Tomo-PIV measurements in the wake of a tethered sphere undergoing VIV

The 3D flow field in the wake of a tethered sphere ( D = 6 mm, m* = 7.77, L* = 4.1) undergoing vortex induced vibrations (VIV) in a water tunnel, was measured using tomographic particle image velocimetry (tomo-PIV). The flow field was investigated for four different reduced velocities ranging between 3.6 < U* < 13.2, corresponding to Reynolds numbers ranging between 382 < Re-D < 1, 392. At U* = 3.6, the sphere did not oscillate and shed vortices resembled a train of hairpin heads having a vertical plane of symmetry. At U* = 5.8, sphere oscillations were highly periodic and omega-shaped vortices having a horizontal plane of symmetry were alternately shed, and transformed into vortex rings further downstream while still connected to upstream structures by a thin vortex filament. At U* = 7.9 and 13.2, the sphere's transverse amplitude response was intermittent and vortices in the sphere's wake were characterized by longitudinal and vertical struts as well as arches, with seemingly little organization. (C) 2019 Elsevier Ltd. All rights reserved.