LES Outlet BC: Convection Boundary
 

Hi Everyone,

I was wondering is the convection boundary condition for LES suggested by Peric and Ferzinger (d(phi)/dt +U*d(phi)/dx=0) has been implemented in OF 2.1

Previous threads have indicated that modifications have been made to OF 1.6; but since it would be a very useful BC, I thought that maybe it has been implemented as part of the standard release.

Currently I have a simulation blowing up with a constant pressure outlet, or an inletOutlet BC. I was would like to try a more physically realistic BC that taken the transient nature of LES into account.

Cheers

Hi,

I think the advective BC is what you are looking for.

Fumiya

reply
 

Hi Fumiya,

From the OpenFOAM documentation advection BC is d(phi)/dt =0. So this isn't really true, since mass conservation isn't achieved.

You could also consider using fixedMeanValue to set a surface averaged pressure. This causes less influence on the velocity field and might solve the 'blowing up' problem your having. Of course you should verify whether this BC is physically applicable to your case...

Cheers,

L

Reply
 

Thanks Lieven, I will give that a try.
I am really confused about why this simulation is being such a bugger....LES I guess!

Reply
 

Hi Lieven,

Is there anything special I have to do (e.g.download and compile) that BC?

Cheers: Adam

p.s. trying pimpleFoam to add stability...still no luck, solution is diverging.

You can find a link to the sources in the following discussion:
http://www.cfd-online.com/Forums/ope...condition.html

Cheers,

L

Update
 

Hi Guys,

I implemented the meanFixedValue pressure outlet BC. I am getting what I think is strange behaviour, check my output. Pressure residual blows up but Co number is stable...anyone seen this before?




Cheers

Question
 

Still having trouble figuring out why this is happening.
The same solution settings work perfectly in Fluent, yet in OpenFoam the pressure residual stabilizes around 0.85, even when the Co number is stable (around 0.035).

Any ideas why? Has anyone else seen anything similar?

Cheers

Very strange indeed.
Unfortunately, I've never seen this type of behaviour before and I don't really have a clue what could be wrong... What are the other BC for the pressure?

Cheers,

L

Reply
 

The outlet BC for pressure is fixedMean Value; meanValue 0;

It's really strange. I am going to post all of my dictionaries. When this type of thing happens it is usually some fundamental mistake...but I can't see it.

I really appreciate all of the help. Its hard to make a direct comparison between FLUENT and OpenFOAM, but I find it strange that OpenFOAM would be giving such odd behavior.

Reply
 

and also the 'p' dictionary.

Cheers

Update
 

This is really confusing me. I have (once again) run the simulation in fluent, using piso, second order discretization in space and time; same initial conditions, same BC for U and p (constant V in, zero gradient out, constant average p out). Fluent is stable, converges nicely, doesn't blow up.

Openfoam won't converge the pressure residual; I have tried many different BCs. Could someone please please look at my dictionaries I posted above? There must be something fundamentally wrong. How can two solvers who have been compared many times be acting so differently on the same mesh?

I have been trying to solve this for so long that I need a second set of eyes...unless there is a bug in OpenFoam i'm not aware of.

Also, just to double check, when monitoring convergence, it's the "initial residual" that matters right? The "final residual" is related to the linear equation solver if I am correct?

Cheers

I think it is DDT(psi) not ddt(psi), so we have ddt(psi)+U.ddx(psi)=0