Effect of viscosity on tornado-like flow driven by a spinning magnet

I. Grants

Institute of Physics, University of Latvia, 32 Miera str., LV-2169 Salaspils, Latvia

Abstract
Tornado-like flow is created by a cylindrical transversely magnetized permanent magnet rotating co-axially below a conducting liquid cylinder. Several geometries are considered numerically, with a focus on the low-Reynolds number regime. The effect of viscosity is discussed in terms of dimensional analysis. The increased viscosity decreases the maximum swirl just slightly as long as the flow remains dominated by inertia. The secondary downdraft slightly increases with the viscosity in this regime, reaching a maximum at the dimensionless electromagnetic torque value around 2×10⁴ in all considered geometries. The maximum is accompanied by a vortex breakdown flow. The downdraft velocity decreases rapidly with the viscosity after the maximum. The respective optimum viscosity is shown to increase proportionally to the setup size provided the skin-effect force is limited. Tables 1, Figs 6, Refs 8.