Low Reynolds number airfoil dynamics: three different flow patterns within a small range of angles of attack

The effect of angles of attack on the flow over of a NACA 65(1)-412 airfoil at low Reynolds number is investigated. Direct numerical simulations in two and three dimensions are performed using a discontinuous-Galerkin spectral element method. Through analysis of lift, drag, and pressure coefficients, average velocity field, and Lagrangian Coherent Structures determined by the Finite-Time Lyapunov Exponent, we show that the flow bifurcates several times in a small range of angles of attack. The flow is in a laminar separated state with transition to turbulence in the wake for angles smaller than 7°. Increasing the angle leads to laminar reattachment of the flow at 8° incidence. Then, for angles of attack larger than 8° the separation bubble bifurcates to a sizable bubble at the leading section of the airfoil, followed by transition to a fully turbulent flow. At an angle of attack of 8°, a high lift coefficient at low drag marks a preferable state in terms of aerodynamic performance of the airfoil.