Coupled FSI Lotus-CalculiX simulation using the preCICE library

Pre-processing

First, the structural model needs to be build. This is done by changing the geom.geo and the generate.py files. Once you have set the correct geometric dimensions, you can generate the mesh using

cd Solid
python generate.py -L 64 -K_B 0.15 -m_r 0.5

which produces the mesh file (geom.inp) required and the calculix.inp setup file. The additional arguments are used to define the length-scale of the problem, the Cauchy number and the mass ratio

This automatically generates the boundary file fixed.nam. But you could specify it manually as well

(DEPRECATED) Boundary conditions for the structural sub-problem

You then have to specify the boundary nodes in the fixed.nam file. The corresponding boundary conditions are set in the calculix.inp file. Here we have a clamped boundary condition on one edge of the plate.

This can be done through the cgx pre-processor.

cgx -c geom.inp

which opens the cgx editor. You can then plot all the nodes with their number with plot na all. To generate a fixed.nam file you have two options, either add by hand the node number that are part of the boundary, or select them by drawing a box around it. This is done by first creating a node list

qadd fixed nam

then pressing a to enter the add mode. To draw a rectangle from which the nodes will be selected press r on the two opposite corners of the desired rectangle. Move the rectangle such that all boundary nodes are inside and the press n+a. This adds all the nodes to the fixed set. You can then exit the selection with q. To write the fixed.nam file type

send fixed abqs nam

This should write the corresponding file.

Running the Simulation

To run the simulations in serial mode simply use the runFSI script in a terminal

runFSI -npF -npS

where npF and npS are the fluid and solid parallel decomposition, default is one for both. Alternatively, you can open two terminals, and go in the the Fluid and Solid directory and run the two following commands

preCICE-Lotus-CalculiX/Inverted-Flag/Fluid$ runFluid -np

preCICE-Lotus-CalculiX/Inverted-Flag/Solid$ runSolid -np

To clean all the files generated, simply clean the repo using

cleanFSI

or the individual folders with cleanSim.

Restarting

You can restart a simulation from the last time step provided a *RESTART,WRITE card was added in your calculix.inp file. Thos will save the last time step and allow to restart from there (or from any time step if you want). To restart from the last time step run

runFluid 1 ./

in the `Fluid`` directory to restart using the latest time step and

runSolid 1 restart

in the solid directory. This will moce the calculix.rout into a restart.rin file with all the restart information, as well as the mesh ans boundary condition information. Restarting requires a restart.inp file, which should have the following format

*RESTART,READ,STEP=

*STEP, NLGEOM, INC=10000000000
*DYNAMIC, ALPHA=0.0
 0.25, 100000

*CLOAD
 Ninterface, 1, 0.0
 Ninterface, 2, 0.0
 Ninterface, 3, 0.0

*NODE FILE,OUTPUT=3D
 U

*END STEP

where the *STEP card and the following entries are identical to the previous input deck, the header now just has a *RESTART,READ card that will open the file restatrt.rin file at the desired step, default is the last one saved.

Results

Here is the results of the inverted-flag problem for a period of oscillation.

We can compute the negative finite time Lyapunov exponent (attracting structures) of the flow field