Mesh-Intro 14.5 L04 Meshing Methods

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

14. 5 Release

Introduction to ANSYS Meshing

Lecture 4 Meshing Methods


Meshing Methods

What you will learn from this presentation

• Meshing Methods for Part/Body Meshing

– Assembly Meshing covered separately

• Methods & Algorithms for;

– Tetrahedral Meshing

– Hex Meshing

– 2D Meshing

• Meshing Multiple Bodies

– Selective Meshing

– Recording Meshing Order


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


Why Multiple Methods?

• Choice can depend on;

– Physics

– Geometry

– Resources

• Mesh could require just one or a combination of methods.

• Example – Typical mesh design based on geometric, physics and resource considerations.

Methods

High aspect ratio cells (Inflation) near wall to capture boundary layer

gradients

Tet (3d) or Tri (2d) cells used here to mesh complex region

Hex (3d) or Quad (2d) cells used to

mesh simple regions

Cells refined around small geometric

details and complex flow


• In the Outline, right click Mesh, Insert > Method

– Select body in Details View

• Or, in the Graphics Window, Select body(s) , right click, Insert > Method

– Body automatically selected in Details View

• Method is selectable using the drop down box

– Select, Automatic, Tetrahedrons, Hex Dominant, Sweep or Multizone

Inserting Methods


Tetrahedrons Method


Method Behavior

• Generates tetrahedral elements - two algorithms are available:

• Patch Conforming

• Patch Independent

Tetrahedrons Method


Method & Algorithm Behavior

• Bottom up approach: Meshing process starts from edges, faces and then volume

• All faces and their boundaries are respected (conformed to) and meshed

• 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

Access

• Insert Method and set to Tetrahedrons

– Additional drop down box for algorithm choice appears - Set to Patch Conforming

Tetrahedrons Method: Patch Conforming


Method & Algorithm Behavior

• Top down approach: Volume mesh generated first and projected on to faces and edges

• Faces, edges and vertices not necessarily conformed to

– 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


Access

• Insert Method and set to Tetrahedrons

– Additional drop down box for algorithm choice appears - Set Patch Independent

Tetrahedrons Method: Patch Independent


Tetrahedrons Method: Algorithm Comparison (Surface Mesh)

Geometry containing

small details

Patch Conforming: All

geometric detail is

captured

Patch Independent: Can

ignore and defeature

geometry


Tetrahedrons Method: Algorithm Comparison (Volume Mesh)

Geometry containing

small details

Patch Conforming:

Delaunay mesh –

smooth growth rate

Patch Independent: Default

Octree Mesh – approximate

growth rate

Smooth Transition option

creates Delaunay mesh


Patch Conforming

• Sizing

– Mesh sizing for the Patch Conforming algorithm is defined by Global & Local Controls

– Automatic refinement based on curvature and/or proximity accessible in Global Controls

• Details of Global & Local Controls covered in separate lectures

– Choice of surface mesher algorithm in global controls

Tetrahedrons Method: Control

Proximity

Curvature


Patch Independent

• Sizing

– Sizing for the Patch Independent algorithm defined in Patch Independent Details

– Automatic curvature & proximity refinement option



Patch Independent

• Defeaturing Control

– Set Mesh Based Defeaturing On

– Set Defeaturing Tolerance

– Assign Named Selections to selectively preserve geometry


Defeaturing Tolerance Off

Named Selection assigned and Defeaturing Tolerance = 0.03m. Features > 0.03m respected.


Patch Conforming

• Clean CAD, accurate surface mesh

Patch Independent

• Dirty CAD, defeatured surface mesh

Tetrahedrons Method: Application Examples


Hex Meshing


Hex Meshing

• Reduced element count

– Reduced run time

• Elements aligned in direction of flow

– Reduced numerical error

Methods Available

• Sweep

• MultiZone

• Hex Dominant (not recommended for CFD)

Initial Requirements

• Clean geometry

• May require geometric decomposition

Introduction

Tet Mesh

Elements: 48K

Sweep Mesh

Elements: 19K


Method Behavior

• Meshes source surface, sweeps through to the target

– Body must have topologically identical faces on two ends, (which act as source and target faces)

• Generates hex/wedge elements

• Side faces must be mappable

• Only one source and one target face is allowed

– Alternative ‘Thin’ sweep algorithm can have multiple source and target faces

Access

• Insert Method and set to Sweep

Sweep Meshing

Source Face Target Face

Side Face(s)

Sweep Path


Source/Target (Src/Trg Selection)

• Automatic

– Source & target faces identified automatically

– Not compatible with inflation

• Manual Source & Manual Source and Target

– User selection (required for inflation)

– Compatible with inflation

• Automatic Thin & Manual Thin

– Multiple source and target faces

– Not compatible with

inflation

Sweep Meshing

Sweep Direction Source Face Target Face


Src/Trg Selection Behaviour

• Automatic selection requires that the application find the Source and Target. Specifying both Source & Target will accelerate meshing

• Inflation

– Must specify at least Source manually when using Inflation & Sweep Method

– 2D inflation defined on source face from boundary edges then swept through volume, source must therefore be specified first

Sweep Meshing

Sweep Mesh No inflation

Automatic Selection

Sweep Mesh with inflation

Manual Selection


Rotational Sweeping

• Sweep meshes can also be created by sweeping a Source around an axis

• Example: Src/Trg Selection - Rotational sweep for sector geometry

– Rotational sweeping requires both Source & Target to be selected

• For both rotational and axial sweeping Source & Target faces are color coded when selected

Sweep Meshing

Sweep Mesh: Wedge

and Hex elements

Manual Source &

Target Selection

Target

Face

Source

Face

Sweep Path


Target

Src/Trg Selection: Automatic Thin & Manual Thin

• Selects an alternate sweep algorithm

• Advantages

– Capable of sweeping multiple Source & Targets

– Can perform some automatic defeaturing

• Disadvantages

– For Multibody Parts only one division across the sweep is allowed

– Inflation & Sweep Bias not allowed

Sweep Meshing

Source

Faces

Source Faces

Imprinted on Target


Sweep Meshing

How to Identify Sweepable bodies

• ANSYS Meshing can identify sweepable bodies automatically

– Rotational Sweep bodies are not identified

• Right click Mesh object in Outline and select Show > Sweepable Bodies

RMB on Mesh to find

sweepable bodies Geometry Sweepable bodies in

green color


Sweep Meshing

How to Ensure Bodies are Sweepable

• Bodies which will not allow sweeping can be decomposed into a number of topologically simpler sweepable bodies

• Decomposition can be performed in CAD/DM

• Example 1

Unsweepable body

Decomposed in

CAD/DM

Sweepable!


Example 2

Sweep Meshing

T Junction Geometry

Unsweepable

Decomposed in

CAD/DM

Sweepable!


Method Behavior

• Based on blocking approach used in ANSYS ICEM CFD Hexa

• Automatically decomposes geometry into blocks

• Generates structured hex mesh where block topology permits

– Remaining region (Free Mesh) filled with unstructured Hexa Core or Tetra or Hexa Dominant mesh.

• Can select source & target faces automatically or manually

– Can have multiple source faces

• Compatible with 3D inflation

Access

• Insert Method and set to Multizone

MultiZone Meshing

MultiZone

Mesh

Target faces should also be selected as “Source” for Multizone Method as mesh is swept from both directions


• Mapped Mesh Type - determines the shape of the elements used to fill structured regions (the default is Hexa).

• Hexa - All hexahedral elements are generated

• Hexa/Prism - For swept regions, the surface mesh can allow triangles for quality and transitioning

• Prism - All prism elements are generated

– This option is sometimes useful if the source face mesh is being shared with a tet mesh, as pyramids are not required to transition to the tet mesh

• Surface Mesh Method – specifies method to create the surface mesh.

• Program Controlled - automatically uses a combination of Uniform and Pave mesh methods depending on the mesh sizes set and face properties

• Uniform - uses a recursive loop-splitting method which creates a highly uniform mesh

• Pave - creates a good quality mesh on faces with high curvature, and also when neighboring edges have a high aspect ratio

MultiZone Meshing

Surface mesh method = Uniform

Surface mesh method = Pave


Example 1

• Single body automatically decomposed into three blocks

• Src/Trg Selection – Automatic

• Results in all hex mesh

• Equivalent to manually decomposing by slicing off upper and lower cylinders to produce three bodies and applying sweep methods

MultiZone Meshing


Example 2

• Blend on central body, Multizone no longer able to create structured block

– Filled according to Free Mesh setting

• Tetra, Hexa Core, Hexa Dominant

• Can specify type of surface mesh using Mapped Mesh Type (Hexa, Hexa/Prism, Prism)

MultiZone Meshing

Free Mesh

Tetra Free Mesh

Hexa Core


Automatic Method


Method Behavior

• 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


Access

• Default Method where not specified

• Can specify by inserting Method and setting to Automatic

Automatic Method


2D Meshing


• Patch Conforming Methods

– Automatic Method (Quadrilateral Dominant) & Triangles

• Patch Independent Methods

– Multizone Quad/Tri

– Full Quad will be generated if "All Quad" is selected as Free Face Mesh Type

• Advanced size functions and local size controls are supported

Methods for 2D Meshing Automatic Triangles

Multizone

Quad

Multizone

Quad/Tri

Multizone Quad/Tri & Multizone Quad Methods were previously called Uniform Quad/Tri and Uniform Quad till R14.0


Mapped Surface Meshes

• Fully mapped surface meshes and specified edge sizing/intervals can be obtained by applying local controls

– Covered in the Local Mesh Controls lecture

2D mesh with Inflation

• Boundary edges are inflated

• Support for global and local inflation controls

2D Meshing Control & Inflation

2D

Mapped

Mesh


ANSYS FLUENT

• For a 2D analysis in FLUENT generate the mesh in the XY plane (z=0).

• For axisymmetric applications y 0 and make sure that the domain is axisymmetric about x axis

• In ANSYS Meshing, by default, a thickness is defined for a surface body and is visible when the view is not normal to the XY Plane. This is purely graphical – no thickness will be present when the mesh is exported into the FLUENT 2D solver

ANSYS CFX

• For 2D analysis in CFX, create a volume mesh (using Sweep) that is 1 element thick in the symmetry direction, i.e.,

• Thin Block for Planar 2D

• Thin Wedge (< 5°) for 2D Axis-symmetric

2D Mesh Solver Guidelines


Meshing Multiple Bodies


Selective Meshing

What is Selective Meshing?

• Selectively picking bodies and meshing them incrementally

Why use 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


Local meshing

• Mesh or clear 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 for context menu

Selective Meshing

Meshing first the block then the pipe Meshing first the pipe then the block


Recording Mesh Operations

• When using selective meshing the order of meshing can be recorded for automated future use

• Right click Mesh in the Outline for Context Menu

• Worksheet is generated recording mesh operations as ordered steps

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

– Example; Meshing cylinder then block

Selective Meshing


Selective Body Updating

• Remeshing only bodies that have changed

• Access option through Tools > Options

– No: All geometry updated, all bodies remeshed.

– Associatively: Accommodates for body topology change (add/delete) (slower)

– Non-Associatively: Assumes no topology change (faster)

• Example; Geometric change to block.

Selective Meshing


Workshop 2 – Introducing Meshing Methods


Appendix

Contents

• Hex Dominant Meshing

• Sweep Meshing Biasing & Complex Geometry

• Surface Meshing with Inflation

• Mesh Connections

• Shell Meshing

• Patch Independent Tetrahedrons - Transition


Hex Dominant Meshing

• The mesh contains a combination of tet and pyramid cells with majority of cell being of hex type

• Useful for bodies which cannot be swept

• Useful for CFD applications not requiring inflation

• Useful for CFD in the range of acceptable Skewness or Orthogonal Quality mesh quality metrics

Access

• RMB on Mesh

• Insert Method

• Definition Method Hex Dominant



• Example:

Geometry with

valve inside

Due to valve sweep

method cannot be

used

Hex Dominant

Mesh generated



Free (unstructured) Face Mesh Types

• Determines the element shape in the free zone (where structured meshing is not possible)

Options

• Quad/Tri

• All Quad

– May insert triangular elements depending on complexity of geometry



Example:

Geometry with

valve inside

Free Face Mesh Type:

Quad/Tri

Free Face Mesh Type:

All Quad

Higher no. of

elements

Lower no. of

elements



Example: Hex Dominant Mesh Elements

Hex Dominant Mesh

He

x

Te

t

Pyra

mid

We

dg

e


Sweep Meshing • Control:

• Free (unstructured) Mesh Type

Sweepable Geometry

Source face

elements: Only

Quad

Source face

elements: Quad

plus Tri

Source face

elements: Only Tri

Type: All Quad

Type: Quad/Tri

Type: All Tri


Sweep Meshing

Control: Type

Element size in

swept direction

2mm

No. of elements in

swept direction: 10

Sweep Element Size

Sweep Num Divisions

Sweepable Geometry

The Number of Divisions can be > 1000. If this number is > 1000 the divisions will not be drawn on the edge


Sweep Meshing

• Control: Sweep Bias Type

Uniform mesh

Cells are

concentrated on

one side

No Bias

With Bias

Sweepable Geometry


Sweep Meshing : Complex geometry


Surface Meshing with Inflation

Triangular Mesh with ‘Inflation’


Mesh Connections For sheet models,

connections can be defined as “Mesh connections”

Mesh connections can be used to extend mesh at mesh level

• Tolerance Value can be specified to close gaps at mesh level

•“Mesh connections” will be created at mesh time (rather than as CE in solver)

Gap

Gap removed at

mesh level Connected Mesh


Shell Meshing: Example

Edge colored by the number of connected faces

• Automatic generation of mesh connections

• Mesh connectivity can be shown also after meshing

• Ability to find mappable faces


Transition

• Effect of Smooth Transition

• Smooth transition uses advanced front meshing technique

Tetrahedrons Method: Patch Independent

Smooth Transition Off (default) Smooth Transition On

Documents

Mesh-Intro 14.5 L04 Meshing Methods