Stiffness issues with OpenFOAM and CalculiX

Using preCICE
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Hi,

I’m currently fiddeling around with some benchmark cases for my thesis.
I came up with this paper and decided to recreate this case because it’s quite similar to the Perpendicular Flap tutorial.

I rebuilt the environment like it’s described in the paper. My simulation is running quite good and I have no problems in stability.
I know I’m working with quite low Reynolds numbers here and I also know there are some Issues with the stiffness of some elements in CalucliX. Therefore I tried the following ones:
C3D8, C3D8I, C3D8R, C3D20, C3D20R

But there is no way I am getting close to any deformation. My deformation is barely visable.
The different elements didn’t change much.

My first guess was that I’ve made a mistake during the setup of my OpenFOAM boundaries but I’ve double checked everything and I guess I did everything like I used to.

In CalculiX, the only thing you can change that makes a difference is the inp file. Since you just have to copy the chracteristics of the material and set up some boundaries there is no much space for mistakes.

Did somone experience similar things? Or am I missing something out?

Any help is grately appreciated! :slightly_smiling_face:

I’m running the case on my cluster that’s why there are some tweaks like the network keyword in the precice config and the run files:

OpenFOAM v1906
preCICE v2.3.0
CalculiX v2.19

For a little shortcut:
precice-config.xml (2.4 KB)
flap.inp (436 Bytes)

The whole case (I’ve only put the coarsed version of the fluid-mesh in here, because the original one is too big to upload):
pf-C3D20R-normal.zip (1.1 MB)

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Hi,

I don’t see any obvious flow at first glance but didn’t run your case yet. A few suggestions to nail down the problem:

  • You can check that openfoam outputs correct forces with preCICE exports Export configuration | preCICE - The Coupling Library. Do they look correct ? What deformation would you expect with such forces ?
  • If the problem lies in CalculiX, maybe try running a CalculiX-only simulation, with for instance constant applied force. This should help deciding if the error is in CalculiX or the coupling configuration.
  • The perpendicular flap tutorial was in 2D and you moved to 3D. Maybe the error lies in mesh setup or the choice of thickness ?

Hope it helps,
Boris

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Hi @boris-martin,

your first tip has brought me to an idea. I searched for the reason why my forces are so low and I found the mistake.

When I entered all the values of the fluid, I did not pay close attention to the preciceDict. There was still a wrong density in it. So the wrong density was given to CalculiX. In addition, I also entered the kinematic viscosity in the preciceDict, like it’s written in the doc.
This solved the problem and the deformation is now correct.

Thank you for your help and your advice, otherwise would not have found the error.

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