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# Best Practices When Performing Fluid Structure Interaction and Thermal Stress Evolution Simulations

Fluid Structure Interaction (FSI) and Thermal Stress Evolution (TSE) models are coupled finite difference (FD)/finite element (FE) methods in FLOW-3D. They compute for fluid flow and solid mechanics at the same time and in the same domain. The fluid governing equations are computed on the customary rectangular Cartesian mesh while the solid stresses and deformations are calculated on a body-fitted FE mesh. The body-fitted FE mesh is generated using the encompassing Cartesian mesh with minimal user input. Here we present a few useful hints and tips for setting up and post-processing FSI/TSE simulations.

### Activating Gravity in a Simulation

Be sure to constrain all FSI and TSE components if gravity is activated in the simulation:

• Solid components (regular or FSI) may be used to define the boundary of FSI components or solidified fluid regions but the fluid domain mesh must be capable of adequately resolving them. If they are not resolved in the FAVOR view, FSI components and solidified fluid regions will not be able to see them either.
• Walls or any other fluid mesh boundary except for symmetry can be used to fix the FSI nodes on the boundary. A symmetry fluid mesh boundary has the effect of a wall boundary with the exception of allowing for sliding along this boundary.

### Displaying FSI Components

Both deformed and undeformed views of FSI components or solidified fluid regions can be displayed simultaneously. Each can be displayed as:

• Wireframe only
• Surface and wireframe
• Surface only

### Visualizing Displacements

Usually, the displacements are orders of magnitude smaller than the dimensions of the geometry. Therefore, in order to be able to visualize the displacements relative to the dimensions of the undeformed geometry a magnification factor can be used.

### Solidified Cast Cooling Simulations

FEA domain removing components can be used to significantly reduce the size of a finite element mesh in order to simulate a solidified cast cooling in air. For TSE problems, the FE mesh generator applies the mesh to all the open cells that can potentially get occupied by fluid. In those circumstances when the user knows that there are certain regions of the domain that are not accessible by the fluid, the user can block those regions by FEA domain removing components. These components have no effect on the solution process; they merely let the FE mesh generator know not to mesh the blocked section.

### Material Property Database

Because of the sensitivity of the FSI and TSE models to elastic properties of the solid, particularly when they are a function of temperature, Flow Science offers a custom version of the Material Property Database (MPDB) which provides thousands of solid materials and their properties. The data exported by MPDB is seamlessly recognized in FLOW-3D. Please contact the Flow Science sales team for more information.

### Computationally Intensive Simulations

If the FE mesh generation process is computationally intensive, remember to check the Simulate with specified mesh file after the satisfactory FE mesh is generated. This eliminates the need for FE mesh generation for subsequent runs.

### Meshing Options

The Use local input mesh option is only functional in the Generate mode. Generate mode makes a temporary copy of the prepin file with the local mesh information and preprocesses this prepin to generate the FE mesh. This local mesh information is stored in the binary FE mesh file. There can only be one set of Cartesian mesh information in the prepin file. Therefore when Preprocess simulation or Run Simulation is selected from the Simulate menu, the global mesh information is always used and the information from the local mesh tree is ignored.

### Download the Fluid Structure Interaction Tutorial

An in-depth tutorial describing how to set up, run and post-process FSI simulations can be downloaded from the FLOW-3D Users Site in the Tutorials section.