Turn the wheel to control the vortex. As the water spins around, the pressure is reduced in the middle. The surface curves down and forms an air tube or vortex.
Water in the Perspex cylinder behaves like the water in your bath when it flows out through the plughole – only on a giant scale! You can make the middle of the whirlpool reach right down to the bottom of the cylinder.
As the water is pumped out of the bottom of the cylinder, it tends to spin around as it flows out of the “plughole”.