Numerical Study of Unsteady Flow Separation on Small Scale Wings Using Vortex Identification Methods

The goal of this work is to develop vortex identification methods for better understanding the unsteady separation using particle-based direct numerical simulation for the unsteady incompressible flow past small wings. It is shown that in flows with Reynolds number 1000, the first vortex identification method, vorticity contour, is used to capture the vortical region behind the surface in which the package of unsteady vortex bubbles is observed in different angles of attack. The second vortex identification method, Q-criterion, has shown the advantages in order to capture the vortical region and track the near-wall separation by recording the vortex strengths of leading edge vortex and trailing edge vortex at high angle of attacks. The time history of the strengths has shown a good agreement to the time history of drag coefficient. The third vortex identification method, Lagrangian coherent structure, has shown its strength to record the most stretching, attracting, and shearing material surfaces that form the skeletons of Lagrangian particle dynamics in the far-field wake region. Accordingly, the tracking of particles in high shear surfaces, which is very important to determine the roadmap of unsteadiness, is well captured.