Analyzing aerodynamic forces of coherent structures on the NACA0012 airfoil at low to moderate angles of attack

Abstract

We investigated the contributions of coherent structures to the aerodynamic forces exerted on a National Advisory Committee for Aeronautics 0012 (NACA0012) airfoil at angles of attack (AoA) of 5°, 10° and 15°. Utilizing Proper Orthogonal Decomposition (POD) in conjunction with vorticity force analysis, we assessed their contributions to lift and drag forces at a chord-based Reynolds number of 50,000. At the smallest $AoA$, the primary source of time-varying aerodynamic forces arises from the detachment of the spanwise vortex at the reattachment point. In this case, the zeroth POD mode (mean flow) has the dominant contribution to the total force, with contributions from the first few non-zero POD modes being indistinct. As $AoA$ increases to 10°, the first POD mode corresponds to an apparent vortical structure located at the second half section of the chord. The clockwise rotation of this vortical structure leads to a strong re-entrant flow on the airfoil’s suction side, resulting in a positive drag contribution due to the intense shear near the boundary. The second POD mode in the case with $AoA=10^{\circ }$ corresponds to a trailing edge vortex (TEV), which counteracts the vortical structure of the first mode, leading to a decrease in drag force. The influence of TEV becomes more pronounced when $AoA$ increases further to 15°. However, it is found that the major contribution of the TEV to aerodynamic forces arises from its interaction with the leading edge vortex, rather than from its own resulting vorticity forces.