# AFSI: Automated Fluid-Structure Interaction Solver Development for Nonlinear Solid Mechanics

## TL;DR

AFSI is an open-source, flexible fluid-structure interaction solver built on FEniCS, capable of simulating large deformations in hyperelastic materials without remeshing, suitable for biomechanical and complex nonlinear applications.

## Contribution

It introduces a novel immersed boundary framework within FEniCS for nonlinear FSI, enabling easy customization and efficient simulation of highly deformable structures.

## Key findings

- Successfully simulates large deformations in hyperelastic materials.
- Provides a modular Python API for rapid setup and modification.
- Achieves efficient performance with hybrid Python/C++ architecture.

## Abstract

AFSI is a novel, open-source fluid-structure interaction (FSI) solver that extends the capabilities of the FEniCS finite element library through an immersed boundary (IB) framework. Designed to simulate large deformations in hyperelastic materials (such as cardiac tissue), AFSI avoids the need for expensive remeshing by coupling a Lagrangian representation of the solid with an Eulerian description of the surrounding fluid. This approach retains the full expressiveness of FEniCS's variational formulations, function spaces, and time integration schemes. Implemented in a hybrid Python/C++ architecture, AFSI allows users to define geometries, constitutive models (e.g., the Holzapfel-Ogden law for myocardium), and strain energy functions directly in Python, while delegating performance-critical tasks such as assembly and linear solvers to optimized C++ backends. Its concise and modular Python API facilitates the setup of FSI simulations, enabling users to easily modify discretization strategies or analyze results using standard FEniCS post-processing tools. By combining the flexibility of FEniCS with a robust immersed boundary formulation, AFSI empowers rapid prototyping of complex nonlinear solid-fluid interaction problems, making it a powerful tool for simulating biomechanical systems and other applications involving highly deformable structures in flow.