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 Lilly July 23, 2013 07:21

species transport-outflow boundary condition

Dear all,

I am simulating a species transport inside a geometry with several outlets and I am using th outflow boundary condition. My question is: does this boundary condition only fix the total flow (like for outflow:0.25-->25% of the total fluid-mixture is passing through this outlet) or is also the amount of species is fixed by the outflow condition (like also exactly 25% of the species is forced to pass this outlet)? I am afraid the latter might lead to wrong results, since depending on the bifurcation the amount of species might be larger or smaller at an outlet...
Does anybody have an idea or a reference?
Thanks a million!
Lilly

 A CFD free user July 28, 2013 13:00

Hey
Off course, the first one is correct, a portion of total mass flow.

 Lilly August 2, 2013 03:24

 vix August 30, 2013 12:22

Dear Lilly,

I am looking to model something similar - contrast agent in blood through a realistic geometry of an artery with a stenosis. I am interesting in seeing the concentration of the agent downstream of the stenosis.
However, I am new to the species transport model.

I have been reading around and so far I understand that I would need an inlet mass fraction for the dye and the diffusion coefficient to use the species transport model. I will assume a zero flux through the wall but I am not sure what outlet bc to use. I only have 1 outlet.

Should I be prescribing a mass fraction at the outlet ( I don't have this information) ? or just use the outflow condition.

Vix.

 Lilly September 4, 2013 04:59

Dear vix,

since I am interested in how the contrast agent bolus shape looks at the outlet I am using the outflow boundary condition and I do not set a bolus shape there. But you could also use a pressure outlet for example. I don't think you have to fix the mass fraction of the species at the outlet there (even though you have the possibility to do so). Do you have information of how your bolus looks there?

Good luck,
Lilly

 vix September 4, 2013 05:58

Hi Lilly,

I do not know how the bolus looks at the outlet so I guess using either an outflow condition or a pressure outlet would be sufficient (without specifying a mass fraction)

I am mainly interested in seeing if there is an increase in the concentration of the species downstream of the stenosis as there is a recirculation zone which occurs there.

Would you be able to help with a few questions I have as I go along this simulation ? I would really appreciate it.

For example, when you select the species transport model, you are supposed to select a mixture. What mixture have you chosen ? or if you have created your own mixture, is it just the contrast agent and the blood. What properties would you need to specify for the two ?

I read somewhere that the agent should be first and the blood should be second since blood is the bulk fluid. Is that correct ?