Your questions touch on key aspects of fluid-structure interaction (FSI) modeling in COMSOL. Here's some guidance to help address the issues:

1. Setting up the “Moving Mesh” and Avoiding Circular Dependencies

The "Circular variable dependency detected" error typically arises from improper coupling or setup of variables between the structural and fluid domains. Here's how to set up the Moving Mesh correctly:

• Ensure Proper Boundary Conditions:When using fsip1.u_solid, fsip1.v_solid, and fsip1.w_solid, ensure these are applied only to the fluid-structure interface boundaries and not mistakenly to solid-only or fluid-only domains. Check that the structural boundaries have been assigned properly in the FSI interface.
• Use the Correct Prescribed Deformation: Assign the solid displacement to the mesh displacement in the Moving Mesh node on the fluid side. For instance:In the Deforming Domain node under Moving Mesh, set ux = fsip1.u_solid, uy = fsip1.v_solid, and uz = fsip1.w_solid. Avoid setting redundant boundary conditions directly in the structural or fluid physics, as these may lead to circular dependencies.
• Manage Solver Dependencies:Circular dependencies often stem from feedback loops in the physics couplings. Ensure the solver sequence is configured so that structural mechanics solve before fluid mechanics in each iteration.
• Initial Mesh Configuration:Ensure that the initial mesh does not overlap or deform incorrectly at the interface, as this can propagate errors during solution steps.

2. Identify Pair vs. Contact Pair in Assemblies

When working with assemblies in COMSOL, the distinction between Identify Pair and Contact Pair is critical:

• Identify Pair:Used when two boundaries are geometrically identical and you want to "glue" them together, effectively treating them as a single entity. This is common in situations where no relative movement or complex interaction is expected at the interface.
• Contact Pair:Used for modeling interactions where boundaries are in contact but can separate or slide relative to each other. Essential for contact mechanics problems or when dealing with interfaces involving friction, separation, or other nonlinearities. For FSI problems, you typically use Contact Pair if the fluid and structure interact dynamically.
• Which to Use for FSI?In most FSI problems, the fluid-structure interface does not involve sliding or separation; hence, Identify Pair is appropriate. Use Contact Pair only if there are additional structural interactions (e.g., between two solid structures that may slide or interact).

General Tips for FSI in COMSOL:

Let me know if you need further clarifications or assistance!