Demo overview:
A sponge is doused in lighter fluid and lit aflame to produce a small column of fire. The sponge is set inside a bin, which is gradually spun on a rotating table. The walls of the container drag the air in a circular fashion, generating a physical vortex. Enhanced by the heat of the fire, the vortex evolves into a tornado-like object which is remarkably stable and striking to see. This demonstration depicts angular momentum, thermodynamics, buoyancy, and fluid mechanics in a physical way, providing insight into the formation of tornadoes. Students of all ages may appreciate this demonstration; however, special care must be taken with younger children to ensure they are not injured from the fire. This demonstration also involves an open flame, and thus must only be conducted with appropriate regard for fire safety.
General Notes
- Lighter fluid should be stowed out of reach of students when not in use
- Make sure that younger students stay at a safe distance and do not touch the flame
- This activity should be performed outdoors, in a well-ventilated area, far from flammable objects
- Strong winds and rain may compete with the vortex outdoors, preventing it from forming
- If the vortex is struggling to form, using more lighter fluid to fuel a strong flame can help the vortex survive
Equipment
- Mesh trashcan
- Flame-resistant bowl
- Small sponge (approx 1 in3)
- Lighter fluid
- Turntable (“lazy susan”)
- Metal lid for the bowl
Setup
- Place the turntable on solid ground
- Place the mesh trashcan on the turntable and the flame-resistant bowl inside the trashcan
- Place the sponge inside the flame-resistant bowl and pour a few drops of lighter fluid on the sponge
- Light the sponge aflame
- Gently spin the trashcan and watch the tornado form
- Place the metal lid over the sponge and flame-resistant bowl to smother the flame, safely extinguishing the fire
Engaging with the physics
Slow moving air can be thought of as a fluid: generally whenever air flows away from a point, other air must rush in to fill the void. Flows often damp out when air molecules bump into one-another, but in special configurations, where the molecules flow in a coordinated fashion, the motion can be longer-living. For instance, vortices are a topologically stable defect where air circulates around an axis. As a mental model, one can imagine air molecules moving azimuthally together; each moving out of the way of the next incoming molecule. Topologically protected objects are subject to growing interest in physics research, including skyrmions in magnetic systems.
Tornado formation requires a strong updraft of air. Wind shear is present because the direction of the wind can be different at different altitudes, which induces rotation into the rising column of air. Flows entering the updraft begin as horizontal, rotating parcels of air which orient vertically through a storm, creating mesocyclones. The mesocyclone only possesses vertical rotation high above ground, but a downdraft in the storm will flow outwards and experience rotational torques, forming rotation near the ground as the air descends. In this demonstration, rotational forces are produced from the rotating trashcan.
Vertical flow of air is influenced by temperature differences. Increasing the temperature of a gas at constant pressure causes it to expand, lowering its density. Bernoulli’s principle states that the buoyant force depends on the density of an object, so warmer air tends to rise while cooler air tends to sink. For this demonstration, the heat of the flame produces updrafts. In contrast, tornado formation requires unstable atmospheres, where higher altitudes are slightly warmer. Thus, small parcels of air moving upwards heat up, becoming less buoyant and experiencing stronger upwards forces. This accelerates the upwards flow of air, contributing to tornado formation.
Sources
https://en.wikipedia.org/wiki/Ouroboros
https://sites.psu.edu/pmarkowski/how-tornadoes-form/
https://phys420.phas.ubc.ca/p420_04/sean/