Rotate the handle several times. A tornado-like structure is formed in water.
Rotating the blades attached to the handle create a low pressure at the bottom. There is a gradual increase in pressure from bottom to top This gradation in pressure is responsible for the tornado-like structure.
In this exhibit, we have an acrylic cylinder closed at the top and an opening at the bottom. The bottom part fixed to a chamber which has a fan fixed right below the opening of the cylinder. This fan is manually operated using the handle provided. The container is filled with water. When the handle is rotated several times and the blades of the fan spin at a sufficient speed, the water also starts to spin and its surface now appears like a funnel as in a typical tornado.
The region where any fluid is moving fast is a region of low pressure. The spinning blades produce a low pressure at the bottom of the tube. As we move up in the liquid column, there is a gradual reduction in the speed at which the liquid spins. As a result, there is a gradual change in the pressure from bottom to the top of the liquid column i.e, low at the bottom and high at the top.
The diameter of the spinning surface of the liquid at any level depends on the pressure difference at that level. Since there is a pressure gradient in this spinning liquid column with high pressure at top and low pressure at bottom, we see that the diameter of the spinning surface is more at the top, gradually decreases as we move down and is the least at the bottom. This gives rise to the funnel shape as seen in a tornado.