Inverted Whirlpools and Drag

[S1 Science and up - Forces]

Ever seen something like this before?

Maybe you have, if you fancied tea and stirred them often before. The drag experienced by the liquid is different, and fairly complex, so think of it like this:

The spirit is less dense (lower Reynolds number), and so it takes less energy to spin and swivel around the container. On the other hand, the coloured water at the bottom is more dense, and takes more energy to start spinning.

The difference in density (which affects drag) thus causes the spirits to travel around the container with less energy required than the water. The work done by the spirits is higher, and because matter occupies space, the lighter spirits that spin faster would occupy more of the region on the outer side of the container. This creates a secondary flow where the spirits are expending their energy outwards faster than the water does, which causes the water to bunch up in the centre. You can think of it as '"inertia for fluids", but it's not entirely accurate either.

It's okay if you don't understand it completely, just acknowledge that it's possible. When the time comes, you understand that it's possible & you can accept it more readily.