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ANSYS Meshing Advanced Techniques

PADT Lunch & Learn Series

17.0 Release

2 © 2016 ANSYS, Inc. April 17, 2017 ANSYS Confidential

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• ANSYS Meshing Overview

• Advanced Tet Methods

• Advanced Hex Methods

• Selective Meshing Methods

• Using Global Meshing Methods

• Using Local Meshing Methods

• Clean Up Tools

• Quality Metrics

• Meshing Tips and Tricks

Overview


ANSYS Meshing is a component of ANSYS Workbench• Meshing platform

• Combines and builds on strengths of preprocessing offerings from ANSYS: – ICEM CFD, TGRID (Fluent Meshing), CFX-Mesh, Gambit

Able to adapt and create Meshes for different Physics and Solvers• CFD: Fluent, CFX and POLYFLOW

• Mechanical: Explicit dynamics, Implicit

• Electromagnetic

Integrates directly with other WB systems

What is ANSYS Meshing


What are the Element Types• Tetrahedral

• Hexahedral

• Polyhedral (CFD)


Advanced Tet Methods


Choosing the proper mesh element type will improve the mesh generation efficiency

Small mesh size on holes need to transition to larger size elsewhere, but transitioning hex mesh can be a problem

Tet mesh can be easily converted to hex mesh, but if the quality is bad,

what’s the point?

1 tet to 4 hex

All Hex

Hexahedral vs. Tetrahedral Elements

• Advantages of tetrahedral over hexahedral:

– Easier to mesh more complex geometry:• Mesh quality is often easier to achieve with tetrahedral (or poly)

mesh

• Mesh transitioning with hex mesh can be problematic


Patch Conforming versus Independent


• Bottom up approach: Meshing process • Edges Faces volume

• All faces and their boundaries are respected(conformed to) and meshed (except with defeaturing tolerance)

• Good for high quality (clean) CAD geometries• CAD cleanup required for dirty geometry

• Sizing is defined by global and/or local controls• Compatible with inflation

To access it• Insert Method

• Set to Tetrahedrons• Set to Patch Conforming

Patch Conforming

• Top down approach: Meshing process • Volume meshed first projected on to faces

& edges• Faces, edges & vertices not necessarily conformed

• Controlled by tolerance and scoping of Named Selection, load or other object

• Good for gross de-featuring of poor quality (dirty) CAD geometries

• Method Details contain sizing controls• Compatible with inflation

To access it• Insert Method

• Set to Tetrahedrons• Set to Patch Independent

Patch Independent

Tetrahedral Methods


Geometry with small details

Patch conforming : details caputredPatch independent : details ignored

Delaunay mesh - smooth growth rate Octree mesh . approximate growth rate

Tetrahedral Method: Algorithm comparison


Advanced Hex Methods


• Advantages of hexahedral over tetrahedral:

– Less elements = Faster solution time with better accuracy

• Naturally anisotropic: Fewer elements required as mesh is aligned with the physics

– Fewer elements for given number of nodes

– 3 mostly parallel sets of faces (improves solution accuracy)

However, this assumes the geometry is such that the hex mesh is more efficient and that the structured mesh aligns to the physics

Hexahedral versus Tetrahedral Elements


Hex Meshing• Reduced element count

– Reduced run time

• Elements aligned in direction of flow

– Reduced numerical error

Initial Requirements• Clean geometry

• May require geometric decomposition

Tetra mesh - 48 000 Cells

Hexa mesh - 19 000 Cells

Hexahedral Mesh


Automatic• Source & Target faces identified automatically

• Requires that the mesher find the sweeping direction

• Manual source & Manual source and target • User selection• Source face colored in red• Target face colored in blue• Rotational Sweeping

Sweep around an axis Requires selection of both - Source & target

Note • Specifying both Source & Target accelerate

meshing

Source & Target selection

Define the numberof intervals on the

side face(s)

Sweep Path

Generation of wedges& hex elements

Sweep Meshing


• Based on blocking approach (ANSYS ICEM CFD Hexa)

• Automatically decomposes geometry into blocks• Generates structured hexa mesh where block

topology permits• Remaining region filled with unstructured

Hexa Core or Tetra or Hexa dominant mesh• Src/Trg Selection

• Automatic or Manual source selection• Multiple source faces • Select Target faces as “Source”

• Compatible with 3D Inflation

To access it• Insert Method Set to Multizone

Mesh Method & Behavior

Multizone Meshing


• Combination of Tetrahedron Patch Conforming and Sweep Method

• Automatically identifies sweepable bodies and creates sweep mesh

• All non-sweepable bodies meshed using tetrahedron Patch Conformal method

• Compatible with inflation

To access it• Default method • Insert method Set to Automatic

Mesh Method & Behavior

Automatic Method


Meshing Options

Global Meshing Parameters

Local Meshing Parameters

Global Settings - Physics Preference- Shape Checking

Global Sizing- Size Functions- Relevance Center

Mesh Method- Tet, Hex,

Multizone, Sweep

Mesh Statistics

Advanced Settings- Number of CPUS

Global Inflation Layers Local Sizings- Edge, Face, Body

Mesh Edit Tools- Node move/merge- Contact Match

Advanced Tools- Inflation, Pinch and match controls

Global Meshing Parameters

Local Meshing Parameters

Global Settings - Physics Preference- Shape Checking

Global Sizing- Size Functions- Relevance Center

Mesh Method- Tet, Hex,

Multizone, Sweep

Advanced Settings- Number of CPUS

Global Inflation Layers Local Sizings- Edge, Face, Body

Advanced Tools- Inflation, Pinch and match controls


Global Meshing Methods


• Global mesh controls are used to make global adjustment in the meshing strategy, which includes sizing functions, inflation, smoothing, defeaturing, parameter inputs, assembly meshing…

• Minimal inputs – Automatically calculates global element sizes

based on the smallest geometric entity

– Smart defaults are chosen based on physics preference

• Makes global adjustments for required level of mesh refinement

• Advanced Size Functions for resolving regions with curvatures and proximity of surfaces Smart defaults !

Global Mesh Controls


• Five options under “Physics Preference”– Mechanical, Nonlinear Mechanical, Electromagnetics, CFD and Explicit

• Three options under “Solver Preference” when CFD is selected– Fluent, CFX and Polyflow

• Mesh setting defaults are automatically adjusted to suit the “Physics Preference” and “Solver Preference”

Defaults

The Nonlinear Mechanical Physics Preference Option has been added in R17.0 and may results in higher mesh quality for FEA users


• Controls the growth and distribution of mesh in important regions of high curvature or close proximity of surfaces

• Five Options: – Adaptive

– Proximity and Curvature

– Curvature

– Proximity

– Uniform

• When CutCell Meshing is active with ‘Proximity’ or ‘Proximity and Curvature’, Proximity Size Function Sources control is displayed to specify the regions of proximity between “Edges”, “Faces” or “Faces and Edges” in the computation of Proximity Size Function (SF)

Sizing: Size Function


SF: Curvature

• Determines the Edge and Face sizes based on Curvature Normal Angle

• Finer Curvature Normal Angle creates finer surface mesh

• Transition of cell size is defined by Growth Rate

SF: Adaptive

• The edges are meshed with global Element Size

• Then the edges are refined for curvature and 2D proximity

• At the end, corresponding face and volume mesh is generated

• Transition of cell size is defined by Transition

SF: Proximity

• Controls the mesh resolution on proximity regions in the model

• Fits in specified number of elements in the narrow gaps

• Higher Number of Cells Across Gap creates more refined surface mesh

• Transition of cell size is defined by Growth Rate

Sizing: Sizing Function Examples


Element Size (only available when the size function is set to Adaptive)

• Element size used for the entire model – This size will be used for meshing all edges, faces and bodies

• Default value based on Relevance and Initial Size Seed– User can input required value as per geometry dimensions

Sizing: Element Size


Local Meshing Methods


Recommended for locally defining the mesh sizes

You can only scope sizing to one geometry entity type at a time • For example: you can apply sizing to a number of edges or a number of faces, but not

a mix of edges and faces

Four Types of Sizing option• Element Size specifies average element edge length on bodies, faces or edges

• Number of Divisions specifies number of elements on edge(s)

• Body of Influence specifies average element size within a body

• Sphere of Influence specifies average

element size within the sphere

Sizing options vary depending on the entity type chosen

Only Element Size type is available for CutCell

meshing

Entity/Option Element Size Number of Divisions Body of Influence Sphere of Influence

Vertices x

Edges x x x

Faces x x

Bodies x x x

Advanced Size

Function in

Global settings

should be

disabled

Requires a

Coordinate

system for

the sphere

Sizing


On Vertex– Available with or without Advanced Size

Functions

– Sets the average element size around the selected vertex

– Inputs:

• Sphere radius and Element size

• Center of the sphere is defined by a model vertex

On Bodies – Available with or without Advanced Size

Functions

– Constant element size is applied within the confines of a sphere

– Use coordinate system to define the center of the Sphere

Sizing: Sphere of Influence


Bodies of influence (BOI) – Lines, surfaces and solid bodies can be used to refine the mesh

– Accessible when ASF is On

– Not available for CutCell meshing

The ‘Body of Influence’

itself will not be meshed

Line BOIs

Surface BOI Solid BOI

Without BOIs

Sizing: Bodies of Influence


If Face Meshing fails, ( ) icon appears

adjacent to corresponding object in the

Tree outline. The mesh will still be created

but will ignore this control.

• Forces Mapped/Pave meshes on selected mappable surfaces

– Face Meshing with advanced control is supported for

• Sweep, Patch Conforming, Hexa Dominant

• Quad Dominant and Triangles

• MultiZone

• Uniform Quad/Tri and Uniform Quad

– RMB on Mesh and Show/Mappable Faces to display all mappable faces

Face Meshing Control


• Sweep/MultiZone (2D and 3D) support for Advanced Vertex options on Face meshing

S

S

Denoted by S, C or E

S

S

S

S

Face Mesh Control: Advanced Vertex Options


• Define periodicity on faces (3D) or edges (2D)• The two faces or edges should be topologically and geometrically the same

• A match control can only be assigned to one unique face/edge pair

• Match controls are not supported with Post Inflation Algorithm

• Match Control with Patch Independent tetrahedrons not supported yet

– Two types of match controls available:• Cyclic and

• Arbitrary

– Not available for CutCell meshing

If ‘Match Control’ fails, ( ) icon appears adjacent to corresponding

object in the outline Tree, however the mesh is created ignoring it

Matching face

mesh

Match Control


• Define mesh periodicity on faces (3D) or edges (2D)

• Faces/edges pairs should be topologically and geometrically identical

• Can be assigned to several face/edge pairs at once

• Not supported with– Assembly Meshing

– Post Inflation Algorithm

– Patch Independent tetrahedrons

– Two types of Symmetry Controls useful for CFD:

• ‘Cyclic’ for rotational periodicity

• ‘Linear Periodic’ for translational periodicity

Matching face

mesh

Symmetry Control


Used to generate prism layers (as explained in Global settings chapter)

• Inflation layer can be applied to faces or bodies using respectively edges or faces as the boundary

Inflation layer grown on edge boundary (red)

Inflation layer grown on face boundary (red)

Inflation


• Connect mesh at mesh level• At R17 only works with tet mesh

• Could avoid problems with shared topology

Contact Match Mesh

Notes:

• Mesh generated as separate parts• Use Contact Match controls to match mesh• Create Named Selection to inspect matched mesh• Node merge can be added as second step to make

mesh conformal


• Nonlinear Adaptive Region• Enables you to change

the mesh during the solution phase to improve precision without incurring a great deal of computational penalties. The Nonlinear Adaptive Region feature is completely automatic.

• Limitations: All 2D elements, Only Linear 3D elements

Nonlinear Adaptive Mesh


Selective Meshing Methods


What is selective meshing?

• Selectively picking bodies and meshing them incrementally

Why selective meshing?

• Bodies can be meshed individually

• Mesh seeding from meshed bodies influences neighboring bodies (user has control)

• Automated meshing can be used at any time to mesh all remaining bodies

• When controls are added, only affected body meshes require remeshing

• Selective body updating

• Extensive mesh method interoperability

Selective Mesh


Clear meshes on individual bodiesGenerate meshes on individual bodies• Subsequent bodies will use the attached face

mesh• The meshing results (cell types) will depend on

the meshing order• Adjust/add controls – able to remesh only

affected body

• Select body(s) • Right click

Local Meshing

Meshing first the pipe then the block

Meshing first the block then the pipe

Selective Mesh


• Use it to record the order of meshing to automate future use

• Right click Mesh in the Outline to access it

• A Worksheet is generated • Record mesh operations as ordered steps

• Named Selections are automatically created for each meshed body for reference in the Worksheet

• We can create Named Selection to define an order

Recording Mesh OperationsExample : Meshing cylinder first and then block

Selective Mesh


• Remeshing only bodies that have changed

• Access option through RMB click on Geometry Properties• No: All geometry updated, all bodies remeshed.• Associatively: Accommodates for body

topology change (add/delete) (slower)• Non-Associatively: Assumes no topology

change (faster)

Selective Body Updating

Example :Geometricchange to block

Selective Mesh


Clean Up Tools


• Node Move: Dynamically pick and drag nodes around (quality plots updated real time)

• History of moves is recorded in Worksheet and allows for “Undo”

Node Merge & Node Move

• Node Merge: enables to merge mesh nodes within a specified tolerance, making the mesh conformal across bodies, parts, and assemblies. Node Merge can be performed on solid, sheet, and line bodies

Note: If you update your mesh (Mesh object>Update), the application maintains your movements in the Worksheet until the mesh is cleared (zero nodes) using the RMB option Clear Generated Data or you refresh data from the CAD source; In these cases all the Node Move movements are lost


Mesh Connections

• Mesh Connections: enable you to join the meshes of topologically disconnected surface bodies that may reside in different parts. This is an alternative to connecting the geometry in SpaceClaim or DM


Contact Matches

• Contact Matches: enable you to match mesh nodes between topologically disconnected solids within a specified tolerance. They are an alternate option to imprinting faces in SpaceClaim or DM.


• Removes small geometry features meeting the tolerances using Pinch or/and Automatic Mesh Based Defeaturing controls in order to improve the mesh quality.

• Automatic Mesh Based Defeaturing (AMBD) when it is ‘On’, features smaller than or equal to the value of Defeaturing Tolerance are removed automatically.

Note: Defeaturing Tolerance can be set maximum up to Global Min Size/2. Use a high value for Global Min Size and use Local Sizing controls for defining Local Min Sizes smaller than this value. Defeaturing will respect local min sizes and will clean only those faces/bodies with no local control. This approach is recommended over Virtual Topology

AMBD Off AMBD On

Defeaturing


• To improve quality Pinch control removes small features (edges or narrow regions) at the mesh level

• The Pinch feature is supported for the following mesh methods:

– Patch Conforming Tetrahedrons

– Thin Solid Sweeps

– Hex Dominant meshing

– Quad Dominant Surface Meshing

– Triangles Surface meshing

– Not supported for CutCell meshing

Pinch


• Pinch control removes small features automatically or manually at the mesh level – Slivers

– Short Edges

– Sharp Angles

• The Pinch feature works on vertices and edges only

• The Pinch feature is supported for the following mesh methods:– Patch Conforming Tetrahedrons

– Thin Solid Sweeps

– Hex Dominant meshing

– Quad Dominant Surface meshing

– Triangles Surface meshing

• Not supported for – CutCell

– Patch Independent

– Multizone & General Sweep

Vertex-Vertex Edge-Edge

before after before after

Pinch


● Close Vertices: Checks for vertices that are very close together:

– Scale can be adjusted depending on model sizes

Diagnostics: Close vertices (New in R17.0)


When to use?• To merge together a number of small (connected)

faces/edges

• To simplify small features in the model

• To simplify load abstraction for mechanical analysis

• To create edge splits for better control of the surface mesh control

Virtual cells modify topology• Original CAD model remains unchanged

• New faceted geometry is created with virtual topology

Restrictions• Limited to “developable” surfaces

• Virtual Faces cannot form a closed region

Without VT With VT

automatically manually

Virtual Topology


In some instances it may be desirable to modify topology to allow application of some desired effect (e.g., mesh control, load, support, …):

• Split face at vertices

• Split Edge

• Add a hard vertex

Virtual Topology


Quality Metrics


Good quality mesh means that…• Mesh quality criteria are within correct range

– Orthogonal quality …

• Mesh is valid for studied physics

– Boundary layer …

• Solution is grid independent

• Important geometric details are well captured

Bad quality mesh can cause• Convergence difficulties

• Bad physics description

• Diffuse solution

User must…• Check quality criteria and improve grid if needed

• Think about model and solver settings before generating the grid

• Perform mesh parametric study, mesh adaption …

Impact of the Mesh Quality


Displays mesh information for Nodes and Elements

List of quality criteria for the Mesh Metric

• Select the required criteria to get details for quality

• It shows minimum, maximum, average and standard deviation

Different physics and different solvers have different requirements for mesh quality

Mesh metrics available in ANSYS Meshing include:

– Element Quality

– Aspect Ratio

– Jacobean Ration

– Warping Factor

– Parallel Deviation

– Maximum Corner Angle

– Skewness

– Orthogonal Quality

For Multi-Body Parts, go to corresponding body in Tree Outline

to get its separate mesh statistics per part/body

Mesh Statistics and Mesh Metrics


Element Quality

• Element Quality:• Composite quality metric that ranges between 0 and 1

• Ratio of the volume to the square root of the cube of the sum of the square of the edge lengths for 3D elements

• 2D quad/tri elements:

• 3D brick elements:

0 1Worst Perfect


2-D:

• Length / height ratio: δx/δy

3-D

• Area ratio

• Radius ratio of circumscribed / inscribed circle

Limitation for some iterative solvers

• A < 10 … 100

• (CFX: < 1000)

Large aspect ratio are accepted where there is no strong transverse gradient (boundary layer ...)

δy

δx

Aspect Ratio

1 ∞Perfect Worst


Jacobian Ratio

• Jacobian Ratio = 𝑅𝑗𝑚𝑎𝑥

𝑅𝑗𝑚𝑖𝑛

• Where 𝑅𝑗 = det(𝐽𝐹) at sampling locations and 𝐽𝐹 =

𝜕𝐹1

𝜕𝑥

𝜕𝐹1

𝜕𝑦

𝜕𝐹2

𝜕𝑥

𝜕𝐹2

𝜕𝑦

in 2D

• Jacobian Ratio ≥ 1

– If 𝑅𝑗𝑚𝑎𝑥 and 𝑅𝑗𝑚𝑖𝑛 have opposite signs then

the Jacobian ratio = -100 and is unacceptable

1 ∞Perfect Worst


Skewness• Skewness: 0 - 1

• Two methods for determining skewness:– Equilateral Volume deviation:

– Skewness =

– Applies only for triangles and tetrahedrons

– Normalized Angle deviation:

– Skewness =

– Where 𝜃𝑒 is the equiangular face/cell (60 for tets and tri’s, and 90 for quads and hexas)

– Applies to all cell and face shapes

– Used for hexa, prisms and pyramids

e

mine

e

emax ,180

max

min

max

optimal cell size cell size

optimal cell size

Optimal (equilateral) cell

Circumsphere

0 1Perfect Worst

Actual cell


Orthogonal Quality (OQ)

Derived directly from

Fluent solver discretization

• For a cell it is the minimum of:

computed for each face i

For the face it is computed as the minimum of computed for each edge IWhere Ai is the face normal vector and fi is a vector from the centroid of thecell to the centroid of that face, and ci is a vector from the centroid of the cell to the centroid of the adjacent cell, where ei is the vector from the centroid of the face to the centroid of the edge

At boundaries and internal walls ci is ignored in the computations of OQ

|||| ii

ii

fA

fA

|||| ii

ii

cA

cA

|||| ii

ii

eA

eA

0 1

Worst Perfect

Orthogonal QualityA1

A2

A3

f1

f2f3

c2

c1

c3

A1

A2

A3

e1

e2

e3

On cell On face


Checked in solver

• Volume Change in Fluent– Available in Adapt/Volume

– 3D : σi = Vi / Vnb

• Expansion Factor in CFX– Checked during mesh import

– Ratio of largest to smallest element volumes surrounding a node

Recommendation:Good: 1.0 < σ <

1.5

Fair: 1.5 < σ <

2.5

Poor: σ > 5 … 20

Smoothness


Mesh quality recommendations

Low Orthogonal Quality or high skewness values are not recommended

Generally try to keep minimum orthogonal quality > 0.1, or maximum skewness < 0.95. However these values may be different depending on the physics and the location of the cell

Fluent reports negative cell volumes if the mesh contains degenerate cells

Skewness mesh metrics spectrum

Orthogonal Quality mesh metrics spectrum

CFD Mesh Quality


Standard Mechanical

- Linear, modal, stress and thermal analysis

Aggressive Mechanical

- Large deformation and nonlinear analysis

Nonlinear Mechanical

- Nonlinear analysis

Mechanical Shape Checking


Geometry problems• Small edge

• Gaps

• Sharp angle

Meshing parameters• Sizing Function On / Off

• Min size too large

• Inflation parameters

– Total height

– Maximum angle

• Hard sizing

Meshing methods• Patch conformal or patch independent tetra

• Sweep or Multizone

• Cutcell

Geometry cleanup in SpaceClaim

or

Virtual topology & pinch in Meshing

Mesh setting change

Mesh setting change

Factors Affecting Quality


• Displays mesh color by quality metrics

• Options to probe quality or show min/max

• Contour band can be adjusted

Display mesh contours

Find Min or Max value

Probe Element Values

Display Option: Color by quality


• Displays Mesh Metrics graph for the element quality distribution

• Different element types are plotted with different color bars

• Can be accessed through menu bar using Metric Graph button

• Axis range can be adjusted using controls button (details next slide)

• Click on bars to view corresponding elements in the graphics window– Use to help locate poor quality elements

Mesh Metric Graph


• Elements on Y-Axis can be plotted with two methods;– Number of Elements

– Percentage of Volume/Area

• Options to change the range on either axis

• Specify which element types to include in graph– Tet4 = 4 Node Linear Tetrahedron

– Hex8 = 8 Node Linear Hexahedron

– Wed6 = 6 Node Linear Wedge (Prism)

– Pyr5 = 5 Node Linear Pyramid

– Quad4 = 4 Node Linear Quadrilateral

– Tri3 = 3 Node Linear Triangle

• Te10, Hex20, Wed15, Pyr13, Quad8 & Tri6 non-linear elements

Mesh Metric Graph Controls

For more information about the different mesh metrics please consult module 05: Mesh Quality


• Solution run with multiple meshes

• Note : For all runs the computed Y+ is valid for wall function (first cell not in laminar zone)

DP 0 DP 3

x8

2%

Grid Dependency Study


• Option 1: Run Convergence Study on Result of interest

• Option 2: Set up convergence study by parameterizing mesh sizing

Grid Dependency Study


Parameterizing Your Model and Mesh


• Create Named Selections for all surfaces that have a load or support applied

• Create Named Selections for all surfaces/bodies that have a mesh setting applied

• Assign loads, boundary conditions, and mesh settings to Named Selections

Named Selections


• Geometry

• Material Properties

• Mesh

• Loads

• Results

• Command Snippets

• UDF’s

Parameterize your Model


Tips and Tricks


• ACT Extensions:

• FE Info 17.0: Locates nodes and elements by IDs

• Iterative Meshing 17.0: Iterated through mesh configurations and reports best mesh

• Mesh Settings 17.0: Export mesh settings to re-use in another project

• Workbench Poly Meshing for Fluent 17.0: Create Fluent meshing polyhedral meshes in the Workbench meshing interface.

• Nonlinear Diagnostics: Identify and display highly distorted elements after solver failure

Tips and Tricks


Questions?

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