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cinwendy April 5, 2013 10:09

Different between species and UDS?

I want to simulate some tracer/reactant. I used species transport and got some results. Now I am looking at UDS, and I get different results. I have a species with properties of pure water (by specifying mass fraction) and a UDS also with diffusivity of water (by specifying value at boundary). These two looks the same for me, so I was expecting I will get the same results. How are they differ from each other?

Thanks a lot in advance!

msaeedsadeghi April 6, 2013 09:06

both species transport equations and UDS are scalars. You should get the same result if you have setted it correctly. I have worked with UDS and UDF so much. It's easy.

cinwendy April 7, 2013 04:57

Hi Saeed,

Thank you. I was comparing the distribution of mass fraction of a mixture with properties of water to that of using UDS. The mixture contain 2 liquid water species, just to differentiate from one another. So I set mass fraction at BCs and apply source term in cell zone. On the other hand, I defined a UDS, set the diffusivity (the only thing I was allow to set in UDS) to the the same as that in the mixture properties. The UDS also has the same "value" (not sure why, but "flux" didn't work well) at BCs and source term in cell zone.

One thing for sure is that pressure outlet setting give me erroneous results. Because setting species mass fraction to 0 and setting UDS value to 0 seems different. I am not sure how to amend this.

Anything else that I missed out?

cinwendy April 9, 2013 02:43


Can anyone tell me which UDF or setting I should use for pressure outlet BC for a scalar which the value and flux are not known? At the BC, FLUENT expect return viod, I can't use DEFINE_SOURCE. But I am clueless looking at the manual...

Thanks a lot in advance!

asal April 9, 2013 04:18


If your flow exposed by the gravity force, which means that if you have buoyancy, then you may have different results from species and UDS.

cinwendy April 9, 2013 09:34

In order to simplify for easier comparison, that my UDS behave exactly like pure liquid water, I didn't use gravity for both cases.

I was able to get similar trend when using UDS, compared to that of using species transport. But the different was big (~1% far away from wall, and totally off at the wall).

What should I do?

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