I simply ramp the application of forces to the structure: i.e. I apply a coefficient to the forces that come from the interface. I usually find it useful or necessary to use small forces at the beginning of the simulation. For example I start the simulation applying only 10% of the force for the first 0.5s then gradually go to 100% (linearly or with a 1-\cos law or in steps) for 0.5 to 1s.
The approach above works much better than time ramping for me. I saw that you use a 1-\cos time ramping in your setup.
Well it is not exactly my own solver: it is developed and maintained at Politecnico of Milan: This solver is mainly used for multibody dynamics. There are many benchmarks for this solver, but not in FSI simulations. I have benchmarked it with FSI2 (as described here: preCICE adapter for MBDyn solver for beam elements - #6 by Claudio) and I think that in that case the results are good.
Your parametrization is interesting and makes the simulation a little better, meaning that I can reach higher values of the coefficient in the ramp. Nevertheless I cannot manage to run a complete simulation for FSI3.
FSI2 is good, it runs smoothly: if I use your setup (preCICE parameters and finer mesh) it runs better, but the results are close to the ones I get with the setup of the tutorial.
FSI3 crashes no matter what. Simulation parameters influence only the time when the simulation crashes. Note that the structural configuration is the same I use for FSI2 (apart from density and Young modulus, of course).
- A finer mesh is better, I also tried an even finer mesh
- I used different a different number of structural elements
- for me the thickness of the domain is important, meaning that the narrower the better
- I even tried with an unrealistic structural damping. In this latter case I manage to apply 100% of the force (which is otherwise impossible) but nevertheless after 0.4s it crashes anyway.
Crash means that at some point the convergence start to become slower and slower up to a point where the simulation blows up. At each coupling iteration the forces become higher and the fluid stops for some floating point exception.
It seems to me that the structural solver is not enough dissipative for this simulation, but I am not sure.
Claudio