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

Abstract

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< ReD<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.