Numerical investigations of tornado-like vortices using two- and three-dimensional simulations
Abstract
Two- and three-dimensional computer models, originally developed by Rotunno (1977, 1984) were extensively modified to continue exploring tornado vortex dynamics. These numerical models were designed to simulate a simpler tornado analog: the tornado-like vortex generated within a Tornado Vortex Chamber (TVC). The primary modifications were: (1) the addition of radial and vertical coordinate transformations to the two-dimensional models; (2) the incorporation of Laser Doppler Velocimeter (LDV) data for setting boundary conditions; and (3) updated pressure and streamfunction solvers for improved computational efficiency. In addition, removal of the flow-restricting “tube” along the central axis in the 3D model was investigated. The modified models were verified through comparisons with previous numerical modeling results and with data previously collected in the chamber using a LDV. The modeling of the later stages of vortex evolution including: vortex breakdown, “drowned vortex jump”, precessing vortex core, two-cell vortex, and multiple vortex phenomenon are discussed. The vortex configuration at low swirl ratios was found to be strongly dependent on the upper boundary condition applied to tangential velocity (v) and the chamber height to inflow depth (H/h) ratio. For a no-slip upper boundary condition on v, H/h < 6 produces an unsteady and complex vortex evolution. For H/h > 6, the same conditions generate a steady state vortex, or at least one that oscillates between two slightly different configurations. Examining the momentum equation terms in and around a vortex breakdown and a drowned vortex jump reveals that the flow is mostly inviscid. Three-dimensional results from a high swirl ratio vortex indicate possible flow along the axes of secondary vortices. This implies that secondary vortices may contain vortex breakdowns, and are capable of producing very high wind speeds for short time periods. Comparing axisymmetric two-cell flow to a multiple vortex flow shows that suction vortices have a significant impact on the mean flow: the radial inflow very near the surface increases, tangential speeds just above surface are stronger, and updrafts and downdrafts associated with the secondary vortices are stronger.
Degree
Ph.D.
Advisors
Snow, Purdue University.
Subject Area
Atmosphere
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