mesh considerations chapter 5. training manual may 15, 2001 inventory #001477 5-2 mesh...

MESH CONSIDERATIONS

Chapter 5

May 15, 2001Inventory

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Training ManualMesh Considerations

• Mesh used affects both solution accuracy and level of computational effort.

• Typical CFD meshes require more elements than typical solid mechanics problems.

• Concentrate mesh density toward steep gradient regions.– Wall boundary layer– Shear layer– Separation regions– Shock waves

• Transitions from high-to-low mesh density regions should be gradual to avoid numerical instabilities.

• 3-D element shapes may be mixed.


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Training ManualFinite Element Mesh

• Rapid Change in Element size is not good.– Can occur with free meshing.

• Aspect Ratio — depends on local behavior of solution.– 10:1 in regions with significant gradients may be high.

100:1 in regions without gradients may be fine.• Included Angles — Approximate recommended limits.

– 25 < Angle < 155– Acceptable included angles depend upon variable gradients in the

region.


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Training ManualFinite Element Model Problems

• Un-referenced nodes– Some geometry with nodes not connected into elements– Free Meshing Difficulty– Generally must clear area/volume & remesh

• Element Errors– Duplicate elements– Overlapping volumes– Degenerate elements– Unsupported element generated– Bad Shapes Generated


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Training ManualExamples of Poor Meshes

Incompatibility - the faces of adjacent elements must line up. The faces of adjacent elements must match up. If they do not, unspecified boundaries result.

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Training ManualAnother Example of Poor Meshing

Change in element size is too great.

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Training ManualFree Meshing

Free meshes are usually not a problem; this one shows too great a change in element size.

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Training Manual

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Triangles and Quadrilaterals

Both of the meshes shown use the same lesize commands. The quadrilateral mesh is preferred because it enables better resolution at a boundary.


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Training ManualPipe Cross Section

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A pipe cross section using a center square.

Using a center square,the pattern allows for mesh refinement towards the pipe walls.

Element sizing near the “center square" is not quite uniform.

A quarter of a circle can be free meshed with all hexes.

May require use of higher order quadrature for element integration


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Training ManualAnother Pipe Cross Section

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This mesh pattern which uses more volumes and enables more uniform spacing near the center.


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Training ManualMixing Triangles and Quadrilaterals

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An example of using triangles to provide a transition in the number of elements in the transverse direction.

Note that the mesh is not as good near the walls in the triangle region.


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Training Manual

Mixing Triangles and Quadrilaterals (continued)

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Using boundary areas preserves the ability to control the boundary layer spacing with a mapped quad mesh.


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Training ManualUsing the Mesh Tool

• The preceding cross sections can be extruded or swept into the third dimension to create a mesh with Hexahedral and, where necessary, Wedge elements.

• It may not always be possible to create a mapped mesh• The mesh tool is useful to mesh irregular areas or volumes

– These can also be extruded into three dimensions


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Training ManualLayered Meshing

• Generates line-graded free meshes in 2D for:– lines with small variation in element size along line– steep transition in element size and number normal to line

• Use Mesh Tool• Specifications

– Desired element size at the wall– Thickness of inner LAYER1 (uniformly sized elements)

• Distance or number of rows– Thickness of outer transitional region - LAYER2

• Size of elements increases by transition factor to global element size


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Training Manual

Layered Meshing Controls

Mesh Tool


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Training ManualControls for a Very Fine Mesh...

• Global Distance between lines: 1.0

• Size at wall: .01• LAYER1 Size factor is 3.0• LAYER2 1.5


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Training Manual

1.0

… Resulting In A Very Fine Mesh


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Training ManualComments on the Layered Mesh...

• Note that this mesh, although quite fine at the walls, is not optimized for this duct configuration. Generally in these cases higher aspect ratio elements would be used near the walls.

• The layered meshing is most appropriate for curved surfaces such as cylinders in cross flow.


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Training ManualAlternate Example

• Same wall element size: .01• Inner Layer 1 element thick• Size factor of 4 for transitional

layer


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Training ManualAlternate Mesh


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Training ManualFree Meshing - Quads

• Smartsizing is available

Settings!


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Training Manual

Circle (r=10) - Smartsize = 6 (Default - no esize)


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Training ManualCircle (R=10) - Smartsize = 1 (Finest)

But this has four elements with included angles near 180o


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Training ManualAdjustments

• Prep7 > Checking Ctrls > Shape Checking: CHANGE SETTINGS

• To use this feature, one must temporarily redefine element as a shell 63….


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Training ManualWith 150 degrees specified


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Training ManualMore Quad Free Meshing

• Half Circle : r = 10• Esize: 0.5• Smartsize : Default (6)


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Training ManualMesh Refinement

• Convenient when mesh is NOT mapped• Take the previous mesh and do a minimum

refinement at the outer edge….• Choose Refine “at Lines”• Click Refine• The Picker Appears (then pick lines)• Generally use minimum refinement in the

resulting dialog box...


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Training ManualResulting refinement...


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Training ManualMesh Tool and Transition elements

• Another pipe cross section – Use two cylinders, one solid and one hollow…. – Glue them together– Further operation necessary to control radial spacing


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Training ManualPreparation

• Rotate the working plane 90 about the X axis and divide the two volumes into four.

• Add the lines together as shown (picture shows only the front lines merged)


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Training ManualSome line sizing

• Set the four azimuthal lines to 20 divisions apiece.

• In the radial direction, use 15 elements with a spacing ratio of 4. Flip the lines if necessary to get small elements near the outer edge.


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Training ManualThe Cross section

• Using the mesh tool, mesh the outer regions with a mapped quadrilateral mesh

• Mesh the inner two areas with a free triangle mesh


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Training ManualSweeping

• Set the number of divisions (and ratio) in the axial direction and then sweep the mesh… • It is best to combine the lines and copy the line divisions to the back areas before doing the sweeping.• You may allow ANSYS to choose the source and target areas.


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Training ManualThe Result

• Quadrilaterals > Hexahedrals• Triangles > Wedges


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Training ManualTransition Elements: More

• In this case it is possible to map mesh hexahedrals directly in the two outer volumes.

• The inner volumes could then be meshed with tetrahedral elements.

• Pyramid elements would be automatically created to provide the transition between the hexahedral and tetrahedral faces.

• The extrusion of the cross section involving wedges is probably preferable if long aspect ratio elements are to be created. (I.e. the pipe is long….)


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Training Manual

Three Dimensional MeshingIntersection of Two Pipes• The World Itself is quite three - dimensional• In this Example two intersecting cylinders are created

– Large annular cylinder – Smaller inlet region

• Goal: Mesh each cylinder with only Hexahedral elements• Techniques illustrated

– Solid Modeling Boolean Operations– Sweeping process

• Display the grid of the working plane. Set the working plane to a spacing of 1.0, a snap increment of 1.0, and have it extend 10 units in all directions


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Training ManualStep 1 : Create The Annular Cylinder

• Choose the isometric viewInner Radius is 3

• Outer radius is 7• Stretch out from z = 0 to

some distance such as z=50.. (“back up” the view as necessary …..)

• The Result…


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Training ManualStep 2: Add the Inlet

• 1 - Move working plane to 0,0,4 .– It is convenient to align with the global

cartesian axes first….• 2 - Rotate WP +90 about Y axis• 3 - Create Solid cylinder with center y=5

units up from origin of working plane, stretch to radius of 2 and then pull to local z = 15


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Training ManualResult of Step 2…..

• At this point, the two cylinders occupy some of the same volume.• The overlap must be eliminated.


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Training ManualStep 3: Split the Small Inlet

• Preprocessor> (Modeling) Operate>Divide> Volume by Area• Pick Volume (OK)• Pick Area (OK)

Note when Using Picker:Keep depressed until correct item is highlighted

The Inlet cylinder is being trimmed by the surface of the middle cylinder


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Training ManualResult of Division

• Adjust the display:– Colors from Utility Menu > Plot

Ctrls > Numbering… (turn on volume numbers)

– Change view via Utility Menu > Device Options: Vector Mode

– Plot Volumes


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Training ManualStep 4: Delete the Extra Volume

• Preprocessor >(Modeling) Delete> Volumes and Below….• Volume 2 has been replaced by Volumes 3 and 4. The volume

inside the big cylinder is now deleted.• Pick the one shown and OK

Note that from revision to revision, changes in the way boolean operations are performed may change entity numbering schemes...


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Training ManualGeometry Modifications

• Divide the annular cylinder into two cylinders:– Move the working plane to z = 8 (Step 5)– This enables meshing most of the annular cylinder with a mapped

mesh– Divide the resulting two annular cylinders at y=0 into a top half and a

bottom half (Step 6)• Using the area of the inlet which projects onto the surface of the

annular cylinder, make a volume which extends back through the cylinder (Step 7)

• Overlap this volume with the cylinder and the Inlet (Step 8)• Delete the resulting extra volume (Volumes and Below) (Step 8)• Details of performing these steps follows…..


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Training ManualStep 5: Divide Cylinder

• Realign working plane (WP) with global cartesian coordinate system and then offset to 0,0,8.

• Then divide the Annular Cylinder by the working plane


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Training Manual

Step 6: Divide the Two annular cylinders• Rotate WP 90Deg about X axis• Divide both new cylinders by WP.• Result: 5 volumes• Turn off wireframe


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Training Manual

Step 7: Make Volume by Projecting Area through Cylinder• Preprocessor >Operate > Extrude >

-Areas- along Normal..• Choose area where the inlet

cylinder has been split in two (use isometric view…)

• Make sure to use a negative value, making sure it is enough to penetrate the annular cylinder completely


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Training Manual

Solid model has now been properly connected

Step 8: Clean up the Volumes

• OVERLAP the two Volumes highlighted

• Delete the “extra” volume (and below)


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Training Manual

The Four Volumes Near the Intersection

This is the region of interest…. The remainder of the annular cylinder is left to the student !There should be 6 volumes all told.


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Training ManualStep 9: Line Combinations

• Select the Two Annular volumes near the intersection• Add lines (LCOMB) - delete existing lines (Operate > Add)

Generally, it will be best to combine the lines to make 180 degree sections where ever two 90m degree segments occur.


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Training ManualStep 10: Line Divisions

• Mesh divisions - Annular Cylinder (Meshtool-Lines-Set)– 20 Azimuthal– 14 Axial– 10 Radial

• Before MeshingDefine Element types2D and 3D– 63,141


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Training ManualStep 11: Mesh sweeping

• Source Area• Target Area

– Larger one is source

• Set ESIZE 0.4

First Mesh the SourceArea, then perform sweep


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• Esize 0.4

• Pick larger area as Source to ensure proper element size…

• Target

• Source

Second Sweep


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Training ManualSweeping Through Annular Volumes..

• Meshtool• Select

– Volume– Source Area– Target Area

• Options Box Available• It is possible to mesh the target

area first to see what mesh will be extruded. It is later deleted.


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Training ManualResulting mesh patterns

• Now the surface of the “plug” will be used to sweep through the inlet volume Q2


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Training Manual

• These two areas must be mapped meshed before the sweeping operation…

• Set number of divisions along cylinder to 18.

(In some cases, it is not necessary to mesh the side areas…)

Step 12: Mesh Division for Small Inlet ….


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Training ManualStep 13: Mesh via Pick Corners...

• The mapped mesh is created through the Mesh Tool - “Pick Corners”

• The same four corners for each of the two areas:


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Training ManualSurface Mesh of the Inlet

• A 2D non-planar element type such as Shell63 should be defined so that a surface mesh can be created.

• Resulting surface Mesh


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Training ManualStep 14: Mesh Inlet via Sweep

• Use Option to Clear Areas afterward

• Meshtool – Sweep– Hex/Wedge– Auto Pick

• In this case, ANSYS can automatically pick the source and target areas…..


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Training ManualResulting Mesh So Far !!!


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Training ManualThe Underside

• The line divisions have already been done….


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Training ManualVoila!!


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Training ManualNotes

• The meshing of the other two volumes of the annular area is straightforward and is left to the student…

• This mesh can now easily be used for a mixing analysis following the application of the appropriate solid model boundary conditions.

• The “the Back End” of the annular cylinder can be made an inlet or a wall, and a velocity applied to the small inlet…

• Challenge to the User– Mesh the same configuration with a combination of hexahedral and

tetrahedral elements….

mesh considerations chapter 5. training manual may 15, 2001 inventory #001477 5-2 mesh...

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