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Jorn July 31, 2007 11:44

problem pure diffusion
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

opaque July 31, 2007 11:53

Re: problem pure diffusion
Dear Jorn,

What type of Additional Variable are you using Poisson, Diffusive, or Transport Equation?

By default, CFX does include diffusion at inlets/opening/outlets for the Transport Equation option..

Hope this helps,


Jorn July 31, 2007 11:56

Re: problem pure diffusion
I'm using the transport equation option... Jorn

Jorn July 31, 2007 12:06

Re: problem pure diffusion
Ok I see now what went wrong... in the 30 sec. the effect of diffusion is small and the mesh at the opening boundaries was too coarse to resolve this.

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