am 12 appendix a

Chapter 1 IntroductionApril 28, 2009 Inventory #002645
Appendix A
Mesh Quality
ANSYS Meshing
Application Introduction
April 28, 2009 Inventory #002645
Training Manual
Skewness
General Considerations
Factors Affecting Mesh Quality
CAD Cleanup
Virtual Topology
Pinch Controls
General Recommendations
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Training Manual
Mesh Metrics are available under
Mesh Options to set and review
mesh metric information and to
evaluate mesh quality
Meshing include:
Element Quality
Aspect Ratio
Jacobian Ration
Warping Factor
Parallel Deviation
Training Manual
Based on the Equilateral Volume deviation:
Skewness =
Based on the deviation from a Normalized Angle deviation:
Skewness =
Where is the equiangular face/cell (60 for tets and tris, and 90 for quads and hexas)
Applies to all cell and face shapes
Used for prisms and pyramids
optimal (equilateral) cell
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Training Manual
Aspect Ratio
Aspect for generic triangles and quads is a function of the ratio of longest side to the shortest side of the reconstructed quadrangles (see User Guide for details)
Equal to 1 (ideal) for an equilateral triangle or a square
aspect ratio = 1 high-aspect-ratio quad
aspect ratio = 1 high-aspect-ratio triangle
Please update or remove this page.
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Training Manual
Mesh Quality Statistics in ANSYS Meshing
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FLUENT requires high quality mesh to avoid numerical diffusion
Several Mesh Quality Metrics are involved in order to quantify the quality, however the skewness is the primary metric
The aspect ratio and cell size change mesh metrics are also very important
In worst scenarios and depending on the solver used (density based or pressure based) FLUENT can tolerate poor mesh quality. However some applications may require higher mesh quality, resolution and good mesh distribution
The location of poor quality elements helps determine their effect
Some overall mesh quality metrics may be obtained in Ansys Meshing under the Statistics object
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Training Manual
The most important mesh metrics for Fluent are:
Skewness
For all/most applications:
For Skewness:
For Hexa, Tri and Quad: it should be less than 0.8
For tetrahedra: it should be less than 0.9
For Aspect Ratio:
It should be less than 40, but this depends on
the flow characteristics
More than 50 may be tolerated at the inflation layers
For Cell Size Change:
Poor mesh quality may
Training Manual
High skewness values are not recommended
Generally try to keep maximum skewness of volume mesh < 0.95. However this value is strongly related to type of physics and the location of the cell
FLUENT reports negative cell volumes if volume mesh contains degenerate cells.
Classification of the mesh quality metrics based on skewness:
* In some circumstances the pressure based solver in Fluent can handle meshes containing a small percentage of cells with skewness ~0.98.
0-0.25 0.25-0.50 0.50-0.80 0.80-0.95 0.95-0.98 0.98-1.00*
Excellent very good good acceptable bad Inacceptable*
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Large cell size change
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Mesh quality requirements are somewhat different for the CFX
solver than for the FLUENT solver due to the difference in the
solver structure for the two codes
Fluent uses a a cell-centered scheme, in which the fluid flow variables are allocated at the center of the computational cell, and the mesh-element is the same as the solver-element
CFX employs a vertex-centered scheme for which the fluid flow variables are stored at the cell vertex, and the solver-element or control volume is a “dual” of the mesh-element. This means that the vertex of the mesh-element is the center of the solver-element
Please complete this slide
Training Manual
The CFX solver calculates 3 important measures of mesh
quality at the start of a run and updates them each time the
mesh is deformed
Maximum Aspect Ratio = 13.5 OK
Maximum Mesh Expansion Factor = 700.4 !
Domain Name: Water Pipe
Global Mesh Quality Statistics :
Good
(OK)
Acceptable
(ok)
Questionable
Training Manual
Orthogonality Factor = n·s, >1/3 desirable
Orthogonality Angle = 90º-acos(n·s), >20º desirable
Are these different than Max/Min Face Angles in CFD Post? YES!
Face angles correspond to angles between edges
One can have an acceptable Face Angle and an unacceptable Orthogonality Angle if an element is skewed in two directions…
Mesh Orthogonality in CFX
Appendix A: Mesh Quality
Training Manual
Expansion factor measures how
Mesh Expansion Factor ≈ ratio of largest to smallest element
volumes surrounding a node,
<20 is desirable
The Mesh Expansion Factor is essentially identical to the Element Volume Ratio in CFD Post
Training Manual
Aspect ratio measures how stretched a control volume is
Aspect Ratio = maximum of the ratio of largest to smallest
ip-areas for each element surrounding a node,
<100 is desirable
The Aspect Ratio is very similar to the Edge Length Ratio in CFD Post
Training Manual
Sources of discretisation error
large mesh expansion introduces errors in storage and source approximations
Amplification of discretisation error
corrections to reduce errors caused by non-orthogonality can create unphysical influences
Difficulties solving linearised equations
(i.e. use of double precision solver)
Why is geometrical mesh quality important?
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Unconnected geometry entities
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Mesh Resolution and Distribution
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Type of Size Function
Inappropriate usage (or no usage at all) of Advanced Size Functions (ASF) may lead to poor mesh quality
Use Curvature ASF for geometries with dominant curvature features
Use Proximity ASF for geometries with gaps or narrow components
Use Curvature and Proximity ASF in geometries having a combination of these features
ASF may be used to avoid this !
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Training Manual
Meshing Method
Inappropriate usage of Meshing Method (Automatic, Tetrahedrons, Sweep, MultiZone and CFX-Mesh) may lead to large skewness
The selection of the Meshing Method depends on the geometry and application
It is a good practice to use Show the Sweepable Bodies under the Mesh object in the Tree Outline
Many applications may take advantage of Patch Conforming and Sweep Meshing Method
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Inflation Option
Inflation parameters
Affected Inflation
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CAD cleanup
Simplify the geometry
Merge small edges
Merge the faces in order to reduce the number of faces
Avoid narrow faces
Decompose the geometry
Remove unnecessary geometries
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Virtual topology
Use VT in order to simplify details at geometry level in AM
Can be added under Model object in the Tree Outline
Mesh may be improved by creating virtual edges/faces
If the resulting surface mesh is distorted consider fixing the geometry issue in DM or CAD
After virtual merging of narrow face with wide face
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Pinch Controls
Intended for Patch-Conforming Tetrahedral Method
When it is defined the small features are “pinched-out” from the mesh when pinch criteria are met
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Sensible Mesh Sizings and Inflation Settings
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General Recommendations
A volume mesh may be considered inacceptable if it satisfies one or more the following conditions:
Very high skewness for FLUENT meshes(> 0.98)
Degenerate cells (skewness ~ 1)
High aspect ratio cells
Improving surface mesh quality
Moving mesh nodes
CAD to fix geometric problems such as sharp angles, small edges, merge faces unite and/or decompose the geometries
Clean-up tools in DM to simplify the geometries and their entities
Different methods, global and local sizings and parameters in the ANSYS Meshing Application
Pinch Controls in the ANSYS Meshing Application to avoid small features
Virtual topology in the ANSYS Meshing Application in order to simplify the geometry
Strategies to Improve Mesh Quality
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Training Manual
Miscellaneous
If the model contains multiple parts or bodies the mesh metric information can be shown by highlighting them under the Geometry object in the Tree Outline
The Body of Influence (BOI) technique may be used also to control the mesh quality and appropriate local resolution
More advanced mesh statistics including histograms can be exhibited by FE Modeler Mesh Metrics in FEM
Different mesh quality metrics
Add more tips here
Virtual Topology for an
Training Manual
Goals
This workshop uses the manifold geometry from workshop 5.2. Recall that this geometry contains many problematic small faces and sharp angles.
In workshop 5.2, the Patch Independent method was used to produce a good quality mesh without modifying the geometry. In this workshop Virtual Topology will be used to “remove” the problematic geometry and then the default Patch Conforming meshing method will be used.
Training Manual
Launch ANSYS 12.0 Workbench
Click on Component Systems in the Toolbox on the LHS of the main panel
Double click the Mesh option to add it to the Project Schematic
In the Project Schematic right-click on Geometry and select Import Geometry > Browse. Select the file Auto-Manifold.agdb
Training Manual
Named Selections
Next, make sure that Named Selections will be brought into Meshing:
Right-click on cell A2 and then select Properties
Ensure Named Selections is checked, and the Named Selection Key is blank
Close the Properties window
Training Manual
Start by suppressing the fluid region and meshing the solid:
Select the Body selection icon from the toolbar
Select the inner fluid region, so
that it is highlighted in green, and
then right-click and select
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Mesh Settings
In the Details view set the Physics Preference to CFD
The assumption here is that heat transfer will be solved in the solid region using a CFD solver
Expand the Sizing section in the Details view and set:
Span Angle Center = Medium
Min Size = 1.0 mm
Max Face Size = 10.0 mm
Max Tet Size = 10.0 mm
Right-click on Mesh in the Outline tree and select Preview Surface Mesh
Since the body is not sweepable, the Patch Conforming method will be applied by default
Training Manual
Examine the Mesh
The Patch Conforming method meshes each individual surface. This produces a poor quality mesh on some surfaces in this geometry. Examine the surface mesh and look for regions of poor mesh quality. By switching between Geometry and Mesh in the Outline tree relate regions of poor mesh quality to the underlying surface geometry. Some examples are shown here:
Training Manual
Virtual Topology allows you to merge adjacent surfaces, removing undesirable surface geometry feature and producing a higher quality mesh
Right-click on Model (A3) in the Outline tree and select Insert > Virtual Topology
A Virtual Topology entry is added to the Outline tree
In the Details view note that the Behaviour is set to Low
Right-click on Virtual Topology in the Outline tree and select Generate Virtual Cells
This automatically creates virtual cells using a “Low” merging strategy. “Medium” and “High” strategies are likely to result in more faces being merged into virtual cells
Training Manual
Virtual Topology
When Virtual Topology is selected in the Outline tree the viewer shows all virtual cells that have been created
Examine the new surface geometry and note that most of the problematic faces have been merged to produce a cleaner surface geometry
In the Details view change the Behaviour to Medium
Right-click on Virtual Topology in the Outline tree and select Generate Virtual Cells
Note that more faces have been merged into virtual cells
Try generating virtual cells using the High option for Behaviour
This does not work as well for this geometry as shown to the right
Switch back to the Medium option and generate the virtual cells again
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Examine Improved Mesh
Re-create the surface mesh and examine the regions that previously showed poor mesh quality
You should find that the surface mesh has been greatly improved
There are still some regions where the mesh quality could be improved. The arrows below shows one of these locations.
If you zoom in and examine the geometry here you will find a kink at the edge of the surface
Training Manual
Adding Virtual Cells Manually
You can manually add Virtual Cells to improve the mesh further
Pick the Face selection icon from the toolbar
Orient the view approximately as shown below (note the X-Y axes)
Check that Virtual Topology is selected from the Outline tree
Select the four faces shown below, then right-click and select Insert > Virtual Cell
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2
4
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You should find an improved surface mesh
You can continue adding Virtual Cells as necessary
In some cases the automatic virtual cell creation may merge faces that you do not want to merge. You can delete individual virtual cells by selecting the Virtual Face from below the Virtual Topology entry in the Outline tree and right-clicking to delete.
Right-click on Mesh and select Generate Mesh to create the final solid mesh
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Viewing the Fluid Body
The next step is to create the mesh for the fluid region
In the Outline tree expand the Geometry > Part section
Right-click on the first solid and select Hide Body to hide the solid region
Right-click on the suppressed (second) solid and select Unsuppress Body
With the second solid selected, in the Details view expand the Graphical Properties section and set the Transparency to 1
Training Manual
Adding Inflation
Select Virtual Topology from the Outline tree
Virtual Cells have already been created on the fluid region from earlier
Check that the automatic virtual cells look reasonable
There should be no small surfaces remaining in the model
The next step is to add inflation to the fluid walls
Right-click on Mesh and select Insert > Inflation
In the Geometry field you need to select the solid body corresponding to the fluid region from the Viewer then click Apply
Once this has been selected click on No Selection in the Boundary field so that the Apply / Cancel buttons appear
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Creating the Fluid Mesh
Now select one of the faces from the model that is not an inlet or outlet
Select Extend to Limits from the toolbar as shown:
All the fluid walls should now be selected
Click Apply in the Boundary field in the Details view
To generate the final mesh right-click on Mesh and select Generate Mesh
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for the FLUENT solver.
Max Skewness would have been considerably
higher
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FLUENT and CFX Mesh
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Goals
This hands on tutorial will demonstrate how the Meshing Application in ANSYS is used to generate a CFD mesh for an internal flow domain
The geometry represents portions of an aerospace valve region, decomposed into 3 bodies
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Launch ANSYS Workbench from the START menu
Click on Component Systems in the Toolbox on the LHS of the WB main panel
Double click the Mesh
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Right click (RMB) on the Geometry button and select Import Geometry (the question mark on the button goes away once a geometry file is imported)
Import the Aero-Valve.agdb file from the tutorial folder
Double click on the Mesh button in the Project Schematic to launch the Meshing Application
Importing the Geometry
Training Manual
Geometry
The original geometry is a Solid part and the Fluid region was extracted out in DesignModeler (DM). Other operations performed in DM;
A parameter was defined for the position of the valve
Some outlet ports were closed
One multi-body part was created and a given the name “Fluid” and the material “Fluid”
Individual bodies were re-named and Named Selection was used to define the Inlet and Outlet
Fillets were added to some
sharp corners to improve
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In the Meshing Options panel select the following meshing options:
Physics Preference
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Click on Mesh to change settings
Verify Defaults
Physics Preference
setting are also presented
On: Curvature
Maintain all other defaults
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Set Inflation parameters
Click drop-list for Use Automatic Tet Inflation and select Program Controlled, leave all others as default
Set Maximum Layers to 4
Activate View Advanced Options
Activate Generate on Refresh
Inflation and Pinch Parameters
Note: Program Controlled Inflation will add inflation on all boundaries that do not have assigned Name Selection. It does not add inflation to Fluid-Fluid interfaces
Note: Smooth Transition provides a transition between the inflation layers and the tetrahedral mesh following the specified Growth Rate
Note: Layer Compression is the default Collision Avoidance
for Fluent and Stair Stepping is default for CFX
Note: When edge length or distance between vertices is less than the pinch tolerance, software will ignore the edge or remove extra vertex during meshing
Note: Pinch Tolerance should be
smaller than Size Function Min Size
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Select Create Pinch Controls
10 Pinch Controls are created (Expand the Mesh button to list the pinch controls)
Pinch Controls
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View the Pinch Controls
Ctrl Left-Mouse-Button – Select the Pinch controls, these will be highlighted in the viewing window
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Sweep Method
Select the bodies (as shown below)
Set the Cursor Mode to Body Selection
Left-Mouse-Button click (Select) one sweepable body
Hold Ctrl key and select the second body
Insert Method
Insert - Method
Select Sweep from the
Training Manual
Click on the Source Selection Field
This will activate the face picker
Hold the Ctrl key and pick both the Inlet and the Outlet face
Apply the Selection
Set Sweep Num Divs; 20
Set Sweep Bias Type; _ __ ___ __ _
Set Sweep Bias; 4
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RMB (Window) Insert-Inflation
Select four edges surrounding the
inlet and outlet faces (marked in red)
Apply the selection
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Right-click on Mesh and select Preview Surface Mesh
This will provide us with feedback about mesh quality and density
The Advanced Size Function creates a very fine mesh in the swept bodies,
We can reduce the size by specifying the edge intervals on the Inlet and Outlet
Initial Surface Mesh
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Edge Sizing
Insert Scoped Edge Size ;
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Right-click on Mesh and select Preview Inflation
View the mesh Statistics, mesh size and max skew is around 310000 and 0.92 respectively
We are ready for volume meshing
Preview Inflation
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RMB (Tree) select Generate Mesh
Again, check the Statistics for the total element count and Max Skewness which will be around 926000 and 0.92 respectively
Volume Mesh with Fluent Settings
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Create a Section Plane:
Click on the Z-Axis at the lower right corner to orient the model
Click the Selection Plane icon
Press and hold the left mouse button while moving along the indicated red arrow then release
The position of the Section Plane can be adjusted by moving the slider bar
Click on “Show Whole Element”
Reselect the rotation button to adjust the view
Using a Section Plane to View Internal Mesh
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RMB (Tree) Show Worst Elements
Note the location; far from the main flow field
Viewing the Worst Elements
Tip: Select ‘Wireframe’ from the ‘View’ menu to help see the element
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Change Solver Preference: CFX
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Meshing application
Double click on Model
Linear Tetrahedron
Max aspect ratio is less than 50
Checking the Quality in FEModeler
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Select File > Close to close FEModeler
In the WB panel select Update
In the WB panel select File > Save Project As… and give the project a name
Exit from ANSYS Workbench by selecting File > Exit
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am 12 appendix a

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