OZEN ENGINEERING, INC.

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MESHING WORKSHOP

Thursday, November 13

th

_{, 2014}

Metin Ozen, Ph.D., ASME Fellow

OZEN ENGINEERING, INC.

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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

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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

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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

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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

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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

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MESHING PROCESS IN ANSYS MESHING

• Physics, Sizing, Inflation, Pinch,

…

• Sizing, Refine, Pinch, Inflation,

…

• Preview surface mesh,

Inflation

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LAUNCHING ANSYS MESHING

–

ANSYS Meshing is launched within Workbench

• 2 ways :

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

From Analysis Systems

Mesh

From Component Systems

Double click Meshin the System or right click and select Edit

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GRAPHICS USER INTERFACE

Mesh Metrics Graphics window Section Planes Manage views Details view Outline Toolbars Units Bar Entity Details Bar

Message window

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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

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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

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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

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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’

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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 unconnectedmesh

Grid interface - Fluent

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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.

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MESHING – 3D GEOMETRY

• Second Meshing Approach

Cut Cell Meshing

Assembly 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

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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

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PREPROCESSING WORKFLOW

Meshing Methods

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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

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PATCH CONFORMING VERSUS INDEPENDENT

Patch Conforming

• Clean CAD, Accurate surface mesh

Patch Independent

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TETRAHEDRONS METHODS

• Bottom up approach: Meshing process

• Edges  Faces  volume

• All faces and their boundariesare respected

(conformed to) and meshed

• Good for high quality (clean) CAD geometries

• CAD cleanup required for dirty geometry

• Sizing is defined by globaland/or localcontrols

• 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

• Set to Tetrahedrons

• Set to Patch Independent

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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 surface mesher algorithm in global controls

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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

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TETRAHEDRONS METHOD : ALGORITHM COMPARISON

Geometry with small details

Patch conforming : details caputred Patch independent : details ignored

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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

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SWEEP MESHING

• Generates hex/wedgeelements

• 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

To access it

• Set to Sweep

Mesh Method & Behavior

Source Face

Target Face

Side Face(s)

Sweep Path

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SWEEP MESHING

Automatic

• Source & Target faces identified automatically

• Requires that the mesher find the sweeping direction

• Manualsource & Manualsource andtarget

• User selection

• Source face colored in red

• Target face colored in blue

• RotationalSweeping

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

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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 sweep

X Inflation not allowed

X Sweep bias not allowed

Source & Target selection

Target Source

Faces

Source Faces imprinted on Target

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SWEEP MESHING

Compatibility with Src/Trg Selection

X



 X X

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

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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 topological shapes

• Perform decomposition in CAD/DM

Making bodies sweepable

Unsweepable

Decompose

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MULTIZONE MESHING

• Based on blocking approach (ANSYS ICEM CFD Hexa)

• Automatically decomposesgeometry 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

• Multiplesource faces

• Select Target faces as “Source”

• Compatible with 3D Inflation 

To access it

• Insert Method Set to Multizone

Mesh Method & Behavior

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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

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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

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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

Mesh Method & Behavior

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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

Mesh Method & Behavior

Automatic Triangles

MultiZone Quad/Tri

MultiZone Quad

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2D MESHING

• Mapped Surface Meshes

• Local mesh controls

• Fully Mapped surface meshes

• Specified edge sizing/intervals

Control

• Boundaryedgesare inflated

• Global& local inflation controls are supported

Inflation

2D Mapped mesh

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2D MESH SOLVER GUIDELINES

• For a 2D analysis in Fluent generate the mesh in the XY plane

• Z = 0

• For axisymmetricapplications 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

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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

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SELECTIVE MESHING

Clearmeshes on individual bodies

Generatemeshes 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

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SELECTIVE MESHING

• Use it to recordthe 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 Operations

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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 : Geometric change to block

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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

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PREPROCESSING WORKFLOW

Meshing Methods

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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

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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

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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

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• 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

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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 of cell size is defined by

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 • Transition of cell size is defined by

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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

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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

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• 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

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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

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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

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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)

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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

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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

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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

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INFLATION: AUTOMATIC INFLATION EXAMPLE

MultiZone

Patch Conforming Tets

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INFLATION : ADVANCED OPTIONS

Collision 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

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INFLATION : COLLISION AVOIDANCE EXAMPLE

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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

On

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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’

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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?

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PREPROCESSING WORKFLOW

Meshing Methods

(68)

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LOCAL MESH CONTROLS

Control the mesh locally

•

Depends on the “Mesh Method” used

Local Mesh Controls are:

The latest control added on a particular entity overrides any prior controls

Non-CutCell meshing local controls

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SIZING

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

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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

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SIZING : EDGES

Bias 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))

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SIZING : EDGES

Behavior

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

(74)

• Element Size

• Defines the maximum element size on the

face

• Element Size

• Defines the maximum cell size on the Body

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• 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

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SIZING : BODIES 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

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MAPPED FACE MESHING

• Creates structured meshes on selected mappable surfaces

(78)

• 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

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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

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MATCH CONTROL: CYCLIC

• Define Rotational periodic

Model is symmetrical at 90°

Full Model Periodic Model

Selected Faces for Match control Matching face mesh

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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

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INFLATION

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)

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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

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PREPROCESSING WORKFLOW

Meshing Methods

Check Mesh

Quality

(85)

(86)

• 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 …

(87)

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

(88)

• 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) Vz_MIN≈-100ft/min Vz_MAX≈400ft/min Vz_MIN≈-90ft/min Vz_MAX≈600ft/min

Large cell size

change

Mesh

2 Mesh

(89)

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

(90)

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)

(91)

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)

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MESH STATISTICS AND MESH METRICS

Displays mesh information for Nodes and Elements

List of quality criteria for the Mesh Metric

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 Aiis the face normal vector and fiis a vector from the centroid of the cell to the centroid of that face, and ciis a vector from the centroid of the cell to the centroid of the adjacent cell, where eiis the vector from the centroid of the face to the centroid of the edge

At boundaries and internal walls

e

A



On face

0 1 Worst Perfect

(94)

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 min e e e max _, 180 max       min



max



optimal cell size cell size

optimal cell size



Optimal (equilateral) cell

Circumsphere

e



0 1 Perfect Worst

Actual cell

(95)

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

(96)

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

(97)

SMOOTHNESS

Checked in solver

• Volume Change in Fluent

– Available in Adapt/Volume

– 3D : σ_i= V_i/ V_nb

• 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

(98)

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

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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

(100)

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

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– The CFX solver calculates 3 important measures of mesh quality at the start of a run and updates them each time the mesh 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

Minimum Orthogonality Angle [degrees] = 32.8 ok Maximum Aspect Ratio = 6.4 OK Maximum Mesh Expansion Factor = 73.5 ! Global Mesh Quality Statistics :

Minimum Orthogonality Angle [degrees] = 20.4 ok Maximum Aspect Ratio = 13.5 OK Maximum Mesh Expansion Factor = 700.4 !

Good

(OK)

Acceptable

(ok)

Questionable

(!)

(102)

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

(103)

FACTORS AFFECTING QUALITY

Geometry problems

•

Small edge

•

Gaps

•

Sharp angle

Meshing parameters

Patch conformal or patch independent tetra

•

Sweep or Multizone

•

Cutcell

Geometry cleanup in Design Modeler or

Virtual topology & pinch in Meshing

Mesh setting change

(104)

VIRTUAL TOPOLOGY

When to use?

•

New faceted geometry is created with virtual

topology

Restrictions

•

Limited to “developable” surfaces

•

Virtual Faces cannot form a closed region

Without VT With VT

(105)

AUTOMATIC VIRTUAL TOPOLOGY

• Automatically creatingVirtual Faces – Left Click Virtual Topologyin Model Tree

– Set Behaviourin 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 Topologyand clickGenerate 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

(106)

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

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ASSEMBLY

MESHING

(108)

(109)

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)

(110)

• 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)

(111)

• 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

(112)

• 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

(113)

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_/

10local size < gap size < ¼ local size : contact sizing can be defined

to close gap

• Gap size < 1_/

10local 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

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– 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

ASSEMBLY MESHING - CONTROLS

Incorrect tessellation may lead to leakage

(115)

STAY AHEAD DURING CHALLENGING TIMES

• To purchase software or for consulting

please contact us:

[email protected]

(116)