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Concentrtion gradient in Time dependent simulation at time t=0

Posted 2014年10月7日 GMT-4 02:04 Chemical Reaction Engineering, Studies & Solvers Version 4.3, Version 4.3a, Version 4.3b 3 Replies

Kishore Gajula
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I have a system in which diffusion takes place from a sphere to a medium(in a cylindrical domain)..i am running time dependent simulation using "Transport of diluted species " physics.
Initial concentration in sphere = C0(some constant value) at t=0
outside sphere = 0 at t=0
1) After the simulation when i checked for concentration profiles in the sphere at t=0 ...it is showing some concentration gradient at the end of the radius of the sphere.Ideally there shouldn't be any gradient at t=0 inside the sphere since i gave some initial concentration(t=0) in sphere and zero concentration outside it.

can anyone tell me how to resolve the issue??
i tried specifying some flux at interface ...but it wouldn't work

Thanks
Kishore Gajula
3 Replies Last Post 2014年10月7日 GMT-4 05:55
Lasse Murtomäki
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Posted: 1 decade ago 2014年10月7日 GMT-4 02:49
At t = 0 the concentration gradient is - in principle - infinite, which numerics cannot obviously handle. But I would not scratch my head too much for that. What I usually do in a problem like yours is that I do the simulation with an exponentially increasing time step. For example, in the simulation time domain I put 10^{range(-3,0.1,3)}. In the very beginning I calculate with a very short time step, because there the concentration changes are the most rapid. I do not have point t = 0 at all, because it is a singularity.
At t = 0 the concentration gradient is - in principle - infinite, which numerics cannot obviously handle. But I would not scratch my head too much for that. What I usually do in a problem like yours is that I do the simulation with an exponentially increasing time step. For example, in the simulation time domain I put 10^{range(-3,0.1,3)}. In the very beginning I calculate with a very short time step, because there the concentration changes are the most rapid. I do not have point t = 0 at all, because it is a singularity.
Kishore Gajula
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Posted: 1 decade ago 2014年10月7日 GMT-4 03:14
Thanks for the reply.

In my case i have to vary the time step linearly not exponentially.
I tried to simulate from t=1e-10 instead from t=0 to certain time t.still i am observing some concentration distribution across the barrier at t=1e-10 which is better than previous simulation in which there was some concentration distribution across the barrier(to end of the radius) at time t=0.

Thanks
Kishore Gajula
Thanks for the reply. In my case i have to vary the time step linearly not exponentially. I tried to simulate from t=1e-10 instead from t=0 to certain time t.still i am observing some concentration distribution across the barrier at t=1e-10 which is better than previous simulation in which there was some concentration distribution across the barrier(to end of the radius) at time t=0. Thanks Kishore Gajula
Lasse Murtomäki
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Posted: 1 decade ago 2014年10月7日 GMT-4 05:55
You can simulate in any time pattern, and present the data at interpolated values.
You can simulate in any time pattern, and present the data at interpolated values.

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