problem pure diffusion

Jorn

Hi, I have a cylindrical domain (1 mm diameter, 5mm length) with opening boundaries defined at the two opposing faces. The value for an additional variable is 1 kg/m3 inside the channel and 0 kg/m3 at the opening boundaries. I have defined water 25C as my fluid inside the channel and switched on kinematic diffusivity for the additional variable with value of 0.5e-5 cm2/sec. There is no convection. I do a transient run with total simulation time 30 sec. At t = 30s, I see not even a slight change in concentration at the edges, i.e. no diffusion out of the domain seems to take place. Anyone a suggestion what I do wrong here?

Thanks, Jorn