By Anupum Pant
Have you ever tried dragging a plate (the one you use to eat food) on the surface of a swimming pool? If you haven’t, you’re missing out on something really cool.
Dragging a plate across the surface of a pool creates a toroidal vortex in the water which, thanks to the combined play of friction and pressure, keeps it going for a long time and long distance. Dolphins, volcanoes and human smokers all do it. It’s a perfect way to create disturbances in fluids which can travel relatively long distances – the whole length of a pool in this case. A vortex travels slow usually in the perpendicular direction of its plane and could take several minutes to traverse, say the length of a pool. But, if done right, it won’t die out easily.
Another interesting thing to note in the plate and pool experiment is that there are two vortices created, each at one end of the plate. Both of them travel together and seem to be rotating in opposite directions. So what really are these?
This is how it works.
Think of it as a curved tornado, a semi circular one created across the whole semicircular part of the plate which is inside the water. Here’s a creative way to see this half toroidal vortex in action.