Meshing Workshop 2014

MESHING WORKSHOP

Thursday, November 13th, 2014

Metin Ozen, Ph.D., ASME Fellow

OZEN ENGINEERING, INC.

www.ozeninc.com

WHAT DO WE DO?

• Ozen Engineering, Inc. helps solve challenging and multidisciplinary engineering problems with

industry leading computational simulation technologies

• We provide advanced

• Multi-Physics FEA

• Computational Fluid Dynamics (CFD) simulations

INTRODUCTION TO ANSYS MESHING

• In this lecture we will learn:

– Process for pre-processing using ANSYS tools

– What is the ANSYS Meshing?

– Meshing Fundamentals

– How to launch ANSYS Meshing?

– ANSYS Meshing interface

– Geometry concepts

– Meshing methods

PREPROCESSING WORKFLOW

Sketches and

Planes

Geometry Import

Options

3D Operations

Bi-Directional

CAD/ Neutral

Geometry

Cleanup and

Repair

Automatic

Cleanup

Simplification,

Mid-surface,

Fluid Extraction

Extrude, Revolve,

Sweep, etc

3D Operations

Booleans,

Decompose, etc.

Import/ Geometry Creation

Geometry Modifications

Meshing Solver

Meshing Methods

Hybrid Mesh: Tet,

Prisms, Pyramids

Hexa Dominant,

Sweep meshing

Global Mesh

Settings

Local Mesh

Settings

Sizing, Controls,

etc.

Assembly

Meshing

WHAT IS ANSYS MESHING

• 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

MESHING FUNDAMENTALS

• Purpose of the Mesh

– Equations are solved at cell/nodal locations

• Domain is required to be divided into discrete cells (meshed)

• Mesh Requirements

– Efficiency & Accuracy

• Refine (smaller cells) for high solution gradients and fine geometric detail.

• Coarse mesh (larger cells) elsewhere.

– Quality

• Solution accuracy & stability deteriorates as mesh cells deviate from ideal shape

MESHING PROCESS IN ANSYS MESHING

• Physics, Sizing, Inflation, Pinch, …

• Sizing, Refine, Pinch, Inflation, …

• Preview surface mesh, Inflation

• Mesh metrics, Charts

LAUNCHING ANSYS MESHING

– ANSYS Meshing is launched within Workbench

• 2 ways :

Fluid Flow (Fluent), Fluid Flow (CFX),

From Analysis SystemsMesh

From Component Systems

Double click

Mesh in the

System

or right click

and select

Edit

GRAPHICS USER INTERFACE

Mesh Metrics

Graphics window

Section Planes

Manage views

Details view

Outline

Toolbars

Units BarEntity Details BarMessage window

Worksheet

OUTLINE

• Three default sections

– Geometry

• Bodies

– Coordinate Systems

• Default global & user defined systems

– Mesh

• Meshing operations (controls & methods)

– displayed in the order in which they are inserted

In the tree

• Right clicking on any object

• launches a context sensitive menu

• Example: contains commands to generate, preview, clear mesh etc.

DETAILS VIEW

• Accessing Object Details

– Select an object (in the Outline)

• Related information to that object are displayed in the Details View below

• Ex: Select a body (“Fluid”) in the Outline

– Details of “Fluid” : contains graphical and geometric details

• To access meshing details

– Click the Mesh object or any of the inserted objects

– The Details View provides options to

• review,

• edit or

• input values for every object in the Tree

GEOMETRY CONFIGURATION – MULTIPLE PARTS

• Geometry composed of Multiple parts

• No connection between parts (no face sharing)

‘Contact Region’ is automatically created between 2 faces

Each part meshed independently

Results in Non-conformal interface.Meshes do not match. No nodes connection.

Independent faces

Grid interface - Fluent

GGI - CFX

GEOMETRY CONFIGURATION – MULTI-BODY PARTS

– Geometry composed of multiple bodies in a part

• Depend on ‘Shared Topology method’ (in DM)

– None » Results in a none connection between the bodies (similar to multiple parts)

– Automatic

Faces in contact imprinted & fused Form a single face shared between the 2 bodies

Results in Conformal mesh

Common face acts as ‘Interior’

GEOMETRY CONFIGURATION – MULTIPLE – BODY PARTS

– Geometry composed of multiple bodies in a part– Imprints

Faces are imprinted on each other ‘like’ faces

Contact Region is automatically created

non conformal interface

For identical mesh on these faces, use ‘Match Control’Results in unconnected mesh

Grid interface - Fluent

GGI - CFX

MESHING – 3D GEOMETRY

• 3D cell Types

• First Meshing Approach

Part/Body based• Meshing occurs at part or

body level.

• Meshing Methods are

scoped to individual bodies.

• Method assignment can be

automatic or manual.

• Bodies contained in one

part are conformally

meshed.

Part/Body Methods• Tetrahedrons.

− Tetras only

• Sweep.− Prisms &

hexahedrons

• MultiZone. − Mainly hexahedron

• Hex Dominant− Not for CFD

• Automatic.− Combines any types

MESHING – 3D GEOMETRY

• Second Meshing Approach

Cut Cell MeshingAssembly Meshing• Meshes an entire model in

one process.− Assembly of parts

• Performs boolean operations.− Volume filling, intersection &

combination

− Does not require prior fluid

body definition or shared

topology.

• Conformal mesh created

across parts.

Assembly Meshing

Methods• Generate mainly

− Hexahedrons

− Tetrahedrons

Part/Body Meshing &

Assembly Meshing

not interoperable

MESHING METHODS

• In this lecture we will learn:

– 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


Sketches and

Planes

Geometry Import

Options

3D Operations

Bi-Directional

CAD/ Neutral

Geometry

Cleanup and

Repair

Automatic

Cleanup

Simplification,

Mid-surface,

Fluid Extraction

Extrude, Revolve,

Sweep, etc

3D Operations

Booleans,

Decompose, etc.



Meshing Solver

Meshing Methods

Hybrid Mesh: Tet,

Prisms, Pyramids

Hexa Dominant,

Sweep meshing

Global Mesh

Settings

Local Mesh

Settings

Sizing, Controls,

etc.

Assembly

Meshing

WHICH METHOD TO CHOOSE?

• Why Multiple Methods?• Choice depends on :

– Physics

– Geometry

– Resources

• Mesh could require just one or a combination of 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

PATCH CONFORMING VERSUS INDEPENDENT

Patch Conforming• Clean CAD, Accurate surface mesh

Patch Independent• Dirty Geometry, defeatured surface mesh

TETRAHEDRONS METHODS

• Bottom up approach: Meshing process • Edges Faces 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

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


• Set to Tetrahedrons• Set to Patch Independent

Patch Independent

TETRAHEDRONS METHOD : CONTROL

• 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 surfacemesher algorithmin global controls

Patch Conforming - Sizing

TETRAHEDRONS METHOD : CONTROL

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

• Automatic curvature & proximity refinement option

Defeaturing Control• Set Mesh Based Defeaturing On• Set Defeaturing Tolerance• Assign Named Selections to selectively preserve

geometry

Patch Independent - Sizing

Defeaturing Tolerance off

Name Selec. assigned & defeaturing Tol = 0.02

Features > 0.02m respected

TETRAHEDRONS METHOD : ALGORITHM COMPARISON

Geometry with small details

Patch conforming : details caputred Patch independent : details ignored

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

HEXA MESH - INTRODUCTION

Tetra mesh - 48 000 Cells

Hexa mesh - 19 000 Cells

• 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

SWEEP MESHING

• Generates hex/wedge elements• Meshes source surfaces Sweeps through to the

target• Body must have topologically identical source

and target faces• Side faces must be mappable

• A sweep path must be identified• Only one source and one target face is allowed

• Alternative ‘thin’ sweep algorithm can have multiple source & target faces


• Set to Sweep

Mesh Method & Behavior

Source FaceTarget Face

Side Face(s)

Sweep Path

Sweep Direction Source face Target face

SWEEP MESHING

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 nbr of intervals on the

side face(s)

Sweep Path

Generation of wedges& hex elements

SWEEP MESHING

Automatic Thin & Manual Thin• Alternate sweep algorithm• Advantages Sweep multiple Source & Target faces Can perform some automatic defeaturing

• Limitations X For multibody parts only one division allowed

across the sweepX Inflation not allowedX Sweep bias not allowed

Source & Target selection

Target

Source

Faces

Source Faces imprintedon Target

SWEEP MESHING

Compatibility with Src/Trg Selection

X

XX

Use of Inflation• Defined on source face ( NOT on target one)• From boundary edges (2D)• Swept through volume

Sweep and Inflation

Sweep Mesh - No Inflation

Sweep Mesh with Inflation

SWEEP MESHING

• Automatic detection of sweepable bodies• Rotational ones are not identified

• Identification method• Right click on mesh object

• Outline tree• Select : Sweepable Bodies

Identifying sweepable bodies

Geometry

Right mouse button

Sweepable bodies in green color

• Decompose bodies into multi-simple topologicalshapes

• Perform decomposition in CAD/DM

Making bodies sweepable

Unsweepable

Decompose

Sweep Mesh

MULTIZONE 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


MULTIZONE MESHING

Determines which elements to use• Hexa

• Default • Only Hexahedral elements are generated

• Hexa/prism• For quality and transition, triangles will be

inserted on the surface mesh (sources)• Prism

• Only prisms will be generated• Useful when the adjacent volume is filled in

with tet mesh

Mapped Mesh Type

Geometry

Hexa

Hexa - Prism

MULTIZONE MESHING

Specify a method to create the surface mesh• 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

• Program controlled• Combination of Uniform and Pave methods• depends on the mesh sizes set and face

properties

Surface Mesh Method

Geometry

Uniform

Pave

AUTOMATIC METHOD

• 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


2D MESHING

• Quadrilateral Dominant & Triangles• Patch conforming methods

• MultiZone Quad/tri• Patch Independent Methods• Associated with face mesh type

• All Tri• Quad/tri• All Quad

• Advanced size function & local size controls are supported


Automatic Triangles

MultiZoneQuad/Tri

MultiZoneQuad

2D MESHING

• Mapped Surface Meshes• Local mesh controls

• Fully Mapped surface meshes• Specified edge sizing/intervals

Control

• Boundary edges are inflated• Global & local inflation controls are supported

Inflation

2D Mappedmesh

2D MESH SOLVER GUIDELINES

• 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 Fluent

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

• Thin Block for Planar 2D

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

ANSYS CFX

SELECTIVE MESHING

• What is ?– Selectively picking bodies and meshing them incrementally

• Why ?– 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 MESHING

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 MESHING

• 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

Recording Mesh OperationsExample : Meshing cylinder first and then block

SELECTIVE MESHING

• 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)

Selective Body Updating

Example :Geometricchange to block

GLOBAL MESH CONTROLS• In this section, we will learn about:

– Introduction to Global Mesh Controls

– Defaults

– General Sizing Controls & Advanced Size Functions

– Global Inflation

– Assembly Meshing Controls

– Statistics


Sketches and

Planes

Geometry Import

Options

3D Operations

Bi-Directional

CAD/ Neutral

Geometry

Cleanup and

Repair

Automatic

Cleanup

Simplification,

Mid-surface,

Fluid Extraction

Extrude, Revolve,

Sweep, etc

3D Operations

Booleans,

Decompose, etc.



Meshing Solver

Meshing Methods

Hybrid Mesh: Tet,

Prisms, Pyramids

Hexa Dominant,

Sweep meshing

Global Mesh

Settings

Local Mesh

Settings

Sizing, Controls,

etc.

Assembly

Meshing


GLOBAL MESH CONTROLS (1)

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

– 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 (2)• Physics Based Settings

– Physics and Solver Preferences• Global Mesh Sizing Controls

• Relevance and Relevance Center• Advanced Size Functions• Smoothing and Transition• Span Angle Center• Curvature Normal Angle• Proximity Accuracy and Cells Across Gap

• Inflation– Inflation Option, Inflation Algorithm– Collision Avoidance– Maximum Angle, Fillet Ratio, Smoothing

• Assembly Meshing– Activation of CutCell/Tetrahedrons Meshing

• Patch Confirming Options– Activation of Advancing Front Method

• Advanced– Numer of CPUs for Parallel Part Meshing– Shape Checking– Element midside nodes

• Defeaturing– Pinch based– Automatic Mesh Based

• Statistics– Mesh statistics, Quality criteria

GLOBAL MESH CONTROLS (3)

DEFAULTS• Four options under “Physics Preference”

– CFD, Mechanical, Explicit and Electromagnetic

• 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”

• Assembly Meshing is active only when Physics Preference is CFD and Solver Preference is Fluent

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

• Five Options:

– Off. Unavailable for Assembly Meshing

– Proximity and Curvature

– Curvature

– Proximity

– Fixed

• When CutCell Meshing is active with ‘Proximity’ or ‘Proximity and Curvature’ Advanced Size Function (ASF), 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 ASF

SIZING : ADVANCED SIZING FUNCTIONS

SIZING : ADVANCED SIZING FUNCTION EXAMPLES

ASF: 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

ASF: Off

• 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

ASF: 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


Growth Rate

Element Size

• 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

Element size option

available when Advanced

Size Function is not used

SIZING : MIN AND MAX SIZE• Min Size

– Minimum element size that the size function will generate

– Some element sizes may be smaller than this size depending on the edge length

• Max Face Size

– Maximum face size that the size function will generate

– Not supported by CutCell meshing

• Max Size

– Maximum element size that can be grown in the interior of volume mesh

Min Size ≤ Max Face Size ≤ Max Size Max Size

Mouse Pointer serves to estimate

mesh sizes

Max Face Size

Min Size

• Define the ratio between sizes of adjacent cells

• On surfaces and inside the volumes

SIZING : GROWTH RATE

Mesh size:

GR = 1.1 : 1,263,297 cells

GR = 1.2 : 587,026 cells

GR = 1.3 : 392,061 cells

Growth Rate =

1.1

Growth Rate = 1.2

(Default)

Growth Rate =

1.3

SIZING : TRANSITION• Controls the rate at which elements grow

• Two level control for transition

• Slow (Default for CFD, Explicit), produces smooth transitions

• Fast (Default for Mechanical and Electromagnetic), produces more abrupt transitions

• Not available for Cutcell meshing

• Hidden for sheet models, ignored for assemblies containing sheets, when ASF is On

Fast Slow

SIZING : SPAN ANGLE CENTER

• Controls curvature based refinement for Edges

• Three options and corresponding span angle ranges are

– Coarse: 91° to 60°

– Medium: 75° to 24°

– Fine: 36° to 12°

• Not available for Cutcell meshing

Coarse Medium Fine

INFLATION• Inflation

– Used to generate thin cells adjacent to boundaries

– Required for capture of wall adjacent boundary layers

– Resolve viscous boundary layer in CFD

– Resolve thin air gaps in Electromagnetic analysis

– Resolve high stress concentration regions in Structures

– Cells are created by ‘inflating’ from the surface mesh into the volume (3d) or inflating from the boundary edge onto the face (2d)

– Options to control growth

INFLATION : AUTOMATIC INFLATION• Three options

• None

– Select this for manual inflation settings using local mesh controls

• Program Controlled

All the faces are selected for inflation except:

– Faces scoped to a Named Selection

– Faces with manual inflation defined

– Faces in contact regions

– Faces in symmetry

– Faces that belong to a part or body that has a mesh method defined that does not support 3D inflation, such as sweep or hex-dominant

– Faces in sheet bodies

• All Faces in chosen Named Selection: can grow inflation layers from faces grouped in one named selection

INFLATION : INFLATION OPTIONS• Five options:

Total Thickness

Maintains constant total height of inflation layer

throughout

First Layer Thickness

Maintains constant first cell height

throughout

Smooth Transition

Maintains smooth volumetric growth between each

adjacent layer. Total thickness depends on the

variation of base surface mesh sizes (Default)

First Aspect Ratio

Controls the heights of the inflation layers by

defining the aspect ratio of the inflations that are

extruded from the inflation base

Last Aspect Ratio

Creates inflation layers using the values of the first

layer height, maximum layers, and aspect ratio

controls

All available for Patch Conformal (PC ) tets and Assembly

meshing

Smooth Transition

INFLATION : INFLATION ALGORITHMS• Two Algorithms

– Post

– Pre

• Patch independent meshes (including Assembly) use Post

Preview Inflation

is available only

with Pre Algorithm

Post

Pre

• Surface mesh is inflated first, then rest of

the volume mesh grows

• Default method for Patch Conforming

Tetrahedrons

• First Tet grows then Inflation process starts

• Tet mesh is undisturbed, if the inflation options are altered

• Default option for Patch Independent Tetrahedrons

INFLATION: AUTOMATIC INFLATION EXAMPLE

MultiZone

Patch Conforming Tets

Cutcell

INFLATION : ADVANCED OPTIONSCollision Avoidance: Control to detect proximity regions and adjust the cells in the inflation layer.

• None

• Does not check for proximity regions

• Layer Compression (Default for Fluent)

• Compresses inflation layers in the proximity regions

• Maintains the given number of layers in the proximity regions

• May stair-step if needed (will give a warning)

• Stair Stepping (Default for CFX)

• Inflation layers are stair stepped in the proximity regions

• Removing layers locally in steps to avoid collisions as well as bad quality at sharp corners

When Cutcell meshing is used, both Layer Compression and Stair Steeping algorithms are used depending on the geometry complexity.

Generates combination of

Pyramids and Tets to fill

the stair step

INFLATION : COLLISION AVOIDANCE EXAMPLE

Example Stair SteppingLayer Compression

DEFEATURING• Removes small geometry features meeting the tolerances using

Pinch or/and Automatic Mesh Based Defeaturing controls in order to improve the mesh quality. Not all meshing methods can take advantage of these controls.

• Pinch Tolerance control removes small features at the mesh level depending on the Pinch Tolerance value provided. ANSYS Meshing offers global and manual pinch controls.

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

AMBD Off AMBD

OnWith Pinch

STATISTICS

• Option to view the mesh quality metric

• Exhaustive list of quality metrics

• Orthogonal Quality mesh quality metrics

• Option to view the Mesh Metric chart

– Intuitive controls available under Mesh Metric Chart

– Various options to explore under the ‘Controls’

• See lecture 7 for details

LOCAL MESH CONTROLS

• In this section, we will learn about:– Local mesh controls (Mesh sizing, Refinement, Match

control, Inflation, etc)

– How to apply local controls?

– Effect of local controls on mesh


Sketches and

Planes

Geometry Import

Options

3D Operations

Bi-Directional

CAD/ Neutral

Geometry

Cleanup and

Repair

Automatic

Cleanup

Simplification,

Mid-surface,

Fluid Extraction

Extrude, Revolve,

Sweep, etc

3D Operations

Booleans,

Decompose, etc.



Meshing Solver

Meshing Methods

Hybrid Mesh: Tet,

Prisms, Pyramids

Hexa Dominant,

Sweep meshing

Global Mesh

Settings

Local Mesh

Settings

Sizing, Controls,

etc.

Assembly

Meshing


LOCAL MESH CONTROLSControl the mesh locally

• Depends on the “Mesh Method” used

Local Mesh Controls are:

• Sizing- For Vertex, Edge, Face and Body

• Contact Sizing - For Edge and face

• Refinement- For Vertex, Edge and Face

• Mapped Face Meshing - For Face

• Match Control - For Edge and Face

• Pinch - For Vertex and Edge

• Inflation - For Edge and Face

Only Sizing and Inflation local controls are available for CutCell meshing

The latest control added on a particular entity overrides

any prior controls

Non-CutCell meshing local controls

CutCell meshing local controls

SIZINGRecommended 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 : EDGES

Sizing Type: Element Size

Sizing Type: Number of Divisions Edge meshed with

constant element size of

60mm

Edge meshed with 10

elements

The Curvature Normal Angle and/or the Growth Rate maybe

not displayed depending on the ASF used

SIZING : EDGESBias Type and Bias Factor

Specify the grading scheme and factor

• Bias Type: grading of elements towards one end, both ends, or the center

• Bias Option:

– Bias Factor: is the ratio of the largest element to the smallest element

– Smooth Transition: defined by Growth Rate which is ratio of size of an element with size of previous element. (Growth Rate = Bias Factor^(1(n-1))

SIZING : EDGESBehavior

Soft: Sizing will be influenced by global sizing functions such as those based on proximity and/or curvature as well as local mesh controls

Hard: Size control is strictly adhered to

Influenced by global

Proximity advanced

size function.

Soft Hard

No influence from other

global settings

• Transition between hard edges (or any edge with bias) and adjacent edge and face

meshes may be abrupt

• Hard edges or edges with bias will override Max Face Size and Max Size properties

Number of Division = 4 Number of Division = 4

• Element Size

•Defines the maximum element size on the face

• Element Size

•Defines the maximum cell size on the Body

SIZING : FACES & BODY (VOLUME)

• 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

SIZING : BODIES OF INFLUENCEBodies 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

MAPPED FACE MESHING• Creates structured meshes on selected mappable surfaces

– Mapped Face Meshing with advanced control is supported for

• Sweep, Patch Conforming, Hexa Dominant

• Quad Dominant and Triangles

– Mapped Face Meshing with basic control is supported for

• MultiZone

• Uniform Quad/Tri and Uniform Quad

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

mappable faces

If Mapped Face Meshing fails, ( ) icon

appears adjacent to corresponding object in

the Tree outline. The mesh will still be created

but will ignore this control.

• If face is defined by two loops, then the “Internal Number of Divisions” field is activated

– User can specify the number of divisions across the annular region

– Also useful for defining number of divisions along sweeping direction for Multizone when there are no side edges

MAPPED FACE MESHING: INTERNAL NO. OF DIVISIONS

Mapped face is swept to create

pure hex mesh

MATCH CONTROL• 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: CYCLIC

• Define Rotational periodic

Model is symmetrical at 90°

Full Model Periodic Model

Selected Faces for

Match control

Matching face mesh

PINCH

• 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

INFLATIONUsed 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)

MESH QUALITY

• In this section, we will learn:

– Impact of the Mesh Quality on the Solution

– Quality criteria

– Methods for checking the mesh quality

– Tools to improve quality in Meshing

– Concept of Assembly Meshing

– Assembly Meshing Methods & Controls


Sketches and

Planes

Geometry Import

Options

3D Operations

Bi-Directional

CAD/ Neutral

Geometry

Cleanup and

Repair

Automatic

Cleanup

Simplification,

Mid-surface,

Fluid Extraction

Extrude, Revolve,

Sweep, etc

3D Operations

Booleans,

Decompose, etc.



Meshing Solver

Meshing Methods

Hybrid Mesh: Tet,

Prisms, Pyramids

Hexa Dominant,

Sweep meshing

Global Mesh

Settings

Local Mesh

Settings

Sizing, Controls,

etc.

Assembly

Meshing

Check Mesh Quality


• 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 physic 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

IMPACT OF THE MESH QUALITY ON THE SOLUTION

• Example showing difference between a mesh with cells failing the quality criteria and a good mesh

• Unphysical values in vicinity of poor quality cells

• Diffusion example

IMPACT OF THE MESH QUALITY ON THE SOLUTION

(max,avg)CSKEW=(0.912,0.291)

(max,avg)CAR=(62.731,7.402)

(max,avg)CSKEW=(0.801,0.287)

(max,avg)CAR=(8.153,1.298)

VzMIN≈-100ft/min

VzMAX≈400ft/min

VzMIN≈-90ft/min

VzMAX≈600ft/min

Large cell size

change

Mesh

2M

esh

1

GRID DEPENDENCY

• 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

• Hexa cells can be stretched in stream direction to reduce number of cells

• Bias defined on inlet and outlet walls

• Bias defined on inlet edges

– 16 000 cells (~DP2)

– Delta P = 310 Pa (~DP3)

HEXA VS. TETRA

• Hexa: Concentration in one direction

– Angles unchanged

• Tetra: Concentration in one direction

– Angles change

• Prism: Concentration in one direction

– Angles unchanged

• Solution for boundary layer resolution

– Hybrid prism/tetra meshes

– Prism in near-wall region, tetra in volume

– Automated

– Reduced CPU-time for good boundary layer resolution

Hexa

Tetra

Prism

Prisms (near wall)

Tetra (in volume)

MESH STATISTICS AND MESH METRICS

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 QUALITY METRICS

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 I

Where Ai is the face normal vector and fi is a vector from the centroid of the cell 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

A1

A2

A3

f1

f2f3

c2c1

c3

A1

A2

A3

e1

e2e3

On cell

|||| ii

ii

fA

fA

|||| ii

ii

cA

cA

|||| ii

ii

eA

eA

On face

0 1

Worst Perfect

MESH QUALITY METRICS

Skewness

Two methods for determining skewness:

1. Equilateral Volume deviation:

Skewness =

Applies only for triangles and tetrahedrons

2. Normalized Angle deviation:

Skewness =

Where is the equiangular face/cell (60 for tets and tris, 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

e

0 1

Perfect Worst

Actual cell

MESH QUALITY

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

ASPECT RATIO

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

SMOOTHNESS

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

SECTION PLANES

Displays internal elements of the mesh

• Elements on either side of plane can be displayed

• Toggle between cut or whole elements display

• Elements on the plane

Edit Section Plane button can be used to drag section plane to new location

• Clicking on “Edit Section Plane” button will make section plane’s anchor to appear

Multiple section planes are allowed

For large meshes, it is advisable to switch to

geometry mode (click on geometry in the Tree

Outline), create the section plane and then go

back to mesh model

MESH METRIC GRAPH

• 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 CONTROLS

• 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

– The CFX solver calculates 3 important measures of meshquality at the start of a run and updates them each time themesh is deformed

– Mesh Orthogonality

– Aspect Ratio

– Expansion Factor

MESH QUALITY CHECK FOR CFX

+--------------------------------------------------------------------+

| Mesh Statistics |

+--------------------------------------------------------------------+

Domain Name: Air Duct

Minimum Orthogonality Angle [degrees] = 20.4 ok

Maximum Aspect Ratio = 13.5 OK

Maximum Mesh Expansion Factor = 700.4 !

Domain Name: Water Pipe




Global Mesh Quality Statistics :




Good(OK)

Acceptable(ok)

Questionable(!)

MESH QUALITY CHECK FOR FLUENT

Grid check tools available

• Check : Perform various mesh consistency checks

• Report Quality : lists worse values of orthogonal quality and aspect ratio

• TUI command mesh/check-verbosity sets the level of details in the report

FACTORS AFFECTING QUALITY

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 Design Modeler

or

Virtual topology & pinch in Meshing

Mesh setting change

Mesh setting change

VIRTUAL TOPOLOGY

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

AUTOMATIC VIRTUAL TOPOLOGY

• Automatically creating Virtual Faces

– Left Click Virtual Topology in Model Tree

– Set Behaviour in Details

• Controls aggressiveness of automatic VT algorithm

• Low: merges only the worst faces (and edges)

• Medium & High: try to merge more faces

– Select if Face Edges shall be merged

– Right Click Virtual Topology and click Generate Virtual Cells

• Manually creating a Virtual Face

– RMB on Model tree and select Insert Virtual Topology

– Select Virtual Topology from the Tree Outline

– Pick faces or edges, RMB and Insert Virtual Cell

•All VT entities created can be seen in different colors if Virtual Topology is selected in Tree Outline

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

Vertex-Vertex Edge-Edge

before after before after

ASSEMBLY 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

CutCell

Tetrahedrons

Note that some

global and local

controls are not

available for

Assembly

Meshing (eg.

Match Control)

• 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)

– Baffles not supported

– High inflation 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

ASSEMBLY MESHING - TETRAHEDRONS

• Controls

– Set Advanced Size Functions

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

• 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

– Define Feature and Tesselation controls (see next slide)

– Apply any required local size controls

– Statistics

• Use Orthogonal Quality for Cutcell

ASSEMBLY MESHING - CONTROLS


Min Size definition– Assembly Meshing is Patch Independent, geometry recovery and leakage

depend on local sizes

– Local sizes are driven by global min sizes and local hard sizing

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

– Local mesh size recommendation to capture 3D features

• Local size < ½ feature size

– Local mesh size recommendation to close gaps

• 1/10 local size < gap size < ¼ local size : contact sizing can be defined to close gap

• Gap size < 1/10 local size : gap closed

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

• Avoid unnecessary geometry details

• Features aligned with Coord. Syst. will be more easily recovered

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 angle to define features to recover

– 0 to capture all

– Tessellation (faceting) refinement

• Program Controlled - default which sets tessellation refinement to 10% of the value of smallest global min size

• Absolute Tolerance – user defined tolerance

– Must be set to 5-10% of smallest size (global min sizes or local hard sizing)

• None - Sets tessellation refinement to the CAD program or DesignModeler default setting


Incorrect tessellation may lead to

leakage

STAY AHEAD DURING CHALLENGING TIMES

• To purchase software or for consulting

please contact us: [email protected](408) 732-4665

mailto:[email protected]

THANK YOU FOR YOUR ATTENTION!

FOR FURTHER INFORMATION, PLEASE CONTACT:OZEN ENGINEERING, INC.1210 E. ARQUES AVE. SUITE: 207SUNNYVALE, CA 94085(408) [email protected]

Documents

Meshing Workshop 2014