{"id":4622,"date":"2017-08-16T07:00:19","date_gmt":"2017-08-16T07:00:19","guid":{"rendered":"http:\/\/www.beanthinking.org\/?p=4622"},"modified":"2017-08-15T09:56:37","modified_gmt":"2017-08-15T09:56:37","slug":"on-rings-knots-sailors-myths-and-coffee","status":"publish","type":"post","link":"https:\/\/www.beanthinking.org\/?p=4622","title":{"rendered":"On rings, knots, myths and coffee"},"content":{"rendered":"
\"vortices<\/a>
Vortices behind a spoon dragged through coffee.<\/figcaption><\/figure>\n

Dragging a spoon through coffee (or tea) has got to remain one of the easiest ways to see, and play with, vortices. Changing the way that you pull the spoon through the coffee, you can make the vortices travel at different speeds and watch as they bounce off the sides of the cup. This type of vortex can be seen whenever one object (such as the spoon) pulls through a fluid (such as the coffee). Examples could be the whirlwinds behind buses (and trains<\/span><\/a>), the whirlpools around the pillars of bridges in rivers<\/span><\/a> and the high winds around chimneys that has led some chimneys to collapse<\/span><\/a>.<\/p>\n

Yet there is another type of vortex that you can make, and play with, in coffee. A type of vortex that has been associated with the legends of sailors, supernovae and atomic theory. If you add milk to your coffee, you may have been making these vortices each time you prepare your brew and yet, perhaps you’ve never noticed them. They are the vortex rings. Unlike the vortices behind a spoon, to see these vortex rings we do not pull one object through another one. Instead we push one fluid (such as milk) through another fluid (the coffee).<\/p>\n

It is said that there used to be a sailor’s legend: If it was slightly choppy out at sea, the waves could be calmed by a rain shower. One person who heard this legend and decided to investigate whether there was any substance to it was Osborne Reynolds (1842-1912)<\/span><\/a>. Loading a tank with water and then floating a layer of dyed water on top of that, he dripped water into the tank and watched as the coloured fluid curled up in on itself forming doughnut shapes that then sank through the tank. The dripping water was creating vortex rings as it entered the tank. You can replicate his experiment in your cup of coffee, though it is easier to see it in a glass of water, (see the video below for a how-to).<\/p>\n