Contrib/SwiftSnap

1 Introduction

snappyHexMesh has developed into a very capable mesher. However, each setup of snappyHexMesh requires some manual work on writing the dictionary and preparing the files. Many people already today employ Blender 3D for generation of the input geometry files for snappyHexMesh. The step is hence not very large to write an add-on for Blender which automatically generates the dictionary and relevant files. The add-on provided here does that, as well as lets the user have full control over how and which feature lines are provided to snappyHexMesh. Further, the addon utilises Blender materials to let the user specify patches’ names, resolution settings, and mesh wall layers. Compatible with snappyHexMesh in OpenFOAM 2.1.

The files are hosted at a git repository https://github.com/nogenmyr/swiftSnap.git

Compatible Blender version

v2.66 - v2.68. Unlikely to work without errors in other versions.

Example case

PipeExampleSnap.tar‎

Note: For older versions, check the chapter Older versions.

For questions, problems, and similar, use the dedicated thread at the www.cfd-online.com forum: SwiftSnap and SwiftBlock, GUIs for OpenFOAM's meshers

2 Getting Started

Untar files into your Blender add-on directory. Then start up Blender

In Blender's User Preferences, enable 'Community' add-ons, and locate SwiftSnap under the OpenFOAM category. Click to enable. Locate STL file export/import and enable. Return to 3D view.

Import / create your geometry in Blender. Keep your geometry in one single object. Use Ctrl-J to join objects

Select your geometry object. Under the object tab in the Properties window, you should now find an entry called SwiftSnap, expand it and click the button to enable SwiftSnap for the selected object.

The SwiftSnap entry will now expand (See right side in the figure below).

Buttons / settings:

Write

Click this button last when finished. When engaged, all files (stl, eMesh, Dict) are created. (As an extra feature, an Engrid Blender file is also written in the same directory as snappyHexMeshDict). Saving the state of Blender for later work is done using Blenders native save option. Note that in the save file dialogue you can click Distribute. When clicked, the triSurface directory will be placed in ../constant/ and blockMeshDict (if enabled) in ../constant/polyMesh.

Convert to Meters

This number converts Blender spatial dimension to meters in the written files. If you want 1 Blender unit to represent 1 mm, set this value to 0.001.

Make base mesh

Click to auto-generate a blockMeshDict for letting blockMesh create a base mesh for snappyHexMesh. This feature uses the objects bounding box, which can be seen in Object Mode using Bounding Box as Viewport shading. When enabled, the base resolution (in meters) should be specified.

Snap mesh / add layers

These two options enable / disable the corresponding settings in snappyHexMeshDict.

Set locationInMesh

Sets the corresponding entry in snappyHexMeshDict to the coordinate of Blender's 3D cursor.

Implicit features

snappyHexMesh can detect features by itself. To use that, enable this. The angle below is used for finding features.

Detect non-manifold

Blender has a function to detect non-manifold edges. It can be enabled here. Good for finding problems and/or features in meshes.

Feature angle

For use with the button below.

Detect features

Selects all edges in-between faces with an angle less than given above.

Feature refinement level

Sets refinement level for edges. For use with the button below.

Mark as level #

Marks selected edges as features to refine to the specified level. The collection of edges are fully determined by the user, but stem frequently from the selection generated with the Detect Features button. When edges are set, they will appear at the bottom of the Feature box, from there they can be selected and modified later. Note that it is important to mark edges as features, even if no extra resolution is wanted. Without feature lines defined, snappyHexMesh will not capture the sharpness of the edges.

Unmark selected

Removes selected edges from the list of feature edges.

Patch name

Here the user can specify a patch name, which is used when the button below is clicked.

Patch type

Set the patch boundary type.

Min/max level

Specifies which refinement levels to have for this patch.

Layers (optional)

Specifies how many layers to create at this patch.

Set patch

In Edit Mode, selected faces will be marked as a patch with individual settings. Patches are internally tracked using Blender's materials, where each material represents a unique patch. At save-out, the object (a copy of it) is split by material, and each patch/material is saved as an ascii-STL file.
Defined patches are listed below. Click the color box if a different color is wanted. Click the text to select faces belonging to the patch.

Refinement region

Refinement regions are defined as triSurfaceMesh in snappyHexMeshDict. Other Blender mesh objects can be used as such surfaces. Create them as separate objects (do not enable SwiftSnap for those objects), and return here and fill in their names. When a valid name is entered, the set button is enabled. Be careful not to have these refinement objects hidden when you press the "Write" button, as Blender then will output empty STL-files. Such files will cause snappyHexMesh to crash.

Level

Sets the refinement level to be used inside/outide of the refinement region.

Distance

Sets the distance from the surface, in which refinement to the level specified above is done. (snappyHexMesh allows for one surface to have several refinement levels at different distance. This is currently not supported by SwiftSnap.)

Refine inside

If clicked, refinement will take place inside the surface.

Set refinement region

When clicked the above given object will be used as a refinement triSurface object in snappyHexMesh with the given settings. If the object is found, it will be added to a list below the button. Click list items to remove them.

2.1 General notes

SwiftSnap frequently switches the users selection and view mode. This is because different operations require different such settings. The user is also likely to jump between Edit Mode and Object Mode (Tab key). In Edit Mode, manipulation on the mesh can be performed, while in Object Mode, manipulation on the object as a whole can be performed. In Edit Mode, the user have good use of switching between Vertex/Edge/Face selection mode (Ctrl-Tab). For instance, use edge selection to modify feature edges, or use face selection to specify patches.

Blender has some functions for making clever selections of mesh components. Refer to Blender's wiki about selection for how to work efficiently. For example:

Ctrl+NumPadPlus

select neighbours

Ctrl+Alt+Shift+F

select based on angle

Alt+RMB

edge loops

Blender cannot currently read typical CAD formats, but the user frequently has to import geometries to Blender as STL files. The user typically has to use some kind of CAD tool to strip complex drawings from unnecessary details before importing the geometry in Blender.

Note

Obviously, the GUI does not give the user access to all the settings available for snappyHexMesh, but many are hard coded into the python code. These are however very easy to alter, just edit the utils.py file. If the user frequently uses different values for some default setting, it will not be too hard to add an entry in the GUI for that setting - just have a look in the code!

3 Example

3.1 T-pipe bend

A T-pipe bend modelled. The grey Cylinder is a refinement region. Yellow color is walls, which will have two layers of boundary cells. Red color is inlet which will be refined two times from the base mesh. The small pipe have an inlet called stream colored in blue. Outlet patch (not seen) is green. Not that due to snappyHexMesh's patch naming convention and Blender's STL exporter, final patch names will be like inlet_Exportedfromblender. A simple sed run,

sed -i 's/_Exportedfromblender//' /path/polyMesh/boundary

executed after snappyHexMesh would solve that problem, though.

4 Final Notes

Feel free to contribute to this page and improvements to the scripts!

For questions, problems, and similar, use the dedicated thread at the www.cfd-online.com forum: SwiftSnap and SwiftBlock, GUIs for OpenFOAM's meshers

5 Older versions

The swiftSnap GitHUb contains the newest version, to work with the newest version of Blender. Older versions of swiftSnap, to work with older versions of Blender, can be found here:

Compatible Blender versions	File
v2.6x, with x<3. (Possibly also for 2.5x, but surely not for earlier).	Swiftsnap.tar.gz Note: has the original user guide.