FTLE and Surface-Pressure Signature of Dynamic Flow Reattachment During Delta-Wing Axial Acceleration

An experimental investigation was conducted on the aerodynamics of an NACA0012 airfoil wing with triangular planform geometry undergoing steady and axial accelerations at Re=310(5) as a model of an unmanned combat air vehicles encountering unsteady environments at pre- and poststall angles of attack (Marzanek, M., and Rival, D., "Separation Mechanics of Non-Slender Delta Wings During Streamwise Gusts," Journal of Fluids and Structures, Vol. 90, Oct. 2019, pp. 286-296.). Ensemble-averaged flowfields, forces, moments, and surface-pressure distributions were measured in the Optical Towing Tank for Energetics Research facility at Queen's University. To characterize the evolution of the flowfield coherent structures around a wing under axial gusts or accelerations, a Lagrangian flowfield analysis including the finite-time Lyapunov exponent (FTLE) was conducted. Results indicate that axial acceleration can induce a dynamic flow reattachment at the poststall angle of attack, and clear FTLE ridges extend from the wing surface and travel down the chord as reattachment progresses. A distinct signature in the surface-pressure distribution tracks closely with the location where the FTLE ridges meet the wing. The timing of how this surface-pressure distribution moves aft is shown to correlate with the relatively large-scale fluctuation in the pitching moment. The current work reveals the correlation between FTLE and surface pressure, which further hints at a potential approach of dynamic estimation by using Lagrangian coherent structures.