Tornado Tower

 

A classic design was replicated for a mist vortex to explore the basic principles of tornados and whirlpools.

Basic Vortex Principles

A vortex (tornado) consist of a spinning column of air that is stable due to the balance of two basic forces. A centrifugal force pulls the outer portion of the column away from the center, creating a low pressure at the core. The pressure difference between the core and the outer spinning air creates an inward force. When these forces balance, the tornado becomes stable, maintaining its structure. 

As a mass of air spins, it tends to maintain its angular momentum, a product of its local mass m, rotational velocity v and distance r from the rotation center (mvr). Air at the outer portions (higher r) rotate more slowly (lower v) than near the core (lower r and higher v). The centrifugal force is then greater near the core, forcing the air to move outward, creating a low-pressure region (the eye of the storm). The funnel shape arises from a slower spin speed at the top compared to the bottom of the column. This can be caused as warm air rises from the ground up to cooler clouds, stretching the column and narrowing r (increasing v) at the bottom. The rising air cools and returns downward outside of the vortex, which helps contain and shape the funnel.

In massive storms, the spin direction is initiated by the Coriolis force resulting from the rotation of the Earth, but smaller tornados and vortices generally start their spin direction as the result of local shear forces. 

Vortex Demonstration

Many science museums exhibit a tornado demonstration featuring a tall swirling funnel cloud that can be manipulated by visitors. A smaller version can be built economically that behaves much the same, which can teach some of the basic principles of vortices. A 2003 paper by Harald Edens provides extensive detail on how to build such a system. His design and many of his methods were used for this project:  https://www.weatherscapes.com/techniques.php?cat=miscellaneous&page=tornado_machine

The vortex that’s generated is about 1 meter tall in a partially open cylindrical case with an exhaust fan at the top and an ultrasonic mist generator in a pool of water at the bottom. The fan pulls air through the top center of the chamber and expels it from a vertical column of holes blowing across the cylinder wall (details in the next section). The mist cloud is drawn upwards and spins in the direction of this wind. With the proper illumination, a white vortex can be seen, varying from a straight smooth column, or turbulent storm with a central dark core, depending on the stillness of the outside air and the rotation speed of the fan. The slightest breath or wave of a hand can disrupt the funnel, but it quickly regains its shape.

In this demonstration, the fan exhaust blows counter-clockwise looking from above, causing the vortex to spin in this direction. Air spins faster near the fan intake hole, resulting in a narrower spinning column at the top compared to the wider and slower spinning air at the bottom, creating an upside-down funnel cloud.

Apparatus Details

A mist generator sits at the bottom of a well in the acrylic base filled with water. The well was produced by pressing a can onto 1/8” thick acrylic softened by a heat gun. All interior surfaces were coated with flat black paint.

Illumination

The top of the vortex has fewer vapor particles due to its smaller radius and higher speed, so it is best to strongly illuminate the top portion. A line of LEDs was extracted from a desk lamp and attached to the upper right edge of the main opening. The control button provides a range of brightness settings.

Blower Fan

Note: A Variac variable transformer was used for speed control, but a lower cost controller used for hearths would also work, such as Midwest Hearth Fan Speed Control Kit.

To couple the blower to the PVC tube, the square flange was removed from the blower and a cloth tube (sock) was duct taped to the blower output and clamped around the PVC. This allows excess pressure to be relieved through the cloth.

Dayton 1TDR3 Model Blower; 273 CFM, 1640 RPM 115V AC

Operation

A puff of air is used to disrupt the vortex after 20 sec to illustrate how it recovers.

At very low speeds, the vortex is a smooth straight column that is easily disrupted. Higher speeds introduce turbulence and exhibit a dark center core (low pressure “eye of the storm”). 

Higher Fan Motor Speed \(\rightarrow\)

The above photo is taken looking down the vortex (in red encircled region) by gently inserting a smartphone camera. The dark region is a low-pressure column with fewer vapor particles and reduced light scattering.