SnappyHexMesh in OpenFOAM: Steps, Tips, and Resources

snappyHexMesh is a meshing tool in OpenFOAM that generates body-fitted meshes for complex geometries. Unlike structured mesh generators such as blockMesh, snappyHexMesh can handle more elaborate shapes by adapting a rectangular background mesh to conform to the details of the imported geometry. This makes it particularly powerful for CFD problems involving real-world geometries.

1. Mesh generation steps

Hereafter are the steps to use snappyHexMesh:

• Create background mesh: typically generated using blockMesh, by setting up a uniform cubic mesh that serves as a base.
• Import geometry: this step involves importing the surface geometry, usually in STL or OBJ format, into the constant/triSurface directory.
• Generate the mesh: snappyHexMesh proceeds through several stages: Castellation: The background mesh is refined near the surface. Snapping: Cells are shifted to align with the surface geometry. Layer Addition: Near-wall layers are added to improve boundary layer resolution.

2. Key features of snappyHexMesh

In the snappyHexMeshDict file, many features are to be defined which could be challenging for a first-time user. Understanding the following aspects is essential for achieving a quality mesh:

• Surface refinement: This step allows you to control the cell size in regions close to the geometry, which helps in capturing features like sharp edges or fine curves accurately.
• Cell quality settings: The quality of the resulting mesh depends on various parameters that ensure the cells are neither too stretched nor distorted. Configuring parameters like maxNonOrtho and maxBoundarySkewness is crucial for ensuring stability in your CFD simulations.
• Boundary Layers: in boundary layer simulations, adding boundary layers is essential, like in aerodynamic studies. snappyHexMesh can generate these layers automatically, although tuning the parameters to avoid collapsing or distorted layers can require iteration and improvement.

3. Practical example

To get started with snappyHexMesh, here is a simple example of meshing a basic model, such as a car geometry:

• Setup the background mesh: by creating a bounding box around the car using blockMeshDict. This generates the initial computational domain.
• Define geometry: importing the car geometry as an STL file (or other supported formats) into the constant/triSurface directory.
• Castellation: setting up refinementSurfaces to determine how the cells near the car should be refined.
• Snapping and adding layers: specifying snapControls to ensure that the mesh conforms to the geometry. And, adding boundary layers to important surfaces, such as the undercarriage of the car, using the addLayersControls feature.
• Running snappyHexMesh: Executing the meshing utility with the command:

snappyHexMesh -overwrite        

The -overwrite flag ensures the final mesh replaces the original background mesh.

Once the mesh is generated, it can be visualized in paraView to check for issues like distorted cells or areas where refinement could be improved.

4. Tutorials and resources for learning snappyHexMesh

Valuable content is available online to explain simply the snappyHexMesh tool. This playlist from O?uzhan KIRIKBAS gathers the main video tutorials about snappyHexMesh, mainly:

• Holzmann CFD: CFD Analysis of a Smoking Pipe | Part 7 | SnappyHexMesh Layer Generation ? OpenFOAM? v8
• Calum Douglas: OpenFOAM SnappyHexMesh Tutorial
• foamDude: OpenFOAM: SnapyHexMesh - Castellated
• József Nagy (OpenFOAM tutorials): Hacking snappyHexMesh - improve your meshing speed

Conclusion

snappyHexMesh is a useful tool in OpenFOAM, capable of generating high-quality meshes for complex geometries. To get started with snappyHexMesh, begin by meshing simple geometries to understand the basics. Always use ParaView to visualize each step; castellation, snapping, and layer addition, to track mesh quality. Use checkMesh to ensure your mesh meets quality standards, as poor cells can destabilize your simulations. Finally, adjust parameters iteratively since each geometry is unique, and achieving optimal results often requires experimentation.