Can I use thermophysicalProperties.water for "compressible" solver such as buoyantSim

hswzzz · July 18, 2015, 11:01

Dear Foamers;

I want to simulate natural convection phenomena. And I do not want to use Boussinesq approximation. (In my problem, there is a large temperature difference)

When I tried with air properties, it seems to work well. (rho max/min : 1.2 1.0)
However, when I changed thermophysicalProperties for air to water..

the "rho max/ min" calculated over 2,000kg/m3.

What is the problem?

Nicole · July 20, 2015, 05:12

Hi Seokwon,

I have a similar problem, I think.

Firstly, have you adjusted rhoMin and rhoMax in fvSolution? (Silly question I know, but I forgot to do this)

Secondly, is your volume closed? If there is no inflow or outflow then you have the same fundamental problem that I do... See my post here:
http://www.cfd-online.com/Forums/ope...ed-volume.html

hswzzz · July 20, 2015, 08:02

1. No, I did not adjust rhoMin/rhoMax...
2. Yes, volume closed problem..

Nicole · July 20, 2015, 08:56

Ok, so the problem with closed volume and incompressible (to explain a bit more than my other post) is that at the start of your simulation, you do not know mow much mass there is in your system. For example, if you start off at 25C, and after 1 iteration the average temperature is 25.5C, the density will have changed, and if the volume is fixed, you cannot conserve mass (m=rho*V) properly.

One solution that I have seen implemented is to maintain a constant average temperature, therefore constant average density and constant mass. This will not, however, work for my case. I think you would have to do a lot of coding in the solver to implement this.

I would like to know if the idea of allowing a small inlet/outlet will still converge to a solution. This would only work for steady state. I am busy working on this at the moment. I will let you know if I find anything.

The other option is to allow the free surface to move/the volume to expand as required to maintain a constant mass. I have no idea how to start implementing this.

hswzzz · July 20, 2015, 22:23

Thanks for your reply!
Actually, I did not think about mass conservation

How can I maintain a constant average temperature?
Do you have any experience to fix the code to solve this kind of problem?

+ my solver is compressible solver..

Nicole · July 21, 2015, 06:01

Which solver are you using? Are you solving for steady state or transient changes?

The only way I can think of maintaining a constant average temperature would require significant modification of the solver code. I recommend you start by spending a lot of time going through the solver code and understanding exactly what it is doing, and when. I am definitely not skilled enough to just tell you off the bat exactly what you need to do.

July 18, 2015, 11:01	Can I use thermophysicalProperties.water for "compressible" solver such as buoyantSim	#1
hswzzz New Member Seokwon Whang Join Date: May 2012 Posts: 25 Rep Power: 13	Dear Foamers; I want to simulate natural convection phenomena. And I do not want to use Boussinesq approximation. (In my problem, there is a large temperature difference) When I tried with air properties, it seems to work well. (rho max/min : 1.2 1.0) However, when I changed thermophysicalProperties for air to water.. the "rho max/ min" calculated over 2,000kg/m3. What is the problem?

July 20, 2015, 08:56		#4
Nicole Member Nicole Andrew Join Date: Sep 2014 Location: Pretoria, South Africa Posts: 58 Rep Power: 11	Ok, so the problem with closed volume and incompressible (to explain a bit more than my other post) is that at the start of your simulation, you do not know mow much mass there is in your system. For example, if you start off at 25C, and after 1 iteration the average temperature is 25.5C, the density will have changed, and if the volume is fixed, you cannot conserve mass (m=rho*V) properly. One solution that I have seen implemented is to maintain a constant average temperature, therefore constant average density and constant mass. This will not, however, work for my case. I think you would have to do a lot of coding in the solver to implement this. I would like to know if the idea of allowing a small inlet/outlet will still converge to a solution. This would only work for steady state. I am busy working on this at the moment. I will let you know if I find anything. The other option is to allow the free surface to move/the volume to expand as required to maintain a constant mass. I have no idea how to start implementing this. hswzzz likes this.

July 20, 2015, 22:23		#5
hswzzz New Member Seokwon Whang Join Date: May 2012 Posts: 25 Rep Power: 13	Thanks for your reply! Actually, I did not think about mass conservation How can I maintain a constant average temperature? Do you have any experience to fix the code to solve this kind of problem? + my solver is compressible solver.. Last edited by hswzzz; July 21, 2015 at 01:51.

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July 20, 2015, 05:12		#2
Nicole Member Nicole Andrew Join Date: Sep 2014 Location: Pretoria, South Africa Posts: 58 Rep Power: 11	Hi Seokwon, I have a similar problem, I think. Firstly, have you adjusted rhoMin and rhoMax in fvSolution? (Silly question I know, but I forgot to do this) Secondly, is your volume closed? If there is no inflow or outflow then you have the same fundamental problem that I do... See my post here: http://www.cfd-online.com/Forums/ope...ed-volume.html

July 20, 2015, 08:02		#3
hswzzz New Member Seokwon Whang Join Date: May 2012 Posts: 25 Rep Power: 13	1. No, I did not adjust rhoMin/rhoMax... 2. Yes, volume closed problem..

July 21, 2015, 06:01		#6
Nicole Member Nicole Andrew Join Date: Sep 2014 Location: Pretoria, South Africa Posts: 58 Rep Power: 11	Which solver are you using? Are you solving for steady state or transient changes? The only way I can think of maintaining a constant average temperature would require significant modification of the solver code. I recommend you start by spending a lot of time going through the solver code and understanding exactly what it is doing, and when. I am definitely not skilled enough to just tell you off the bat exactly what you need to do.