A laboratory investigation of forced vortex motion

Abstract

Vortex motion was generated in a cylinder of 30-in. diameter and 8-in. depth. The cylinder, with a transparent lateral boundary and a glass window in the top, was equipped with a box attached to its base. Chemicals in this box supplied smoke through small holes for visual and photographic tracing of the vortex motion. The vortex motion was induced by either a vacuum motor or a heat source applied to a 4-in. pipe inserted in the top center of the cylinder. The otherwise solid lateral boundary of the cylinder was broken with two diametrically-opposite vents designed to impart cyclonic vorticity to the incoming air. The converging air flow of the vortex motion was controlled by regulating the diameter of the exit pipe. Pressure measurements were made with manometer tubes inserted in the base of the cylinder, and a Flow Corporation HWB2 hot-wire anemometer was used to measure the magnitudes of the velocity components at various levels in the cylinder. The direction of the velocity vectors had to be determined by application of a chemical smoke tracer and photographic analysis. Theoretical velocity and pressure distributions also were computed for a steady-state, non-rotating, frictionless system under the assumption of incompressibility. These were compared with the observed distributions. Both the theoretical and observed distributions were found to resemble that of a Rankine combined vortex in which a central core of quasi-solid rotational motion is surrounded by irrotational potential flow. In general, it was found that as the diameter of the exhaust pipe was decreased the diameter of the smoke column decreased, but flow speeds and the horizontal pressure gradient increased. Good dynamic similarity was found to exist between Scottsbluff, Nebraska tornado of 1955 and the experimental vortex on the basis of the Froude and Euler numbers. A discussion is given suggesting under what circumstances conditions in the atmosphere might duplicate the laboratory situation and this be conducive to tornado formation.