Fluid-Structure Interaction — ALE Formulation & Coupling
One-way and two-way FSI, Arbitrary Lagrangian-Eulerian (ALE) formulation, mesh deformation, partitioned coupling, and added mass instability.
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Quick Explainer
What is the ALE formulation and why is it needed for FSI?
ALE allows the mesh to move independently of the fluid — not fixed in space (Eulerian) and not tied to material points (Lagrangian). When a structure deforms and the fluid mesh must follow the boundary, ALE deforms the interior mesh smoothly while maintaining good quality. Without ALE, large structural deformations would invert the fluid mesh.
What is the added mass instability in strongly-coupled FSI?
When fluid and structural densities are comparable (blood in arteries, water around marine structures), the fluid exerts force proportional to structural acceleration. Weakly-coupled schemes pass loads once per time step, ignoring this feedback — leading to numerical instability. Strongly-coupled schemes iterate within each time step until convergence.