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Geometry and Mesh Update

Experimenter
Experimenter

I've updated my idealized part to simplify model. I then went back to my .fem file and selected the polygon body and clicked on "Recreate." Then I re-meshed it using the 3D tet mesh option.

 

The mesh in the area that I've updated doesnt seem to match my polygon geometry. When I tried using mesh controls, it doesnt pick up the edge that I want. Am I missing something? See attached images.

 

Thanks in Advance!

2 REPLIES

Re: Geometry and Mesh Update

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Hello!,

When you mesh any 2-D or 3-D geometry the NX AdvSim mesher performs automatically the "geometry abstraction" on the polygon geometry in order to eliminate issues with the CAD geometry that can cause undesirable results when you mesh your 2-D or 3-D model.

 

Whether you choose to perform the abstraction during meshing or by using Auto Heal Geometrycommand, the abstraction process is the same. In both cases, the software searches your model for geometric features that are so small that they can prevent the software from being able to mesh or solve your model.

 

Here you are a simply example: a piston rod.

 

rod-for-tet-meshing.png

 

I will mesh with TET10 elements using the default options: please note the SMALL FEATURE TOLERANCE under MODEL CLEANUP OPTIONS that specifies the percentage of the element size to use to characterize small features. The software uses the small feature tolerance to determine which features to eliminate during the abstraction process that precedes meshing. The default value is 10.0%. For example, if the element size is 10 mm, the software ignores features such as holes or faces that are 1 mm or less in size.

 

tetmesh-cleanup-options.png

rod-tet10-mesh.png

 

If I hide the mesh then you can see the EXCELENT geometry abstraction job done by the TET mesher of NX AdvSim V10:

 

rod-tet10-mesh-abstraction-job.png

 

Also you can use the Reset command to restore abstracted polygon geometry to its original state. Reset lets you remove changes you have made to the polygon geometry with the geometry abstraction tools, such as Split Face and Match Edge. When you use Reset, the software returns the portion of the polygon geometry you select to its original state prior to any modifications.

 

Here you are the result after specifying a zero value for the small feature tolerance, the software will attempts to mesh all features and does not perform any abstraction.

 

rod-for-tet-meshing-no-abstraction.png

 

Enjoy!.

Best regards,

Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Geometry and Mesh Update

Siemens Phenom Siemens Phenom
Siemens Phenom

As Blas mentioned, the meshing process is abstracting the geometry to remove small features that would lead to poor element quality. Your example has an isthmus condition between the planar face and the cylinder. The abstraction process will reduce isthmus conditions to edges (essentially collapsing an area to a curve). It chose to move a portion of the planar edge to the cylindrical edge. You can preserve the planar face in NX 10 by using the Freeze Geometry command. The What's New portion of the NX 10 online help has this to say about Freeze Geometry.

 

Use the new Freeze Geometry command to designate faces that should be excluded from most automatic or manual geometry abstraction processes. When you use the Freeze Geometry command on a set of faces:

  • The software does not perform any automatic abstraction or geometry cleanup operations on those faces during 2D or 3D meshing.

  • You cannot select those faces for use with most polygon geometry commands. For example, you cannot use the Stitch Edge command to modify a face on which you have previously used the Freeze Geometry command.

You can continue to use the Split Edge and Merge Edge commands on faces that you select with the Freeze Geometry command.

 

The alternative to NOT collapsing the isthmus region would be elements of very poor quality or very small size.

 

Regards,

Mark

 

Mark Lamping

Simulation Product Management

Simulation and Test Solutions

 

Siemens Industry Sector

Siemens Product Lifecycle Management Software Inc.

 

mark.lamping@siemens.com

www.siemens.com/plm