper has the aim to provide a contribution to the understanding of this phenomenon. Large eddy simulations of flows around airfoils are proposed with an extensive analysis of riblet performance, obtained by a proper slip flow boundary condition. It is shown that riblets reduce the boundary layer displacement thickness inducing small but significant modifications to the pressure distribution, in particular in the adverse pressure gradient region. The reduced thickening of the equivalent body is the reason for the reduced form drag.