IQN fails for quickly changing forces during wetting in a free-surface FSI problem

[uekerman]

@JessicaMarino

See the video

https://drive.google.com/file/d/1GUvul8tab72HvaYIqBgiMpLdvkIIXzlR/view?usp=sharing

The link is unfortunately private. I guess you have to adjust some sharing option in Google Drive.

@JessicaMarino

For this point with the acceleration methods. How can decide correctly the value for the filter, the number of reused iterations, and time reused?

There is no simple to answer to this. Some guidelines:

• The more past information you use the better the performance in general (as long as you don’t use too much, then the condition gets bad). But then the coupling scheme has also more troubles to react to sudden changes. An interesting future direction of research would be to make IQN more adaptive.
• The filter should work, meaning not filter out too little, but also not too much. You can check the number of filtered out columns per timestep in the “iterations” file of the second participant.

@JessicaMarino

• If the time is bigger, Calculix converges very fast and the simulation is more stable but Fastest diverges because the multiphase model does not support very large time-steps.
• If the time is smaller and appropriate for the multiphase fluid, Calulix diverges after a few coupling interactions.

This sounds like an intersting use-case for the waveform relaxation we want to implement in preCICE, @BenjaminRodenberg’s PhD project. Preliminary results here, but might still take a year till we have this directly in preCICE. At the moment, there is no good solution for this.

@JessicaMarino

How can be a good proportion between the fluid and structure elements at the fluid-structure interface?

Well, hard question again. The black-box philosophy would say that both solvers should use what is best for them, so to balance the error between both. And then the data mapping has just to cope with those meshes. But, of course, the meshes also have an influence on the mapping error.
Maybe some hope here: we are currently working on an artificial testing environment for data mappings. So that you can test mappings between your meshes without needing to run the complete coupled simulation. Because in the end, things like this simply need a lot of testing and comparison.

@JessicaMarino

This test case is a 2D case (0.8mx0.8m fluid domain, 0.004mx0.09m rubber beam), I only have one volume on the z-axis (for FASTEST) and in the mapping setup z-dead=true. However, I do not know what is the appropriate width for the grid in the z-direction. On the one hand, if I use a very small value, for example, 0.002 or 0.006, the grid aspect ratio for the fluid domain is okay, but the RBF mapping diverges and to works requires at less 2 elements (C3D20R or C3D8i) are defined in the z-direction of the structure.

This is odd. If you have z-dead="true" the RBF mapping should not see the width in z-direction at all. Could CalculiX have troubles with too small aspect ratios?

@JessicaMarino

My question is how can I decide the appropriate width for a quasi2D case? and how can I avoid these unequal displacements?

You do give force values in z direction. Then, CalculiX also computes a deformation in z direction and, thus, you get different displacements along the z axis. Could an easy solution be to set the z components of the forces to 0?

@JessicaMarino

Finally, in this case, the structure is a rubber beam (E=3.5MPa, nu=0.5), I am still using the elastic model (with nu=0.49) and the non-linear effects are considered (NLGEOM). Do you think that it is adequate? or the hyperelastic model should be used? However, for the hyperelastic model, I do not have the complete parameters that require these models. Maybe do you have more information about the constants for a rubber material?

That is, unfortunately, beyond my expertise. Maybe @KyleDavisSA knows more. Or you could ask on the CalculiX mailing list.