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-   -   What are the Units for Souce Term? (http://www.cfd-online.com/Forums/phoenics/52060-what-units-souce-term.html)

Kali Sanjay May 7, 2006 04:07

What are the Units for Souce Term?
 
Hi! As per PHOENICS documentation, source term should be Kg/Sec if 'Cell' is specified in Patch. But I am solving for concentration and my equation is

dc/dt = Source Term

where 'c' is concentration not mass and units of dc/dt are coming to be moles/Volume-Sec. Will the Source term I specify in PHOENICS should be moles/Sec or Moles/Volume-Sec for 'Cell' type Patch?

Thanks in Advance!

Sanjay

Kali Sanjay May 7, 2006 04:30

Re: What are the Units for Source Term?
 
Actually I tried to solve a simple diffusion equation for concentration and equation I formulated is

dc/dt = d(D*dc/dx)dx with fixed boundary conditions.

I tried to solve the problem using diffusion active in terms and secondly I tried to solve the RHS part as source term and I gave patch as 'East' and 'West'. I gave the patches as 'FIXFLU' and gave D*dc/dx at east wall as one source and -D*dc/dx at west wall as another source for all cells.....units of the source here are moles/area*sec and since I gave 'East' and 'West' in patch the units will be moles/Sec.....but the results are not matching for 2 cases.....

What am I doing wrong?

Thanks in advance!

Sanjay

Mike May 9, 2006 12:17

Re: What are the Units for Souce Term?
 
If you are solving for a concentration, say C1, with units of mol/m^3, how have you eliminated the density from the built-in transient term? If convection and diffusion are absent, setting the density to unity is probably sufficient.

TYPE=cell means that PHOENICS will not apply any geometrical multiplier to your specified source term, which means that you must then supply the source (via CO & VAL) for each cell in units of mols/s. If it is a volumetric source, then one could use TYPE=VOLUME, and then set the source(via CO & VAL) in units of mols/m^3/s.

Kali Sanjay May 9, 2006 12:46

Re: What are the Units for Souce Term?
 
Thnaks Mike! If I make RHO1 as unity will it not take care of density of diffusion and convection terms? Then my source can be moles/sec where I am solving for C1 whose units are moles/volume.

Mike May 10, 2006 04:47

Re: What are the Units for Souce Term?
 
Yes, it will take care of the convection and diffusion terms in the generic built-in default transport equation, but my uncertainty is that I don't know the form of the convection and diffusion terms in your transport equation for the molar concentration. Well, not without resorting to a text book.

Kali Sanjay May 10, 2006 05:30

Re: What are the Units for Souce Term?
 
Thank you Mike! My values calculated by diffusion active and solving same problem with diffusion as source term are matching provided RHO1 is set to unity.


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