Detecting Vortex Formation and Shedding in Cylinder Wakes Using Lagrangian Coherent Structures

The wake behind a circular cylinder is studied to investigate the complex vortex shedding physics in the near-wake region. Both the Q criterion and a Lagrangian coherent structure analysis are applied to flowfields acquired from a numerical simulation, as well as from experimental particle image velocimetry to determine the properties of the wake. A rate-of-strain filter is applied to the finite-time Lyapunov exponent field to filter out ridges corresponding to local shear, and yields ridges along which fluid trajectories separate hyperbolically. This strain filter reveals a sudden loss of hyperbolicity along a Lagrangian coherent structure as a new vortex begins to form. The Lagrangian coherent structures are also shown to identify and track topological Lagrangian saddle points in the cylinder near wake. This information characterizes the behavior of the vortices as they form, shed, and convect downstream. In particular, a Lagrangian saddle point is observed to remain attached to the cylinder surface until the vortex separates, and then consequently accelerates downstream with a similar track in both numerical and experimental results. The present approach provides a novel criterion for the identification of vortex shedding.