CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM

rhoPimpleFoam/rhoPisoFoam-compressible flows

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

Reply
 
LinkBack Thread Tools Display Modes
Old   December 11, 2011, 11:41
Default rhoPimpleFoam/rhoPisoFoam-compressible flows
  #1
Member
 
achinta
Join Date: May 2010
Location: Sydney
Posts: 66
Rep Power: 7
achinta is on a distinguished road
Hi all,
I would like to learn about rhoPimple/rhoPisoFoam code. I have few questions.

1) The code solves rhoEqn first. Then it solves UEqn and hEqn. pEqn is solved at the end. Is it necessary to follow this order. Could we solve rhoEqn and other equations like scalar transport eqn after pEqn?

2)What is 'psi' in the pEqn of rhoPimple/PisoFoam? Could we directly use fvc::ddt(rho) instead of fvm::ddt(psi,p)? Whats the advantage of using the latter notation?

Thank you,
AV
achinta is offline   Reply With Quote

Old   December 12, 2011, 06:29
Default
  #2
Senior Member
 
Bernhard Linseisen
Join Date: May 2010
Location: Magdeburg/Geneva
Posts: 178
Blog Entries: 1
Rep Power: 7
Linse is on a distinguished road
I am not too deep into the particular solvers, but my take on the questions is:

1) It depends if the equations included later depend on the equations included before. If you see any reference to any part of the previous equations, it MIGHT be problematic for the equation to find the preconditions.
If there is no reference to the other equations, from the side of the solver there should not really be a problem.
But maybe that problem does not exist at all if the compiler deals fine with that problem. Just give it a try!

2) If I remember correctly, psi has to do with the flux and already contains division by time. So if I am right you would have a different dimensionset for the approach you offer, which - given OpenFOAM's dimensionality control - would cause the simulation to fail due to conflicting dimensionsets on the different equation terms.
Even if I am wrong on the resulting dimensionset (which is quite possible!), you need to be sure it doesn't matter if you define an explicit or an implicit equation term, because once you use fvm(ethod) and the other time fvc(alculus)...


Sorry for not giving perfect answers, but I guess at least I am pointing out some directions that might be helpful. ;-)
Linse is offline   Reply With Quote

Old   December 19, 2012, 18:57
Default
  #3
Senior Member
 
Join Date: Nov 2012
Posts: 168
Rep Power: 4
hz283 is on a distinguished road
If I didnot make s mistake, the density for output in rhoPimpleEqn is from thermo.rho(). So here why is the rhoEqn should be solved? Just for correction? Does anybody know something about this?

Thank you!
hz283 is offline   Reply With Quote

Old   December 20, 2012, 16:21
Default
  #4
Senior Member
 
Karl-Johan Nogenmyr
Join Date: Mar 2009
Location: Linköping
Posts: 255
Rep Power: 12
kalle is on a distinguished road
fvc::ddt(rho) would be an explicit formulation of the density derivative. You want something implicit, as you want to find pressure at the new timestep, such that continuity equation is fulfilled. rho is however equal to p*psi (for ideal gas, and where psi is compressibility). By writing fvm::ddt(psi, p) (which means implicit time derivative of the product of p and psi) you can get density at the new time step's pressure. You cannot, for instance, write fvm::ddt(rho), as the linear solver would not know how to get density at the new time step, while it does know how to get psi*p, as psi is a constant, while p at the new time step is the variable solved for (iteratively).

Hope it is a bit more clear,
K
kalle is offline   Reply With Quote

Old   December 20, 2012, 16:33
Default
  #5
Senior Member
 
Join Date: Nov 2012
Posts: 168
Rep Power: 4
hz283 is on a distinguished road
Hi Kalle,

Thank you so much for your reply. but what do you mean by 'the linear solver would not know how to get density at the new time step'? Could you please write more about this? Thank you in advance.

Merry Christmas!
h
hz283 is offline   Reply With Quote

Old   December 21, 2012, 03:11
Default
  #6
Senior Member
 
Karl-Johan Nogenmyr
Join Date: Mar 2009
Location: Linköping
Posts: 255
Rep Power: 12
kalle is on a distinguished road
The solver is solving to find p at the new time step. Rho at that time step is not known yet, as it depends on p. The solver can however still caluclate the ddt(rho) implicitly, as rho=p*psi, which consists of know quantities (psi) and the quantity solved for (p)

K
kalle is offline   Reply With Quote

Old   December 21, 2012, 09:07
Default
  #7
Senior Member
 
Join Date: Nov 2012
Posts: 168
Rep Power: 4
hz283 is on a distinguished road
Hi Kalle,

Thank you very much for your reply. About the rhoPimpleFoam, based on the method that the density is updated, i.e. rho=pressure/(RT). As far as I know, this pressure for the EoS is from Poission equation, so this is the hydrodynamc pressure. From this point, rhoPimpleFoam is a fully compressible solver. However, when it solves the governing for rho, rho*U, it seems that it still use the Simple or Piso method, they are methods for incompressible flows. I am a little confused about this. Can you give any comments about this?

Merry Xmas!
hz283 is offline   Reply With Quote

Reply

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
SIMPLE pressure correction in compressible flows Maarten de Jong Main CFD Forum 7 April 30, 2012 02:23
Using pressure based solver for compressible flows aamer Main CFD Forum 2 June 15, 2011 12:45
Using Compressible Solver (sonicFoam) to solve subsonic flows ezsoal OpenFOAM 0 October 27, 2009 10:13
Case check - Velocity-Inlet not compatible with compressible flows fluent-user FLUENT 2 September 15, 2009 08:46
PDF Model in compressible flows with shock waves Hanxsi FLUENT 0 February 26, 2006 21:51


All times are GMT -4. The time now is 22:38.