Characterization of wake vortices behind tethered spheres based on curvature and torsion

The flow field in the wake of tethered spheres undergoing vortex-induced-vibrations is complex and depends strongly on the reduced velocity, amongst others. Here, we propose a methodology borrowed from the analysis of porous media to characterize the vortex structure measured in the wake of heavy, tethered spheres. The measurements were performed using tomographic particle image velocimetry. Our results indicate that in the so-called lock-in region where vortex shedding frequency and structural frequency are synchronized, shed vortices can be classified into three categories: open and enclosed hairpins, and ring vortices. Beyond the lock-in region, distinct, pronounced vortices are no longer shed. Instead, complex structures comprising the three categories emerge. The here proposed methodology uses skeletonization and applies shape decomposition of the vortical structures from which a database of the curvature and torsion statistics of each of the vortex types can be created. Based on this the vortical strcutures in the wake of the tethered spheres are characterized. We show here that this methodology can be used to statistically analyze complex turbulent flows in the wakes of bluff bodies and we believe it may be extended to other turbulent flows.