Exploratory Model Study of Tornado-Like Vortex Dynamics

S. J. Ying Dept. of Space Science and Applied Physics, The Catholic University of America, Washington, D. C.

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C. C. Chang Dept. of Space Science and Applied Physics, The Catholic University of America, Washington, D. C.

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Abstract

A laboratory model of the tornado-like vortex near the ground is developed and studied. The circulation is produced by a rotating cylindrical screen and the updraft is produced by an exhaust fan at the opening of the top hood. By means of kerosene smoke, the vortex core and a reverse flow zone were observed in the experiment. The profiles of velocity and pressure were measured at three different circulation strengths. The maximum inward radial velocity in the boundary layer is approximately proportional to the circulation strength. Outside the vortex core, the top hood and ground boundary layers, the flow is a potential vortex flow with a very small inward radial velocity. The vertical velocity distribution generally has a Gaussian profile except that it is slightly downward in the annular reverse flow region. The diameter of the reverse flow region is controlled by the opening size of the outlet on the top hood. The reverse flow region extends to the top of the ground boundary layer only when the circulation is strong enough. The maximum downward flow speed observed in the experiments was less than 30 cm sec−1. A minimum pressure occurs at 1.27 cm from the ground on the vortex axis and shows the complexity of the flow in the conjunction region of the vortex core and the ground boundary layer.

Abstract

A laboratory model of the tornado-like vortex near the ground is developed and studied. The circulation is produced by a rotating cylindrical screen and the updraft is produced by an exhaust fan at the opening of the top hood. By means of kerosene smoke, the vortex core and a reverse flow zone were observed in the experiment. The profiles of velocity and pressure were measured at three different circulation strengths. The maximum inward radial velocity in the boundary layer is approximately proportional to the circulation strength. Outside the vortex core, the top hood and ground boundary layers, the flow is a potential vortex flow with a very small inward radial velocity. The vertical velocity distribution generally has a Gaussian profile except that it is slightly downward in the annular reverse flow region. The diameter of the reverse flow region is controlled by the opening size of the outlet on the top hood. The reverse flow region extends to the top of the ground boundary layer only when the circulation is strong enough. The maximum downward flow speed observed in the experiments was less than 30 cm sec−1. A minimum pressure occurs at 1.27 cm from the ground on the vortex axis and shows the complexity of the flow in the conjunction region of the vortex core and the ground boundary layer.

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