I have been trying for a long time to get my 2-way FSI using a Mechanical and fluent model of an patient-specific carotid artery to work but it keeps crashing with errors before finishing the first time step. I cannot get even an extremely simplified model to work for any material softer than Young’s modulus of 50 MPa.

I have looked at all relevant tutorials (best practices for FSI, oscillating plate, etc). The model I am trying to solve is quite complex with linear elastic isotopic material and a coded time-dependent velocity inlet. It works completely fine when solving the individual solvers.  However, I can simply not solve a 2-way FSI simulation with this geometry no matter how much I simplify it, except when solving for structural steel material properties which are defaulted in engineering data.

Every time I get an “excessive deformation” error. In cases where the material is still pretty hard, it might not crash but it cannot converge. I have tried to correct the geometry and the geometry is perfectly good. but I can still not solve a 2-way FSI with a softness to human arterial tissue.

I have tried the following things (and checked if the individual solvers and 1-way FSI work):

-         GEOMETRY

-         Structural: idealized carotid artery. Internal diameter: 5.0.9 mm, thickness: 0.07 mm, length: 47 mm

-         Fluid:  same diameter as the internal diameter of the artery

-         I have checked the geometries and they completely match (no scaling issues during import)

-         MATERIALS

-         The only material I can solve the 2-way FSI simulation for is structural steel

-         I tried working with linearly elastic materials with different Young's Modulus. Since I am working with small pressures and velocities, the artery is not deforming visibly even for Young’s modulus of 100 MPa in the mechanical solver (deformation around e-8 m ).

-         Tring different Poisson Ratios, 0.49,0.45,0.42

-         MESHING

-         Working with fine meshes generated in Ansys Meshing (good values for min angle, quality, skewness, and orthogonality). Tetra or Hex. Homogenous or with inflation near the data transfer surface

-         Alternatively working with fine tetrahedral meshes. Smoothed to ensure good values for min angle, quality, skewness, and orthogonality.

-         BOUNDARY CONDITIONS

-         Pulsatile inlet velocity and 0 pressure at the outlet. Still crashes in the first timestep even though there are basically no forces applied to the system.

-         Various types of support (fixed on the ends or cylindrical or displacement or elastic etc)

-         SOLVER

-         Trying the “System Coupling” and “Fluid-Structure Interaction” interfaces

-         Using the Ramping option in System Coupling for the data transfers

-         Trying different Windows workstations

-         Building the model in different versions of Ansys (R2024 R1 and R2023 R2)

-         Trying out small timesteps of e-5 and less but stability is not improved and convergence is not accelerated

Many researchers are solving very complex FSI models with Fluent and Mechanical on patient-specific stented arteries with different pressure outlet boundary conditions. so I am very confused that even when following their tips and advice I cannot even get such a simple model to work. Any advice would be highly appreciated. Thank you!