Model to simulate a sink (drain) will it rotate?
 

When you observe the flow out of e.g. a bath tub you notice that the water starts to rotate when the fluid level becomes low enough. That has of course nothing to do with the coriolis acceleration but will it nevertheless begin to rotate? Which forces would be responsible for it or is this phenomenon due to instabilities?

Need I the model of free surfaces? What is with surface tension?

Peter