# 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 generate_model.py which writes a system/blockMeshDict.model dictionary. Another script, generate_domain.py 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

#!/bin/bash

# see https://damogranlabs.com/2020/10/runfunctions-a-quick-cheatsheet/
. $WM_PROJECT_DIR/bin/tools/RunFunctions .$WM_PROJECT_DIR/bin/tools/CleanFunctions

# generate blockMeshDict of the model (not domain!)
# see https://github.com/damogranlabs/classy_blocks
./generate_model.py # writes system/blockMeshDict.model
./generate_domain.py # 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

...