Measurement of the Velocity Field in a Simulated Tornado-Like Vortex Using a Three-Dimensional Velocity Probe

Abstract

A three-dimensional velocity probe, incorporating hot-film anemometry, is developed to study the mean flow field of a simulated turbulent tornado-like vortex interacting with a smooth, flat boundary. The flow characteristics are essentially controlled by a dimensionless parameter ϵc and a Reynolds number Ret. The parameter is the ratio of sink strength to the product of the free-stream circulation and the vortex core radius. Flow fields for two different ϵc (0.022 and 0.51) are studied, with Ret>105 in both cases. It is found that. 1) the tangential velocity profiles at more than three core radii from the axis are similar to those of a turbulent flow over a flat plate; 2) the radial velocity close to the boundary is directed inward due to a positive radial pressure gradient, while it changes direction and approaches a constant at a height of two core radii from the boundary; 3) the vertical velocity depends strongly on ϵc, and is directed downward near the vortex axis in case 1 and upward in case 2, the magnitude of the vertical velocity becoming quite small far from the axis in both cases; 4) the streamline pattern in the meridian plane has a two-celled structure for case 1 and a one-celled structure for case 2; 5) as a result of radial and axial flow, the circulation distribution is appreciably different from that of a potential vortex due to the stretching of the vortex tube; 6) zones of maximum wind speed form an annular region near the ground boundary; and 7) the maximum pressure drop, occurring at the axis, exceeds twice the core dynamic head for case 1 and 15 times for case 2 due to higher sink strength. Velocity data obtained from a real tornado are compared with the present results. Good agreement was found with case 2.