Detection and tracking of vortex phenomena using Lagrangian coherent structures

The formation and shedding of vortices in two vortex-dominated flows around an actuated flat plate are studied to develop a better method of identifying and tracking coherent structures in unsteady flows. The work automatically processes data from the 2D simulation of a flat plate undergoing a 45∘ pitch-up maneuver, and from experimental particle image velocimetry data in the wake of a continuously pitching trapezoidal panel. The Eulerian Γ₁, Γ₂, and Q functions, as well as the Lagrangian finite-time Lyapunov exponent are applied to identify both the centers and boundaries of the vortices. The multiple vortices forming and shedding from the plates are visualized well by these techniques. Tracking of identifiable features, such as the Lagrangian saddle points, is shown to have potential to identify the timing and location of vortex formation, shedding, and destruction more precisely than by only studying the vortex cores as identified by the Eulerian techniques.