FSI Solution Strategy

Exchange of Coupling Data

The concept of coupling data exchange in Simcenter STAR-CCM+ can be summarised in the table below:

The underlying concepts relating to the construction of this table are explained in the subsequent sections.

Fluid to Solid

Data Transfer

The information flowing from the fluid to the solid is the fluid traction. The fluid traction is automatically interpolated from the fluid side of the interface to the solid side where it is being applied as a load. In order to avoid the loss of information during the interpolation step you are advised to apply a similar mesh resolution on both sides of the interface. The interpolation scheme supports conformal and non-conformal interfaces.

Consistency

In general the force consistency is an important part of an FSI simulation. In certain cases the force consistency can be broken intentionally in order to reduce simulation time by omitting information transfer from the fluid to the solid. Moreover, Simcenter STAR-CCM+ allows you to improve the simulation stability by ramping or clipping the fluid traction.

If the fluid traction is enabled and no ramping or clipping is applied, the forces across the interface are consistent.

For rigid 6-DOF bodies special force consistency conditions apply — see Deformable DFBI Bodies.

Solid to Fluid

Consistency

As mentioned above, in Simcenter STAR-CCM+ the interface deformation is directly linked to the mesh motion. The following kinematic consistency is enforced across the interface during solution initialization:

• Consistency of frames — meaning that both regions must use the same reference frame.
• Consistency of Interface Deformation — meaning that the mesh on both sides of the interface must deform in the same way.

Consistency of interface deformation can be achieved if the boundaries on both sides of the interface remain stationary, move as a rigid body, or deform consistently. For simulations containing rigid solid motion, the interface is either stationary or moves rigidly according to the motion of the solid, hence the solid does not deform (Interface not Deformed). Conversely, if the solid region Motion Specification is set to Solid Displacement, the solid deforms and so does the interface (Interface Deformed).

Solid Motion ↓	Fluid Motion
	Rigid	Morphing
Rigid	Interface not Deformed	Interface not Deformed
Solid Displacement	Invalid	Interface Deformed

Beside the consistency requirement across the interface, be aware that the interface deformation is directly linked to the transfer of information from the solid to the fluid. Because of this dependency there is no explicit option to enable the coupling from solid to fluid. Instead the coupling of solid to fluid is implicitly controlled by the solid motion. For this reason the respective FSI Coupling Specification at the fluid region interface boundary is a read only property and is controlled by the motion of the coupled solid region.

Solution Stabilization

The purpose of the Dynamic is to increase the performance of the FSI simulation by predicting the fluid tractions resulting from the change of the interface position. It is assumed that a certain volume of fluid moves with the FSI boundary acting as an added mass. This method is recommended for two-way coupled problems.

For Constant Displacement Under-relaxation you specify a fixed under-relaxation factor in the FSI solver which is then applied to the change of the interface displacement between consecutive iterations within a time step. The Under-relaxation factor is a single global value.