Error in solver? (buoyantPisoFoam)

fijinx · January 29, 2010, 18:51

This is for you smart ones out there (or at least smarter than me)! I have a simplified problem where I have a 1 meter cubed box with an inflow of 1kg/sec (trying to model filling a tank with gas). after 15 seconds I check the values, and the pressure is very much off (factor of 12ish). I have check, re-checked, and re-re-checked my values in all the files, but they all look accurate to me. Based off hand calcs, I'm expecting a pressure of around 1.724 MPa after 15 seconds, but it only gets to 135 kPa. Anyone out there that can crack this? I am using buoyantPisoFoam to solve the problem.

http://rapidshare.com/files/343125087/meterBox.zip.html

fijinx · January 29, 2010, 18:56

The link above is the full case file.

fijinx · February 1, 2010, 14:27

Anyone have a chance to look at this yet? I'm still pulling my hair out.

matejfor · February 2, 2010, 03:43

To save some of your boldness for other cases, just without much compressible experience and source checking, I would say, that this buoyant solver, although compressible, is only weakly compressible. It means it does compute the buoyancy force from density difference and not the Boussinesq approximation, but is not suited for real compressible case like yours.

I would suggest to try rhoPisoFoam or some other real compressible solver.

I hope someone will correct me if I'm wrong.

Good luck

matej

Graham81 · February 2, 2010, 07:00

Hey James,

Since Im waiting on a simulation, Im trying to think with you....though Im rather newbish to openFOAM.

I downloaded your case, but was wondering where you prescribed the inflow of gass.

Pieter

Graham81 · February 2, 2010, 07:05

My bad, I see it now in the BC for U.

fijinx · February 2, 2010, 15:19

Thank you matejfor! You were correct, using rhoPisoFoam gave me almost exact results to my hand calculations. I do however have one delima now.

Does rhoPisoFoam support gravity, and how would it be implimented?

--James

matejfor · February 2, 2010, 15:58

Good to hear!

now the gravitation. Before turning most of my brain on, do you really need it?
You are filling a volume quite fast and compress it a lot. Does the gravity really plays important role in your analysis.
Is yes, then let's have a look how the gravity is implemented.
Let's have a look into: $FOAM/applications/solvers/heatTransfer/buoyantPisoFoam/buoyantPisoFoam.C

line 49 is important ( in my 1.6.x installation) with
#include "readGravitationalAcceleration.H"

then the gravitational force is taken into account as a source of momentum in UEqn.H
and pEqn.H

especially with a pressure equation it is not going to be copy&paste as I bet the pressure equation looks different in the fully compressible case. But with a pen and pencil, DoNotDisturb sign and mug of tea, you should found out how to do it.

hope this helps

matej

January 29, 2010, 18:51	Error in solver? (buoyantPisoFoam)	#1
fijinx Member James Baker Join Date: Dec 2009 Posts: 35 Rep Power: 17	This is for you smart ones out there (or at least smarter than me)! I have a simplified problem where I have a 1 meter cubed box with an inflow of 1kg/sec (trying to model filling a tank with gas). after 15 seconds I check the values, and the pressure is very much off (factor of 12ish). I have check, re-checked, and re-re-checked my values in all the files, but they all look accurate to me. Based off hand calcs, I'm expecting a pressure of around 1.724 MPa after 15 seconds, but it only gets to 135 kPa. Anyone out there that can crack this? I am using buoyantPisoFoam to solve the problem. http://rapidshare.com/files/343125087/meterBox.zip.html

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January 29, 2010, 18:56		#2
fijinx Member James Baker Join Date: Dec 2009 Posts: 35 Rep Power: 17	The link above is the full case file.

February 1, 2010, 14:27		#3
fijinx Member James Baker Join Date: Dec 2009 Posts: 35 Rep Power: 17	Anyone have a chance to look at this yet? I'm still pulling my hair out.

February 2, 2010, 03:43		#4
matejfor Senior Member matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 182 Rep Power: 17	To save some of your boldness for other cases, just without much compressible experience and source checking, I would say, that this buoyant solver, although compressible, is only weakly compressible. It means it does compute the buoyancy force from density difference and not the Boussinesq approximation, but is not suited for real compressible case like yours. I would suggest to try rhoPisoFoam or some other real compressible solver. I hope someone will correct me if I'm wrong. Good luck matej

February 2, 2010, 07:00		#5
Graham81 Member Join Date: Dec 2009 Posts: 57 Rep Power: 17	Hey James, Since Im waiting on a simulation, Im trying to think with you....though Im rather newbish to openFOAM. I downloaded your case, but was wondering where you prescribed the inflow of gass. Pieter

February 2, 2010, 07:05		#6
Graham81 Member Join Date: Dec 2009 Posts: 57 Rep Power: 17	My bad, I see it now in the BC for U.

February 2, 2010, 15:19		#7
fijinx Member James Baker Join Date: Dec 2009 Posts: 35 Rep Power: 17	Thank you matejfor! You were correct, using rhoPisoFoam gave me almost exact results to my hand calculations. I do however have one delima now. Does rhoPisoFoam support gravity, and how would it be implimented? --James

February 2, 2010, 15:58		#8
matejfor Senior Member matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 182 Rep Power: 17	Good to hear! now the gravitation. Before turning most of my brain on, do you really need it? You are filling a volume quite fast and compress it a lot. Does the gravity really plays important role in your analysis. Is yes, then let's have a look how the gravity is implemented. Let's have a look into: $FOAM/applications/solvers/heatTransfer/buoyantPisoFoam/buoyantPisoFoam.C line 49 is important ( in my 1.6.x installation) with #include "readGravitationalAcceleration.H" then the gravitational force is taken into account as a source of momentum in UEqn.H and pEqn.H especially with a pressure equation it is not going to be copy&paste as I bet the pressure equation looks different in the fully compressible case. But with a pen and pencil, DoNotDisturb sign and mug of tea, you should found out how to do it. hope this helps matej