FTLE analysis of unsteady flow around pitching airfoils of high amplitude

The finite-time Lyapunov exponent (FTLE) method was used to analyze the unsteady flow around NACA 0012 airfoil pitching at high amplitude (±40°), with reduced frequencies of k = 0.2 − 0.6 and Rec = 12000. Phase-averaged particle image velocimetry (PIV) was used to obtain 14 windows of the two-component velocity field, that were then mirrored to represent the opposite half of the period, as the airfoil motion was symmetric. The fields of view were stitched together in post-processing to obtain a window that was approximately 2.37 c by 1.65 c. Due to the temporal and spatial resolution of the data set, both a primary and secondary LEV were resolved in the FTLE results. The vortex tracking via FTLE was compared with Eulerian tracking methods, and then with the calculation of LEV circulation. The interactions of the distinct vortices near the airfoil surface were also investigated using FTLE, to determine the behavior of the Lagrangian attracting and repelling ridges as vortices shed and/or merge.