CFD Online Logo CFD Online URL
Home > Forums > FLUENT

Species source and sink

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   August 25, 2010, 12:17
Default Species source and sink
New Member
Join Date: Aug 2010
Posts: 2
Rep Power: 0
froniawi is on a distinguished road
Hi all,

My problem is concerning a sphere in a closed box and a fluid (initially of species A) flowing in between them. Now, I would like the sphere to take up one species (A) and excrete another (B) in a way that the mass-fluxes scale by A_in:B_out=2:1.

As you can see, there is a difference in mass-flux. Theoretically, the difference in mass is taken up by the sphere, which would thus grow in mass and volume. This is not yet implemented, since: it is not essential to me (except, maybe for mass-balance) and I have no idea how to model it.

Could you please help me with these topics:
- Is mass-balance conserved in my actual situation? So, without the difference in mass taken up by the sphere. (In my opinion: No)
- If not, is there a way to model this? Solutions involving a static sphere as well as a growing sphere are welcome.

By the way. Cheating with the densities of the species is not possible, since this problem involves buoyancy.

Thanks in advance for reading and helping. It is greatly appreciated!

Best regards
froniawi is offline   Reply With Quote

Old   August 25, 2010, 12:42
Senior Member
Join Date: Aug 2010
Posts: 125
Rep Power: 8
xrs333 is on a distinguished road
Hi, Froniawi,
IMHO, CFD cannot simulate a fiction. How does the transportation of species A and B take place? Would you please describe the real world phenomenon of your problem?
xrs333 is offline   Reply With Quote

Old   August 25, 2010, 13:53
Allan Walsh
Join Date: Mar 2009
Posts: 55
Rep Power: 8
Allan Walsh is on a distinguished road
If I understand your question correctly, there is a simple solution in Fluent. Define the volume as a separate fluid zone (i.e. in Gambit), specify that you want to include source terms, and specify the mass sink. If A and B have different properties, then you can specify the species and have a sink of A and source of B.
The means by which you implement is a bit different between Fluent 6 and Fluent 12, which you can look up in help. Good luck.
Allan Walsh is offline   Reply With Quote

Old   August 25, 2010, 17:23
New Member
Join Date: Aug 2010
Posts: 2
Rep Power: 0
froniawi is on a distinguished road
First of all, thanks to the both of you! I really appreciate your help.

@xrs333: As far as I know my research is not classified, but I will keep it a little vague to be sure. Sorry for that.
Imagine me modeling a simplification of a static living being (the sphere) consuming species A and excreting species B. Some of species A is used for self-maintenance and growth. Please note that I am only interested in the resulting flow surrounding the living being (the sphere).

@Allen_Walsh: That solution also came to my mind. It is indeed possible to make a model like that in Fluent and I know how. However, what I am wondering/asking is: Will the mass-balance be satisfied and will the results be "true"? The problem I have with that solution is as follows:
Lets make another simplification and assume that species A and B have the same density. Now. The sphere is consuming two mass-amounts of A and excreting one mass-amount of B (which is equal to one mass-amount of A). This means that one mass-amount of A just vanishes. As a result, the surroundings of the sphere is then decreasing in mass and constant in volume, which means that the density is decreasing while my model is assumed to be incompressible. Hopefully you can prove me wrong!

Alternative solutions I have been thinking about:
- The most straightforward solution: Modeling the growth of the sphere, for instance by remeshing the model at every timestep. Unfortunatly I have no idea how to implement this and it will probably be extremely time consuming.
- Modeling a normal sphere and somewhere far away from it a fictional anti sphere consuming and excreting the exact opposite. This seems like a reasonable solution, but: On the short run, it will require me to investigate the interactions between the normal sphere and anti sphere. On the long run, this solution will become impossible as the model becomes more and more complex.
- Also mesh the inside of the sphere, fill it with species B and apply mass-fluxes and/or a porous zone on the boundary to regulate the mass-fluxes. This also seems like a reasonable solution, but: On the short run, it will only work for short simulations. On the long run, this solution will also become impossible as the model becomes more and more complex.
- Cheating with the densities to make the model mass-conserving. The problem is, my model involves buoyancy.
- Decoupling the momentum and species equations with use of the Boussinesq-approximation? First solving the momentum equations with Boussinesq approximation, then solve the species equations. This is just a crazy idea which is probably not possible, but I believe you should always say what is on your mind during a brainstorm-session.

Last edited by froniawi; August 26, 2010 at 03:57. Reason: Added another alternative solution.
froniawi is offline   Reply With Quote

Old   August 30, 2010, 07:49
Byron Smith R J
Join Date: Mar 2009
Location: India
Posts: 46
Rep Power: 8
byronsmith is on a distinguished road
from what you are saying it appears to me that the sphere is in a mixture of A and B and it consumes A and excretes B at a ratio. if that is the case i suggest you to create a fluid zone around the surface of the sphere. you will have to write a UDF for the rate of consumption of A and excretion of B and apply them as sink and source at this zone. as far as tracking the increasing particle size you may have to use the discrete phase model with UDF to solve it. hope could throw some light on the problem
byronsmith is offline   Reply With Quote


mass conservation, mass sink, mass source, species

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
accessing the source and sink per cell in patchs Noel Phoenics 15 July 10, 2010 01:45
mass source and mass sink for multi-component frank CFX 0 May 14, 2008 11:55
species mass source in porous media ? PK FLUENT 0 February 16, 2007 12:12
Solid body becoming source or sink zonexo Main CFD Forum 2 June 28, 2006 23:22
source or sink?? frederic felten Main CFD Forum 2 August 8, 2000 12:35

All times are GMT -4. The time now is 02:22.