Mesh-Intro 14.5 L07 Assembly Meshing

© 2012 ANSYS, Inc. January 15, 2013 1 Release 14.5

14. 5 Release

Introduction to ANSYS Meshing

Lecture 7 Assembly Meshing


What you will learn from this presentation

• Concept of Assembly Meshing

• Assembly Meshing Methods

– CutCell

– Tetra

• Controls

• Virtual Bodies

• Fluid Surfaces

• Contacts

• Sharp Angle Tool

• Leakage

Assembly Meshing


Preprocessing Workflow

Sketches and

Planes

Geometry Import

Options

3D Operations

Direct CAD/Bi-

Directional CAD

Geometry

Cleanup and

Repair

Automatic

Cleanup

Merge, Connect,

Projection, Flow

Volume

Extraction, etc

Extrude, Revolve,

Sweep, etc

3D Operations

Boolean, Body

Operations, Split,

etc

Meshing

Methods

Hybrid Mesh: Tet,

Prisms, Pyramids

Hexa Dominant,

Sweep meshing

Global Mesh

Settings

Local Mesh

Settings

Sizing,

Body/Sphere of

Influence, Match

Control, etc

Geometry Creation OR

Geometry Import

Geometry Operations

Meshing Solver

Assembly

Meshing


Meshing Process in ANSYS Meshing (Assembly Meshing)


Behavior

• Meshes an entire model as single process

– Mesh Methods covered so far are part or body based methods

– Not compatible with part/body methods

• Two Algorithms available

– CutCell & Tetrahedrons

Access

• Assembly Meshing is accessible only when Physics and Solver Preferences are set to CFD and Fluent respectively

• To activate, replace None by Cutcell or Tetrahedrons

Assembly Meshing


• Operates on parts, multi-body parts, etc.

– Tolerates overlapping bodies

• Creates conformal meshes across parts in contact

– Eliminates the need for multi-body part generation in CAD

• Ability to define Virtual Bodies for fluid flow from a “closed” set of bodies (sheet or solid)

– Eliminates the need for Boolean/Fill operations in CAD

• Patch Independent

– Eliminates the need for pinch control and VT operations

• Mesh elements size driven by Size Functions

Assembly Meshing - Characteristics

Note that some global and local controls are

not available for Assembly Meshing (eg.

Match Control)

CutCell Assembly Mesh


CutCell Behavior

• Cartesian meshing method designed for the ANSYS FLUENT solver

• Generates a majority of hex cells

– Some wedges, tets and pyramids at boundaries to capture geometry

– During transfer to Fluent hexa cells at size transition are converted into Polyhedra

• Supports Inflation

– Post-inflation (TGrid algorithm)

• Inflation on zero thickness baffles not supported

• Thick inflation layers may fail

– Cutcell mesh generated first, inflation generated second (Post)

Assembly Meshing - CutCell


Tetrahedrons Behavior

• Generates a Patch Independent tetra mesh with automatic defeaturing

• Following steps occur in background

– Generate CutCell

– Delete volume mesh

– Triangulate surface mesh and improve

– Fill with tetra mesh

• Compatible with inflation

– Pre-inflation

• Algorithm similar to Tetra Patch Conformal

• Has the benefits of cutcell (no requirement on solid bodies, patch independent, etc) but with the flexibility of the bottom up approach i.e. “pre” inflation rather than post

Assembly Meshing - Tetrahedrons


• Set Advanced Size Functions

– Proximity SF Sources : 'edges', ‘faces’ or ‘edges and faces’

– ‘Faces’ is v. expensive and edges is recommended

– Define correct Min Size (details next slide)

• Inflation defined by Global or Local controls

– Combined Global & Local not supported

– Program Control acts on Fluid bodies only

• Bodies can be set as Fluid in Body properties

– For Virtual Bodies, only automatic Program Controlled inflation can be used (wetted surfaces may only be defined during meshing)

• Define Feature and Tessellation controls (see next slide)

• Apply any required local size controls

• Statistics

– Use Orthogonal Quality for Cutcell meshes

Assembly Meshing - Controls


Min Size definition

• Assembly Meshing is Patch Independent and starts with a Cartesian overlaid grid which finds closed regions

• Geometry recovery and leakage between regions depend on local sizes, i.e. if Cartesian size is too small near a gap you will get leakage between regions

• Local sizes are driven by Size Function definitions

– ‘Min Size’ and ‘Prox Min Size’ must be set with care

– Prox Min Size can be smaller than Min Size to avoid over-refinement in curved areas

• Local mesh size recommendation to capture 3D features

– Local size < ½ important feature size

• Local mesh size recommendation to close gaps

– 1/10 local mesh size < gap size < ¼ local mesh size : apply hard sizing to force gap closed

– Gap size < 1/10 local mesh size : gap closed


Example2 . Doubling

the Min Size closes the

gap

Example 1. Min Size

too large compared to

the size of the

geometric detail


• Feature Capture

– Program Controlled : default which sets feature angle = 40

– Feature Angle : user defined angle to define feature level to recover

• 0 to capture all curves in model

• Tessellation (faceting) refinement

– Assembly meshing is applied to a faceted representation of the geometry via TGrid libraries

– Program Controlled - default which sets tessellation refinement to 10% of the value of smallest mesh size

– Absolute Tolerance – user defined tolerance

• Must be set to 5-10% of smallest mesh size

– None - Sets tessellation to the CAD program or Design Modeler default setting

• Note DM faceting control is in Options GraphicsDesign Modeler Graphics Facet Quality


Tessellation problems may

lead to leakage


• Images above show the tessellated representation of the geometry in the workshop which is passed to the TGrid libraries for cutcell meshing

• With no refinement the geometry is coarsely faceted but with the recommended refinement of 0.1*min-size a good curvature of geometry is captured and will give better geometric fidelity in the mesh

Tessellation Refinement

Tess. Ref. = None Tess. Ref. = 0.1mm


Assembly Meshing - Virtual Bodies

• Ability to mesh fluid regions without prior CAD manipulations

• Definition of fluid flow from a “closed” set of bodies (sheet or solid)

– Capping faces might be necessary to close internal volume

• Use of virtual body (material point) to define meshed region to be kept

Material point

Capping Face



1

Using Virtual Bodies

1. Define a new Coordinate System inside the Fluid Void

– Easy creation based on entity selection, e.g. Select 2 points, RMB > Create Coordinate System

2. RMB on Geometry and Insert a Virtual Body

3. Assign the Coordinate System to the Material Point in the details of the Virtual Body

2

3 Material

Point



Using Virtual Bodies

• When using Virtual Bodies, user can choose to keep or discard the solid mesh

• ‘Keep Solid Mesh’ option in the global controls


Assembly Meshing - Fluid Surfaces Creating Fluid Surfaces for Virtual Bodies

• Description

– If possible, the user can improve robustness by picking all faces that make up the wetted surfaces of the flow volume

• Can take advantage of “Extend to Connection” (see appendix)

– Only specified fluid surface faces will be used for mesh generation (solids ignored)

• Applications

– Used when only flow volume is needed

• ‘Keep Solid Mesh’ NO

• Advantages

– Faster

– Less memory

– Reduction of leakages

– Reduction of unnecessary refinement

• Usage

– Fluid Surfaces will ignore baffles

– Fluid surfaces recommended when Virtual Body is bounded by surface bodies only


Assembly Meshing - Sharp Angle Tool

• A special cell cutting algorithm has been developed to properly capture sharp 3D angles

• Can be used to improve feature capturing in general

• Similar to “thin cuts” in ICEM CFD

• Right click on Mesh, Insert ‘Sharp Angle’.

• Select surfaces adjacent to edges at sharp locations. Can select multiple areas.

Sharp angles in the Flow volume of a drill bit model

Mesh without Sharp Angle Tool

Mesh with Sharp Angle Tool


Find leaks using material points

• When using material points (for internal flow) and the mesh is leaking to the outside, the leak-path will be automatically displayed together with the surface mesh. User can try to repair gap/hole based on path.

Assembly Meshing - Leakage Path

Example; Gap between pipe flange surfaces. Message indicates leakage. Leakage path displayed in Graphics Window.


Contents

• Contacts

• Sharp Angle Notes

• Mesh Groups

• Important General Notes

Appendix


Using Contacts for Assembly Meshing

• Workbench has extensive capabilities with respect to the detection of contacts (interfaces or connections) between parts

• When meshing intersecting bodies using assembly meshing, the intersection lines can have poor feature capture

• Assembly Meshing Contact Tools

– Find Contacts

• Features at contact pairs will be well preserved in the mesh

• Enable use of ‘Extend to Connection’ selection tool

– Find Thin Sections

• Check thin bodies to set a corresponding size

– Closing of small gaps using Contact Sizing

Assembly Meshing - Contacts

For example, in this image the circled edges would be removed without contact defined since the feature angle is below the default (40 degrees)



Finding Contacts

• By default connections are created automatically in WB, so to start with delete all the old connections

• To create contacts, RMB on Mesh and ‘Find Contacts’

– This will generate contacts for the whole assembly

• Face - Edge and/or Face – Face

– Make sure Size Functions and min-size are set to appropriate levels for the geometry


Finding Thin Sections

• Assembly meshing can leak through very thin plates

• Assembly Meshing produces better quality meshes if thin baffles and fins are well resolved

• By using the Find Thin Sections tool, thin geometry can be found in advance and appropriate sizing can be applied (< half the thickness of the thin section)




#1

#2

#3

Defining Contact Sizings

• Contact Sizings can be used to avoid leaks between parts

– Closing Gaps smaller than ~¼ of min size

• How to set Contact Sizings 1. Insert a contact with the entities

between which there is a gap • Face/Face or Face/Edge

2. Drag and drop the contact on top of the Mesh Icon • Creates a Contact sizing

3. Adjust Contact sizing • Should be bigger than the gap

4. Generate Mesh

Example; Pick the face of the valve plug (blue) and the edge of the valve seat (red)



• Selection helper ‘Extend to Connection’ can be used to pick wetted surfaces

– User pick one surface, upon click on Extend to Connection all surfaces between neighboring contacts are selected

• Contacts must be defined beforehand

• Picking Fluid Surface with Extend to Connection

– Start by picking all capping faces

• Use the Tree to confirm the number of capping faces (Surface Bodies)

• In large models, hide all 3D-bodies!

– Select faces in the fluid region away from contacts

• Extend to connections, does not work if the face is already in “contact”

– Check that all the faces are picked, if not, meshing will leak

• To see missing faces, hide all bodies and look at the picked faces in the mesh group

• Faces could extend outside the flow volume (as they are not split at contacts)

• Note: Fluid Surfaces will ignore baffles


This should

be resolved in

the CAD

Contact Sizing Limitations

• The features at a Face-Face location are often not well resolved since there are not features on both side of the gap

• Avoid using inflation at contact size locations

– Exclude the faces around the gap using Named selection

• Do not combine Contact sizing with sharp angle tool

– Contact sizing tries to “walk over” thin gaps

– Sharp angle tools try to resolve small gaps


No Feature Features


• Sharp angle tools does not work in following cases

– Between baffles

– In combination with contact sizing

• Using sharp angle tool can lead to reduced quality

– Ignoring inflation at sharp angle faces often improves quality

• Sharp angle tool to resolve thin plates

– If you have a big thin plate and only edge proximity size function, CutCell might fail and leak right through the plate

– However, instead of resolving the plate using face proximity (which is time consuming), you can add sharp angle tool on both side of the plate

• Note that if the plate is meshed, the ends need to be well resolved

Assembly Meshing - Sharp Angle Notes


Without sharp angle tool

With sharp angle tool

Assembly Meshing - Sharp Angle Notes

very thin plate


Using Mesh Groups

• Mesh Groups can be defined to identify bodies that should be grouped together for assembly meshing algorithms

• This approach may be useful if the fluid domain was decomposed into multiple volumes

• With CutCell, decomposed volumes are no longer needed. Grouping tells the mesher to treat certain solid parts as one part

• The mesh generated on the combined parts (i.e., the group) will be associated with the mesh of the selected master body

Assembly Meshing - Mesh Groups


• Cannot be used in combination with other Meshing Methods

• Feature recovery limitations

– Sharp trailing edges and acute edges will produce a jagged mesh

– Recovering some features might lead to bad quality

• Prior to meshing the user is advised to resolve geometry features properly CAD/DM

– Avoid unnecessary geometry details

– Min size should be ½ of smallest 3D feature, check and adjust if required

• Interoperability

– Inflation not supported on baffles or internal walls

– Ignored symmetry condition for meshing

• Failure in the CutCell meshing algorithm is almost always related to faceting issues in relation to the value of Min Size. Make sure that the value of Min Size truly represents the smallest size that you want the curvature and proximity size function to capture

Assembly Meshing – Important General Notes


• ANSYS Meshing Gives the Warning: “The tolerance of the geometry is larger than the applied tessellation refinement tolerance. This might lead to an uneven mesh and/or to poor geometric accuracy of the mesh.”

• Background: For assembly meshing a tessellated version of the geometry is generated and passed to TGrid libraries where the mesh is generated. This is very much like an STL representation.

• Meaning of warning: In certain models, some CAD curves may not accurately follow the topologically connected surface(s). The warning above happens when the tolerance describing the mismatch between curves and surfaces is greater than the refinement tolerance. This mismatch can lead to holes being created in the faceting.

• A post-tessellation refinement “hole-fixing algorithm” subsequently tries to patch any holes created with an algorithm but hole closing itself can lead to poor representation of the geometry. Hole closing can also fail, leading to leakage in the Assembly Meshing algorithms (cutcell and tetrahedra) inside ANSYS Meshing.

• Example given on next slide.

Assembly Meshing – Common Warning 1


2. Tessellation Refinement will refine

facets to improve situation but can open

holes

1. No Tessellation Refinement – sloppy

CAD surfs don’t match curves

3. Hole fix step done automatically after

tessellation refinement can distort

geometry or fail leaving holes, causing

cutcell to “leak”

Hole opened

Hole Fixed



• What can the user do about it?

• This is a warning only and it may be that there are no ill effects caused by it – patches could all be very clean. Check the final mesh.

• User should inspect their geometry to check for problems around areas of curvature and fix if possible. Use CAD/DM to simplify.

• Do not use merging operations for faces in DM for Assembly Meshing – it is patch independent and these are not required.

• Do not use virtual topology for Assembly Meshing for the same reason.

• Usually poor quality will show up if this is a problem or there will be some poor feature capture

• If cutcell “leaks” and the final surfaces of the mesh look nothing like the original geometry try switching off refinement to see if this helps and then trying larger user defined tolerances (recommend 1/10th min size in the mesh)


Mesh-Intro 14.5 L07 Assembly Meshing

Documents

Transcript of Mesh-Intro 14.5 L07 Assembly Meshing