Twister in a bottle

Vortices are difficult to describe. It is because they are often formed as a result of turbulent flow. For low turbulent flows, the surface of vortex can be described with satisfactory approximation with the means of rotational hyperbola. In case of high turbulence flows, vortices multiply themselves. In a big vortex a smaller one is formed, in which even smaller one is formed and so on, as in nesting dolls (matrioshka), in a word - chaos. Apparently, Werner Heisenbeg said on his death bed, he would ask God two questions when he met Him: "Why relativity? Why turbulence?". After a while he added, "I really believe He will have an answer only to the first one." (J. Gleick, "Chaos. Narodziny nowej nauki." Wyd. Zysk i S-ka, Poznañ 1996).

A vortex in a bottle allows for overcoming viscosity forces: were it not there, the air would squeeze through a small aperture between the bottles in small bubbles. Centrifugal force in swirling water pushes it to the edge of the aperture, allowing for a smooth passage of a stream of air through the centre of the aperture at the top.