ANSYS User’s Group Non-Linear Adaptive Meshing...

1 © 2017 ANSYS, Inc. October 10, 2018

ANSYS User’s GroupNon-Linear Adaptive Meshing (NLAD)

19.2 Release

Sriraghav SridharanApplication Engineer, ANSYS [email protected]


• Background on Rezoning

• Introduction to Mesh Nonlinear Adaptivity

• Defining Nonlinear Adaptivity Criteria

• Demonstration

• Limitations

• Learning resources

• Questions

Topics


Many applications can benefit from the ability to strategically modify a mesh during solution, based on certain criteria to:

• Simulate challenging large deflection/distortion which otherwise cannot be solved.

• Improve the accuracy of simulation results.

• Distortion of mesh is severe and nonrecoverable

• Mesh refinement is inadequate

Background on Rezoning


• Unlike manual rezoning, mesh nonlinear adaptivity is completely automatic, requiring no user input during solution.

Automatic During Solution

Introduction to Mesh Nonlinear Adaptivity


• Supported in MAPDL:

─ NLADAPTIVE

─ NLMESH

• Supported in WB-Mechanical GUI



• Nonlinear adaptivity (NLAD)– Support 10 node tetrahedral elements: Solid 187 and Solid 227

– New option to refine elements of Plane182, Plane222, Solid187 and Solid227 through general remeshing

– Allow combined criterion options CONTACT/ENERGY/BOX/MESH in the same input

– Support 3D fluid penetration

• 2D to 3D analysis– MAP2DTO3D

• Support distributed ANSYS

New Features At and Beyond 19.0

2D PLANE 182/282 (Structural/Coupled-field)

3D SOLID 187/227 (Structural/Coupled-field)


• Support high order 10 node tetrahedral elements Solid187/Solid227

– Remesh the regions of distorted elements caused by large deformation

• Solve the large deformation problems otherwise can not be solved

• Only remesh the regions of distorted elements Identified by

NLADPTIVE,,,MESH,SKEW,VAL1,VAL2

– Where VAL1 is skewness and VAL2 is Jacobian ratio

• Regenerate contact, target and surface elements automatically if necessary

NLAD: to fix mesh distortion

Rubber Sealing simulation


• All boundaries, loads and solutions mapped to the new mesh completely

• Allow partial and whole domain remeshing and follow the initial mesh size gradients

• Support self-contact and fluid penetration

• Can have 3D fluid penetration

• Built-in acceptance criteria to accept the new mesh or continue to solve with current mesh if no

improvement

• Work with most of material models, including history dependent materials

• Support distributed ANSYS solve

• Maximum number of remeshings: 99; doesn’t allow dropping of mid-side nodes

NLAD: to fix mesh distortion


• Elasto-plastic materials with 13 remeshing cycles

NLAD: Cross-bar impression


• Visco-plastic material using PEIRCE rate law: 12 remeshings

NLAD: Glass Blowing


NLAD: Threading Simulation


• Very large elongation with 93 remeshings

NLAD: Metal Extrusion


ExampleLiner Hanger

Casing

Packer

Expander

Credit: Texas Iron Works

Distortion of mesh is severe and nonrecoverable

Mesh refinement is inadequate


Nonlinear adaptivity is based on criteria checking at the end of each specifiedsubstep after it converges. After generating the new mesh, the program moves to the next substep.

Three inputs from user are necessary to define Mesh Nonlinear Adaptivity:

• Defining Element components

• Defining Nonlinear Adaptivity Criteria

• Defining Criteria Checking Frequency



• To use Nonlinear Adaptivity most effectively, it is very important to understand the Criterion that will trigger mesh modification.

─ Energy Based.

─ Position Based.

─ Mesh –Quality Based.

─ Contact Based (available via MAPDL only).

Defining Nonlinear Adaptivity Criteria

NLADAPTIVE, Component, Action, Criterion, Option, VAL1, VAL2,VAL3

Criterion:

- Contact

- Energy

- Box

- Mesh



Energy Based Criterion

Criterion for remeshing Element Strain Energy ≥ (Factor) X (Mean Strain energy)

• Refines the mesh in regions of high concentration of stress exists and elements are too large.

• Applies to current technology 2D elements and 3-D linear tetrahedral elements (SOLID285).

Position Based Criterion

Criterion for remeshing All nodes of an element are within a user defined region, the element is split and remeshed.

• For example, a small cavity filled by the deformation of a compressed seal.

• Applies to current technology 2D elements and 3-D elements



Mesh Based Criterion

Criterion for remeshing Element Skewness ≥ (User defined threshold) – The element is remeshed

• Vel is the volume of the element under calculation.

• Vreg is the volume of the standard tetrahedral element

Contact Based Criterion (MAPDL)

Criterion for remeshing Number of contact elements with target element ≤ User defined value

• Contact region is remeshed to follow the geometry of targets more accurately.

• Contact Surface Wear can also be modeled in MAPDL


When Criterion is defined as ‘Mesh’, additional mesh control options become available in the ‘Details Windowof Analysis Settings Branch ’.

• Refer to NLMESH command for details.



Result


• The following materials properties are not supported:

– Cast Iron

– Concrete

– Cohesive Zone

– Damage

– Microplane

– Shape Memory Alloy

– Swelling

Nonlinear Adaptivity Limitations


• Mesh nonlinear adaptivity via splitting or refinement, in general cannot repair an already distorted mesh.

– In some cases, it can exacerbate mesh distortion by creating smaller elements.

– To reduce the effect, the code uses built-in morphing and topology repair after splitting; even so, mesh quality improvement is slight.

– Avoid splitting or refinement in highly distorted regions, therefore, as doing so may lead to convergence problems.

Original Mesh After 1st Mesh Refinement



• Unstable Structures

– For some geometries and loads, a deformation may cause a "snap-through," or local buckling.

– Rubber seal problemsKT=0

A C

B

D

O

– Such behavior can also manifest itself as a mesh distortion, but one that nonlinear adaptive region cannot repair.

– The effect can be detected by closely checking the deformed region or the load-versus- time (displacement) curve.



• ANSYS Basic Structural Non-linearities course

➢ Instructor led or Self-paced learning

➢Register via the ANSYS Learning Hub

• ANSYS Workbench Documentation – Detailed reading

Learning resources


Questions?


APPENDIX

18.2 Release


• R13: Rezoning for 3d geometries (SOLID185, SOLID186)

• R16: Nonlinear adaptivity introduced – mesh splitting (SOLID181)

• R17: NLAD – full remeshing (SOLID181)

• R18: NLAD – full remeshing (SOLID285)

Background on Rezoning


Contact-Based Criterion (MAPDL only) : Number of Contacting Elements

• If the number of contact elements in contact with the target element is less than the defined value, the underlying element is split. Contact elements on the faces of the solid elements are regenerated as well.

• If contact elements are moving away from target, no splitting occurs.

• This criterion is used to allow a contact region to follow the geometry of targets more accurately.

• Component that bears the criterion consists of target elements only.

… Defining Nonlinear Adaptivity Criteria


Contact-Based Criterion : (cont’d)

Contact Surface Wear (MAPDL only) :

During a substep, when the amount of wear at a contact element exceeds a user defined critical value, the mesh is morphed at the interface to improve the quality of the mesh.

• Only applies to contact elements having surface wear, specified via the TB,WEAR command.

• User defines critical ratio of magnitude of wear at the contact element to the average depth of the solid element underlying the contact element.

Wear distorting elements at interface

Adaptive Mesh Morphing improves mesh

… Defining Nonlinear Adaptivity Criteria


• MAP2DTO3D– Support distributed ANSYS now for performance

• Map 2D axisymmetric analysis results to 3D

• Migrate all BCs and Loads to 3D

• Regenerate contact and surface elements

• Rebalance the 2D solutions on 3D domain

2D to 3D Analysis

MAP2DTO3D


– Some transition layers are created automatically to connect the refined regions to the unrefined regions.

– For 3-D tetrahedral element, some topology changes and morphing occur during splitting to improve the quality of the new mesh.

– For 2-D meshes, morphing is only done after the element-splitting operation.

• Splitting applies to Energy-based, Position-based and Contact-based criterion.

Procedure

• During splitting, the current elements are divided into elements having a half-edge length.

• When the defined criteria are met, mesh modification occurs either by a combination of splitting followed by morphing or by general remeshing:


Create User Defined Results (using the PNUMELEM Expression) to view the new elements that have relatively larger element identities than the original element identities.

08.06 Procedure


For each mesh modification, mesh quality metrics stats are recorded in the Solver output comparing previous mesh with new mesh.

• Source Column represents previous mesh.

• Target Column represents modified mesh.

• In general, lower Skewness and Aspect Ratio values for the Target mesh, indicate improved quality.

• If these stats indicate a deterioration of the mesh, try adjusting NLMESH options.

… Procedure


• Cannot be used in combination with the following features/conditions on the same part:

– Cyclic Symmetry

– Contact Formulations: Normal Lagrange (3D), MPC, and Beam

– Contact Behaviors: Auto Asymmetric

– Point Mass, Beam Connection, Joints, Spring, and Bearing

– Remote Force, Remote Displacement, Moment, Thermal Condition, and Remote Point

– Spatially varying boundary conditions

• Cannot be used in combination with the following boundary conditions:

– Coupling

– Constraint Equation



• Nonlinear Adaptivity cannot resolve numerical instabilities.

– The constraints can include kinematic constraints such as applied displacements, couplings, and constraint equations, and volumetric constraints introduced by fully incompressible material in mixed u-P elements.

– In many cases, numerical instability is apparent even in the early stages of an analysis.

… Nonlinear Adaptivity Limitations


• Combinations of criteria might be required for applications involving complex geometry under complex loading (i.e. local deformations in buckling or rubber seal problems).

– The contact-based criterion should be used only when solid elements touch target elements and more elements/nodes are necessary to simulate details of contact boundaries and filling.

– The position-based criterion should be used for refining elements moving in to particular regions (for example, small cavities).

– Without a fine enough mesh and a sufficient number of degrees of freedom, the simulation might not accurately predict the behavior of material moving into such regions.



• When used during a restart, the Nonlinear Adaptive Region object does not support Named Selections if your model contains a mesh change prior to the restart point.

• If analysis failed to converge and you are adding a new Nonlinear Adaptive Region object, it is necessary that the contact object property, Behavior, was set to either Symmetric or Asymmetric for the initial solution that was processed.


ANSYS User’s Group Non-Linear Adaptive Meshing...

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