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

Solver-guidance: Dilatable or fully compressible?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   September 21, 2012, 04:46
Default Solver-guidance: Dilatable or fully compressible?
  #1
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
Dear all,

at the moment I am starting some simulations which certainly will become compressible at some point (Ma >> 0.3).

If I am informed correctly, there are two ways density can be influenced, either via the ideal gas law or via velocity in addition. The first case would be called dilatable, the second one fully compressible.

Unfortunately also the existing documentation does not tell which solvers are working dilatable and which one work fully compressible. Thus at least here I would like to establish a small list which solvers fulfill which criteria. Please help out as well, if possible!
I will update the list with information in this first post.

The list of socalled compressible solvers I put below is compiled from both versions of OpenFoam, the 1.6-ext and the 2.0.x releases...

Solvers are:

rhoCentralFoam
rhoPimpleFoam
rhoPisoFoam
rhoPorousMRFPimpleFoam
rhoPorousSimpleFoam
rhopSonicFoam
rhoSimpleFoam
rhoSimplecFoam
rhoSonicFoam
sonicFoam
sonicFoamAutoMotion
soncLiquidFoam

Cheers,
Bernhard
Linse is offline   Reply With Quote

Old   September 21, 2012, 05:00
Default How to know?
  #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
As I certainly do want to contribute as well: Can someone tell me definite criteria for the solver to be dilatable or fully compressible?

Of course the best would be to get a short description why a specific solver should be regarded fully compressible or dilatable only.
For me the default assumption would be that any solver not proven fully compressible is dilatable...
Linse is offline   Reply With Quote

Old   September 24, 2012, 06:02
Default
  #3
Member
 
Franco Marra
Join Date: Mar 2009
Location: Napoli - Italy
Posts: 52
Rep Power: 8
francescomarra is on a distinguished road
Dear all,

I too do not have the answer, but maybe I can try to provide some indications.

Probably it is necessary to check how the different solvers treat the propagation of sound waves. Strictly speacking, fully compressible solvers should be able to fully resolve sound waves. This is usually the case for so called density based solvers (rhoSonicFoam ?) that explicity integrate in time the density equation, recovering the pressure from the equation of state.

This approach, very commonly adopted for supersonic flow, is not suitable when dealing with many cases of weackly (Mach < 1) compressible flows, as it would require too small time steps with respect to the time scale of the phenomena and because of numerical issues arising from the disparity of flow velocity with respect to sound speed.

Now, several approach exists to deal with this type of problems. If the Mach number is very low (Mach << 0.3) but large density gradients occurs because of the presence of strong heat addition (like in many combustion systems) compressibility effects (from pressure waves) are negligible with respect to thermal expansion, in the sense that compressibilty equilibrates much faster than thermal expansion.
Some solvers use this property to solve a different set of equations based on asimptotic expansion with respect to the Mach number. They are therefore unable to include real compressibility, but they solve the thermal expansion with a great gain in computational demand and stability. I think that fireFoam is probably based on these assumptions.

Otherwise, in the case that both effects are present (thermal expansion and sound wave compressibility, Mach <1), there are solvers that, without modifying the original formulation, use an iterative procedue that solve the numerical issues arising from the disparity of scales. I am not really sure, but I understood that, by deriving an elliptic equation for the pressure, you are able to find a pressure field that equilibrate the flow at a time step longer than that required to fully resolve the sound wave propagation. This probably means that this class of solvers, if used with small enough time steps, should be able to recover a fully compressible solution, while, if adopted with larger time steps, allow for an efficient computation of weakly compressible flows. If my ideas are correct (plese help me to check this issue) many of the solvers based on the solution of an elliptic pressure equation like rhoSimpleFoam and rhoPimpleFoam, should fall in this class.

I am sorry I do not have a definitive answer, but I hope the this can contribute to form a scheme for the classification of the solvers.

My best regards to all,

Franco
francescomarra 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
Solver for compressible Navier-Stokes equation treima OpenFOAM Running, Solving & CFD 3 May 30, 2012 05:25
a question about compressible solver xck1986 OpenFOAM 3 April 16, 2011 04:27
Steady compressible solver smart OpenFOAM Running, Solving & CFD 8 August 20, 2010 07:46
Liquid instead of gas in a compressible solver !? iggor OpenFOAM 15 October 14, 2009 11:58
Can a compressible solver solve incompressible ? jinwon park Main CFD Forum 17 December 13, 2007 06:00


All times are GMT -4. The time now is 03:24.