Vortex breakdown in the polar region between rotating spheres

Abstract

The flow field in the polar region between a rotating and stationary, co- or counter-rotating sphere was studied numerically by a steady-state axisymmetric finite-element code and experimentally by flow visualization using a photochromic dye excited by a UV laser. The experimental results show that at a critical Reynolds number a steady axisymmetric vortex breakdown bubble forms at the polar axis, as predicted by the computation. At higher values of the Reynolds number, and depending on the gap, the eccentricity of the spheres along the axis of rotation, and their rotation ratio, further unsteady structures are observed experimentally, viz., an axially oscillating bubble, or a three-dimensional irregular structure.