Fluid-Structure Interaction (FSI)

Fluid-Structure Interaction (FSI) is a surface coupled problem where the state of the fluid model depends on the state of the solid model and vice versa. This dependency can be symmetric or asymmetric. Asymmetric problems are often referred to as one-way coupled problems, which means that one of the models is independent whilst the other is dependent.

An FSI simulation is governed by the principle that corresponding fluid and solid particles at the coupled interface move with the same kinematics (position, velocity, and acceleration) and experience the same tractions. This principle results in the consistency of kinematics and consistency of forces as explained in FSI Solution Strategy. In Simcenter STAR-CCM+ this pair of consistencies are used to model the mutual influences as follows:
  • The information transferred from the fluid to the solid is the fluid traction which is composed of the fluid pressure and the wall shear stress. This transfer happens over the coupled wall boundaries (fluid-structure interface).
  • The information transferred from the solid to the fluid is the deformation of the solid, specifically the deformation of the fluid-structure interface.

In general, FSI simulations are consistent both in their kinematics and their forces. These cases are denoted as two-way coupled problems. Under certain conditions this full consistency is broken and the simulation can be considered as a one-way coupled problem. For example, if the impact of the solid deformation on the fluid is negligible, the simulation can be considered as a one-way coupled fluid to structure simulation. In this case the information is considered to be transferred only in the fluid to structure direction. Equally, you can consider that the fluid model is independent while the solid model is dependent with regards to the fluid traction.

If the impact of the fluid traction on the solid is negligible then the simulation is considered as a one-way coupled structure to fluid problem. In this case the information is transferred only in the structure to fluid direction. Equally, you can consider that the solid model is independent while the fluid model is dependent with regards to the solid deformation.

Cases in which the solid deformation does not impact the fluid and the fluid traction does not impact the solid are considered to be uncoupled. For these cases there is no transfer of information in either direction. Simcenter STAR-CCM+ can also simulate scenarios where the current position of the undeformed rigid or stationary solid is passed to the fluid while the fluid traction is passed to the solid.

A two-way coupled FSI problem within Simcenter STAR-CCM+ is perceived as the sum of exchanges from the fluid to the solid and from the solid to the fluid. This designation allows you to split the modeling of the two-way FSI problem into its two respective parts. While a two-way coupled problem requires a concurrent solution strategy, a one-way coupled problem can be solved either concurrently or sequentially. A sequential procedure is described in One-way Coupling with Prerecorded Flow Solution. In the sequential procedure the independent model is solved first and the interface solution is recorded to a file. The dependent solution is then solved taking into account the previously recorded interface solution of the independent model.