5 Replies Latest reply on Oct 14, 2014 7:54 PM by Jared Conway

What is the purpose of fluid subdomain if I have to set inlet flow?

Vicente Centelles Oct 14, 2014 10:34 AM

Hi,

I've been reading about fluid subdomains and I have understood how to create them and how to edit their parameters but I have still one question left.

In my case (a heat exchanger) I define both subdomains. One would be water and the other one will be a mixture of gases which I define by setting 34% of air, 33% of methane, 20% of methane and 13% of Nitrogen. Then in the LIDs inlets I would have to set again that from one is entering water and from the second one I repeat and enter again that mixture...

So, what difference would it be if I simply entered the parameters for the inlets without creating the fluid subdomains?

(It is useful for me because it helps me to see if both subdomains are isolated, but that's all, after that I erase them).

Thanks in advance

Re: What is the purpose of fluid subdomain if I have to set inlet flow?

Chris Michalski Oct 14, 2014 10:53 AM (in response to Vicente Centelles)

IIRC: If you do not use subdomains your "fluid" cells are either all gas or all liquid. If you want to include both gases and liquids in the simulation, you need subdomains in order for Flow to differentiate between liquids and gases.
Even if they are physically separated regions, without actively telling Flow they are subdomains they are all part of the same domain and therefore must be the same state of matter.
Like Show 1 Likes(1)

Actions
- Re: What is the purpose of fluid subdomain if I have to set inlet flow?
  
  Vicente Centelles Oct 14, 2014 10:59 AM (in response to Chris Michalski)
  
  Oh! I understand now. So in the case in which my heat exchanger worked with air on one side and the mixture I've said on the other, there would be no need of subdomains?
  
  What happens if I say that muy subdomain has 100% of air and 0% of the other gases and I introduce a mixture of gases? What would be the composition of that volume?
  
  Like Show 0 Likes(0)
  
  Actions
  - Re: What is the purpose of fluid subdomain if I have to set inlet flow?
    
    Chris Michalski Oct 14, 2014 11:14 AM (in response to Vicente Centelles)
    
    Yes, with air on one side vs a mixture of gases on the other side there is no need for subdomains because they are all gases.
    If you define the initial condition has having a concentration of 100% air, and then inject a mixture of gases you can trace how uniformly or how fast the air is displaced by the mixture.
    I often use fluid volumes as initial conditions for mixtures (or initial temperatures to reduce time to convergence). So you could define one volume as air, and another as the mixture without using subdomains.
    I've never tried using a subdomain to set initial conditions of a volume so I can't be 100% sure about your 2nd question. But I would guess that it would simply make that volume air, and then exchange it with the gaseous mixture. There may be other benefits of using a subdomain in this situation that others could highlight.
    
    Like Show 1 Likes(1)
    
    Actions
    - Re: What is the purpose of fluid subdomain if I have to set inlet flow?
      
      Vicente Centelles Oct 14, 2014 11:18 AM (in response to Chris Michalski)
      
      Fantastic. Thank you very much!
      
      Like Show 0 Likes(0)
      
      Actions
      - Re: What is the purpose of fluid subdomain if I have to set inlet flow?
        
        Jared Conway Oct 14, 2014 7:54 PM (in response to Vicente Centelles)
        
        just remember
        in the wizard you are defining basically the "default" fluid subdomain and the initial conditions of all those domains
        fluid subdomains and local intial conditions override the defaults
        and then the boundary conditions override the initial conditions
        it is essentially a hierarchy
        
        Like Show 0 Likes(0)
        
        Actions

Go to original post