INTRODUCTION to ICEM-CFD

M4 – ICEM-CFD INTRODUCTION.

INTRODUCTION to ICEM-CFD

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by Ing.Ivone Valle

http://www.grupossc.com/


Introduction to ANSYS ICEM CFD

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by Ing.Ivone Valle



• Ansys ICEM CFD is a general purpose grid generating program – Grids for various solver types can be produced

• Fluid analysis (CFD)

• Structural analysis (FEA)

• Electromagnetic, and more

– The terms “mesh,” “grid,” and “discretizing the domain” all refer to the same thing

• The purpose of this course is to teach the basic tools and methods for generating meshes with Ansys ICEM CFD – Geometry import/editing

– Meshing (surface meshing, tetra, prism, hexa, hybrid meshes)

– Mesh Editing (smoothing, error checks, individual element editing)

– Solver setup

• Best meshing practices will be shown as well as the tools needed to produce them

3 by Ing.Ivone Valle


M4 – ICEM-CFD INTRODUCTION. 4

Ansys ICEM CFD Features

• Geometry Creation/Repair/Simplification

– Including Mid-Plane Extractions/Extensions

– Most geometry intended to be imported

• Powerful Meshing tools

– Tetra/Prism from CAD and/or existing surface mesh

– Shell meshing: structured, unstructured

– Hex-dominant, swept, Structured hexa, Extruded quads, Body-fitted and stair-step Cartesian, hexa-core

– Hybrid meshing (merging, multi-zone)

• Advanced mesh editing

• Solver Setup

by Ing.Ivone Valle



GUI and Layout

Data Entry

Panel

Model

Tree

Message

Window

Utility

Icons Function Tabs

Histogram

Window

Utility Menu

Selection Toolbar

Display Triad

by Ing.Ivone Valle



• Use of many files – Not one large common database – For faster input/output

• All files can optionally be associated within a Project – Establishes working directory when opened or

saved – Settings (*.prj) file contains associated file names

• Primary file types: – Tetin (.tin): Geometry

• Geometry entities and material points • Part associations • Global and entity mesh sizes • Created in Ansys ICEM CFD or CAD Interface

– Domain file (.uns) • Unstructured mesh

– Blocking file (.blk) • Blocking topology

– Attribute file (.fbc, .atr) • Boundary conditions, local parameters & element

types – Parameter file (.par)

• solver parameters & element types – Journal and replay file(.jrf, .rpl)

• Record of performed operations (echo file)

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File and Directory Structure

.prj

.uns

.tin .blk

.fbc

.par .jrf

.rpl

by Ing.Ivone Valle



• To open/save/close – Projects

• Will open/save/close all associated files including

– Geometry (*.tin) – Mesh (*.uns) – Blocking (*.blk) – Attributes…

(*.fbc, *.atr) – Parameters

(*.par) – All file types can be

opened/saved/closed independently

• Also to – Import/Export Geometry/Mesh

• Exit

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File menu

• Icons

– Many menu items duplicated by utility icons:

• Several common functions are duplicated as utility icons:

Save frequently to new file names so you can go back!

Open Project Save Project

by Ing.Ivone Valle



Typical ICEM CFD Workflow:

Create/open new project

Import/Create geometry

Build topology/Clean geometry/Create geometry

Mesh model (Possibly Hex Blocking)

Check/edit mesh

Output to Solver

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Workflow

General Order of Workflow

by Ing.Ivone Valle



• Ansys ICEM CFD can be inserted into the Workbench project schematic from the Component Systems and linked with other systems

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Workbench Project Schematic

by Ing.Ivone Valle



• File > Import Geometry (parasolid or acis)

• Show menu locations

• Mouse controls

• Change display options – Solid/Wire, simple/full

– Views/Save view

• Create Part – Parts concept

• Overview process – Create/open/Save Project

– Build topology

– Define global and local mesh sizes

– Create volume mesh

– Edit/check quality, smooth

– Set up and write out to solver

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Let’s Explore the GUI with a simple practice part

by Ing.Ivone Valle



Geometry Handling


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by Ing.Ivone Valle



Geometry handling

This Jet engine model was built solely with ICEM CFD geometry tools

ANSYS ICEM CFD was designed to mainly import geometry, not create complicated geometries, although many geometry tools are provided

An accurate solution reflects the underlying geometry. To get such, ICEM CFD provides:

Geometry import

– From CAD package

– 3rd party formats (step, acis, etc…)

– Via Workbench/ Design Modeler

Surface geometry kernel

– Imported solids are converted to surfaces

Many internal CAD tools

– Geometry creation

– Geometry modification

– Geometry fixing

by Ing.Ivone Valle



CAD from just about any source

• Workbench Readers – for most CAD imports

– The Import Model option allows you to use the Workbench Readers to import many kinds of files into ANSYS ICEM CFD.

– Requires a Workbench installation!

• 3rd-party import

– ACIS (.sat)

– DWG/DXF

• Direct CAD Interfaces

– Legacy interfaces which are not updated. Use Workbench Readers instead for current CAD versions

– Set up ICEMCFD/AI*E meshing requirements within CAD environment

• Saved within CAD part for parametric geometry changes

– Directly write out ICEM formatted geometry (tetin file)

• No 3rd party exchange (clean!)

– ProE, Unigraphics, Solidworks, Catia V4, IDEAS (IDI)

– ProE, UG, and Solidworks imports require CAD libraries; CAD software and licensing must be available


Geometry Import

• Parasolid

• STEP/IGES

• GEMS



When CAD is not available, an old legacy model or x-ray scan of the part can be imported as geometry. This input is a collection of facets (triangulated surfaces).

• Faceted Data – Nastran

– Patran

– STL (most common)

– VRML

– Other solver formats (indirectly from mesh conversion, after importing mesh)

• Formatted Point Data – Auto curve/surface creation from regular table

of points forming ordered U or V curves of a surface


Geometry Import - other sources



Open Geometry

• Geometry saved as “tetin” (*.tin file)

– Legacy name as an abbreviation of “tetra input.”

– Surface geometry kernel

• Any imported solid models are represented as a series of watertight surfaces

– Surfaces are internally represented as triangulated data

• Resolution or approximation of true bspline surface data set by Triangulation Tolerance in settings>model

• Smaller value = better resolution, curvature

• 0.001 works best for most models

• Use a high tri tolerance to work with a large model, but lower the tolerance when it comes time to compute the mesh

• Not used if surfaces are already facetized (e.g. STL, VRML) Tri tolerance = 0.1 Tri tolerance = 0.001

by Ing.Ivone Valle



• The first 3 icons to create Geometry are:

• It includes points in different ways like Screen

Select, Explicit Coordinates, Base Point and Delta,

Center of 3 Points/Arc, Based on 2 Locations,

Curve Ends, Curve-Curve Intersection, Parameter

along a Curve, Project Point to Curve, Project

Point to Surface.

• For lines, it could be created by points, an

intersection, projection of this, segment lines by

points or concatenated, extract an interpolated,

etc.

• And the surfaces can be created by curves and

modify whit operations like curtain, extend, untrim

or some simplification even you can create some

surfaces to obtain a shape like a sphere, a box,

etc.

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Geometry Creation Tools

First 3 icons to create Geometry

by Ing.Ivone Valle



Create Body

• Material point and body

– Material point used by tetra octree to instruct which volume regions to keep

• Volume elements will be in the same part as the material point

– Used in hexa blocking as a part for placing blocks

– Material point method is most robust

– By Topology method automatically creates a material point in every closed volume

• Requires build diagnostic topology first to determine connectivity

• Can save you the work of creating a lot of material points for each region

• Any regions not completely closed (yellow curves indicating gaps/holes) will not get a material point so this is less robust

by Ing.Ivone Valle


M4 – ICEM-CFD INTRODUCTION. 18 18

Geometry Handling – Other Tools

• Build Diagnostic Topology

• Check Geometry

• Close Holes

• Remove Holes

• Stitch/Match Edges

• Split Folded Surfaces

• Adjust varying Thickness

• Modify surface normals

• Bolt hole detection

• Button detection

• Fillet detection

• Translate

• Rotate

• Mirror

• Scale

• Translate & Rotate

– Three Points

– Curve to Curve

• Restore Dormant Entity

• Curves/points originally made inactive - ignored by meshing tools

• Restore to activate again -seen as constraints by meshing tools

Transformation Tools

Repair Geometry

• Delete

• Points

• Curves

• Surfaces

• Bodies

• Any Selected Entity

Build topology with filtering

by Ing.Ivone Valle



Tolerance

Edge 1

Edge 2

• Geometry -> Repair Geometry -> Build Diagnostic Topology

• To diagnose potential geometry problems

– Shows potential leakage (tetra octree) before meshing

– Shows where surface mesh may not be connected

– Patch dependent surface mesher requires build topology

– Tolerance

• Specifies allowable gap between surfaces

• Size should be set big enough to ignore small gaps, but not too big to ignore leakage (tetra octree) or remove important features

• Default is 1/2500th of the diagonal of the bounding box

• Connectivity is set up between surface edges that meet within the tolerance

• Filtering should be off when using to determine connectivity


Building Topology – Determine Connectivity



Color coding • Topology curves are color coded to indicate their surface connection

status – green = unconnected, yellow = single, red = double, blue =

multiple, Grey = dormant (filtered out)

– Turn color coding off/on in Model tree > Geometry > Curves > Color by count

– Red curves indicate two surfaces meet within the tolerance, This is what you want for a solid model.

– Yellow curves will usually indicate some repair is required


Building Topology – Color Coding

Red curves indicate that surfaces meet within the tolerance setting

Can you spot the hole in the solid?

Now you can find the hole

Yellow curves indicate that the surface is probably missing or the gap is greater than the tolerance

Build Topology



• Filter by angle (default 30 degrees)

• Filter Points: Points between two curves whose tangency is below the feature angle will be “filtered out” (made dormant)

• Filter Curves: Curves between two surfaces whose tangency is below the feature angle will be “filtered out” (made dormant)


Build Topology – Extract Curves and Points

Filtering No filtering

Tetra octree and patch dependent surface

mesher enforce nodes on the curves

• Automatically extracts curves and points from the surfaces

Needs smaller mesh size at

fillets



• Automatically segments all surfaces where curves either make a complete loop on the surface or span across the surface

• Turn Split surface at T-connections off to turn off segmenting


Build Topology – Segment Surfaces

Check off to disable segmenting

You can then delete any surfaces you don’t want

Build topology



• Set adequate tolerance!

– Example: some multiple (blue) edges. This indicates that more then two surfaces meet within the tolerance setting

– Turning on the surfaces reveals one surface is now missing.

– In this case, the tolerance (0.2) was set to greater than

the thickness (0.1). One of the surfaces was seen as a

“duplicate” within the tolerance and removed. UNDO

– You will need to exercise care not to damage your model with build topology

• Too small is safer but indicates more gaps

• Too big can alter the model in bad ways

– Rule of thumb: tolerance should be about 1/10th smallest foreseen mesh size or smallest feature that you wish to capture

– Build topology will delete duplicate geometry because its tolerance is zero


Tolerance setting 0.1



– New Part Name • Inherit Part: Default: new curves and

points will inherit the part names from surfaces they are extracted from

– Check off Inherit Part to type a new name or choose from the list

– Single curve cleanup • Merges single edge curves with a second

tolerance while resolving sliver surfaces (normally larger than base tolerance)

– Split Surface at T-connections • Resulting mesh will conform to common

edge even though the surface is not split into two separate surfaces

• Will also split a surface into separate surfaces if the curves form a closed loop or span across the surface

– Split Surface at Interior Curves • Surfaces trimmed along curves that

don’t span surface or form a closed loop

• Resulting mesh will conform to curve


Building Topology – Other Options



• Workshop -Fluid_Manifold

– Build topology

– Remove holes

– Project Point to Curve

– Delete entities

– Segment and close Curve

– Filter points


Workshop

• Watch the instructor demo – Mid surfacing

– Build topology

– Filtering curves and points

– Close multiple holes



Shell Meshing


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by Ing.Ivone Valle



• Usages of shell meshing: – Thin sheet solid modeling (FEA) – stamped parts

– 2D cross sectional analysis (CFD)

– Input for volume meshing (FEA/CFD) – Delaunay, Advancing Front, T-grid

– Filling a surface mesh is faster than tetra octree but requires well-connected geometry

• Procedure

– First need to decide mesh setup parameters

• Mesh method

– Algorithm used to create mesh • Mesh type

– quad/tri/mix • Mesh sizes

– Small enough to capture physics, important features

– Large enough to limit grid size (number of elements)

» Memory limitations

» Faster mesh/solver run

– Set mesh sizes on parts, surfaces, and/or curves

– Based on edge length

– Can have different types/methods set on different surfaces

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Introduction to Shell Meshing

by Ing.Ivone Valle



• Mesh Setup Icons

• Global Mesh Setup

– To change defaults globally for size, method and type

– For entire model – For Shells – For Volume – For Prism – To set periodicity

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Global Mesh Setup

• Global Mesh Size

– For entire model

– Scale factor

• Global setting by which many local settings are multiplied

• Good for scaling overall mesh

– Global Element Seed Size • Maximum possible element size in model

• Default size if don’t wish to set local sizes

– Curvature/Proximity Based Refinement

• Automatically creates smaller element size to better capture geometry

• Only for Patch Independent method and tetra octree

Mesh tab

by Ing.Ivone Valle



• Global Shell Meshing Parameters – From Global Mesh Setup tab – Set surface mesh parameters globally

• Defaults for the selected mesh method

– Methods • Autoblock • Patch dependent • Patch independent • Shrinkwrap • Delaunay

– Type • All Tri, Quad w/one tri, Quad dominant, All

quad – Options for different mesh methods – Global types and methods can be overridden by

• Surface Mesh Setup – Local settings

• Compute Mesh

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Global Shell Meshing Parameters

by Ing.Ivone Valle



• Quad layers grown from curves (e.g. rings around holes), use these 3 parameters: – Height: First layer quad height on curves

– Height ratio: growth ratio which determines the heights of each subsequent layer (multiplication factor)

– Num layers: Number of rings/inflation layers

• For quad layers, the minimum required to be set is height (for 1 layer) or numlayers (height = max. size)

• If done in the Part Mesh Setup spreadsheet you must toggle on Apply inflation parameters to curves

• Part Mesh Setup (pop up spread sheet) – Set mesh parameters on all entities within part – Max. size

• Multiplied by global Scale Factor = actual size

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Part Mesh Setup

Or set on individual

curves by Ing.Ivone Valle



Local Surface Mesh Setup

• Surface Mesh Setup

– Same parameters as part mesh setup but also includes:

• Mesh type

• Mesh method

– Select surfaces first from screen, set sizes/parameters and Apply

– Mesh method/type will override global shell mesh settings for selected surface(s)

– Will override Part Mesh Setup settings if set afterward

– Display mesh sizes

• Right mouse, select in Model tree on Surfaces > Tetra/Hexa Sizes

– One Icon appears for each surface

– Gives you a visual estimate of prescribed max. size and height

by Ing.Ivone Valle



Mesh Methods Algorithm used to create mesh

• Patch Dependent

– Based on loops of curves surrounding patches

– Best for capturing surface details and creating quad dominant mesh with good quality

• Patch Independent

– Robust octree algorithm

– Good for dirty geometry, ignoring small features, gaps, holes

• Autoblock – Based on 2D orthogonal blocks

– Best for mapped meshing, mesh follows contours of

geometry

• Shrinkwrap

– Automatic defeaturing

– Quick Cartesian algorithm

– Allows ignoring of larger features, gaps and holes

• Delauney (beta options)

– Allows for transition in mesh size

• Coarser towards surface interior

– Tri only

• Set in Global Mesh Setup or locally using Surface Mesh Setup

by Ing.Ivone Valle



Patch Dependent Mesher - Boundary option

• Boundary – Protect given line elements

• Keeps existing line elements which are smaller than the Ignore size

• Grayed out unless Respect line elements is on

– Smooth boundaries • Smoothes mesh boundaries after mesh

generation. Ignores curve mesh settings – Offset type

• Interior – Force mapping

• Forces mapped mesh on regular (4 sided) surfaces to desired degree (0-1)

• Adjusts the number of nodes on opposite sides (0.2 = change number nodes by 20%)

– Project to surfaces • Interior nodes project to surface rather

than interpolate position – Adapt mesh interior

• Allows transition to larger element size in the interior of the surface (uses surface max size )

Standard

Simple

Would require too many nodes increased from original setting based on force mapping setting by Ing.Ivone Valle



• Uses robust Octree method

– Volumetric tetra elements created around geometry

– Faces projected to surfaces

– Only surface mesh is retained

– Discussed in more detail in Volume Mesh lecture

• Mesh sizes defined on surfaces and curves

• Can walk over small details, thin gaps, small holes

– Relative to mesh size

• Nodes and edges don’t have to be lined up with surface edges

– Only lined up where curves exist

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Patch Independent

Volume around is

first meshed

Nearest nodes projected to surface and only surface mesh is left

Matches up with previously meshed

surfaces

by Ing.Ivone Valle



• Surface (2D) blocks are created automatically from each surface – Internal, blocks aren’t recognized or visible

– For further description of blocking, refer to Hexa chapter

• Blocks structurally connected – Conformal mesh between blocks and surfaces

• Structured blocks result from 4-sided surfaces – For regular or four-sided blocks, structured

(mapped) mesh follows contours of geometry

• Best for recognizing rounds or fillets

• Irregular (non-4 sided) or trimmed surface patches may be unstructured

Mesh sizes set on surfaces or curves

• Options

– Ignore size

– Mapped or free (unstructured as in patch dependent)

• Build Topology MUST be run beforehand

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Autoblock

by Ing.Ivone Valle



• Cartesian (rectilinear) method – Can ignore larger features, gaps, holes

• Cube faces partially projected to geometry

• Quickest method for creating surface mesh

• Can’t recognize sharp features – Currently in development phase

• Best for “wrapping” complicated geometry – Quick and dirty surface meshing of complex

geometries

• For “solid” models – Not recommended for thin sheet solids

• Options – No. of smooth iterations

• To improve grid quality

– Surface projection factor

• To fully project to original geometry (1.0), to not project at all (0.0), or partially (0.0 < factor < 1.0)

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Shrinkwrap

by Ing.Ivone Valle



• Once sizes, methods and types are set – ready to compute!

• Select Mesh > Compute Mesh > Surface Mesh Only – Most of the time can just select Compute at

bottom of panel which will create shell mesh for entire model (Input = All)

– Other options – Overwrite Surface Preset/Default Mesh

Type/Method • To quickly override global and local settings • Avoid going back to other Mesh Setup

menus to change parameters – Input

• Can mesh All (default – entire model) • Visible – only visibly displayed

surfaces/geometry • Part by Part

– Parts meshed separately – Mesh will be non-conformal between

parts • From Screen

– Select surfaces or a loop of curves to mesh from screen

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Compute Mesh

by Ing.Ivone Valle



Workshops

• Workshop - Wingbody – Global element size

– Mesh setup by parts

– Different mesh method on selected surfaces

– Curve parameters – dynamic method

– Deleting elements – flood fill

– Try harder setting – patch independent

– Respect line elements

– Remesh only selected surfaces


• Watch the instructor demo. – Build topology

– Set Curve Mesh Sizes

– Set Shell Meshing Parameters and Compute



Unstructured Volume Meshing


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by Ing.Ivone Valle


M4 – ICEM-CFD INTRODUCTION. 40 by Ing.Ivone Valle

Introduction to Volume Meshing

• To create 3D elements to fill volumetric domain

– Generally termed “unstructured” mesh

• Mainly tetra

– Full 3D analysis

• Where 2D approximations don’t tell the full story

– Internal/External flow simulation

– Structural solid modeling

– Thermal stress

– Electromagnetics

– Many more!



General Procedure • Load/create surface mesh

– As in Shell Meshing chapter

– For Delauney, Advancing Front, ANSYS TGrid, Hex-Dominant

• Either of these types run from geometry will automatically create surface mesh using global and local Shell Mesh settings without any user input/editing

• If in doubt, run Shell Mesh first, then from “existing mesh”

• Compute Mesh – Mesh > Compute Mesh >

Volume Mesh

• Define density regions (optional)

• Applying mesh size within volume where geometry doesn’t exist

• First decide global volume mesh parameters – Global Mesh Setup > Volume

Meshing Parameters

– Select Mesh Type

– Select Mesh Method for selected Type

– Set options for specific Methods

• Set mesh sizes – Globally

– Locally

• Part/Surface/Curve Mesh Setup

• For From geometry:

– Octree

– Cartesian

• Define volumetric region – Material points

– Typically for octree on complex models

– Multiple volumes possible

• Compute Prism (optional) – As separate process

– Also option to run automatically with tetra creation

by Ing.Ivone Valle



Body/Material Point

• Define Volumetric Domain

– Mainly used for tetra octree (optional) • Recommended for complex geometries

• Or multiple volumes

– Geometry -> Create Body

– Material Point

• Centroid of 2 points

– Select any two locations whose mid-point is within volume

– Preferred, because more robust than By Topology method • At specified point

– Define volume region at a “point” within volume – By Topology

• Defines volume region by set of closed surfaces

• Must first Build Diagnostic Topology to determine connectivity

– Will fail if gaps/holes in body • Entire model

– Automatically define all volumes • Selected surfaces

– User selects surfaces that form a closed volume by Ing.Ivone Valle



Mesh Types

• Tetra/mixed

– Most used type

1. Pure tetra

2. Tetra with prism layers • Prism is discussed in a later presentation

• Prisms from tri surface mesh

• Hexas from quad surface mesh

• Pyramids to cap off any quad faces from prism sides or hex prism layers

3. Tetra with hex core

• Can also have prisms

• Hexa filling majority of volume

• Tetra (from Delaunay algorithm) used to fill between surface mesh or top of prism layers and hex core

• Pyramids to make conformal between tetra and hexa quad faces

– A Hybrid mesh can be created by merging a tetra mesh with a structured hex mesh

Pure tetra

Tetra/Prism

Tetra/Prism/Hexcore

by Ing.Ivone Valle



Mesh Types - Continued

• Hexa-Dominant

– Uses existing quad mesh

– Good quality hex near surface

– Typically poor interior quality

– Typically not suited for CFD, but good enough for static structural analysis

– Not covered in detail here

• Cartesian

– Methods available in Cartesian

– Body fitted

– Staircase

– Hexa-Core

– Automatic pure Hexa

– Rectilinear mesh

– Fastest method for creating volume mesh

– Not covered in detail here

Poor quality interior

by Ing.Ivone Valle



Mesh Methods - Octree

• Type - Tetra/Mixed

– Method - Robust (Octree)

• Same as Shell Meshing > Patch Independent

– Except volumetric tetras are kept

• Good choice for complex and/or dirty geometry

• Good if you don’t want to spend too much time with geometry cleanup

• Good if you don’t want to spend too much time with detailed shell meshing

• Good if you don’t want to spend time defeaturing geometry

• Just set appropriate mesh sizes on geometry

– Global sizes (max size, curvature/proximity based)

– By parts (Part mesh setup spreadsheet)

– Surfaces

– Curves

– Review Shell Meshing chapter

•Part/Surface/Curve Mesh Setup

by Ing.Ivone Valle



Octree Method Characteristics

Geometry

Mesh detail

Mesh

• Octree process

– Volume first generated independent of surface model

– Tetras divided near regions where sizes are set smaller

– Nodes are projected to model surfaces, curves and points

– Surface mesh is created when outside tetras are cut away

• Resulting mesh is independent of the underlying arrangement of surfaces

– Not all surface edges need to be captured!

– Surfaces edges only captured if curve exists there

• Delete curves to ignore hard edges

• Or filter points/curves under Build Diagnostic Topology

Sliver ignored

by Ing.Ivone Valle



Octree Tetra Process

The Octree process creates an initial mesh of “Maximum size” elements which fills a bounding region completely encapsulating the geometry.

Initial conditions

– Geometry including surfaces, curves and points (from Build Topology)

– Mesh size set globally and/or on surfaces/curves/densities

– Optional material point could also be created

– All saved in the tetin file

by Ing.Ivone Valle



Tetra Process, Cont’d • Smooth

– Octree mesh is initially composed of regular right angle tetras

– Since nodes are simply moved to nearest geometry, smoothing is necessary

– Smoother can be set to run automatically to improve quality in Global Mesh Setup

– Or run afterwards: Edit Mesh -> Smooth Mesh Globally

by Ing.Ivone Valle



Geometry Requirements for Octree Tetra

• Tetra requires a reasonably enclosed surface model

– Run Build Diagnostic Topology to find gaps/holes

– Octree can tolerate gaps smaller then the local element size (approximately 1/10th the element size or less)

• Keep points and curves at key features and hard edges

– Filter curves and points by angle with Build Diagnostic Topology

• Create Material points to define volumes

– Will material point(s) if none exists (named CREATED_MATERIAL#)

• Set Global, Part, Surface, Curve Size Parameters

– Similar to Shell Meshing section Missing inlet surface

Geometry Repair tools quickly locate and fix

these problems.

Hole highlighted by yellow single edge curve

by Ing.Ivone Valle



Using Points and Curves with Tetra Octree

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 curves and surfaces

• Curves and points affect which features are captured by the mesh! • Build Topology easily creates the necessary points and curves easily with filter by angle

by Ing.Ivone Valle



Tetra Octree - Options

• Global Mesh Setup > Volume Meshing parameters – Run as batch process

• Runs as a separate process (tetra.exe). GUI will stay interactive. – Fast Transition

• Allows for a slightly faster transition in element size from finer to coarser

• Reduced element count – Edge Criterion

• Split elements at a factor greater than set value to better capture geometry

– Define Thin cuts • Tool for resolving thin gaps, sharp angles • User selects pairs of opposing parts • Resolves elements jumping from one side to another

– Smooth • Automatically smoothes after grid generation process

– Coarsen – Fix Non-manifold

• Automatically tries to fix elements that jump from surface to another surface

– For a more detailed description go to Help > Help Topics > Help Manual > Mesh > Global Mesh Setup > Volume Meshing Parameters > Tetra/Mixed > Robust (Octree)

by Ing.Ivone Valle



Compute Mesh – Tetra Octree

• Run options: Compute Mesh > Volume Meshing Parameters – Create Prism Layers

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

– Create Hexa-Core • Will retain tri surface mesh (or tri and prisms), throw

away tetra mesh and regenerate volume • Fill volume interior with Cartesian hexas • Cap off hexas with pyramids • Map tetra to tri or top prism face with Delaunay filling

algorithm – Input

• Select Geometry – All, Visible – Part by Part

•Meshes each part separately •Mesh not conformal between parts

– From file •Select tetin file (save memory by not loading it)

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

existing surface mesh by Ing.Ivone Valle



Curvature/Proximity Based Refinement

• Curvature/Proximity Based Refinement

– Octree and Patch Independent Surface Mesher

– 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 Factor and is the minimum size allowed for the automatic subdivision

– Used primarily to avoid setting up meshing parameters specifically for individual entities thus allowing the geometry to determine the mesh size

– Convenient for geometry with many fillets of varying curvature

Prescribed size is adequate here

Prescribed element size: Surface/Curve Max. Element Size times Scale Factor

Auto subdivision at tighter radius of curvature

Min Size Limit: multiplied by Scale Factor = global minimum

by Ing.Ivone Valle



Curvature Based Refinement

• Refinement

– Approximate number of elements along curvature if extrapolated to 360o

– To avoid subdivision always to global minimum which would otherwise result in too many elements

– Subdivision will stop once number of elements along curvature is reached

– Approx max number of edges along 360o

– Won’t exceed global minimum set by min size limit value

• Example – Specified refinement achieved with

larger elements

– Global minimum (min size limit) not realized, not necessary to capture curvature

Prescribed size Min size limit

Refinement = 12

by Ing.Ivone Valle



Proximity Based Refinement

• Elements in Gap

– Number of cells desired in narrow gaps

– The default is one element in the gap to just model the feature

• Can specify more than one element in gaps

– Will not make elements smaller than the Min size limit

• Example

– Only one element in gap

– Can’t go smaller than Min size limit

– Have to set smaller Min size limit to allow the 5 elements in gap

Prescribed size Min size limit

Cells in Gap = 5

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

Cells in Gap = 5

by Ing.Ivone Valle



Edge Criterion • Edge criterion

– For tetra octree only

– A number 0 – 1

– 0.2 (the default) means if more than 20% of an edge crosses a surface or curve, then split the edge

– Has an effect similar to globally applying a thin cut

– Smaller numbers will cause more splitting. The closest node will be projected to the surface, modeling smaller features

– Use prudently. Too small a number can result in random clusters of refined mesh

Split edge and project new node to surface

> 0.2

by Ing.Ivone Valle



Tetra Octree Workflow Recommendation

1. Check geometry accuracy and integrity

– Visually scan surfaces to see if curvature is too choppy or if gaps are large

– Can use build diagnostic topology with no “filtering” to indicate gaps and holes by the yellow curves

• Must use a reasonable tolerance, indicating a maximum allowable gap

– Large gaps also show after running tetra if leakage is reported

2. Move surfaces into intended part names

– Possibly move curves and points later, but surfaces are most important

3. Obtain necessary curves and points

– Needed for nodes to align at hard corners

– Build topology with “filtering” feature angle (30 - 45) or extract curves from surfaces and points from curves, then delete excess

– Scan model and delete curves/points at more gradual angles where surfaces meet

4. Create material point(s)

– Defines volumes to keep mesh (all regions initially meshed)

– Material point must be INSIDE volume, not on surfaces

– ORFN point is a reserved name which instructs to cut away that region

by Ing.Ivone Valle



Tetra Octree Workflow Recommendation

5. Set mesh sizes

‒ Globally first

– Maximum size

– Curvature/proximity refinement

‒ Locally

– Part mesh setup spreadsheet

– Surface mesh setup

– Curve Mesh setup

– Densities

6. Compute mesh

7. Smooth

‒ Approximately 0.2 minimum quality for tetra

8. Check errors and possible problems

‒ See ICEM CFD help for descriptions of each check

‒ Also discussed in Mesh Preparation presentation

‒ Edit mesh directly to fix problems or change a mesh size and remesh

by Ing.Ivone Valle



Mesh Methods - Delaunay

• Type - Tetra/Mixed – Method - Quick (Delauney)

• Start from a good quality, closed surface mesh

– Can be quad and tri elements

– From Shell Mesh

– From Octree

– From imported surface mesh

• Setup Options:

– Delaunay Scheme

• Standard: Delaunay scheme with a skewness-based refinement

• TGlib: TGrid Delaunay volume grid generation algorithm that utilizes a more gradual transition rate near the surface and faster towards the interior

– Use AF: TGrid Advancing Front Delaunay algorithm which has smoother transitions than the pure Delaunay algorithm.

– Memory Scaling Factor: To allocate more memory than originally

– Spacing Scaling Factor: Growth ratio from surface (1 – 1.5 typically)

– Fill holes in volume mesh: Use to fill holes/voids in existing volume mesh . E.g. if bad quality region is deleted

– Mesh internal domains: For multiple sets of closed volumes in one model

– Flood fill after completion: For multiple volumes – Will assign tetras within closed volume to Part designated by Body or Material Point

– Verbose output: For troubleshooting

Initially distributes nodes so as the centroid of any tetra is outside the circumsphere of any neighboring tetra

by Ing.Ivone Valle



Mesh Methods – Advancing Front

• Type - Tetra/Mixed – Method - Smooth (Advancing Front)

• Same as Quick (Delauney) but

• Uses advancing front method that marches tetras from surface into interior

• Algorithm from GE/CFX

• Results in more gradual change in element size – “Better” but finer mesh, more elements than Delaunay

– Elements grow slowly for first few layers from surface, then growth rate increases into volume more

– Input surface mesh has to be of fairly high quality

• Setup Options: Do Proximity Checking

– Check to properly fill small gaps

– Longer run time

• Can create pyramids from quads

– Quads need to be a 10 aspect ratio or less

– Delaunay can handle much higher quad aspect ratios

• Respects densities

by Ing.Ivone Valle



Compute Mesh – Delaunay, Adv. Front, TGrid

• Run Options: – Similar options as octree except cannot mesh to partial geometry and

partial mesh (option: Use existing mesh parts)

– Create Prism Layers available for both

– Hexa-Core not available for Advancing Front

– Volume Part Name • For newly created tetras

• Can choose Inherited to use material points

– Input

• All Geometry

– Will run shell mesh first with no user input/editing

– Using parameters from Model/Part/Surface/Curve Mesh Setup

– Review Shell Meshing chapter

– If doubtful as to shell mesh quality, run Shell Mesh first, then use Existing Mesh Input method

• Existing Mesh

– Most common method. Surfaces already meshed

• Part by Part

– Meshes each part separately. Nodes are not connected

• From File

– Saves memory. Surface mesh does not need to be loaded

by Ing.Ivone Valle



Comparison O

ctre

e

Del

aun

ey

Ad

v.fr

on

t

Note the 2:1 growth ratio and square volume elements of the octree method

The 3 filling methods (Delaunay, Adv Front, Tgrid) allow a smoother growth (< 2:1) than octree

by Ing.Ivone Valle



Workshop

• Workshop – Flux_viewer – Build Topology

– Set parameters

– Different size on surfaces

– Global Mesh Setup

– Active curvature/proximity




Prism Meshing


by Ing.Ivone Valle

64



Prism Meshing

• Inflation layers – To better simulate boundary layer effects

– Mesh orthogonal to surface with faces perpendicular to boundary layer flow direction

• Procedure – Set Global Prism Parameters

– Select Parts to grow layers from

• Typically wall boundaries and holes – Set Local Parameters for each part

• Local overrides global

• Zero or blank entries will defer to global settings – Run mesher

• From existing mesh

– Extrude into tetra/hexa mesh

– Extrude from surface tri mesh, then fill volumes

• Run automatically during Volume Mesh creation

by Ing.Ivone Valle 65



Prism - Global Parameters

• Global Prism Parameters – Growth law

• exponential: height = h(r)(n-1) [n is layer #]

• linear: height = h(1+(n-1)(r-1))

• wb-exponential: height = h*exp((r-1)(n-1)) – Initial height of first layer – (h) in formulae above

• Auto calculated if not specified

• Based on factor of edge length of base triangle/quad

• Height determined so that top layer volume is slightly less than that of tetra/hex just above it

– Number of layers (n)

– Height ratio (r) – multiplied by 1st layer to determine 2nd layer

– Total height - of all layers

– Usually specify 3 of the above 4 parameters

• Compute params will calculate the remaining parameter (total height usually left blank)

– Or specify only Height ratio and Number of layers for auto calculation of initial height

– Individual height/ratio/layers set on surfaces and curves will override these global defaults if set

Other global parameters explained later

Initial height (h)

Total height

Height ratio (r)




Growth Law Comparison

• The growth rate of Wb-exponential is slightly greater than exponential

• The growth rate of exponential is much greater than linear

Linear Exponential Wb-Exponential




Setting Prism Parameters on Surface Parts

• Prism extrusion areas defined by the parts – Mesh > Part Mesh Setup – Toggle on Prism for parts where inflation layers are desired

• Surface mesh (tri/quad) gets extruded into prisms

– Set Height, Height Ratio, Num Layers

• Will use global defaults if not set or zero

Applying these settings causes these parameters to be applied to each individual surface within each part If Apply inflation parameters to curves is toggled on, they will also be set on each curve within each part




Setting Prism Parameters on Volume Parts

•Normally toggle prism on only for parts that contain surfaces (becomes surface mesh)

•Can also toggle on prism for parts that contain material points (becomes volume mesh)

– For interior surface mesh, this defines the allowable volumes for extrusion

– Selecting no volume parts has the same result as selecting all volume parts

Only one volume part

selected

Both or no volume parts selected

Edge of Interior surface

VOL1

VOL2

VOL1

VOL2




Setting Prism Parameters on Surfaces

• Mesh > Surface Mesh Setup

• You can specify different local height and ratio on any selected surface without moving the surface to a new part (num. layers ignored)

• Usually set height and/or ratio smaller on specific surfaces to avoid collision

Height = 0.2 Height on part = 0.4

Collisions occurred when the height was 0.4 on all surfaces

No collisions after setting height to 0.2 on selected surfaces by Ing.Ivone Valle 70



Setting Prism Parameters on Curves

• Mesh > Curve Mesh Setup • You can get Prism to transition linearly across a

surface by not setting a height (height = 0) on the surface, but instead set a different height on each curve on the opposite sides of the prism surface

• Height ratio and Num. of layers have no affect on prism for curve settings

• Global height must also be zero

Height = 0.01

Height = 0.003

Height = 0 on surface




Run Prism • Can run separately

– Mesh > Compute Mesh > Prism Mesh

– The Select Parts for Prism Layer button pops up the same menu as the Part Mesh Setup, except non-prism related columns aren’t displayed

– Input

• Existing Mesh

• From File (saves memory by not not loading mesh)

• Or run automatically linked into volume mesh

– Toggle on Create Prism Layers when tetra meshing

– Not advisable if this is the first mesh for a particular geometry

– Must be confident about setup parameters and sizing

– Running prism separately allows you to smooth and error-check the tri or tetra mesh first.




Smooth Tetra/Prism Transition

• Leave initial height as “0” globally and locally – This causes the initial height to float in order to

reduce the volume change between the last prism and adjacent tetra.

Initial height specified

Initial height = 0




Prism – Global Parameters Continued

• Quality Control Options

‒ Fix marching direction – Maintains normal from surface

– Can cause intersections with other mesh

‒ Min prism quality – Either re-smooth directionally or cap/replace with pyramids if quality not met

(minimum allowed = 1x10-6)

‒ Ortho weight – Weighting factor for node movement from 0 - improving triangle quality, to 1 -

improving prism orthogonality

‒ Fillet ratio

‒ Max prism angle

‒ Max height over base

‒ Prism height limit factor

‒ Ratio multiplier (m) – For varying exponential growth: height = h(r)(n-1) (m)(n-1)

See next slides




Prism Options – Fillet Ratio

– Blends prism grid lines around sharp corners • 0 = no fillet

• 1 = fillet radius equals last prism height

– Improves angles further away from the corner

– Orients prisms more in direction of flow

– If meshing tight spaces with tight curves (less than 60o), may not have space for a fillet radius

Fillet Ratio = 0.0 Fillet Ratio = 0.5 Fillet Ratio = 1.0

r h

Fillet Ratio = r/h




Prism Options – Max Prism Angle

– Controls prism layer growth around bends or adhering to adjacent surfaces

– If the Max (internal) Prism Angle is not met, the prism layers will end and be capped off with pyramids in those locations

– Usually set in the 120o to 179o range

– Experience pays off here. If extruding from one part and not its neighbor, and the angle between the two surfaces is greater than the Max Prism Angle, the prisms will detach and be capped off with pyramids. This prevents bending the prisms that might create lower-quality internal angles. However, the pyramids are usually of lower quality, too.

– It’s usually better to run prism along adjacent surfaces until it can meet at a smaller angle, leaving quad faces. Pyramids will be avoided.

This may be better

Original mesh Max prism angle = 180o

Max prism angle = 140o

160 o

Pyramids




Prism Options – Max Prism Angle - Continued

• A high (up to 180o) Max Prism Angle keeps the prism layers connected around tight bends. – Set this at 180 to prevent pyramids wherever possible

Max Prism Angle = 140 Max Prism Angle = 180




Prism Options – Max Height Over Base

– Restricts prism aspect ratio

– Prism layers stop growing in regions where prism aspect ratio would exceed specified value • Number of prism layers would not be preserved locally

– Mesh is made conformal with pyramids at prism boundaries

– Acceptable values vary widely (typically 0.5 – 8)

Height (h)

Base (b)

h/b

Largest height over smallest base length

Max Height Over Base = 1.0 Max Height Over Base not set

Pyramids




Prism Options – Prism Height Limit Factor

– Restricts prism aspect ratio • Prism height will not expand once this factor is met

– Uses the same height over base factor as the previous metric except prism layers are not capped off with pyramids

– Preserves the specified number of prism layers

– “Pulls in” the prism layers to reduce heights

– Will fail if sizes of adjacent elements differ by more than a factor of 2

– Acceptable values vary widely (typically 0.5 – 8)

Limit factor = 0.5 Limit factor not set

Height (h)

Base (b)

h/b

Largest height over smallest base length




Prism Options-Part Control

• New volume part – Can specify new Part for prism elements

• Must specify if extruding from surface-only mesh

• If extruding into volume mesh, prism will inherit tetra volume part if this field is left blank

• Side part – For quad faces on prism side boundary

• Top part – For tri faces capping off top of last prism layer

• Extrude into orphan region – Extrude prisms away from existing volume, not

into it

– Must specify new volume, side and top part, or they’ll be in ORFN

Leaving these parts blank will inherit the names from the current mesh




Prism Options - Smoothing

• Prepares tri/tetra for best prism quality

– Set surface/volume steps to 0 if only extruding one layer or if tri/tetra mesh is already smoothed

• Otherwise defaults adequate

• Value depends on model/user experience

– Triangle quality type

• Laplace typically best for eventual prism quality

• Other types may be better when marching directions condense at inside corners

– Max directional smoothing steps • Redefines extrusion direction based on initial prism

quality

• internally calculated for each layer

– First layer smoothing steps

• Used for the first layer only when “Fix marching direction” is on




Advanced Prism Meshing Parameters

• These are best left at default for most models

• Auto Reduction

– Achieves layer compression

– Produces very thin layers where otherwise collisions would result in pyramids

– Must redistribute prism layer afterward

• Intermediate write

– Writes “prism.uns” after each layer in case the process crashes

– Open prism.uns and inspect for problems

• Blayer 2d

– For running prism on a 2D model

– Select curve parts instead of surface parts for inflation

– Must select surface parts in place of volume parts

• Stair Step

– Controls gradual or immediate reduction where the number of layers must be reduced

Stair step

No stair step




Advanced Parameters - Interpolate Heights

• Affected by any Parts with height set to zero when the global height is also zero.

– The default is OFF, which causes these parts to float the height to get a smooth transition at the last prism layer, as shown in previous slide 11

– With Interpolate Heights ON, the height will transition across the part to match the heights set on parts at its ends

Interpolate heights OFF

Interpolate heights ON

Height=0

Height=0

Height = 0.01

Height =0.002

Global height must also be zero




Prism Parameters File

• Read a Prism Parameters File

– To set all prism values from a prism settings file (*.prism_params)

– Written to the working directory every time prism is run

Typical prism parameters file




Checking for Problems

• Pyramids form where prism has problems

– Wherever prism layers stop forming, the sides are capped off with pyramids

– Collisions are the most common cause, and can make prism.exe crash if severe enough. Need to locally reduce heights, ratios, or num layers.

– Other common causes are the min prism quality or max prism angle is violated, or a geometric formation cannot have continuous prisms (scissors junction), or other problems.

• The first thing to do after the prism process is turn on only pyramids

• Inspect for what is wrong. View a cut plane through these regions

Pyramids

Cut plane

2 layers missing because collisions




Splitting Prism Layers

– If many prism layers are desired, it is faster, but less robust – to create a few “fat” layers and then split them with mesh editing

– Edit Mesh > Split Mesh > Split Prisms – Fix ratio: The layer is split such that its

resulting layers employ the given growth ratio (height is free variable to adjust)

– Fix initial height: The layer is split such that its first sub-layer is of the given height (ratio is free variable)

– Specify the number of layers to result from each existing layer

– Can split specified or all existing layers




Workshops

• Workshop – Tetra Meshing - Valve – Build Topology

– Create Parts

– Global Mesh Size

– Surface Mesh Size

– Curve Mesh Size


• Workshop – airfoil – Build Topology

– Set Curve Mesh Sizes

– Set Shell Meshing

– Modify Curve Mesh Sizes


Documents

INTRODUCTION to ICEM-CFD