Relating surface pressure to Lagrangian wake topology around a circular cylinder in cross-flow

The locations of Lagrangian saddles, identified as non-parallel intersections of positive and negative-time finite-time Lyapunov exponent (FTLE) ridges, have been shown to indicate the timing of von Kármán vortex shedding in the wake of bluff bodies. The Lagrangian saddles are difficult to track in real-time however, since future flow field data is needed for the computation of the FTLE fields. In order to detect the topological changes in the wake without direct access to the FTLE field, the saddle dynamics are related to measurable surface quantities on a circular cylinder in cross-flow. The Lagrangian saddle found upstream of a forming and subsequently shedding vortex has been shown to accelerate away from the cylinder surface as the vortex sheds. In previous numerical results for a two-dimensional flow at Re = 150, this acceleration coincides with a minimum in the static pressure at a location slightly upstream of the mean separation location. In the current work, this result is compared with results from three-dimensional numerical data at Re = 400, as well as two-dimensional experimental data at Re = 19, 000, with similar findings for each case. This successful validation provides a strategy for locating sensitive regions on the cylinder surface where vortex shedding could be detected using simple pressure transducers.