Anti-centrifugal effect

[Gerenuk]

I have a pot with water and some large particles lying on the bottom. When I stirr the water in a circular motion and then let the particles settle, they all gather in the center of the pot. Why is that so?

 

[A.T.]

I have a pot with water and some large particles lying on the bottom. When I stirr the water in a circular motion and then let the particles settle, they all gather in the center of the pot. Why is that so?

http://en.wikipedia.org/wiki/Secondary_flow#Circular_flow_in_a_bowl_or_cup

 

[Gerenuk]

Thanks for the solution!

Btw, that reminds me of another simpler effect: Apparently when a van with helium balloons accelerates, the ballons move to the front :) Bouyancy I suppose.