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

Oodles and pressure oscillation

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 4, 2007, 04:48
Default Hi everybody, I'm trying to i
  #1
New Member
 
Nicolas Coste
Join Date: Mar 2009
Location: Marseilles, France
Posts: 11
Rep Power: 8
nikos is on a distinguished road
Hi everybody,
I'm trying to investigate airflow around a rectangular cylinder.
The choosen mesh is an hexaedral unstructured mesh appropriate for a low reynolds number modelling at wall. I've tested this case under turbfoam and everything is OK.
I've started the computation with smagorinsky approach and same parameters as those provided in pitzdaily case conserning the numerical approach.
A turbulentinlet is provided at inlet and pressure outlet at outlet.
Around the body, the airflow is developping fine, however I can see a pressure oscillation in time (from -0.2 to 0.2 in a p* formulation ) travelling from inlet to outlet where p* remains 0.
Solver is GAMG with GS smoothing.
CFL max is 2 and mean value is quasi 0.
I've tried to reduce the timestep but similar phenomena occurs.

Can it be due to delta coefficients, because my mesh is relatively coarse at inlet and I have an agressive refinement around the body ?

I've also tried to increase non orthogonal corrections but nothing has changed ?

What do you think of using GAMG as preconditionner for PCG approach because there is a large time reduction during time step ?

ps : i've applied modified backwarddtscheme provided and recompiled.
nikos is offline   Reply With Quote

Old   June 4, 2007, 06:29
Default what do you mean a "turbulentI
  #2
Senior Member
 
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 12
eugene is on a distinguished road
what do you mean a "turbulentInlet" is provided at the inlet?

CFL max should be 1. Anything larger can destabilise the solution.

Increasing non-orthogonal correction will make any problem worse. More stability is gained by decreasing non-orthogonal correction, i.e. Gauss linear limited 0.333;
eugene is offline   Reply With Quote

Old   June 4, 2007, 06:57
Default Thank you for responding so qu
  #3
New Member
 
Nicolas Coste
Join Date: Mar 2009
Location: Marseilles, France
Posts: 11
Rep Power: 8
nikos is on a distinguished road
Thank you for responding so quickly Eugene

I've setup a turbulentInlet condition with low velocity fluctuation scale (0.005) even it is not the best condition ... but for a sample test it's better than nothing

I've forgot to say that in the region where mesh is refined quickly, there are particularly elevated values for the Nusgs

When you say destabilize do you mean diverges ? because in this case there is no order changes on all variables, there are pretty good except that pressure around the cylinder is modulated by the global pressure oscillation travelling in the domain.
nikos is offline   Reply With Quote

Old   June 4, 2007, 07:30
Default 1. Skip the turbulentInlet, it
  #4
Senior Member
 
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 12
eugene is on a distinguished road
1. Skip the turbulentInlet, it is worse than nothing.

So you have pressure oscilations traveling from your inlet to your outlet messing up the pressure measurements as they pass?

I never use Smagorinsky so I cant help you there. Try SpalartAllmaras or oneEqEddy.
eugene is offline   Reply With Quote

Old   June 4, 2007, 07:52
Default Ok , but I've tried without an
  #5
New Member
 
Nicolas Coste
Join Date: Mar 2009
Location: Marseilles, France
Posts: 11
Rep Power: 8
nikos is on a distinguished road
Ok , but I've tried without and set up a classical inlet ... same problem

Concerning oneq eddy, if I don't init my case with smagorinksy during few time steps computation won't start, but after that using solution coming from smagorinsky, I have the same behaviour with oneeqeddy

DES is doing the job on a coarse mesh.

I will try it changing the discretisation scheme as proposed

If I find where te problem comes from I will post a message

thank you Eugene

did someone use turbulence generator like works done in Chalmers ?
nikos is offline   Reply With Quote

Old   June 5, 2007, 08:46
Default Hello Eugene, where is the or
  #6
Member
 
Ralph
Join Date: Mar 2009
Posts: 40
Rep Power: 8
ralph is on a distinguished road
Hello Eugene,
where is the orthogonal correction used?
Just in the laplacianSchemes and the snGradSchemes, or anywhere else?

Ralph
ralph is offline   Reply With Quote

Old   June 5, 2007, 08:50
Default Yes, the snGradScheme correcti
  #7
Senior Member
 
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 12
eugene is on a distinguished road
Yes, the snGradScheme correction feeds into a lot of other stuff though.
eugene is offline   Reply With Quote

Old   June 5, 2007, 08:56
Default Thanks
  #8
Member
 
Ralph
Join Date: Mar 2009
Posts: 40
Rep Power: 8
ralph is on a distinguished road
Thanks
ralph 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
Oodles and more r2d2 OpenFOAM Running, Solving & CFD 10 September 23, 2007 01:54
Oodles vs turbFoam rolando OpenFOAM Running, Solving & CFD 9 June 4, 2007 05:42
Parallel oodles sriharsha OpenFOAM Running, Solving & CFD 0 December 27, 2006 18:25
Pressure oscillation in time Hao Meng Main CFD Forum 3 June 5, 2005 15:35
pressure oscillation!! DJ Main CFD Forum 1 August 19, 2004 11:59


All times are GMT -4. The time now is 05:49.