Coherent structures in the flow field generated by rigid flapping wing in hovering flight mode

Abstract

Galilean invariant vortex identification methods and proper orthogonal decomposition (POD) are used to capture the coherent structures (CS) in the flow field generated by rigid wing in main flapping motion. Experiments are conducted for a square rigid wing at a flapping frequency f = 1.5 Hz. Tests are performed in water as a fluid medium in hover mode. The main flapping mechanism executes asymmetric lower-upper stroke of 1:3 ratio single degree of freedom motion. Two dimensional particle image velocimetry (PIV) technique is used to generate the phase locked velocity field at each discrete flapping angle by illuminating the mid chord plane of the wing. Three different Galilean invariant methods namely λ2 criterion, Q criterion and Δ criterion are used for vortex identification. These methods were found to be consistent in detecting swirling vortices or coherent structures (CS). Proper orthogonal decomposition is used to exhibit the most energetic modes of the flow field. The captured modes were identified to be in-connect with vortex identification methods. The combination of these tools were found to be more effective in comparison with velocity field data for achieving a deeper understanding of the complex flow produced by the flapping wing.