Finite-time Lyapunov exponent and force partitioning method in the wake of a pitching airfoil

Unsteady flow around a pitching airfoil at Re= 1000 is investigated using both finite-time Lyapunov exponent (FTLE) and the force and moment partitioning method (FMPM). FTLE is used to analyze the kinematics of the flow by defining a scalar field based on the amount of Lagrangian stretching experienced by the fluid within the field. The FMPM quantifies the dynamical impact of flow kinematics by partitioning the loads induced on the airfoil by different physical mechanisms, including distinct vortex structures. The study uses simulation data around a sinusoidally pitching NACA0015 airfoil which exhibits dynamic stall vortex shedding. Maximizing ridges of the positive and negative-time FTLE fields have been shown to outline distinct regions of unsteady and vortex dominated flow fields, and their evolution correlates with important dynamic phenomena, such as flow separation, vortex shedding, and reattachment. Here, the FTLE ridges, saddles and scalar fields are compared with the FMPM influence field and the force/moment contributions from distinct vortex structures. Combining these kinematic and dynamic approaches further expands our understanding of the nonlinear relationship between flow field features and mechanical performance of immersed bodies is unsteady flows.