Characteristics of Tornado-Like Vortices as a Function of Swirl Ratio: A Laboratory Investigation

Abstract

The investigation of tornado vortex dynamics by means of a laboratory simulation is described. Based on observations from nature and an examination of the Navier-Stokes equations, a laboratory simulator of the Ward type has been constructed. This simulator generates various vortex configurations as a function of swirl ratio, radial Reynolds number and aspect ratio. Configurations which are described are 1) a single laminar vortex; 2) a single vortex with breakdown bubble separating the upper turbulent region from the lower laminar region; 3) a fully developed turbulent core, where the breakdown bubble penetrates to the bottom of the experimental chamber; 4) vortex transition to two intertwined helical vortices; and 5) examples of higher order multiple-vortex configurations that form in the core region. Hot-film anemometry measurements of the magnitude of the velocity vector and inflow (swirl) angle have been obtained in a sequence of flows characterized by progressively increasing values of swirl ratio. These data include measurements in both the quasi-irrotational outer flow and the more complex core region. Due to the similarity that exists between the model and tornadoes, these observations provide insight into the flow fields likely to be encountered in real events. Particularly significant findings include the mapping of the transition points at which the flow converts from a single to a double helical vortex configuration, and from a double to a triple pattern, as a function of system parameters. Additionally, the velocity measurements show the development of a cylindrical shear zone at the outer edge of the core, which, through inertial instability, may lead to the multiple-vortex phenomenon.