Different between species and UDS?

cinwendy · April 5, 2013, 10:09

Hi,

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

Hi,

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

Hi!

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?

April 5, 2013, 10:09	Different between species and UDS?	#1
cinwendy Member Join Date: Feb 2013 Posts: 31 Rep Power: 13	Hi, 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!

April 9, 2013, 02:43		#4
cinwendy Member Join Date: Feb 2013 Posts: 31 Rep Power: 13	Hi, 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! Last edited by cinwendy; April 9, 2013 at 09:32.

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April 6, 2013, 09:06		#2
msaeedsadeghi Senior Member SSL Join Date: Oct 2012 Posts: 226 Rep Power: 14	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.

April 7, 2013, 04:57		#3
cinwendy Member Join Date: Feb 2013 Posts: 31 Rep Power: 13	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?

April 9, 2013, 04:18		#5
asal Senior Member Astio Lamar Join Date: May 2012 Location: Pipe Posts: 186 Rep Power: 13	Hi! 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.

April 9, 2013, 09:34		#6
cinwendy Member Join Date: Feb 2013 Posts: 31 Rep Power: 13	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?