Analysis of flows past airfoils at very low Reynolds numbers

Abstract

This article presents the special features of airfoil flows at very low Reynolds numbers, which are of interest for unmanned micro-air-vehicles. Airfoil flow solutions at low Reynolds numbers are obtained with an efficient numerical analysis based on a pseudo-time integration method using artificial compressibility for solving accurately the Navier—Stokes equations. The flow problem is solved in a rectangular computational domain obtained by a coordinate transformation from the physical flow domain around the airfoil at incidence. This method uses a second-order central differencing approach on a stretched staggered grid. A special decoupling procedure using the continuity equation reduces the problem to the solution of scalar tridiagonal systems of equations, which enhances substantially the computational efficiency of the method. The pressure distribution, lift, and drag coefficients are presented for several NACA airfoils at various incidences and low Reynolds numbers between 400 and 6000. Streamline contours for the flow with separations past several airfoils at low Reynolds numbers have also been generated and compared. For a better understanding of the complex flow separation phenomena in the viscous flows past airfoils at very low Reynolds numbers, the onset of separation and reattachment locations have been calculated, and a detailed study is presented on the influence of the Reynolds number, angle of attack, relative thickness and camber, and the maximum camber position along the chord.