A flow visualization study of vortex interaction with the wake of a sphere

A study of the interaction between an initially columnar intake vortex ther einafter referred to as the primary vortex) and vortex structures in the wa ke of a single sphere (hereinafter referred to as secondary vortices) is pe rformed using a series of flow visualization experiments, in which a sphere is towed toward a stationary vortex with ambient axial flow and gradually decelerated to rest at a specified distance from the vortex core axis. Both the primary vortex core and the boundary layer of the sphere are visualize d using laser-induced fluorescent dyes of two different colors, which are i lluminated either with a laser sheet or a laser volume. The strength of the primary vortex is measured using particle-image velocimetry, which is also used to measure the strength of the secondary vortices in select cases. Th e form of the sphere wake in the presence of the primary vortex and the eff ect of the induced velocity from the secondary vortices on the primary vort ex are studied for different values of the vortex-sphere separation distanc e, the sphere diameter, and the primary vortex core radius. Weak secondary vortices are observed to induce formation of small-amplitude waves of varyi ng core area on the primary vortex, eventually forming a turbulent sheath s urrounding the primary vortex. Stronger secondary vortices have an increasi ngly strong effect on the primary vortex, including large-amplitude variati on in core area, outward ejection of fluid from the primary vortex core, an d breakdown of the primary vortex. [S0098-2202(00)01403-6].