Three-dimensional flow field measurements in the wake of a tethered sphere crossing the onset of vortex induced vibrations

The changing three-dimensional vortex shedding topology of a heavy, tethered sphere exposed to uniform flow crossing the onset induced vibrations (VIV) is reported together with simultaneously tracked positions. Transient upstream conditions were imposed by increasing reduced velocity from $U^{*} = 2.2$ 4.5 (‘lock-in’, mode I). Three-dimensional under these transient strongly resembled those at steady same $U^{*}$ . Changes in wake accompanying VIV indicated several well-defined stages associated instantaneous location deficit centroid and wake's symmetry plane's orientation. Before VIV, appearance vortices led lock-in streamwise oscillations prior transverse direction. Shortly after ( \approx 3.6$ ), preferential (wake plane aligned tether) was lost. Upon reaching 4.5$ , flow–structure interaction reorganised motion, resulting state some delay. At this stage, perpendicular tether pairs alternately shed, single hairpins exerted forcing on sphere. 7.2$ (mode II, flow), double shed per oscillation period maximum force coefficients that higher than While present Reynolds numbers $Re$ ) chosen low, different identified are also expected be relevant range.