The washbasin vortex
Put water into a washbasin, pull the plug and watch it swirl out. Is the
flow rotational? or irrotational?
It depends. Sometimes you can make parts of the flow irrotational. See
the sequence of pictures below---the sequence in time is down the fisrt
column and then down the second column. They show water flowing out of my
washbasin.
But I have put some lego blocks to float on the surface to act as tracers.
Because of their positions at different times, they show that the water
flows anticlockwise around the central vortex. However, they also show
the local vorticity by the rotation, or lack thereof, of their orientation.
- Observe that the yellow block maintains virtually the same orientation
in all the frames. It is in a region of irrotational flow, a region where
the vorticity is zero.
- However, the other blocks rotate their orientation. They are in
regions of rotational flow, the vorticity is non-zero.
These photographs are taken 1 second apart.
Prepared by Tony Roberts, aroberts@usq.edu.au, 9
March 1999.
Dept Maths & Comput, University of Southern Queensland,
Toowoomba, Qld 4350, Australia.
