On the onset of negative lift in a symmetric airfoil at very small angles of attack

The variation of the lift coefficient in a symmetric airfoil (NACA 0012 profile) at very small angles of attack is studied experimentally and numerically for a range of low chord-based Reynolds numbers, Re. Experimentally, the non-linearity around the zero angle of attack leading to a switch in sign on the lift was observed for a big enough aspect ratio at Re = 40 x 10(3). The existence of the negative lift for wing models with the biggest aspect ratio suggests that the three-dimensional effects are negligible. Therefore, two-dimensional simulations were performed to understand the cause of the negative lift. For the cases with the negative lift, the flow displays an interesting feature of pre-alignment with the chord upstream of the profile. Furthermore, it was found that the negative lift is directly related to the positive net circulation (anti-clockwise) around the airfoil.