1 Charles A. Doswell, National Severe Storms Laboratory, Norman OK
2 Thomas P. Grazulis, The Tornado Project, P.O. Box 3-02, St. Johnsbury, VT. 05819
phone: (802)-748-2505 Fax: (802)-748-2543 e-mail: tornproj@plainfield.bypass.com
Various aspects of vortex dynamics not previously explored in laboratory models are demonstrated in a new variant on the vortex simulator. The simulator is 2 m tall and 1 m in diameter, and is simple and inexpensive. The design consists of eight pressurized vertical tubes (5 cm in diameter) set at regular intervals around the perimeter of a circular base. The open ends of the tubes are set in a plenum, which can be pressurized by either hair dryers or duct fans. By this means, air is forced out of the tubes, at a variable angle to the penmeter, through evenly spaced holes that run the full length of the tubes. The simulated updraft is provided by a box window fan, but the vorticity imparted to the flow by the fan is decoupled from the simulator. Condensing vapor from dry ice in hot water and/or the mist from an ultrasonic humidifier is used as a tracer.
By covering the holes in the tubes, the circulation in the simulator can be made to vary with height. If all the holes in the tubes are open, a very stable single-cell vortex over the full height of the simulator can be created. By covering the holes in the lower third of tube, the vortex can still extend to the surface, but the extended vortex is not as stable as when the vorticity is more uniform with height.
If the holes are covered beyond about a third of the simulator's height, the vortex cannot be made to "penetrate" to the surface. This configuration is also used to show the need for a lower surface in order to have vortex formation, which can be demonstrated by introducing vapor into the upper part of the model (having no vortex) and then introducing a solid surface into the center of the model. It also appears that when the holes are covered in the middle third of the apparatus, no vortex forms, in spite of strong rotation in the upper and lower third.
Other demonstrations are possible, including standard simulator vortex behavior, such as vortex breakdown, multiple vortices, etc. These are created by opening and covering various numbers of holes in the tunes on all or opposing sides of the chamber, and at various heights in the chamber.