An analysis of the unsteady wake behind a circular cylinder using lagrangian coherent structures

The unsteady wake behind a circular cylinder was experimentally measured and com- pared with numerical results. The location and evolution of coherent structures, or vortices, in the flow were analyzed to facilitate the understanding of the complex vortex shedding physics in the near wake region. Two-dimensional phase averaged velocity data was collected using a 2D DPIV measurement system for multiple Reynolds numbers and cylinder diameters. The Eulerian Q-criterion, along with a Lagrangian coherent structures (LCS) analysis, was applied to the numerical and experimental data to determine the properties of the wake. An FTLE strain restriction was applied to the LCS to filter out ridges that identify regions of shear instead of hyperbolic separation. The strain restriction exposes the loss of hyperbolicity of the LCS, causing a break in the material line, as a new vortex begins to form behind the cylinder.