A Flow Visualization Study of Vortex Interaction With the Wake of a Sphere

Abstract

A study of the interaction between an initially columnar intake vortex (hereinafter referred to as the primary vortex) and vortex structures in the wake of a single sphere (hereinafter referred to as secondary vortices) is performed 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 visualized using laser-induced fluorescent dyes of two different colors, which are illuminated 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. The form of the sphere wake in the presence of the primary vortex and the effect of the induced velocity from the secondary vortices on the primary vortex are studied for different values of the vortex-sphere separation distance, the sphere diameter, and the primary vortex core radius. Weak secondary vortices are observed to induce formation of small-amplitude waves of varying core area on the primary vortex, eventually forming a turbulent sheath surrounding the primary vortex. Stronger secondary vortices have an increasingly strong effect on the primary vortex, including large-amplitude variation in core area, outward ejection of fluid from the primary vortex core, and breakdown of the primary vortex. [S0098-2202(00)01403-6]

External turbulence interaction with a columnar vortex

“…These loops are aligned primarily in the azimuthal direction, but they also have a significant component in the radial direction which arises from the difference in self-induced velocity between the head region and the legs of the vortex loops. A detailed experimental study of this process of entrainment of vortex loops by a columnar vortex is given by Sun & Marshall (2000), and additional information is given in Marshall & Beninati (2000) and Gossler & Marshall (2001). Contours of the different vorticity components are plotted in figure 4 in an x-y slice of the flow, where the nominal location of the lateral core surface is indicated by dashed lines.…”

Helicopter tail rotor orthogonal blade vortex interaction

“…The turbulence external to the vortex, formed from vorticity shed from the blade, gradually eats into the outer part of the columnar vortex core, and over time can have a significant impact on the vortex axial flow. This principle was demonstrated by Sun and Marshall [56] in an experiment in which a sphere was towed to within a specified position outside of the vortex core and then stopped. The subsequent interaction of the sphere wake with the columnar vortex was then examined.…”