SnappyHexMesh

1 Introduction

In a nutshell, snappyHexMesh is a mesh generator that takes an already existing mesh (usually created with blockMesh) and chisels it into the mesh you want. But for this, it requires:

• A very well defined dictionary, namely system/snappyHexMeshDict.
• Good geometrical definitions, such as:
  • STL/OBJ files with well defined surfaces;
  • eMesh feature edge files, that define feature edges that have describe sharp corners.

Valid versions:

Note: feature edges only available in:

Configuring both the dictionary file and providing good geometries can be a very arduous task. Therefore, at the moment of this writing (11:06, 18 August 2012 (CEST)), this wiki page will try to point you in the right direction, without describing each and every detail related to using snappyHexMesh. Feel free to contribute to this page!

2 Tutorials and Guides

Here are listed links to tutorials and guides on using snappyHexMesh:

• Official OpenFOAM User Guide, chapter 5.4 - Mesh generation with the snappyHexMesh utility

• A Comprehensive Tour of snappyHexMesh - 7th OpenFOAM Workshop (25 June 2012):
  • Original source: EugenedeVilliers-TrainingSlides.tgz
  • Local copy here at openfoamwiki.net: Final-AndrewJacksonSlidesOFW7.pdf

• Tutorials by Jens H. at personal blog:
  • snappyHexMesh Tutorial (part 1)
  • snappyHexMesh Tutorial (part 2)
  • Snap edges with snappyHexMesh

• Tutorials by T. Holzmann at personal homepage:
  • old snappyHexMesh Tutorials
  • new snappyHexMesh Tutorials

• Meshing tutorials for multi-region meshing by T. Holzmann:
  • exhaust-heat-exchanger meshing tutorials

3 Tips

On this chapter are listed links and direct tips on using snappyHexMesh.

3.1 Generate 2D meshes

Instructions/examples on how to generate 2D meshes using snappyHexMesh can be found in at least two places:

• Here at the wiki: Main_ContribExamples/2DsHM
• In the tutorial incompressible/pimpleDyMFoam/wingMotion/wingMotion_snappyHexMesh

Other tutorials:

• This is an awesome tutorial: Dynamic Mesh for a Gear Pump.

3.2 Multi-region meshes

There are a few instructions/examples on how to generate multi-region meshes with snappyHexMesh:

• See the tutorial heatTransfer/chtMultiRegionFoam/snappyMultiRegionHeater.
• Instructions on how to remove an external unwanted region: Background Mesh in snappy with multi domain (CHT) post #3
  • Note: this happens whenever the base mesh doesn't coincide with the outer limits of the target surface geometry. The tutorial heatTransfer/chtMultiRegionFoam/snappyMultiRegionHeater is such an example where the outside of the original STL files coincides with the base mesh.
• Discussion on this topic for simplified multi-tubes connected between two boxes: multiple regions at cfd-online.com.
  • The resulting example case is provided here: Exhaust gas heat exchanger (in German)
• Not 100% multi-region, but more specifically a single mesh domain with a porous region: Multiple regions with a porous zone - post #5 - at cfd-online.com

3.3 Running in parallel

Currently there are several official tutorials on how to use snappyHexMesh in parallel:

• incompressible/windSimpleFoam/turbineSiting - Valid versions:
• incompressible/simpleFoam/turbineSiting - Valid versions:
• incompressible/pisoFoam/les/motorBike/motorBike - Valid versions:

Other tutorials:

• Modified motorBike tutorial to be used as a benchmark case for running in parallel: Runtimes of OpenFOAM vs blueCFD - Valid versions:

3.4 GUI (Graphical User Interfaces)

Currently there are several interactive ways (using GUI) of generating meshes with snappyHexMesh. Here will only be listed the ones that are open-source:

Discretizer 

Check the Screencasts page for more information.

SwiftSnap 

A very useful plug-in for Blender for configuring snappyHexMeshDict. There is SwiftBlock for blockMeshDict as well!

Helyx^OS 

A GUI for using OpenFOAM. See the video tutorials:

• Helyx^OS - Example: External Aerodynamics Setup
• Helyx^OS - Engys' free-to-download Open Source native GUI for OPENFOAM®

3.5 Another way of projection of a mesh on an STL surface

Adding boundary layers with snappyHexMesh can sometimes be very stressing, specially when it sometimes stops the layers in the wrong places. Trying to improve this feature was one of several works made in the MSc/PhD course in CFD with OpenSource software, 2011 at Chalmers, namely:

• Christoffer Järpner: Projection of a mesh on an stl surface. Slides, Report, Code, Case

Other important links:

• Thread dedicated to discussing on this subject: [1]
• Adjusted code for working with the latest OpenFOAM versions: [2]

3.6 Isolate and Conquer

Since snappyHexMesh has so many mesh quality parameters, these sometimes to get in the way of the meshes you're trying to generate. The solution apparently is simple:

1. Disable all quality controls.
2. Diagnose visually the resulting mesh.
3. Gradually turn back on one quality indicator at a time, while checking the resulting mesh with both checkMesh and ParaView.

For more on this, read the following posts/threads:

• Troubles with layer adding in my mesh - post #3
• (I know there are more threads at cfd-online.com, but currently I'm unable to find them -- Wyldckat (talk) 22:04, 3 September 2012 (CEST))

3.7 Valid and invalid solid names

There are 2 details to always keep in mind when exporting to STL:

• Always export to STL in ASCII or "coded" format (this means it'll be plain text).
• Edit the resulting file with a text editor and look for the lines that start with "solid". Next to the word "solid" is the name of said solid; that name should be a single C/C++ word.

Examples of valid words/names:

• wall
• ladder
• part_001_73_

Invalid names:

• This was generated by productnamehere
• part 001-73:
• 70 inch wall

3.8 Handling inclined surfaces

Q: Do you have a geometry that has an inclined surface and gets a very bad mesh, due to the cells not adjusting properly to the geometry, namely the inclined surfaces?

A: Check the following thread at CFD-Online: How to get a snapped mesh with SnappyHexMesh? post #4 and #5