The formation and intensification of supercritical tornado-like vortices - a laboratory study

Previous laboratory measurements on tornado-like vortices have been significantly compromised by the spatial wandering of the vortex during the measurement process. Innovative techniques developed during this study have greatly improved the situation, allowing for the acquisition of quasi-instantaneous velocity profiles throughout the core region for varying degrees of swirl. This paper focuses on the structure of supercritical (i.e. one-cell or "suction") vortices, and examines them over the entire range of flow conditions in which they exist, from their earliest development and intensification to their subsequent evolution to two-celled structures.

A single hot-film sensor, attached to a scanning arm and sampling at a high rate, obtained a velocity profile during each sweep through the vortex core. For each value of swirl the measurements were repeated at several heights, ranging upwards from about 1mm above the surface. The sensor data was processed using LabVIEW software. This procedure was repeated for each specific swirl ratio; first over an aerodynamically smooth surface and later over surfaces having different degrees of roughness. A model was adopted which enabled resolution of the single sensor data into the respective velocity components, as well as estimates of core radius and vertical vorticity.

Notable results from this study include the following: supercritical vortices exist over a range of swirl ratios ~0.15-0.40. At the lowest swirl the vortex develops out of a region of separated swirling flow, and appears only sporadically until there is sufficient swirl for it to become firmly established in a region of favorable pressure gradient. The velocity profile is jet-like, with a vertical velocity maximum on-axis. Under high-swirl conditions, where the vortex breakdown is very close to the surface, this axial velocity approaches a value of 20 times the average vertical velocity of the background flow. The maximum value in the tangential velocity profile is 60-70% of the axial maximum. As the swirl increases the core radius shrinks, in the limit to less than 1% of the radius of the updraft region. Values of maximum vertical vorticity are on the order of 10,000/sec. A vortex formed over a moderately rough surface is similar to that over a smooth surface at lower swirl, namely lower velocities and broader cores