Disclaimer
“This offering is not approved or
endorsed by OpenCFD Limited, the
producer of the OpenFOAM software
and owner of the OPENFOAM® and
OpenCFD® trade marks.”
Introductory OpenFOAM® Course
University of Genoa, DICCA
Dipartimento di Ingegneria Civile, Chimica e Ambientale
Your Lecturer
Joel GUERRERO
[email protected]
[email protected]
Matteo BARGIACCHI
[email protected]
[email protected]
This lecture is about meshing and mesh quality
assessment for CFD computations
Generating high quality meshes is a critical step for CFD computations.
Depending on the quality of the mesh you can get very different results,
which can made post-processing and interpretation of the solution a
difficult task, due to contrasting or misleading results because of meshing
issues. This holds independently of the solver used.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
This lecture is about meshing and mesh quality
assessment for CFD computations
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Meshing and mesh quality
I do not know how much I will stress this, but try to always
use a good quality mesh.
Have always in mind, garbage in - garbage out.
As I am a really positive guy, I rather say,
Today’s lecture
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
1. Meshing preliminaries
2. Mesh quality assessment
3. Mesh conversion and manipulation utilities
4. Mesh generation using blockMesh and
snappyHexMesh
5. Mesh generation using open source tools
6. Hands-on session
Today’s lecture
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
1. Meshing preliminaries
2.
Mesh quality assessment
3.
Mesh conversion and manipulation utilities
4.
Mesh generation using blockMesh and
snappyHexMesh
5.
Mesh generation using open source tools
Meshing preliminaries
• Mesh generation consist in dividing the physical domain into a finite
number of discrete regions, called control volumes or cells in which the
solution is sought (domain discretization).
• Meshes used for the FVM method can consist of tetrahedras, pyramids,
prisms, hexahedras or any kind of polyhedral element (or a mix of all of
them).
• The meshes can be unstructured or structured. In our discussion, when
we talk about unstructured or structured meshes we refer specifically to
the method used to generate them.
Meshing preliminaries
• The data structure of the meshes used in the FVM is represented by the
points and faces that make up each control volume.
• The connectivity information of each cell (how faces and cells are
connected) and cell/face neighbor information is also needed for FVM
unstructured meshes.
• Meshes used for the FDM method are made of hexahedra and they are
known as single and/or multi-block structured meshes.
• The connectivity information of the meshes used in the FDM is expressed
as a two or three dimensional array, this is highly memory efficient as we
do not need to store all the connectivity information of the faces and cells.
Meshing preliminaries
• In the next slides, I will show a few meshes generated by using different
meshing methods.
• Also, I will present a table with a comparison of four commonly used
mesh generation methods.
• The figures of merit used to compare the meshes are related to the
method used to generate the mesh and not to the flow solver.
• All the methods that we are going to talk about, can generate a valid
mesh to be used with the FVM. Remember, when we pass the mesh
information to an unstructured FVM solver, we pass the data structure
and connectivity information (points, faces and cells information).
Meshing preliminaries
Meshing preliminaries
Meshing preliminaries
Meshing preliminaries
Meshing preliminaries
Meshing preliminaries
STRUCTURED
UNSTRUCTURED
CARTESIAN
OVERLAPPING
Geometric Flexibility/
Adaptation
Grid Adaptation/
Local Refinement
Viscous Computation
Moving/Deforming
Meshes Quality
Interpolation/
Conservation
Grid generation
easiness
Memory
Requirements
CPU Requirements
Meshing preliminaries
Meshing preliminaries
A structured mesh requires as input
the blocking definition (blue lines in
the figure).
For complicated geometries, it can
be extremely difficult arrive to the
right blocking, it requires a lot user
experience.
After defining the blocking, the mesh
generation time is quite fast, in the
order of seconds or minutes.
Unstructured meshes, only requires
as input the element size on the lines
and surfaces that define the
geometry.
After defining the element size, the
meshing process can be quite time
consuming and memory expensive.
Meshing time is in the order of
minutes or hours, even days.
Meshing preliminaries
Unstructured Hybrid Mesh
(tetras, prisms and hexs)
Cell count:
approx. 5 000 000
Structured Mesh
(hexahedrals)
Cell count:
approx. 5 000 000
Meshing preliminaries
Unstructured Hybrid Mesh
(tetras, prisms and hexs)
Cell count:
approx. 5 000 000
Structured Mesh
(hexahedrals)
Cell count:
approx. 5 000 000
Meshing preliminaries
Unstructured Hybrid Mesh
(tetras, prisms and hexs)
Cell count:
approx. 5 000 000
Structured Mesh
(hexahedrals)
Cell count:
approx. 5 000 000
Meshing preliminaries
Unstructured Hybrid Mesh
(tetras, prisms and hexs)
Cell count:
approx. 5 000 000
Structured Mesh
(hexahedrals)
Cell count:
approx. 5 000 000
Meshing preliminaries
Unstructured Hybrid Mesh
(tetras, prisms and hexs)
Cell count:
approx. 5 000 000
Structured Mesh
(hexahedrals)
Cell count:
approx. 5 000 000
Meshing preliminaries
Unstructured Hybrid Mesh
(tetras, prisms and hexs)
Cell count:
approx. 5 000 000
Structured Mesh
(hexahedrals)
Cell count:
approx. 5 000 000
Meshing preliminaries
Unstructured
Vs.
Structured
meshes,
Who wins?
•
Each mesh type has its advantages and disadvantages.
•
At the end of the day, the mesh you use must has a good
overall quality and must be smooth.
•
The mesh density should be high enough to capture all
relevant flow features.
•
Wait, but what is a good mesh?. Mesh quality and
smoothness will be studied in the next slides.
Today’s lecture
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
1.
Meshing preliminaries
2. Mesh quality assessment
3.
Mesh conversion and manipulation utilities
4.
Mesh generation using blockMesh and
snappyHexMesh
5.
Mesh generation using open source tools
What is a good mesh?
•
A standard rule of thumb is that the elements shape and distribution should
be pleasing to the eye.
•
There is no written theory when it comes to mesh generation. Basically, the
whole process depends on user experience.
•
The user can rely on grid dependency studies, but they are time consuming
and expensive.
•
No single standard benchmark or metric exists that can effectively assess
the quality of a mesh, but you can rely on suggested best practices.
•
Hereafter, I will present you the most common mesh quality metrics:
•
Orthogonality.
•
Skewness.
•
Aspect Ratio.
•
Smoothness.
Mesh quality metrics. Mesh orthogonality
• Mesh orthogonality is the angular deviation of the vector
S
(located at the
face center
f
) from the vector d connecting the two cell centers
P
and
N
.
• Affects the gradient of the face center
f
.
• It adds diffusion to the solution.
Mesh quality metrics. Mesh skewness
• Skewness is the deviation of the vector d that connects the two cells
P
and
N
,
from the face center
f
. The deviation vector is represented with
and is the point where the vector d intersects the face
f
.
• Affects the interpolation of the cell centered quantities to the face center
f
.
• It adds diffusion to the solution.
Mesh quality assessment
Mesh quality metrics. Mesh aspect ratio AR
• Mesh aspect ratio AR is the ratio between the longest side and the
shortest side .
• Large AR are fine if gradients in the long direction are small.
• High AR smear gradients.
Mesh quality assessment
x
Mesh quality metrics. Smoothness
• Smoothness, also known as expansion rate, growth factor or uniformity,
defines the transition in size between contiguous cells.
• Large transition ratios between cells add diffusion to the solution.
• Ideally, the maximum change in mesh spacing should be less than 20%:
Mesh quality assessment
Smooth transition
Steep transition
y
2
Mesh quality assessment
f
=
P
+
⇥
⇥y
y +
O( y
2
)
f
=
P
+
⇥
⇥y
y +
O( y
2
)
P
f
P
f
Element type close to the walls -
Cell/Flow alignment
• Hexahedrals, prisms, and quadrilaterals can be stretched easily to resolve
boundary layers without losing quality.
• Triangular and tetrahedral meshes have inherently larger truncation error.
• Less truncation error when faces aligned with flow direction and gradients.
Striving for quality
• For the same cell count, hexahedral meshes will give more accurate
solutions, especially if the grid lines are aligned with the flow.
• The mesh density should be high enough to capture all relevant flow
features. In areas where the solution change slowly, you can use larger
elements.
• To keep cell count low, use non-uniform meshes to cluster cells only where
they are needed. Use local refinements and solution adaption to further
refine only on selected areas.
• In boundary layers, quad, hex, and prism/wedge cells are preferred over
triangles, tetrahedras, or pyramids.
• If you are not using wall functions (turbulence modeling), the mesh adjacent
to the walls should be fine enough to resolve the boundary layer flow. This
will rocket the cell count and increase the computing time.
Striving for quality
• Use hexahedral meshes whenever is possible, specially if high accuracy in
predicting forces is your goal (drag prediction) or for turbo machinery
applications.
• For complex flows without dominant flow direction, quad and hex meshes
loose their advantages.
• Keep orthogonality, skewness, and aspect ratio to a minimum.
• Change in cell size should be smooth.
•
Remember, a poor quality mesh will generate
inaccurate solutions and/or will slow down
solution convergence.
Mesh quality metrics in OpenFOAM®
• In WM_PROJECT_DIR/src/OpenFOAM/meshes/primitiveMesh/
primitiveMeshCheck/
primitiveMeshCheck.C
you will find the
quality metrics used in OpenFOAM®. Their maximum (or minimum) values
are defined as follows:
•
Foam::scalar Foam::primitiveMesh::closedThreshold_ = 1.0e-6;
•
Foam::scalar Foam::primitiveMesh::aspectThreshold_ = 1000;
•
Foam::scalar Foam::primitiveMesh::nonOrthThreshold_ = 70; // deg
•
Foam::scalar Foam::primitiveMesh::skewThreshold_ = 4;
•
Foam::scalar Foam::primitiveMesh::planarCosAngle_ = 1.0e-6;
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Checking mesh quality in OpenFOAM®
• OpenFOAM® comes with the utility
checkMesh
which checks the validity of
the mesh.
•
checkMesh
will look for/check for:
• Mesh stats and overall number of cells of each type.
• Check topology (boundary conditions definitions).
• Check geometry and mesh quality (bounding box, cell volumes,
skewness, orthogonality, aspect ratio, and so on)
• If for any reason
checkMesh
finds errors, it will give you a message and it
will tell you what check failed.
• It will also write a set with the faulty cells, faces, points. These sets are
saved in the directory constant/polyMesh/sets/
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Checking mesh quality in OpenFOAM®
• Mesh topology and patch topology errors must be repaired.
• You will be able to run with mesh quality errors such as skewness, aspect
ratio, minimum face area, and non-orthogonality. But remember, they will
severely tamper the solution accuracy and eventually can made the solver
blow-up.
• Unfortunately,
checkMesh
does not repair these errors. You will need to
check the geometry for possible errors and generate a new mesh.
• To visualize the failed sets you can use the utility
foamToVTK
. This utility
converts the failed sets to VTK format.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Checking mesh quality in OpenFOAM®
• To visualize the failed faces/cells/points in
paraFoam
you will need to
proceed as follows:
•
foamToVTK
-set_type name_of_sets
where
set_type
is the type of sets ( faceSet, cellSet, pointSet,
surfaceFields) and
name_of_sets
is the name of the set in the directory
constant/polyMesh/sets (
highAspectRatioCells
,
nonOrthoFaces
,
wrongOrientedFaces
,
skewFaces
,
unusedPoints
)
• At the end,
foamToVTK
will create a directory named VTK, where you will find the
failed faces/cells/points in VTK format. At this point you can use paraFoam to
visualize the failed sets.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Checking mesh quality in OpenFOAM®
We will now check the mesh quality of a sample case. From now on follow me.
•
Go to the directory $ptofc/mesh_conversion_manipulation/M2_wingbody. In the
terminal type:
•
cd
$ptofc/mesh_conversion_manipulation/M2_wingbody
•
fluent3DMeshToFoam ./mesh/ascii.msh
•
checkMesh
At this point check the output of the utility
checkMesh
. To write the failed sets to VTK format:
•
foamToVTK
-faceSet nonOrthoFaces
•
mv
VTK VTK1
•
foamToVTK
-pointSet unusedPoints
Now you can visualize the faulty sets by using paraFoam.
Remember, each time you use the utility
foamToVTK
it will overwrite the directory VTK
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Non orthogonal faces (green) and unused points (yellow)
By following the instructions, you should get something like this
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
By following the instructions, you should get something like this
Mesh quality assessment
Today’s lecture
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
1.
Meshing preliminaries
2.
Mesh quality assessment
3. Mesh conversion and manipulation utilities
4.
Mesh generation using blockMesh and
snappyHexMesh
5.
Mesh generation using open source tools
Mesh conversion and manipulation utilities
• It is also possible to export a mesh generated with a third party software and
use it in OpenFOAM®. Some of the utilities available for mesh conversion
are listed below:
•
ansysToFoam
: converts an ANSYS input mesh file OpenFOAM® format.
•
cfx4ToFoam
: converts a CFX 4 mesh to OpenFOAM® format.
•
fluent3DMeshToFoam
: converts a Fluent mesh to OpenFOAM® format.
•
gambitToFoam
: converts a GAMBIT mesh to OpenFOAM® format.
•
gmshToFoam
: reads .msh file as written by Gmsh.
•
ideasUnvToFoam
: I-Deas unv format mesh conversion.
•
plot3dToFoam
: plot3d mesh (ascii/formatted format) converter.
•
star4ToFoam
: converts a STAR-CD (v4) PROSTAR mesh into OpenFOAM®
format.
•
tetgenToFoam
: Converts .ele and .node and .face files, written by tetgen.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh conversion and manipulation utilities
• OpenFOAM® also comes with many mesh manipulation utilities. Some of
them are listed below:
•
checkMesh
: checks validity of a mesh.
•
refineMesh
: utility to refine cells in multiple directions.
•
renumberMesh
: renumbers the cell list in order to reduce the bandwidth,
reading and renumbering all fields from all the time directories
•
transformPoints
: transforms the mesh points in the polyMesh directory
according to the translate, rotate and scale options.
•
mirrorMesh
: mirrors a mesh around a given plane.
•
setSet
: manipulate a cell/face/point/ set or zone interactively.
•
refineWallLayer
: utility to refine cells next to patches.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh conversion and manipulation utilities
• In OpenFOAM® it is also possible to manipulate the geometries in STL
format. Some of the utilities available are listed below:
•
surfaceTransformPoints
: transform (scale/rotate) a surface. Like
transformPoints
but for surfaces.
•
surfaceCheck
: checks geometric and topological quality of a surface.
•
surfaceClean
: removes baffles and collapses small edges, by removing
triangles. Converts sliver triangles into split edges.
•
surfaceFeatureExtract
: extracts and writes surface edge features to file.
•
surfaceMeshInfo
: miscellaneous information about surface.
•
surfaceMeshTriangulate
: extracts triSurface from a polyMesh.
Geometry and mesh manipulation in OpenFOAM®
We will now manipulate a STL geometry and a mesh. From now on follow me.
•
Go to the directory $ptofc/mesh_conversion_manipulation/
M6_ahmed_body_transform
In the terminal type:
•
cd
$ptofc/mesh_conversion_manipulation/M6_ahmed_body_transform
•
blockMesh
•
surfaceCheck
./constant/triSurface/
ahmed_body.stl
•
surfaceTransformPoints
-rollPitchYaw '(0 0 15)' ./constant/
triSurface/ahmed_body.stl ./constant/triSurface/
rotated.stl
•
surfaceTransformPoints
-translate '(0 0.12 0)' ./constant/
triSurface/rotated.stl ./constant/triSurface/
translated.stl
•
surfaceTransformPoints
-scale '(0.9 1.1 1.3)' ./constant/
triSurface/translated.stl ./constant/triSurface/
scaled.stl
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Geometry and mesh manipulation in OpenFOAM®
We will now manipulate a STL geometry and a mesh. From now on follow me.
•
Go to the directory $ptofc/mesh_conversion_manipulation/
M6_ahmed_body_transform
In the terminal type:
•
cd
$ptofc/mesh_conversion_manipulation/M6_ahmed_body_transform
•
surfaceSmooth
./constant/triSurface/scaled.stl 0.3 100 ./constant/
triSurface/
smooth.stl
•
surfaceFeatureExtract
•
snappyHexMesh
-overwrite
•
transformPoints
-rollPitchYaw '(0 10 -15)'
•
transformPoints
-scale '(2 1 1)’
•
checkMesh
•
paraFoam
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
By following the instructions, you should get something like this
STL surface manipulation
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
By following the instructions, you should get something like this
Mesh conversion and manipulation utilities
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
By following the instructions, you should get something like this
Mesh conversion and manipulation utilities
Today’s lecture
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
1.
Meshing preliminaries
2.
Mesh quality assessment
3.
Mesh conversion and manipulation utilities
4. Mesh generation using blockMesh and
snappyHexMesh
5.
Mesh generation using open source tools
Mesh generation using blockMesh and snappyHexMesh
blockMesh
For simple geometries, the mesh generation utility
blockMesh
(supplied with
OpenFOAM®), can be used. The
blockMesh
utility creates multiblock meshes.
The mesh is generated from a dictionary file named
blockMeshDict
located in
the constant/polyMesh directory.
snappyHexMesh
For complex geometries, the mesh generation utility
snappyHexMesh
(supplied
with OpenFOAM®), can be used. The
snappyHexMesh
utility generates 3D
meshes containing hexahedra and split-hexahedra from a triangulated surface
geometry in Stereolithography (STL) format. The mesh is generated from a
dictionary file named
snappyHexMeshDict
located in the system directory
and a triangulated surface geometry file located in the directory constant/
triSurface.
Mesh generation using blockMesh and snappyHexMesh
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
blockMesh block topology
Mesh 1. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh.
From now on, follow me.
In the terminal window type:
•
cd
$ptofc/sHM_1/M1_cyl
•
blockMesh
•
snappyHexMesh
(And by the way, take a look at the
snappyHexMeshDict
dictionary)
•
checkMesh
•
paraFoam
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 1. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh.
From now on, follow me.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh generation using blockMesh and snappyHexMesh
Mesh 1. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh.
From now on, follow me.
So… What did we do?
or
What did
snappyHexMesh
do?
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
So… What did we do?
SnappyHexMesh mesh generation process
Mesh generation with the snappyHexMesh utility
• The process of generating a mesh using
snappyHexMesh
will be described using
this figure.
• The objective is to mesh a rectangular shaped region (shaded grey in the figure)
surrounding an object described by and STL surface (external aerodynamics)
• You can also generate the mesh for an internal aerodynamics simulation.
• Note that the schematic is 2D to make it easier to understand, even though
snappyHexMesh
is a 3D meshing tool.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Creating the background hexahedral mesh
Before
snappyHexMesh
is executed the user must create a background mesh of
hexahedral cells that fills the entire region as shown in the figure. This can be done by
using
blockMesh
.
So… What did we do?
SnappyHexMesh mesh generation process
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Cell splitting at feature edges
Cell splitting is performed according to the specification supplied by the user in the
snappyHexMeshDict
dictionary. The splitting process begins with cells being selected
according to specified edge features as illustrated in the figure.
So… What did we do?
SnappyHexMesh mesh generation process
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Cell splitting at surfaces
Following feature edges refinement, cells are selected for splitting in the locality of
specified surfaces as illustrated in the figure. The surface refinement (splitting) is
performed according to the specification supplied by the user in the
snappyHexMeshDict
dictionary.
So… What did we do?
SnappyHexMesh mesh generation process
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Cell removal
Once the feature edges and surface splitting is complete, a process of cell removal
begins. The region in which cells are retained are simply identified by a location vector
within the region, this information is supplied by the user in the
snappyHexMeshDict
dictionary.
So… What did we do?
SnappyHexMesh mesh generation process
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Cell splitting in specified regions
Those cells that lie within one or more specified volume regions can be further split by a
region (in the figure, the rectangular dark shaded region). This information is supplied by
the user in the
snappyHexMeshDict
dictionary.
So… What did we do?
SnappyHexMesh mesh generation process
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Snapping to surfaces
After deleting the cells in the region specified and refining the volume mesh, the points
are snapped on the surface to create a conforming mesh.
So… What did we do?
SnappyHexMesh mesh generation process
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh layers
The mesh output from the snapping stage may be suitable for simulation, although it can
produce some irregular cells along boundary surfaces. There is an optional stage of the
meshing process which introduces boundary layer meshing in selected parts of the
mesh. This information is supplied by the user in the
snappyHexMeshDict
dictionary.
So… What did we do?
SnappyHexMesh mesh generation process
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 2. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh with feature edge
refinement.
From now on, follow me.
In the terminal window type:
•
cd
$ptofc/sHM_1/M2_cyl_er
•
blockMesh
•
surfaceFeatureExtract
(This utility uses
surfaceFeatureExtractDict
dictionary which is located in the system directory)
•
paraview
(Take a look at the
.obj
files created by
surfaceFeatureExtract
, we use
paraview
instead of
paraFoam
because by default
paraFoam
reads the mesh generated by
blockMesh
. The
.obj
files are located in the directory
constant/extendedFeatureEdgeMesh)
•
snappyHexMesh
•
checkMesh
•
paraFoam
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 2. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh with feature edge
refinement.
• The utility
surfaceFeatureExtract
will create a file named
*.eMesh
(e.g.
banana.eMesh
), this file will be saved in the directory constant/triSurface
This file contains the information of the feature edges. By the way, try to read the file,
it is easy to understand.
• Next, you should tell to
snappyHexMesh
that you want to use refinement in the
feature edges, and this is done by pointing to the
*.eMesh
file
in the
snappyHexMeshDict
dictionary
// Explicit feature edge refinement
features
(
{
file “banana.eMesh”;
level 4;
}
);
•
snappyHexMesh
knows that the
*.eMesh
file is in constant/triSurface
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 2. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh with feature edge
refinement.
• At this point, you should have in your case directory two time directories 1 and 2 (if
you chose a time step of 1 in your
controlDict
).
• In the time directory 1, you should have the mesh with the castellated mesh.
• In the time directory 2 you should have the mesh with the conforming mesh (the
mesh after snapping).
• Additionally, if you chose to generate the boundary layer mesh, you should have a
time directory 3, where you should have the mesh with the inflation layer.
• We are going to deal with boundary layer meshing later on.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 2. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh with feature edge
refinement.
• Now you must copy the mesh from the time directory 2 (or 3) to your constant
directory. In the terminal type:
•
mv
constant/polyMesh constant/polyMesh.org
(To rename the background mesh folder, this is optional, but I highly recommended it)
•
cp
-r 2/polyMesh constant/polyMesh
(To copy the new mesh to your constant/polyMesh directory, this is compulsory)
•
rm
-r 1
(If you do not want to keep this time folder)
•
rm
-r 2
(If you do not want to keep this time folder)
• Now you are ready to run.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 2. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh with feature edge
refinement.
• Alternatively,
snappyHexMesh
can automatically overwrite the original background
mesh contained in constant/polyMesh. In the terminal type:
•
snappyHexMesh
-overwrite
• If you use this option, you will not be able to visualize the intermediate steps.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 2. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh with feature edge
refinement.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh generation using blockMesh and snappyHexMesh
Mesh 2. 3D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh with feature edge
refinement.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh generation using blockMesh and snappyHexMesh
Feature edge refinement
No feature edge refinement
Mesh 3. Mixing elbow (internal mesh).
Initial mesh
.
Let us generate the mesh by using SnappyHexMesh.
From now on, follow me.
In the terminal window type:
•
cd
$ptofc/sHM_1/M3_mixing_elbow1
•
blockMesh
•
surfaceFeatureExtract
(It will use an
includedAngle
of 130 degrees as defined in the dictionary)
•
paraview
(Take a look at the
.obj
files created by
surfaceFeatureExtract
, we use
paraview
instead of
paraFoam
because by default
paraFoam
reads the mesh generated by
blockMesh
. The
.obj
files are located in the directory
constant/extendedFeatureEdgeMesh)
•
snappyHexMesh
•
checkMesh
•
paraFoam
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 3. Mixing elbow (internal mesh).
Initial mesh
.
Let us generate the mesh by using SnappyHexMesh.
• Alternatively,
snappyHexMesh
can automatically overwrite the original background
mesh contained in constant/polyMesh. In the terminal type:
•
snappyHexMesh
-overwrite
• If you use this option, you will not be able to visualize the intermediate steps.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 3. Mixing elbow (internal mesh).
Initial mesh
.
Let us generate the mesh by using SnappyHexMesh.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh generation using blockMesh and snappyHexMesh
An
includedAngle
of 130 degrees does not resolve well the intersection between the
two pipes.
Mesh 4. Mixing elbow (internal mesh).
Improved mesh
.
Let us generate the mesh by using SnappyHexMesh.
From now on, follow me.
In the terminal window type:
•
cd
$ptofc/sHM_1/M3_mixing_elbow2
•
blockMesh
•
surfaceFeatureExtract
(It will use an
includedAngle
of 150 degrees as defined in the dictionary)
•
paraview
(Take a look at the
.obj
files created by
surfaceFeatureExtract
, we use
paraview
instead of
paraFoam
because by default
paraFoam
reads the mesh generated by
blockMesh
. The
.obj
files are located in the directory
constant/extendedFeatureEdgeMesh)
•
snappyHexMesh
•
checkMesh
•
paraFoam
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 4. Mixing elbow (internal mesh).
Improved mesh
.
Let us generate the mesh by using SnappyHexMesh.
• Alternatively,
snappyHexMesh
can automatically overwrite the original background
mesh contained in constant/polyMesh. In the terminal type:
•
snappyHexMesh
-overwrite
• If you use this option, you will not be able to visualize the intermediate steps.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 4. Mixing elbow (internal mesh).
Improved mesh
.
Let us generate the mesh by using SnappyHexMesh.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh generation using blockMesh and snappyHexMesh
By increasing the
includedAngle
to 150 degrees we are able to resolve well the
Mesh 5. Sphere (external mesh).
Let us generate first the geometry using blender (or whatever you want to
use) and then the mesh by using SnappyHexMesh.
From now on, follow me.
For this tutorial you will first need to generate the geometry using blender or whatever
you want to use, I will use blender. In the terminal type:
•
cd
$ptofc/sHM_1/M4_sphere_sHM
(The blender file and STL file are in the directory geometry)
•
blender
(For geometry modeling.
Do not forget to export the geometry in ascii STL format
)
•
blockMesh
•
snappyHexMesh
•
checkMesh
•
paraFoam
•
cp
-r 3/polyMesh constant/polyMesh
(To copy the new mesh to your constant/polyMesh directory, this is compulsory
•
rm
-r 1,
rm
-r 2,
rm
-r 3
(If you do not want to keep these time folders)
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 5. Sphere (external mesh).
Let us generate first the geometry using blender (or whatever you want to
use) and then the mesh by using SnappyHexMesh.
• Alternatively,
snappyHexMesh
can automatically overwrite the original background
mesh contained in constant/polyMesh. In the terminal type:
•
snappyHexMesh
-overwrite
• If you use this option, you will not be able to visualize the intermediate steps.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 5. Sphere (external mesh).
Let us generate first the geometry using blender (or whatever you want to
use) and then the mesh by using SnappyHexMesh.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh generation using blockMesh and snappyHexMesh
Mesh with no inflation layer.
Mesh 5. Sphere (external mesh).
Let us generate first the geometry using blender (or whatever you want to
use) and then the mesh by using SnappyHexMesh.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh generation using blockMesh and snappyHexMesh
Mesh with inflation layer.
Mesh 5. Sphere (external mesh).
Let us generate first the geometry using blender (or whatever you want to
use) and then the mesh by using SnappyHexMesh.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh generation using blockMesh and snappyHexMesh
Mesh 5. Sphere (external mesh).
Inflation layers with different control parameters
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 6. 2D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh.
From now on, follow me.
In the terminal window type:
•
cd
$ptofc/sHM_1/M5_2d_cylinder
•
blockMesh
•
snappyHexMesh
•
checkMesh
•
extrudeMesh
(Take a look at the dictionary
extrudeMeshDict
, this dictionary tells OpenFOAM® to create the 2D mesh)
•
checkMesh
•
paraFoam
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 6. 2D Cylinder (external mesh).
Let us generate the mesh by using SnappyHexMesh.
• Alternatively,
snappyHexMesh
can automatically overwrite the original background
mesh contained in constant/polyMesh. In the terminal type:
•
snappyHexMesh
-overwrite
• If you use this option, you will not be able to visualize the intermediate steps.
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 6. 2D Cylinder (external mesh).
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 6. 2D Cylinder (external mesh).
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 6. 2D Cylinder (external mesh).
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 7. 3D Cylinder with periodic boundary conditions
(external mesh).
Let us generate the mesh by using SnappyHexMesh.
From now on, follow me.
In the terminal window type:
•
cd
$ptofc/sHM_1/M8_periodic_cylinder
•
blockMesh
•
snappyHexMesh
•
checkMesh
-latestTime
•
createPatch
(Take a look at the dictionary
createPatchDict
, this dictionary will create two patches named periodic1 and
periodic2
, which contain the information regarding to the periodic bcs)
•
checkMesh
-latestTime
•
paraFoam
Note: the
createPatch
utility will create a new mesh in the time directory 3 (if you chose a deltaT equal to 1 in
controlDict
).
This mesh has the periodic patches, now you can copy it to the folder constant/polyMesh
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 7. 3D Cylinder with periodic boundary conditions
(external mesh).
• Alternatively,
snappyHexMesh
can automatically overwrite the original background
mesh contained in constant/polyMesh. In the terminal type:
•
snappyHexMesh
-overwrite
• If you use this option, you will not be able to visualize the intermediate steps.
• In the same way,
createPatch
can overwrite the final mesh. In the terminal type:
•
createPatch
-overwrite
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 7. 3D Cylinder with periodic boundary conditions
(external mesh).
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 7. 3D Cylinder with periodic boundary conditions
(external mesh).
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 7. 3D Cylinder with periodic boundary conditions
(external mesh).
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 7. 3D Cylinder with periodic boundary conditions
(external mesh).
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 8. Meshing in parallel using snappyHexMesh.
BWB body (external mesh)
From now on, follow me.
In the terminal window type:
•
cd
$ptofc/sHM_1/M9_bwb_parallel
•
cp
constant/triSurface/N2A_Hybrid.stl constant/triSurface/
surfacemesh.stl
(The geometry was generated using
openvsp
)
•
surfaceTransformPoints –scale ‘(0.3048 0.3048 0.048)’
constant/triSurface/N2A_Hybrid.stl constant/triSurface/
surfacemesh.stl
(The geometry was generated using
openvsp
)
•
surfaceFeatureExtract
•
blockMesh
•
decomposePar
(Take a look at the dictionary
decomposeParDict
, here you need to specify the decomposition method and the number
of partitions. Later on we are going to deal with running in parallel)
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 8. Meshing in parallel using snappyHexMesh.
BWB body (external mesh)
From now on, follow me.
In the terminal window type:
• mpirun –np 8
snappyHexMesh
–parallel –overwrite
Notice the syntax used to run
snappyHexMesh
in parallel:
mpirun -np 8
snappyHexMesh
–parallel –overwrite
where mpirun is a shell script to use the mpi library (you need to install mpi), -np
is the number of processors you want to use, -parallel is a flag that you shall
always use if you want to run in parallel, and –overwrite
is an option specific to
snappyHexMesh
.
•
paraFoam
–builtin
(To visualize the decomposed mesh)
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 8. Meshing in parallel using snappyHexMesh.
BWB body (external mesh)
From now on, follow me.
In the terminal window type:
• mpirun –np 8
checkMesh
–parallel
(To run
checkMesh
in parallel)
•
reconstructParMesh
–mergeTol 1e-06 –constant
(Reconstruct the mesh using geometry information only)
At this point you are ready to run the simulation, but first you will need to transfer the
boundary and initial conditions to the decomposed mesh, to do this
•
decomposePar
-fields
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 8. Meshing in parallel using snappyHexMesh.
BWB body (external mesh)
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”
Mesh 8. Meshing in parallel using snappyHexMesh.
BWB body (external mesh)
“This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks.”