Slip length based boundary condition for modeling drag reduction devices

Riblets are one of the most interesting passive drag reduction techniques. They essentially consist of streamwise grooved surfaces. Despite interest in these devices, the literature does not offer numerical simulations of riblets in the presence of pressure gradient or more generally around complex aeronautical configurations because, due to their scale length (microns), direct simulations are unfeasible. In the present paper an accurate and efficient method for the numerical simulation of riblets around complex aeronautical configurations is proposed. Riblets are modeled by a proper wall boundary condition based on the slip length concept. Slip length concept has been used by many authors in different fields; in the present paper a relation between the slip length and riblet height valid also in nonlinear range of drag reduction curve is proposed for the first time. This new boundary condition has been implemented in a standard code solving the Reynolds averaged Navier–Stokes equations. Validation tests in subsonic and transonic flows are performed comparing the results with the available experimental data. The analysis provides an interesting insight into riblets performance in the presence of the pressure gradient. Drag reduction improvement evidenced in some experiments is still debated because it seems to contradict the fact that riblet effect is confined very near to the surface (i.e., it is a local effect) and an explanation was never proposed until now. The present analysis provides a first contribution to understand why riblet performance can improve in the case of airfoil flow.