AutoVolumeMeshingV12

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4-1ANSYS, Inc. Proprietary

2009 ANSYS, Inc. All rights reserved.July 2009

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Chapter 4

Volume Meshing

ICEM CFD IntroductoryCourse


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Training ManualIntroduction to volume meshing

To automatically create 3D elements to fill

volumetric domain Generally termed unstructured

Mainly tetra

Full 3D analysis

Where 2D approximations dont tell the fullstory

Internal/External flow simulation

Structural solid modeling

Thermal stress

Many more!

Standard procedures

Start from just geometry

Octree tetra

Robust

Walk over features

Cartesian

Fastest

Have to set sizes

Start from existing shell mesh

Delauney/T-grid

Quick

Advancing Front

Smoother gradients, sizetransition

Hex Core

Hex Dominant

Both geometry andshell mesh

Portions of modelalready meshed

Inflation layers

Prism


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Training ManualGeneral Procedure

Compute Prism (optional)

As separate process

Also option to runautomatically following

tetra creation

First decide volume mesh parameters

Global Mesh Setup > Volume Meshing

Parameters

Select Mesh Type

Select Mesh Methodfor selected Type

Set options for specific Methods

Set mesh sizes

Globally

As in Shell Meshing

Locally

Part/Surface/Curve Mesh Setup

As in Shell Meshing

ForFrom geometryonly

Octree

Cartesian

Define volumetric region

Typically for octree on complex models

Multiple volumes

Define density regions (optional)

Applying mesh size within volumewhere geometry doesnt exist

Load/create surface

mesh As in Shell Meshing

chapter

ForDelauney, AdvancingFront, T-grid, Hex-Dominant

Either of these types runfrom geometry will

automatically createsurface mesh usingglobal and local ShellMesh settings withoutany user input/editing

If in doubt, run ShellMesh first, then fromexisting mesh

Compute Mesh

Mesh > Compute Mesh >Volume Mesh


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Training ManualBody/Material Point

Define Volumetric Domain Optional

Recommended for complex geometries

Multiple volumes

Geometry -> Create Body

Material Point Centroid of 2 points

Select any two locations whose mid-point is within volume

Preferred

At specified point

Define volume region by point within volume

By Topology

Defines volume region by set of closed surfaces Must first Build Diagnostic Topology

Entire model

Automatically define all volumes

Selected surfaces

User selects surfaces that form a closed volume


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Training ManualMesh Types

Tetra/mixed

Most used

Tetra

With hex core

Hexa (cartesian) filling majorityvolume

Tetra (from delauney algorithm) used

to fill between surface or top ofinflation layers and hex core

Pyramids to make conformal betweentetra tri and hex quad faces

With inflation layers

Prisms from tri surface mesh

Hexas from quad surface mesh Tetra and/or hex core filling interior

Pyramids to cap off any quad faces

Of hex core or hex inflation layers

Merged hybrid with structured hexmesh

Pure tetra

Tetra/Prism

Tetra/Prism/Hexcore


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Training ManualMesh Types

Hexa-Dominant

From existing quad mesh

Good quality hex near surface

Somewhat poor in interior

Typically good enough forstatic displacement

Not covered in detail here

Cartesian

Automatic pure hexa

Rectilinear mesh

Staircase or

Body fitted

Fastest method for creatingvolume mesh

Not covered in detail here


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Training ManualMesh Method

Tetra/Mixed

Robust (Octree)

Same as Shell Meshing > Patch Independent

Retains volumetric tetras

If you have complex and/or dirty geometry

Dont want to spend too much time with geometry

cleanup Dont want to spend too much time with detailed

shell meshing

Dont want to spend time defeaturing geometry

Just set appropriate mesh sizes on geometry

Parts

Surfaces

Curves

Review Shell Meshingchapter

Part/Surface/Curve Mesh Setup


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Training ManualOctree Method

Geometry

Mesh detail

Mesh

ICEM Tetra uses patch-independentOctree method

Volume first generated independent of surface model

Nodes are projected to model surfaces, curves and points

Surface mesh is created

Resulting mesh is independent of the underlying

arrangement of surfaces

Not all surface edges need to be captured!

Make curves/points dormant

Delete

Filter points/curves under Build Diagnostic Topology

Sliver ignored


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Training ManualGeometry Requirements forOctree Tetra

Tetra requires an enclosed

surface model Run Build Diagnostic Topology

to find gaps/holes

Octree can tolerate gaps

smaller then the local element

size

Recommend points and curves

at key features

Recommend Material points to

define volumes

Will create a material point if

none exists

Set Global, Part, Surface, Curve

Size Parameters

As in Shell Meshing

Missing inlet surface

Geometry Repair

tools quickly locate

and fix these

problems.

Hole highlighted by

yellow single edge curve


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Training ManualUsing Points and Curves

Coarse mesh walks over detail

in surface model

Mesh captures detail

Curves and points not included

Mesh size specified only on surfaces

Curves and points included

Mesh size specified on curvesand surfaces

Curves and points

affect which

features arecaptured by the

mesh!

Build Topology

easily creates the

necessary points

and curves


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Training ManualOctree - Options

Setup options:

Global Mesh Setup > Volume Meshing parameters Run as batch process

Runs as a separate process. GUI will stay interactive.

Fast Transition Allows for a faster transition in element size from finer to coarser

Reduce element count

Edge Criterion

Split elements at a factor greater than set value to better capturegeometry

Define Thin cuts Tool for resolving thin gaps, sharp angles

User selects pairs of opposing parts

Resolve elements jumping from one side to another

Smooth Automatically smooths after grid generation process

Coarsen

Fix Non-manifold

Automatically tries to fix elements that jump from surface to another

For a more detailed description go to Help > Help Topics > Help

Manual > Mesh > Global Mesh Setup > Volume Meshing

Parameters > Tetra/Mixed > Robust (Octree)


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Training ManualOctree -Options

Run options: Compute Mesh > Volume MeshingParameters Create Prism Layers

Will create prisms marked underPart Mesh Setup Immediately after tetra calculation Prism layers grown into existing tetra mesh

Create Hexa-Core Will retain tri surface mesh (or tri-prism), throw away tetra

mesh Fill volume interior with cartesian Cap off with pyramids Map tetra to tri or top prism face with delauney algorithm

Input Select Geometry

All, Visible Part by Part

Meshes each part separately Mesh not conformal between parts

From file Select tetin file

Use Existing Mesh Parts Select Parts that are already surface meshed Uses Make Consistentto match octree volume mesh to

existing surface mesh


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Training ManualCurvature/Proximity Based Refinement

Curvature/Proximity BasedRefinement

Octree only

Automatically subdivides to create elements that are

smaller than the prescribed entity size in order to capture

finer features

Min size limit value entered is multiplied by the global Scale

Factorand is a lower limit for the automatic subdivision

Used primarily to avoid setting up meshing parametersspecifically for individual entities thus allowing the

geometry to determine the mesh size

Prescribed size

is adequate here

Prescribed element size: Surface/CurveMax. Element

Size times Scale Factor

Auto subdivision

at tighter radius

of curvature

Min Size Limit: multiplied by

Scale Factor = global minimum


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Training ManualCurve Based Refinement

Refinement

Number of elements alongcurvature if extrapolatedto 360o

To avoid subdivisionalways to global minimumwhich would otherwiseresult in too manyelements

Subdivision will stoponce number ofelements along curveis reached

Wont exceed globalminimum set by naturalsize value

Example Specified refinement

achieved with largerelements

Global minimum (Size) notrealized, not necessary tocapture curvature

Prescribed size Min size limit

Refinement = 12


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Training ManualProximity Refinement, Elements in Gap

Elements in Gap

Number of cells desired innarrow gaps

To avoid subdivision

always to global minimum

which would otherwise

result in too many

elements

Subdivision will stop

once number of cells

in gap is reached

Will not override global

minimum (Min size limit)

Example

Only one element in gap

Cant go smaller than Min

size limit

Have to set smaller Min

size limit

Prescribed size Min size limit

Cells in Gap = 5

Prescribed size Min size limit (1/5th smaller)

Cells in Gap = 5


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Training ManualMesh Methods

Tetra/Mixed Quick (Delauney)

If already have

good quality

surface mesh

From Shell

Mesh

From Octree

From imported

surface mesh

Uses initial point

cloud; distributed

so that the

centroid of anytetra is outside

circumsphere of

any neighboring

tetra

SetupO

ptions: Memory scaling factor

To allocate more memory

Usually does so automatically

Expansion factor

Growth ratio from surface

Fill holes in volume mesh

Use to fills holes/voids inexisting volume mesh

e.g. if bad quality region isdeleted

Mesh all domains

For multiple sets of closedvolumes in one model

Usually

Flood fill after complete

For multiple volumes Will assign tetras within closed

volume to Part designated byBody orMaterial Point

Verbose output

For troubleshooting


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Tetra/Mixed Smooth (AdvancingFront)

Same as Quick (Delauney) but

Uses advancing front method that marches tetrasfrom surface into interior

Algorithm from GE/CFX

Results in more gradual change in element size

Better but finer mesh

Inut surface mesh has to be of fairly high quality

Setup Options: Do Proximity Checking

Check to properly fill small gaps

Longer run time

Tgrid

Volume grid generation algorithm from Fluent Uses somewhat different delauney approach

Gradual transition near surface

Faster transition toward interior

Setup Options: Use Background Mesh


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Run

Options:

Same forDelauney, AdvancingFront, TGrid

Create Prism Layers/Hexa-Core

Same as forOctree

Hexa-Core not available forAdvancingFront, TGrid

Input

AllG

eometryWill run shell mesh first with no user input/editing

Using parameters from Shell/Part/Surface/Curve

Mesh Setup

Review Shell Meshingchapter

If doubtful as to shell mesh quality, run Shell Mesh

first, then use Existing Mesh

Existing Mesh Part by Part/From File

Same as forOctree

Volume Part Name

For newly created tetras


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Training ManualComparison

Octree Delauney Adv.front


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Training ManualSet Mesh Sizes

Create Mesh Density

Define volumetric region with certain meshsize where no geometry exists, e.g. wake

region behind a wing

Not actual geometry!

Mesh nodes not constrained to density object

Can intersect geometry

Can create densities within densities

Always subdivides to smallest set size

Set Size

As for surface/curve multiplied by global Scale

Factor

Ratio expansion ratio away from density object

Width Number of layers from object

Type

Points Select any number of points Size and Width (number of layers) will detemine thickness of

volume if number of points selected is 1-3

4-8 creates polyhedral volume

Entity bounds define region by bounding box

of selected entities


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Training ManualPeriodicity

Define Periodicity

Forces mesh alignment across periodic sides

For meshing and solving only one section ofsymmetrically repeatable geometry

Rotational Periodic

EnterBase,Axis, andAngle

Translational Periodic

Enter dX, dY, dZ offset

Tip: Place

material point

close to mid-plane


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Training ManualWorkshops

Workshop 1 Engine Block Model

Build diagnostic topology

Octree mesh

Smooth mesh

Curvature/proximity refinement

Delaunay mesh

Workshop 2 Valve Model

Build diagnostic topology

Create parts, Create body

Set sizes Global, Surface, Curves, density

Octree mesh


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Training ManualWorkshops

Workshop 3

Import Design Modeler file into ICEM

Create Parts from Subsets

Global, Part mesh set up

Using ANSYS ICEM CFD Hexa to create a Cartesian initial grid with biasing

Computing the mesh

Viewing cut-planes

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AutoVolumeMeshingV12