Flow Dynamics of a Pitching Foil by Eulerian and Lagrangian Viewpoints

Abstract

The dynamic behavior of the flow past a two-dimensional pitching foil is simulated. Based on the Lagrangian coherent structures, it was observed that the leading-edge vortex can trigger a reverse flow from the lower surface, and this reverse flow is the source of the ensuing vortex around the middle of the upper surface. Then, the reverse flow directly drags the fluid flow near the lower surface to form the trailing-edge vortex. During the shedding of the leading-edge vortex at angle of attack of 18.8 deg, the leading-edge vortex is separated into two parts by a new Lagrangian coherent structure. The rear part will leave the foil, and the front part will stay and participate in the generation of the secondary vortex. The trajectories of the particle groups around this Lagrangian coherent structure highlight the evolution of the fluid flow inside the leading-edge vortex during the shedding process. The current study shows that the Lagrangian coherent structure approach together with the Eulerian method can help to have better understandings of the flow physics of a pitching foil during the dynamic stall and the evolution of the secondary vortex.