VORTEX IDENTIFICATION AND PROPER ORTHOGONAL DECOMPOSITION OF RIGID FLAPPING WING

The present study is focused on vortex identification and proper orthogonal decomposition (POD) of the flow field generated by a rigid wing in main flapping motion. Experiments are conducted for a rigid rectangular wing of aspect ratio (AR) 1.5 at two different flapping frequencies f = 1.5 and 2 Hz with water as the fluid medium. The main flapping mechanism executes asymmetric lower-upper stroke of 1:3 ratio single degree of freedom motion. Two-dimensional particle image velocimetry (PIV) measurement technique has been used to generate the 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, have been used for vortex identification. Limited comparison has been made with a new omega method. These methods were found to be consistent in detecting swirling vortices or coherent structures (CS). Proper orthogonal decomposition was 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 was more effective in comparison with velocity field data for achieving a deeper understanding of the complex flow produced by the flapping wing.