# Inertial Forces in OpenFoam-Calculix coupling

Hello,
I was looking for information about the way the CalculiX - OpenFOAM coupling transfers the inertial force. I try to simulate a dynamic case, where inertial forces are necessary to achieve a proper solution.

From what I found it seems that these forces are not taken into consideration in the coupling. As I understand, for each timestep the forces from fluid mesh are transferred to solid mesh, but the inertial forces resulting from the previous timestep are not added to these forces. So for example if the structure gained a velocity in the previous timestep, the new timestep starts with the velocity of the structure equal to zero?

Am I wrong? Or how can I set the coupling to take this data in consideration.

I was looking for information about the way the CalculiX - OpenFOAM coupling transfers the inertial force.

The inertial forces are usually not directly transferred in FSI scenarios. If your Fluid faces inertial forces such as gravitational acceleration, the corresponding force contribution is already captured in the force calculation on the Fluid side itself.

but the inertial forces resulting from the previous timestep are not added to these forces.

As mentioned above: if your Fluid undergoes acceleration, the force contribution will be captured in the force calculation.

So for example if the structure gained a velocity in the previous timestep, the new timestep starts with the velocity of the structure equal to zero?

I don’t understand what you exactly mean here. Of course it might happen that your velocity becomes zero in some time-step (although it was nonzero before). In general, these kind of things are numerically mostly dependent on the time-integration scheme used in CalculiX.

I was wondering about something similar, if my interpretation of the question in the OP is correct: Given the state of the solid structure at time t, Calculix will receive force data from OpenFOAM and compute the new state of the solid structure at time t + dt. But does the solid state at t+dt depend on the velocity of the solid structure at time t, or does it always assume that the solid structure was at rest at time t when computing the new state?

In my case, Calculix uses a *DYNAMIC analysis. Does the FSI coupling neglect the current velocity of the solid structure when it computes the positions at the next time step?