Aerodynamic characteristics of flapping airfoils at low Reynolds number

Unsteady flows around flapping airfoils are simulated at low Reynolds number, by solving incompressible Navier-Stokes based on pseudo compressibility method. The effect of maximum thickness variations is investigated using uncambered NACA four-digit airfoils range from 4% to 12% thickness. For pure pitching motion, the results show that average drag coefficient decreases as airfoil thickness is reduced. But for pure plunging motion, it was found that the thickness variations have a different effect on aerodynamic performance compared with that of pitching motion. Average drag coefficient increases as airfoil thickness is reduced. An analysis of periodic flow streamlines shows that it is probably due to the effect of leading edge vortex. As airfoil thickness is increased, the average pressure drag, affected by leading edge vortex is reduced even to be negative, and meanwhile friction has little increase. For Reynolds-number, as it increases, average drag coefficient is reduced slowly. The research work of this paper is expected to be useful for the design of flapping micro air vehicles.