[Moslem]

Hi everyone,
I tried to run a parallel simulation in OpenFOAM. Although decomposition of fields using the decomposepar seemed fine, the processor boundary faced a problem after the first BC update. Problem occured in the processor boundary in the Temperature field. It returned zero (or very small ) values for the elements on the processor boundary while it should return values close to the initial value of 300 K.
Does anyone have any idea how to solve this problem?

Thanks in advance.
Moslem

[Neka]

Hello M. Sabouri,

I also have problems with processor boundaries. If you use the variable time step for your calculations, then this thread would probably help you:
https://www.cfd-online.com/Forums/openfoam-bugs/94085-strange-processor-boundary-behavior-linearupwindv.html

I posed my problem already there, but nobody answered so far. So, I decided to post my problem here, because it suits to this thread as well.

I've observed a very strange behavior at the processor boundaries.
I work with foam-extend- 4.0 and use a kind of a mix between reactingFoam and sonicFoam to simulate chemical kinetics at supersonic velocities. My test case is a 2D wedge in a supersonic flow. I heat the gas locally on the surface of the wedge which leads to a decomposition of molecular nitrogen into atomic nitrogen.


When I run my test case serial then everything is fine. I get my dissociation of N2 into N at the location, where I heat the gas. But after I switched from serial to parallel (here 4 processors) the N mass fraction started moving along the processor boundary – see in the attached image a strip on the left side. What you see is the upper side of the wedge where I heat the gas. For visualizing I reduced the maximum of the color scale.


I tried to update mass fractions after the Yi-equation with Yi.correctBoundaryConditions(). I tried to change the div and laplacian schemes which showed some changes in the behavior but the behavior was still there.


When I deactivate the convective and diffusive terms in the Yi-equation then this phenomenon does not occur which means the problem is with the div and laplacian schemes. Correct?


I don’t use adjustTimeStep but a fixed time step. My div scheme in the Yi-equation is Gauss limitedLinear01 1 and my laplacian scheme is Gauss linear uncorrected.


Does anybody know what happens with my mass fractions? Has anyone observed the same error?


Any suggestion or advice are welcome!

Regards,
Alex

[Moslem]

Hello Neka,

I am not sure if this can help you. The problem in my simulations occurred when I used a boundary condition (e.g. the maxwellSlipU in rhoCentralFoam) which calculates the gradient of a variable when applying the BC. As described in the following link, it would overwrite the processor buffer values.

Please see the following link.

https://bugs.openfoam.org/print_bug_page.php?bug_id=659

[Neka]

Hello M. Sabouri, hello everyone,

First, thanks for your reply M. Sabouri.

Second, I guess I solved the problem but I don’t know why it works.

I simulate supersonic flows at very high velocities, meaning, at deltaT of about 5.0e-9 I have a max Courant Number of about 0.2.
So, I have to work with very small deltaT values.

I use the following solver conditions for my Y-Equation:

Solver BiCGStab;
preconditioner DILU;
tolerance 1e-08;
relTol 0;

If I reduce the tolerance to 1e-07, the strange behavior at the processor boundaries disappears! I don’t know why it happens.
Anyhow, the proper combination of deltaT and tolerance are needed.

Does anybody know why it happens?

Regards,

Alex

[Neka]

Ok, it turns out I haven’t solved the problem!

When I reduce the tolerance from 1e-08 to 1e-07, the strange behavior at the processor boundaries disappears, but simultaneously the Y-equation stops convecting, meaning the mass fractions are frozen and don’t move anymore. I probably forgot to mention that my mass fractions of N (atomic nitrogen) are very small (e.g. Y_N = 1.0e-08), so I work with very small numbers here.

Any suggestion or advice are welcome!

I’ll continue this discussion in this thread:

https://www.cfd-online.com/Forums/openfoam-bugs/94085-strange-processor-boundary-behavior-linearupwindv.html#post659135