Diffusion simulation
 

Hi all,

I want to simulate diffusion in a 2D geometry. The geometry is a simple circle with a diameter of 500µm. I want to put water in it and then simulate a tracer (which is also water, but with another color), which I patch with a diameter of 50µm, which will move to the walls. That way I can see how long it takes before it reaches the walls and then I can calculate diffusion and see whether fluent simulates it right.

The problem is the geometry. When I define the outeredge of the circle as a pressure outlet and when I patch the tracer in the middle and start the simulation, it won't move at all. So I think I need some kind of entry (maybe a velocity-inlet) in the middle, but I'm not sure, because when I give the entry a velocity then it will influence the diffusion.
So anyone knows how to solve this problem?

Grtz
Momo