blockMesh for External Flows

The Old-Fashioned blockMesh

BlockMesh is the weapon of choice for directly generating rather simple meshes of channels, pipes, 2D-stuff etc. Those are mostly internal flows. With external flows, life gets tough much quicker. Consider a very simple case of a 3D flow past a cube. You now need this many blocks:

  • if the cube sits on the floor: 17,
  • when the flow surrounds the whole body: 26,
  • a cube, tilted in any direction: [sorry, can’t even wrap my head around that one.]

But the cube itself is as simple as it gets – a single block!

I am aware that one could mesh a single block and then remove the cube from the domain. Since a cube doesn’t have a very interesting engineering application, try doing that with a cylinder (or anythine else, for that matter).

Using blockMesh as a External Flows Model Generator

The model in all of the above cases is still very simple. If it was meshed internally, it would take a single block. And that is the exact idea to use for external flows:

  • Create an internal mesh with blockMesh
  • export patches of the model as .stl
  • use the exported geometry for some proper meshing with snappy or whatever you use.

A Sample Script

In a short summary, this script first calls which writes a system/blockMeshDict.model dictionary. Another script, generates a blockMeshDict.domain. blockMesh is run for both dicts, STL files are extracted from generated geometry and fed into snappyHexMeshDict for a proper mesh generation.

Note that this is a made-up example. For a real-life application, check out this blog post: A Parametric CFD Model: Wind Turbine from Pipes


# see
. $WM_PROJECT_DIR/bin/tools/RunFunctions
. $WM_PROJECT_DIR/bin/tools/CleanFunctions

# generate blockMeshDict of the model (not domain!)
# see
./ # writes system/blockMeshDict.model
./ # writes system/blockMeshDict.domain

# generate 'model' mesh and extract STL surface from it
runApplication -s model blockMesh -dict system/blockMeshDict.model
runApplication surfaceMeshExtract -constant constant/triSurface/model.stl

# similarly, generate the 'domain' mesh and extract surface from it
runApplication -s domain blockMesh -dict system/blockMeshDict.domain
surfaceMeshExtract -constant -patches "(inlet outlet walls)" constant/triSurface/domain.stl

# the constant/triSurface now contains STL files for snappy.
# continue with your usual snappy routine
runApplication surfaceFeatureExtract
runApplication decomposePar -force
runParallel snappyHexMesh -overwrite


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  1. Pingback: A Parametric CFD Model: Wind Turbine from Pipes | Damogran Labs

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