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ICEM Mesh for CFD Analysis
2
Index
1.
Introduction to ICEM
2. Geometry Handling
3. Shell Meshing
4. Volume Meshing
5. Prism Meshing
6. Mesh Preparation Before Output to Solver
7. Output to Solver
8.
ICEM CFD Hexa
3
1. Introduction to ICEM
What is a Mesh?
4
• Mesh
– Volume comprised of elements used to d iscretize a domain for numerical solutio n
• Structural
• Fluid dynamics
• Electromagnetics
• Other
– Elements
• 0D – Node element
– Point mass
– Constraint, load location
• 1D – Lines
– Bars, beams, rods, springs
– 2D mesh boundary
• 2D – Surface/Shell
– Quads
– Tris
– Thin sheet modeling
– 2D volume
– 3D mesh boundary
• 3D - Volume
– Tetra
– Pyramid
– Penta (prism)
– Hexa
– Solid modeling
– 3D fluid modeling
• Formats
– Unstructured
– Block Structured
– Nodes
• Point locations of element corners
5
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
– Hybrid meshing (merging, multi-zone)
•
Advanced mesh editing
•
Solver Setup
•
Output to 100+ Solvers
•
Scripting … and much more…
6
Utility
Icons
GUI and Layout
Utility Menu
Selection Toolbar
Model
Tree
Data Entry
Panel
Function Tabs
Message
Window
Histogram
Window
Display Triad
7
File and Directory Structure
• 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
– 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)
.fbc
.tin
.par
.uns
.rpl
.prj
.blk
.jrf
8
Mouse Usage
• ‘Dynamic’ viewing mode (click and drag)
– left:
– middle:
– right: rotate (about a point) translate
– Wheel
• Selection mode (click) zoom (up-down) screen Z-axis rotation (sideways) zoom
– left:
– middle:
– right: select (click and drag for box select) apply operation unselect last selection
•
F9
toggles the mouse control to Dynamic mode while in Select mode
–
Toggle Dynamics
button also
– does this
• Spaceball allows for dynamic motion even while in select mode
9
Utility Menus
File Menu
(file i/o)
Edit Menu
View Menu
Info Menu
Settings
Menu
(preferences)
Help Menu
10
File menu
• To open/save/close
– Projects
• Will open/save/close all as sociated files including
• Geometry (*.tin)
• Mesh (*.uns)
• Attributes… (*.fbc, *.atr)
– All file types can be opened/sa ved/closed independently
• Also to
– Import/Export Geometry/Mesh
– Invoke scripting
• Exit
Save frequently!
•
Several common functions are duplicated as utility icons:
Open Project Save Project
Open/Save/Close
Geometry
Open/Save/Close
Mesh
Open/Save/Close
Blocking
11
Other Commonly Used Utilities
• Edit > Undo/Redo
Also here
• View
– Fit
•
Fit visible entities into screen
– Box Zoom
– Standard views
•
Top, Bottom, Left, etc.
•
Can also select X, Y, Z axis of display triad in lo wer right hand corner of main view screen to ori ent to standard views, e.g. selecting “X” will orie nt “right”
•
Isometric – select blue dot within triad
• Measure
– Distance
– Angle
– Location
– Local Coordinate System
• Used by:
• Select location
• Measuring
• Node/point movement/creation
• Alignment
• Loads
• Transformation
– Surface display
• Wireframe
• Solid
• Transparent
Help
• Menu Driven
– Searchable
– Includes tutorials
– Programmers guide (for ICEM
CFD/Tcl scripting procedures)
• Hyper-link to specific topic
12
•
Bubble explanation with cursor positioning
Common Function Tabs
Geometry
Mesh
Blocking
Create/Modify geometry
Set mesh sizes, types and methods
Set global mesh options
Auto create Shell, Volume, Prism meshes
Initialize blocking
Split/modify blocks
Generate structured hexa mesh
Edit Mesh
Check errors/problems, Smooth, Refine/Coarsen,
Merge, repair mesh, Transform, etc.
Set Boundary Conditions and Parameters
Write mesh for 100+ solvers.
13
Output
14
Structural Function Tabs
• Only available when solver is set to Abaqus, Ansys, Autodyn, LS
-Dyna, or Nastran
•
Settings->Product
must also be set to an FEA version
Properties
Create, read, write out material properties
Apply to geometry/elements
Constraints
Set constraints, displacements, define contacts, initial velocity, rigid walls
Set force, pressure and temperature loads
Loads
Solve options
Set parameters, attributes, create subcases, write out input file, run solver
15
Selection Toolbar
•
During select mode, popup selection toolbar appears
– Some tools are common to all, others are contextual
– Linked to select mode hotkeys
– Filtering of entities and mass-selection methods
Geometry
Polygon
Select By Part
Select all
Cancel
Only visible
Flood fill to Curve
Flood fill to Angle
Entity Filter
Mesh attached to Geometry
Mesh
Blocks
Toggle Dynamic
Mode (F9)
Circle
By Subset
Entire/Partial toggle
Faceted
Geometry
Segments
Set Flood Fill angle
All Shells
Toggle between mesh and geometry
From
Corners
In between segments
16
Model Tree
• To toggle on/off various sections of the model
• Main Categories are:
– Geometry, Mesh, Blocking, Parts
– Local Coord Systems, Element Properties,
Connectors, Displacements, Loads and Ma terial Properties
• Toggle check boxes to blank/unblank
– Blanked/inactive
– Visible/active
– Partially visible/active: some sub members turned on, some turned off
• Click on plus sign to expand tree
– Expose sub members
• Right mouse click for display options
17
Model Tree: Parts
•
Parts
– Grouping of mesh, geometry, and blocking entities
• Based on boundary condition/property
• Based on mesh size (can set mesh size by part)
• Based on material property
• Just to partition large model
– Select to blank/unblank all entities within part
– Color coded: Part name matches entity screen display
–
Right Mouse Button
on
Parts
to access:
•
Create Part
•
Create Assembly
•
Delete Empty Parts
•
Etc.
•
RMB
on specific part names allows options to modify or delete th e parts
•
Properties
are shown as a sub branch of the part
– Double Left Click or RMB > Modify to modify element pro perties
Workflow
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
18
General Order of Workflow
Accessing from Workbench
• Ansys ICEM CFD 14.0 is not fully linked inside Workbench
– Export files from Mechanical Model (Simulation) or Meshing Application to open in ICEM CFD
19
• Some ICEM CFD capabilities have been integrated into the Meshing
Application
– Tetra octree (patch independent)
– 3D blocking fill (Multizone)
– Autoblock (2D, uniform quad)
– Body fitted cartesian
20
Workbench Interactive Link
• Ansys ICEM CFD can be accessed from Workbench from certain mesh methods
• Insert a meshing method
– MultiZone
– Patch Independent tetrahedrons
• Set
Write ICEM CFD Files
to
Interactive
•
Generate mesh
• Edit or remesh within ICEM CFD,
save project, then exit ICEM CFD
– Don’t edit geometry in ICEM CFD
21
2.
Geometry Handling
Geometry handling
ANSYS ICEM CFD was designed to mainly import geometry, not create complicated geometries, although many geometry tools are provided
22
An accurate solution reflects the underlying geometry. To get such, ICEM CFD provides:
Geometry import
– From CAD package
– 3 rd 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
This Jet engine model was built solely with
ICEM CFD geometry tools
23
Geometry Import
CAD from just about any source
• Workbench Readers – for most CAD imports
– Anything that Workbench can import can also be imported into I
CEM CFD using Workbench readers
– Requires a Workbench installation!
• 3 rd
-party import
– ACIS (.sat)
– DWG/DXF
•
Parasolid
•
STEP/IGES
•
GEMS
• Direct CAD Interfaces
– Legacy interfaces which are not updated. Use
Workbench Reader s
instead for current CAD versions
– Set up ICEMCFD/AI*E meshing requirements within CAD envir onment
• Saved within CAD part for parametric geometry changes
– Directly write out ICEM formatted geometry (tetin file)
• No 3 rd party exchange (clean!)
– ProE, Unigraphics, Solidworks, Catia V4, IDEAS (IDI)
– ProE, UG, and Solidworks imports require CAD libraries; CAD s oftware and licensing must be available
24
Geometry Import - other so urces
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 coll ection of facets (triangulated surfaces).
• Faceted Data
– Nastran
– Patran
– STL (most common)
– VRML
– Other solver formats (indirectly from mesh conversion)
• Formatted Point Data
– Auto curve/surface creation from regular table of points
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
• 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
25
Tri tolerance = 0.001
Geometry Creation Tools
First 3 icons to create geometry
•
Screen Select
•
Explicit Coordinates
•
Base Point and Delta
•
Center of 3 Points/Arc
•
Based on 2 Locations
•
Curve Ends
•
Curve-Curve Intersect ion
•
Parameter along a Cu rve
•
Project Point to Curve
•
Project Point to Surfa ce
•
From Points
•
Arc Through 3 Points
•
•
•
•
•
•
•
•
•
•
Arc from Center Point/2 Points on Plane
•
From Curves
Surface Parameter
•
Curve Driven
Surface-Surface Intersection
•
Sweep Surface
Project Curve on Surface
•
Surface of Revolution
Segment Curve
Concatenate Curves
•
Loft Surface Over
Several Curves
Surface Boundary Extraction
•
Offset Surface
Modify Curves
•
Midsurface
Create Midline
•
Segment/Trim Surface
Create Section Curves
•
Merge/Reapproximate
Surface
26
•
Untrim Surface
•
Curtain Surface
•
Extend Surface
•
Geometry
Simplification
–
Convex Hull
–
Cartesian
Shrinkwr ap
•
Create Std
Geometry
–
Sphere
–
Box
–
Cylinder
–
Plane
–
Disc
–
Trim normal to curve
27
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
Faceted Geometry Handling
Create/Modify Faceted
28
•
Convert from Bspline
•
Create Curve
•
Move nodes
•
Merge nodes
•
Create segment
•
Delete segment
•
Split segment
•
Restrict segments
•
Move to new curve
•
Move to existing curve
•
•
Convert from B-spline
•
Create Triangles
Coarsen Surface
•
Delete Triangles
•
Create new Surface
•
Split Triangles
•
Merge Edges
•
Split Edges
•
Restrict Triangles
•
Delete Triangles
•
Swap Edges
•
Move Nodes
•
Move to new Surface
•
Merge Nodes
•
Move to new Surface
•
Merge Surfaces
•
Align Edge to Curve
•
Close Faceted Holes
•
Trim by Screen Loop
•
Trim by Surface Loop
•
Repair Surface
•
Create Curve
Facetted (triangulated) surfaces
Geometry Handling
Repair Geometry
Transformation Tools
29
•
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
•
Delete
•
Points
•
Curves
•
Surfaces
•
Bodies
•
Any Entity
Build topology with filtering
30
Building Topology – Determine Connectivity
•
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 reasonably to ignore small gaps, but not ignore le akage (tetra octree) or remove important features
• Default is 1/2500 th of the diagonal of the bounding box
• Connectivity is set up between surface edges that meet within the to lerance
• Filtering should be off when using to determine connectivity
Edge 1
Edge 2
Tolerance
31
Building Topology – Color Coding
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
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
Red curves indicate that surfaces meet within the tolerance setting
Build Topology
Build Topology – Extract Curves and Points
•
Automatically extracts curves and points from the surfaces
–
Filter by angle
(default 30 degrees)
•
Filter Points:
Points between two curves whose tangency is belo w the feature angle will be “filtered out” (made dormant)
•
Filter Curves:
Curves between two surfaces whose tangency is bel ow the feature angle will be “filtered out” (made dormant)
No filtering
Tetra octree and patch dependent surface mesher enforce nodes on the curves
32
Filtering
Needs smaller mesh size at fillets
Build Topology – Segment Surfaces
•
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
You can then delete any surfaces you don’t want
Build topology
33
Check off to disable segmenting
34
Tolerance setting
• Set adequate tolerance!
– Example: some multiple (blue) edges. This indicates tha t more then two surfaces meet within the tolerance setti ng
– Turning on the surfaces reveals one surface is now missi ng.
– In this case, the tolerance (0.2) was set to greater than th e thickness (0.1). One of the surfaces was seen as a “dup licate” within the tolerance and removed. UNDO
–
You will need to exercise care not to damage your model with buil d 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 1/10 th smalle st foreseen mesh size or smallest feature that you wish t o capture
–
Build topology will delete duplicate geometry because its tolerance is zero
0.1
35
Building Topology – Other Options
–
New Part Name
•
Inherit Part:
Default: new curves and points will i nherit the part names from surfaces they are ext racted from
– Check off
Inherit Part
to type a new name o r choose from the list
–
Single curve cleanup
• Merges single edge curves with a second toleran ce while resolving sliver surfaces (normally large r than base tolerance)
–
Split Surface at T-connections
• Resulting mesh will conform to common edge e ven though the surface is not split into two sepa rate surfaces
• Will also split a surface into separate surfaces if the curves form a closed loop or span across th e surface
–
Split Surface at Interior Curves
• Surfaces trimmed along curves that don’t span s urface or form a closed loop
• Resulting mesh will conform to curve
36
Building Topology – Other Options
– Method
•
All parts
, default method
•
Only visible parts
– Build topology is only run on active
Parts
in the model tree
– Inactive Parts are not affected
•
Selection
– Build topology on one or more surface entities
– Part by part
• Build topology is run on one part at a time
• Use with assemblies to keep parts separate
• Otherwise build topology may fix gaps, create T-connections or rem ove duplicates across Parts
– Delete unattached curves and points
• Removes unattached curves (green) and points after running build t opology
• Easy clean-up of unwanted curves/points
• Users may, however, wish to keep these curves/points for constructi on purposes (turn option off)
37
3. Shell Meshing
38
Introduction to Shell Meshing
• 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 reduce 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
39
•
•
Global Mesh Setup
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
• Parameters relative to scale factor
– Max size
– Min size limit
– Max deviation
Mesh
tab
•
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
40
Global Shell Meshing Parameters
•
Shell Mesh Setup
– 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 methods
– Global types and methods can be overridden by
•
Surface Mesh Setup
– Local settings
•
Compute Mesh
•
Part Mesh Setup
Part Mesh Setup
(pop up spread sheet)
– Set mesh parameters on all entities within part
–
Max. size
• Multiplied by global
Scale Factor
= actual size
• 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
–
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
Or set on individual curves
41
42
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
• Right mouse, select in Model tree on
Surfaces > Tetra/Hexa Sizes
– 1 Icon appears for each surface
– Gives you a visual estimate of prescribed max. size
43
Local Curve Mesh Setup – General
•
Curve Mesh Setup
–
General
• Same as
Surface Mesh Setup
• But also can prescribe
Number of nodes
– Instead of element size
• Also includes node biasing along curves
Side 2
Arrow shows side
1 and side
2
Side 1
– Initial spacing from either curve end
– Bunching laws
– Expansion ratios from either curve end
– Matching of node spacing to adjacent curves
– For a better description, refer to the
Hexa chapter –
Edge Parameters
• Select curves first, middle mouse to accept selection, then type in parameters/ sizes -
Apply
Display
• Right mouse select in Model Tree,
Curves -> Curve
Tetra/Hexa Sizes
or
•
Curve Node
Spacing
Tetra sizes
Node spacing
Local Curve Mesh Setup – Dynamic and Copy
•
Curve Mesh Setup
–
Dynamic
• Adjust mesh parameters on screen
• Interactively toggle displayed values near curve with left (to increase)/right mouse (to decrease) keys
–
Copy Parameters
• Copy parameters set on one curve to others
• e.g. parallel curves downstream
–
Curve Mesh Setup
will override
Part Mesh Setup
parameters if set afterward
Left mouse to increase
Right mouse to decrease
44
45
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
46
Patch Dependent Method
• Patch defined by a closed loop of curves
– Typically each surface defines a patch
• Loop defined by boundary curves
• Curves automatically created by
Build
Diagnostic Topology
- a must!
– Can remove or filter out curves to define multisurface patches
• Delete curves
• Turn on filter points/curves when building topology
• Only uses curve sizes (curve nodes seed loop perimeter)
• Paving algorithm used to fill interior of loop
– Interior nodes typically projected to surface
– Adjacent loops share nodes at common edge making mesh conformal throughout
• Default method, fastest method
Filtered or deleted curves
(dormant) loop 1 loop 2 loop 3
Build topology MUST be done first to build surface connectivity and curves
47
Patch Dependent – Common Options
• All method options set from
Global
Mesh Setup -> Shell Meshing P arameters
section
•
General
–
Ignore Size
• Small features, such as sliver surfaces smaller than defined value are ignored. Merges loops behind the scenes
• Will override max. size setting if smaller
Surrounding mesh done afterwards is conformal to existing mesh
–
Respect line elements
• Line elements (bars) on existing mesh are respected
• Maintains conformal mesh between newly created mes h and existing mesh on adja cent surfaces
Sliver Surface,
0.6 mm wide
Ignore size =1,
Sliver surface is ignored
48
Patch Dependent Mesher - Boundary option
•
Boundary
–
Protect given line elements
• Keeps existing line elements which are small er than the Ignore size
• Grayed out unless
Respect line elements
is on
–
Smooth boundaries
• Smoothes the mesh boundaries after mesh ge neration. May not respect the initial node spa cing set on curves
–
Offset type
•
Interior
–
Force mapping
• Forces mapped mesh on regular (4 sided) sur faces to desired degree (0-1)
• Adjusts the number of nodes on opposite side s (0.2 = change number nodes by 20%)
–
Project to surfaces
• Interior nodes project to surface rather than i nterpolate 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
49
Patch Dependent Mesher - Repair option
•
Repair
–
Try harder
• For loops that fail with requested paving algorithm
• Levels (0-3) to make further attempts to create grid
•
0
- No further attempts, failed surface(s) marked and put into a subset
•
1
- Simple triangulation of surface, converted to requested type
•
2
- Same as 1, but dormant curves activated
•
3
- Run octree, same as patch independent
–
Improvement Level
• Levels (0-3) to improve mesh quality
•
0
- Laplace smoothing only
•
1
- STL tri mode, with conversion to quads (if requested)
•
2
– tri to quad conversion, splitting of bad quads
•
3
- allow nodes to move along boundary
•
Other options and fuller descriptions may be found in the
H elp
menu.
Patch Independent
• Uses robust Octree method
– Volumetric tetra elements created around geometry
– Faces mapped 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 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
50
Matches up with previously meshed surfaces
Volume around is first meshed
Nearest nodes projected to surface and only surface mesh is left
51
Autoblock
• 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
52
Shrinkwrap
• 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” geometry
– Quick and dirty surface meshing of complex geo metries
• 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), t o not project at all (0.0), or partially (0.0 < f actor < 1.0)
53
Mesh Types
• Mesh Types
– Set in
Global Mesh Setup > Shell Mesh Parameters
or
Surface Mesh Setup
(local upon selected surface entities
• Global defaults overridden by local settings or
Compute Mesh
options
Global settings
–
All Tri
–
Quad w/one Tri
• Almost all quad except with one tri per surface
• Single tri allows transition between uneven mesh distribution on loop edges
• Where pure quad will fail
–
Quad Dominant
• Allows for several transition triangles
Local
• Very useful in surface meshing complicated surfaces where a pure quad mesh may have poor quality surface settings
–
All Quad
• These mesh types will look different with the different mesh methods
All quad, autoblock
Examples done with patch dependent mesher
54
Compute Mesh
• 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
(In put = 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 entities to mesh from screen
55
4.
Volume Meshing
Introduction to Volume Meshing
56
• To automatically create 3D elements to fill volumetric domain
– Generally termed “unstructured”
• Mainly tetra
– Full 3D analysis
• Where 2D approximations don’t tell the full story
– Internal/External flow simulation
– Structural solid modeling
– Thermal stress
– Many more!
• Standard procedures
– Start from just geometry
• Octree tetra
– Start from existing shell mesh
– Robust
– Walk over features
• Cartesian
– Fastest
• Have to set sizes
– Both geometry and shell mesh
• Delauney/T-grid
• Octree tetra
– Quick
• Advancing Front
– Portions of model already meshed
– Smoother gradients, size transition
• Hex Core
– Prism layers
• Hex Dominant
– Set sizes on rest
• “Prism”
57
General Procedure
• First decide 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
• As in
Shell Meshing
– Locally
•
Part/Surface/Curve Mesh
Setup
• As in
Shell Meshing
• For
From geometry
:
– Octree
– Cartesian
• Define volumetric region
– Typically for octree on complex models
– Multiple volumes possible
• 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
• Compute Prism (optional)
– As separate process
– Also option to run automatically following tetra creation
58
Body/Material Point
• Define Volumetric Domain
– 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
Mesh Types
59
•
Tetra/mixed
– Most used type
– Pure tetra
– With prism 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 from prism sides, hex core, or hex prism layers
– With hex core
• Available in Cartesian type too
• Hexa filling majority volume
• Tetra (from Delauney algorithm) used to fill between surface or top of prism layers and hex core
• Pyramids to make conformal between tetra and hex quad faces
– Hybrid mesh can be created by merging with a structured hex mesh
Tetra/Prism
Pure tetra
Tetra/Prism/Hexcore
Mesh Types - Continued
• Hexa-Dominant
– Uses existing quad mesh
– Good quality hex near surface
– Somewhat poor in interior
– Typically good enough for static structural analysis but not CFD
– Not covered in detail here
60
• Cartesian
– Methods available in Cartesian
–
Staircase
–
Body fitted
–
Hexa-Core
– Automatic pure Hexa
– Rectilinear mesh
– Fastest method for creating volume mesh
– Not covered in detail here
61
Mesh Methods - Octree
• Type -
Tetra/Mixed
– Method -
Robust (Octree)
• Same as Shell Meshing > Patch Independent
– Retains volumetric tetras
• 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
62
Octree Method Characteristics
• 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
Mesh detail
• 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
Geometry
Mesh
Octree Tetra Process
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
63
The Octree process creates an initial mesh of “Maximum
size” elements which fills a region around and through a bounding region completely encapsulating the geometry.
64
Tetra Process, Cont’d
– Mesh then subdivided to meet the entity size parameters
– Factor of 2 in 3-dimensions, hence the name Octree
– Nodes are adjusted (projected) and edges are split/swapped to conform to the geometry
– Automatic “flood fill” process finds volume boundaries
– Initial element assigned to part name of material point
– Adjacent layers added to same part until boundary surfaces are reached
– Multiple volumes are supported for multi-region or multi-material problems
– Elements outside the domain are marked into a reserve part name called
ORFN
, then deleted
Flood fill
•User defined volumes kept
•
ORFN
region is discarded
Material point
65
Tetra Process, Cont’d
• Smooth
– Octree mesh is initially composed of regular right angle tetras
– Smoother can be set to run to improve quality
– Or run afterwards:
Edit Mesh -> Smooth Mesh Globally
66
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 (1/10 th 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 create a material point if none exists
(named
CREATED_MATERIAL#
)
• Set Global, Part, Surface, Curve Size Parameters
– Similar to
Shell Meshing
section
Geometry Repair tools quickly locate and fix these problems.
Missing inlet surface
Hole highlighted by yellow single edge curve
Using Points and Curves with Tetra Octree
• Curves and points included
• Mesh size specified on curves and surfaces
Mesh captures detail
67
• Curves and points not included
• Mesh size specified only on surfaces
Coarse mesh ‘walks over’ detail in surface model
• 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
68
Tetra Octree - 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
• 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)
69
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 have it loaded)
–
Use Existing Mesh Parts
• Select
Parts
that are already surface meshed
• Merges nodes to preexisting surface mesh
• Uses
Make Consistent
to match octree volume mesh to existing surface mesh
70
Curvature/Proximity Based Refinement
•
Curvature/Proximity Based Refinement
–
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 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
Min Size Limit: multiplied by Scale
Factor = global minimum
Prescribed element size: Surface/Curve Max. Element Size times Scale Factor
Prescribed size is adequate here
Auto subdivision at tighter radius of curvature
71
Curvature Based Refinement
•
Refinement
– Approximate number of elements along curvature if extrapolated to 360 o
– 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
– 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
Refinement = 12
Min size limit
72
Proximity Refinement, Elements in Gap
•
Elements in Gap
– Number of cells desired in narrow 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
– Can’t go smaller than Min size limit
– Have to set smaller Min size limit
Prescribed size
Cells in Gap = 5
Prescribed size
Min size limit
Min size limit (1/5 th smaller)
Cells in Gap = 5
73
Thin Cuts
•Define thin Cuts
– Only works with Tetra Octree
– To avoid ‘holes’ in thin solids/narrow gaps when mesh size is much larger than gap
– Define thin cuts by selecting two parts and then Add them to the list of defined Thin cuts
• The two sides of the thin cut must be in different parts
If the face of a tetra element has a surface/line node on part
“
A
” then it may not have a surface/line node on part “
B
”
B
A c
Note; If the surfaces of the two parts,
A
and
B
, meet, then the contact curve must be in a third part,
c
, or the thin cut will fail.
74
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 closeest node will be projected to the surface
– Use prudently. Too small a number results in strange globs of refined mesh
>
0.2
Split edge
75
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
Initially distributes nodes so as the centroid of any tetra is outside the circumsphere of any neighboring tetra
• 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
Mesh Methods – Advancing Front
76
• 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
Mesh Methods – ANSYS TGrid
77
•
Tetra/Mixed
–
ANSYS TGrid
• Runs Tgrid through an extension module
• Good mesh quality
• Fast mesh generation
• Setup Options:
•
Flood fill after completion
: Same as octree Flood fill
•
Verbose output
: This option writes more messages to help in debugging any potential problem
• Will not respect densities
• Will not mesh to quads. It converts them to triangles
• Similar to Advancing Front, but does not group elements as close near surface
78
Compute Mesh – Delaunay, Adv. Front, TGrid
• Run Options:
– Similar options as octree except cannot mesh to part geometry and part mesh (option:
Use existing mesh parts
)
–
Create Prism Layers
available for both
– Hexa-Core not available for
Advancing Front, ANSYS TGrid
–
Volume Part Name
• For newly created tetras
• Can choose
Inherited
to use material
–
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
•
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
79
Comparison
80
Density Region
•
Create Mesh Density
–Define volumetric region with smaller mesh size 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
• Max size within – multiplied by global
Scale Factor
•
Ratio
– expansion ratio away from density object
•
Width
– Number of layers from object before mesh size is allowed to growth
–
Type
•
Points
– Select any number of points
–
Size
and
Width
(number of layers) will determine
“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
Density from 2 points makes a line.
The
width
defines the radius of the cylinder
81
Periodicity
•
Define Periodicity
• Forces mesh alignment across periodic sides
• For meshing and solving only one section of symmetrically repeatable geometry
–
Rotational Periodic
• Enter
Base
,
Axis
, and
Angle
–
Translational Periodic
• Enter dX, dY, dZ offset
Tip: Placing material point close to midplane makes tetra octree obey periodictiy easier
82
5.
Prism Meshing
83
Prism Meshing
•
Inflation layers
–
To better simulate boundary layer effects
–
Mesh orthogonal to surface with faces perpendicular to bou ndary 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
84
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 t han that of tetra/hex just above it
–
Number of layers
n
–
Height ratio
r
–
Total height
- of all layers
– Usually specify 3 of the above 4 parameters
•
Compute params
will calculate the remaining parameter (
total height
usua lly left blank)
– Or specify only
Height ratio
and
Number of layers
for auto calculation of init ial height
– Individual surface/curve height/ratio/layers will override these global de faults if set
Other global parameters
Total height explained later
Height ratio
(r)
Initial height
(h)
Growth Law Comparison
•
The growth rate of
Wb-exponential
is greater than
exponential
•
The growth rate of
exponential
is greater than
linear
85
Linear
Exponential
Wb-Exponential
86
Smooth Tetra/Prism Transition
• Leave initial height as “0”
– 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
87
Setting Prism Parameters on 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
88
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
Edge of Interior surface
Both or no volume parts selected
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
• Usually set
height
and/or
ratio
smaller on specific surfaces to avoid collision
Height on part = 0.4
Height = 0.2
89
Collisions occurred when the height was
0.4 on all surfaces
No collisions after
90
s
Setting Prism Parameters on Curve
•
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
Height =
0.01
Height = 0 on surface
Height = 0.003
91
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 col umns 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 geome try
–
Must be confident about setup parameters and sizing
–
Running prism separately allows you to smooth and error-c heck the tri or tetra mesh first.
92
Input as Surface or Volume Mesh
•
Input can be a surface mesh or volume mesh
– Surface mesh
– Must be a closed boundary mesh
– Must specify a volume part
– Use tetra fill methods after:
–
Delaunay
–
Advancing Front
–
Ansys TGrid
– Volume mesh
– Moves and reconnects tetras
Delaunay fill
Prism extrudes into existing tetras
93
Prism – 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
See next slides
•
Prism height limit factor
• Ratio multiplier (m)
– For varying exponential growth: height = h(r)
(n-1)
(m)
(n-1)
94
Prism Options – Fillet Ratio
– Blends prism grid lines around sharp corners
• 0 = no fillet
• 1 = fillet ratio 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 60 o
), may not have space for a fillet ratio
Fillet Ratio = r/h r h
Fillet Ratio = 1.0
Fillet Ratio = 0.0
Fillet Ratio = 0.5
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 120 o to 179 o 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 lowerquality 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.
95
160 o
Original mesh
Max prism angle = 180 o
Pyramids
Max prism angle = 140 o
.
96
Prism Options – Max Prism Angle - Continued
• A high (up to 180 o
)
Max Prism Angle
keeps the prism layers connected around tight bends.
– Set this at 180 to prevent pyramids where possible
Max Prism Angle = 140
Max Prism Angle = 180
97
Prism Options – Max Height Over Base
– Restricts prism aspect ratio
– Prism layers stop growing in regions where prism aspect ratio would exc eed specified value
• Number of prism layers would not be preserved locally
Base (b)
– Mesh is made conformal with pyramids at prism boundaries
– Acceptable values vary widely (typically 0.5 – 8)
Height
(h)
h/b
Largest height over smallest base length
Pyramids
Max Height Over Base not set
Max Height Over Base = 1.0
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
Base (b)
– Preserves the specified number of prism layers
– Will fail if sizes of adjacent elements differ by more than a factor of 2
– Acceptable values vary widely (typically 0.5 – 8)
Height
(h)
h/b
Largest height over smallest base length
98
Limit factor not set
Limit factor = 0.5
99
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 not specified
•
Side part
– For quad faces on 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 the y’ll be in ORFN
Leaving these parts blank will inherit the names from the current mesh
100
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
– Set surface smoothing steps to zero for a tri/tetra mesh that is already smoothed
– 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
•
Other Advanced Prism Meshing Parameters
– Detailed in Help menu (usually left default)
101
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
102
Smoothing a Tetra/Prism Mesh
After generating prisms:
Edit Mesh > Smooth Mesh Globally
– Prisms are smoothed during prism generation
– If input mesh was a tetra mesh, the tetras adjacent to the last prism layer will be messed up
– First smooth only the tetras and tris
• Set
PENTA_6
to
Freeze
• Don’t want to modify the prism layers at this point
– Once tetra and tri elements are as smooth as possible, smooth all elemen ts
1 st step
– Set
PENTA_6
to
Smooth
– Decrease the
Up to quality
value so as not to distort prism elements t oo much
2 nd step
The prisms get compromised a bit when everything is on smooth
103
Splitting Prism Layers
– If many prism layers are desired, it is faster, but less ro bust – to create “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 e mploy the given growth ratio (height is free variable)
–
Fix initial height:
The layer is split such that its first sub-la yer is of the given height (ratio is free variable)
– Specify the number of layers to result from each existi ng layer
– Can split specified or all existing layers
104
Redistributing Prism Layers
Redistribute prism layers after splitting
•
Edit Mesh > Move Nodes > Redistribute Prism Edge
–
Fix ratio:
The initial height and subsequent layer heights will be adjusted to achieve this growth ratio
–
Fix initial height:
The growth ratio is the variable that will be a djusted to achieve this initial height
– The total prism thickness remains fixed and layers are adjust ed within this thickness
105
6.
Mesh Preparation Before Output to
Solver
•
106
Mesh Preparation Before Output to Solver
What will you learn from this presentation:
Checking and Improving the quality of the mesh
Manipulating the elements
Subsets
Usage of Edit Mesh tools
• To diagnose and fix any problems and improve mesh quality
• To convert element types
• Refine and/or coarsen mesh
• Manual and automatic tools
• For imported as well as internally created mesh
107
Mesh Checks
•
To diagnose mesh connectivity problems
–
Errors
– most likely to cause problems in:
•
Solver translation
•
Solver input
•
Solution convergence/run
–
Possible Problems
– “Unclean surface mesh”
•
Unwanted elements
•
Unwanted holes/gaps
•
May result in incorrect solution
–
Can check any combination of errors/possible problems at any one time
•
Individually select
•
Clicking on
Error
or
Possible Problems
headings will select all options in column – selecting again will de-select all
•
Set Defaults
will select the most common checks for the current mesh type (2D or 3D)
–
Check Mode
•
Create Subsets
– creates a subset of elements for each problem found (will run through all selected checks)
•
Check/Fix Each
– offers automatic fixing of indicated problem (needs user decision after each problem found)
108
Mesh Checks - Mesh Errors
•
Duplicate Elements
–
Elements that share all nodes with other elements of the same type
•
Uncovered Faces
–
Volumetric element faces that are neither attached to the face of another volumetric element nor to a surface element (boundary face)
•
Missing Internal Faces
–
Volumetric elements that are adjacent to another of a different part with no surface element between them
•
Periodic Problems
–
Inconsistency in the pattern of nodes/faces between periodic sides
–
Special check for rotating (sector) or translational periodic grids
–
Select pairs of parts to check
•
Volume Orientation
–
Left handed elements due to incorrect connectivity (node numbering of cell)
•
Surface Orientations
–
Surface
elements whose attached volume elements share part of the same space
•
Hanging Elements
–
Line (bar) elements with a free node
(node not shared by any other element)
•
Penetrating Elements
–
Surface element(s) that intersect or penetrate through other surface elements
•
Disconnected Bar Elements
–
Bar elements where both nodes are unattached to any other elements
109
Mesh Checks - Possible Problems
•
Multiple Edges
–
Surface elements with an edge that shares three or more elements
–
Can include legitimate T-junctions
•
Triangle Boxes
–
Groups of 4 triangles that form a tetrahedron with no actual volume element inside
•
2 -Single Edges
–
Surface element with 2 free edges (not shared by another surface element)
•
Single-Multiple Edges
–
Surface element with both free and multiple edges
•
Stand-Alone Surface Mesh
–
Surface elements that don't share a face with a volumetric element
•
Single Edges
–
Surface elements with a free edge
–
Can include legitimate hanging baffles
–
2D-only mesh boundaries are single
•
Delaunay Violation
–
Tri elements with nodes that are within the circumsphere of adjacent tri elements
– legacy quality criteria
•
Overlapping Elements
–
Continuous set of surface elements that occupy the same surface area (surface mesh that folds on to itself within a small angle)
•
Non-manifold vertices
–
Vertices whose adjacent surafce elements’ outer edges don't form a closed loop
–
Typically found in tent-like structures where surface elements jump from one surface to another across a narrow gap or sharp angle
•
Unconnected Vertices
–
Vertices that are not connected to any elements
–
Can always be deleted
110
Mesh Checks – Check Mode
•
If
Create Subsets
was selected
–
Will go through all checked criteria without interruption
–
Elements that have a particular error/problem are put into a subset with the same diagnostic type name
–
Subsets activated in Model Tree
•
Turn off all parts or shells to view subsets
•
If
Check/Fix Each
was selected
–
Will be prompted with options one criteria at a time
•
Fix:
Automatically fix the error/problem
–
Recommended only for
Duplicate Elements,
Uncovered Faces, Missing Internal Faces,
Volume Orientations, Unconnected Vertices
•
Create Subset
•
Ignore
–
For example, multiple edges may be legitimate t-junctions; single edges may be legitimate free edges
Valid multiple edges usually form closed loops
111
Mesh Quality Display
•
A diagnostic check of individual element quality
–
Mesh types to check
– Allows you to select the mesh types to check
•
1D (Line elements)
•
2D (Tri and/or Quad)
•
3D (Tetra, Penta, Hexa and/or Pyramid)
–
Elements to check
– By part and subset
•
All
•
Active parts
•
Visible subsets
•
Visible subsets and active parts
–
Refresh Histogram
– Refreshes the histogram displayed
–
Quality type
– Specifies the Quality criterion for display
•
48 quality criteria available
•
Some checks don’t apply to all element types
Selecting a histogram bar will display the elements in that range
112
Mesh Smoothing
•
Automatically improve element quality
–
All element types
–
Necessary to have geometry loaded
–
Nodes are moved to improve the element quality
•
Automatic node movement constrained by node projection type to geometry type – e.g. curve nodes will be constrained to move only on curves
–
Histogram is automatically displayed/updated after smoothing
•
User chooses:
–
Criterion
–
Up to value
–
Smooth Mesh Type
•
Smooth
: Element types actively smoothed; quality of type appears as part of histogram.
•
Freeze
: Nodes are held in place during the smoothing process.
These elements not shown in histogram
•
Float
: Nodes can be moved along with adjacent smoothed elements, but quality ignored; not shown in histogram
•
Example: Freeze Prisms and Pyramids while smoothing Tetra.
Float surface elements
113
Mesh Smoothing
•
Advanced Options
–
Smooth Parts/Subsets
•
Smooth all parts, visible parts (activated in model tree), or visible subsets
•
Quick, local smoothing
–
Laplace smoothing
•
Gives more uniform mesh size relative to neighboring elements and equal angles
•
Recommended for TRI only – smooth Tetra after with
Laplace turned off and tri’s froze
•
Recommended prior to prism generation
–
Not just worst 1%
•
Factors in all elements instead of only the worst 1% of those beneath quality value and their neighbors
•
Can improve quality but takes much longer
–
Violate geometry
•
Unconstrain nodes slightly from geometry within user defined tolerance – absolute or relative to minimum edge length of mesh elements
114
7.
Output to Solver
Output to Solver - Selecting Solver
•
Selecting solver
–
Use the red toolbox in the
Output
tab to select the solver format
–
This same menu is accessed with
Settings > Solver
–
123 solver formats available
–
All output formats can be set with the
Output Solver
pulldown
–
Help on each solver format can be found on the Ansys website: http://www.ansys.com/Products/Other+Products/ANSYS+ICEM+CFD/Outp ut+Interfaces/Output+Interfaces+TOC
–
Also found in
Help >
Output Interfaces
ABAQUS
ADINA
AUTOCFD
CFD++
ACE-U AcFlux ACRi
AIRFLO3D ALPHA-FLOW ANSYS
BAGGER CEDRE
CFL3D CFX-4
CGNS CHAD
CONCERT3D CRSOL
DSMC-SANDIA DTF
C-MOLD
CRUNCH
EM
CFD-ACE
CFX-5
COBALT
CSP
EXODUS
ACUSOLVE
ATTILA
COMCO
DATEX
FANSC
FASTEST-3D FASTU
FLEX
FENFLOSS
FLOTRAN FLOWCART
FIDAP FIRE
FLOW-LOGIC FLUENT V4
FLUENT V6 GASP GLS3D(ADH) GMTEC GSMAC-DF
VULCAN
WIND
CFDesign WINDMASTER
CFX-TASCflow ZEN
SCRYU
STARCD
TDF
SC/Tetra
STARS USM3D
USMKV3V
VECTIS
TGRID
SpecElem
VRML STL
VSAERO/USAERO TLNS3Dmb
Trio_U SPECTRUM-CENTRIC
GUST
ICU
KIVA-3
MACS
HAWK
IDEAS
KIVA-4
MAGREC
HDF
IMPNS
LAURA
MAZe
N3SNATUR NASTRAN NEKTON
NSU3D
PATRAN
NS3D NUMECA
PHOENICS PLOT3D
IBM-BEM
INCA
ICAT iPLES
TSAR
UGRID
LL-DYNA3D LS-DYNA3D
MOUSE MULTIBLOCK
UH3D
USA
NOPO NPARC SAUNA
PAB3D
PMARC
PARC
POLYFLOW
SPLITFLOW
TNO
TEAM
TRANAIR
POLY3D
POPINDA
PRECISE
RADIOSS
RTT
115
116
Mesh Formats
i j k
•
There are 2 types of mesh formats that solvers read
–
Unstructured
•
Most solvers use unstructured formats
•
Some examples are
Fluent, Ansys CFX, CFD++, Abaqus, Ansys
•
Nodes have an ID and location
•
The ICEM CFD unstructured mesh has a
*.uns
extension and can be made from any mesher in ICEM CFD
–
Multiblock structured
•
Some older CFD solvers require this format
•
Examples are
Plot3D, CFX-TASCFLOW, KIVA-3V
–
CGNS
supports both unstructured and multiblock structured
•
Nodes have IJK index designation and location
•
The ICEM CFD structured mesh has many files:
–
Project.1
,
project.2
,
project.3
, etc… for each mesh domain
–
A topology file,
topo_mulcad_out.top
, that describes how each domain is connected to the other domain.
–
A dummy file,
Project.multiblock
, that enables selection of all the multiblock mesh files of that project name by selecting this one file
•
Only an ICEM CFD hexa blocking is capable of being written out in multiblock structured format
117
Boundary Conditions
P
art highlights white when selected
BC’s shown for solver Fluent_V6
•
Both CFD and FEA solvers allow boundary conditions to be set
–
Output > Boundary Conditions
•
BC’s are set on mesh elements grouped by part names
•
Tree structure organizes part names by dimension
(2D, 3D, etc) of geometry and mesh in the part
•
Any part containing mixed dimensions (ex. curves and tri’s) will be grouped into
mixed/unknown
•
Use
File> Attributes > Save Attributes As…
to save the BC file (*
.fbc
and *
.atr
extensions)
118
Parameters
•
Some structural solvers have global parameters
–
Output > Edit Parameters
•
If the solver type requires global parameters, you must bring up this menu before writing out and press
Accept
, even if not changing any parameters
•
Then use
File > Parameters > Save Parameters As…
to save the file (*
.par
extension)
Example parameters shown for ANSYS solver
119
Write Input
•
Output > Write Input
to write the solver file
–
It will ask you to save a boundary condition file
(*
.fbc
) and the project
–
It’s always safe to save these when asked, but not necessary if you know you saved them and didn’t make any changes since then
–
It will then ask for the ICEM CFD mesh file to translate
•
*
.uns
file if the solver requires an unstructured mesh
•
*
.multiblock
file if the solver requires a structured mesh
–
A final menu will pop up that has specific options for the solver translation being used
•
The example on the left is for
Fluent_V6
–
All translators will require a boundary condition file even if no BC’s are set
•
A empty BC file will contain just the part names
–
There will always be a name for the output file
120
Special Structural Solvers
•
5 Special structural solvers
–
There are 5 structural solvers that have extended boundary condition setup – called loads and constraints – which are set using the
Common
Structural Solver
dropdown
–
These are
Nastran
,
Ansys
,
LS-Dyna
,
Abaqus
, and
Autodyn
–
These are set using the 4 extra tabs;
Properties
,
Contraints
,
Loads
, and
Solve Options
Nastran
Ansys and Abaqus
LS-Dyna and Autodyn
•
Constraints tab
–
All functions are only available for
LS-Dyna
and
Autodyn
–
“
Define contact
” is allowable with
Ansys
and
Abaqus
–
Nastran
only allows the first two functions
121
Special Structural Solvers - Procedure
•
Define which elements to write out
–
First define a
material
in the
Properties
tab
–
Then define an element property on the element types and parts you want to write out
•
3D property for volume elements
•
2D property on shells
•
1D property on line elements
•
0D property on node elements
•
Define any
Loads
and
Constraints
in the other tabs (optional)
•
Use the
Solve Options
tab to write the mesh to the solver
–
Solve Options > Write/View Input File
•
Can submit solver run if the corresponding environment variable is set to find the solver executable
–
For Ansys, it is ANSYS_EXEC_PATH
122
Special Structural Solvers – Write Input
•
If using the
Properties
,
Loads
, and
Constraints
tabs for one of the 5 special structural solvers, then use the
Solve Options
tab to write the file, not the
Output
tab
–
The
attribute
file (*
.atr
) is the BC file used with these special structural solvers
–
The
attribute
file is replaced with the boundary condition file (*
.fbc
) when using the
Output
tab
–
Any elements set to
All
or
None
will override any properties set in the
Properties
tab if not set to
Defined
–
To communicate with the
Output
tab, click on
Advanced
, and then click
Create Attribute
& Parameter Files
•
You can then
Edit Parameters
and
Edit
Attributes
here or in the Output tab
•
The attributes will then be the same as the boundary conditons
–
View input file here or in your own text editor
123
8.
ICEM CFD Hexa
124
What is Blocking?
• A hexa mesh is created by first making a “blocking”
– A blocking breaks down a geometry into large brick-shapes and structures the directio n of grid lines by the arrangement of the blocks
– Each “block” is easily meshed with a pure Cartesian mesh
• Some blocks can be defined as “swept” and be unstructured along one face
– Block entities (faces, edges, and vertices) are projected onto the geometry
– The blocking is saved to an independent file, and can be loaded onto a different geom etry
Mesh with projection
Blocking
Mesh without projection
Geometry
Approach – Top Down and/or Bottom Up
Block structure is created independent of geometry
– “Top down” topology creation
• The user as sculptor instead of brick layer
• One-step creation of advanced topologies (O-grid)
O-grid
– “Bottom up” topology creation
• Blocking is built up like “laying bricks”
– Create blocks
– Extrude face
– Copy topologies
125
– Combinations of top-down and bottom-up methods can be used
Geometry Requirements for Hexa
Use same geometry (tetin) as used with Tetra
– Does not necessarily need to be a completely enclosed volume
Associating
Face to Surface
to a dummy point family or
Interpolation
can effectively mesh where geometry doesn’t exist
Blocking
126
Geometry
– Points and curves are not required but are very useful
• Use
Build Diagnostic Topology
to quickly build all curves and points
– Block structure is projected to geometry
• Surfaces – automatically with manual override
• Curves and points – manually projected
Geometry/Blocking Nomenclature
• Geometry
– Point
– Curve
– Surface
– Volume
• Blocking
– Vertex
– Edge
– Face
– Block
Vertex
Edge
Curve
Point
Face
Surfaces
Note: “curve” refers to lines, arcs, and splines (1D geometry)
127
Block
Blocking process
• Structure blocking to capture the shape of the geometry
– Top down
• split and discard unused blocks
– Bottom up
• create block by extrusion, creating, copying
• Associate blocking to geometry
– Usually just edges to curves
• Move vertices onto geometry
– Manual and automatic methods
• Assign mesh sizes
– Quickly by setting sizes on surfaces and/or curves
– Fine tune by setting edge distributions
• View mesh and check/improve quality
• Write out mesh
128
129
Initialize Blocking-3D or 2D
• New Blocking
–
3D Bounding Box
• One 3D volume block encompassing selected entities
–
2D Planar
• One 2D block on XY plane around entire 2D geometry
• First rotate geometry to XY plane
• No surfaces required
–
2D Surface Blocking
• Discussed next slide
130
Initialize Blocking-2D Surface
•
2D Surface Blocking
– Each surface becomes one 2D block
– Free blocks – fully unstructured
• Most robust
–
Mapped
– structured
• Aligned mesh along 4 block boundary edges
Geometry
2D blocking
Unstructured
– Must
Build Diagnostic Topology
first
• It needs the connectivity information
– Can always convert blocks between free and mapped afterward:
Edit Block > Convert Block Type
131
Structuring Blocking to Fit Geometry
Top down approach
Split the block to capture the underlying shape
Start with one block which encloses the entire geometry
Delete unused blocks
Note: Deleted blocks are put into the part
VORFN by default, so they can be re-used later if wanted
132
Associate Blocking to Geometry
• Associate blocking to geometry
– Usually just Edges to Curves
– In the final mesh, edges will take the shape
(be projected to) these curves
– Right click on
Edges > Show Association
in the model tree to display the association arrows
133
Move Vertices onto geometry
• Move vertices to better represent the shape
– All visible or selected vertices can be projected to the geometry at once
– Can be moved individually along the geometry
– Single or multiple at a time
– Along fixed plane or line/vector
Moving Vertices of Different Association
134
• Color indicates type of association and how a vertex will move
(edges also follow these colors, except red)
– Red
• Constrained to a point
• Can’t be moved unless association is changed
– Green
• Constrained to a curve
• Vertex slides along that particular curve
– White (black on light background)
• Constrained to surfaces
• Vertex will slide along any ACTIVE surface (surface parts which are turned on in the model tree)
• If not on a surface, it will jump to the NEAREST ACTIVE surface when moved
– Blue
• Free (usually internal) vertex
• Select NEAR
(
not on) the vertex on the edge to move along edge direction
135
Assign Mesh Size
• Assign mesh sizes
– Hexa sizes can be assigned on surfaces and curves for a quick mesh
– Need to
Update sizes
to apply to surface/curve sizes to edges
– Or set edge-by-edge for fine tuning
• Automatic
Copy to parallel edges
136
Edge Parameters
Spacing 2 = 1.0
Side 2
Ratio 2 = 1.5
17 meshing laws
Side 1 (base of arrow) params
Ratio 1 = 1.5
Spacing 1 = 1.0
Requested
Side 2 (head of arrow) params
Side 1
Actual
Arrow indicates side 1 and side 2
Spacing can be linked to another edge
Spacing 1
– distance between first two nodes on side 1
Ratio 1
– growth ratio from side 1 toward center
Spacing 2
– distance between first two nodes on side 2
Ratio 2
– growth ratio from side 2 toward center
Max Space
– Maximum element length along edge
137
View Pre-Mesh
• Pre-Mesh
– Create mesh at any stage of the process
– Mesh with different projection methods
– Use
Project faces
(default) to fully represent the geometry
– View only certain surface mesh by turning on only that
Part
in the model tree
– Use
Scan planes
to view internal mesh (covered later)
No projection
Face projectio n
Checking Quality
• Using the Quality Histogram
– Determinant
• Measurement of element deformation (squareness)
• Most solvers accept > 0.1
• Shoot for > 0.2
– Angle
• Element minimum internal angles
• Shoot for >18 degrees
– Aspect ratio
– Volume
– Warpage
• Shoot for < 45 degrees
– Many more metrics
You can display elements in a given range by selecting the histogram bar
138
139
Write Mesh
• Convert pre-mesh to a permanent mesh
– Two formats depending on what your solver takes
• Unstructured: Cells defined by node numbers (connectivity)
• Structured: Multiblock – cells defined by i, j, k index
– Blocking changes will no longer affect this mesh
Using the
File>Blocking>Save
… menu only writes the mesh to disk
Pre-Mesh>Convert to
…” a mesh from the model tree saves and immediately loads the mesh
Ogrid Definition
• An O-grid is a series of blocks created in one step which arranges grid lines into an
“O” shape or a wrapping nature
• 3 basic types created through the same operation all referred to as “O-grids”
• O-grid
• C-grid (half O-grid)
O-grid C-grid
• L-grid (quarter O-grid)
• Reduce skew where a block corner must lie on a continuous curve/surface
L-grid
• Cylinders
• Complex geometries
• Improves efficiency of node clustering near walls for CFD applications
O-grid
140
No O-grid
Creating Ogrid
• Select blocks for O-grid
– Can select by visible, all, part, around face, around edge, around vertex, 2 corner method, or individual selection
5 blocks in 2D
7 blocks in 3D
141
Select specific blocks or around face, edge, or vertex
Note: Internal block has all internal (blue) edges and vertices
Ogrid – Adding Edges/Faces
• Adding faces during O-grid creation
– O-grid “passes through” the selected block faces
– In general, add faces on the “flat parts”
– Adding a face actually adds blocks on both sides of the face
O-grid passes through this face
Half O-grid
(C-grid)
• Examples of uses
– Pipe ends
– Symmetry planes
– Complex geometries
142
O-grid passes through this face
143
Ogrid – Adding Multiple Edges/Faces
• Any number of faces can be added around a selected block
– If all the faces are added around a block, the result is no change since the
O-grid passes through all the faces
Quarter O-grid
(L-grid)
Quarter O-grids can be used to block triangular shapes
Seen as a C-grid in one direction and an L-grid in another direction
144
Ogrid – Around Blocks
• Select
Around block(s)
to create the O-grid around the selected blocks
– Useful for creating wrap-around grid around a solid object
– Examples
• Flow over a cylinder
• Boundary layer resolution around an airplane or car body
145
Scaling Ogrid
• O-grids can be re-sized after or during creation
– By default the O-grid size is set to minimize block distortion
– You are actually scaling all parallel O-grid (radial) edges to the selected edge
– The selected edge is given a factor of 1
– Numbers < 1 will shrink the edge and thus create a larger inner block
Factor = 0.3
Selected edge factor = 1
146
Why Create an Ogrid?
Before O-grid
This mesh can be improved by using an O-grid
– An example of bad mesh in the block corners
Right mouse click on the histogram to access options like
show
,
replot
, or
done
147
Index Scheme
• All blocks and vertices are defined with a global index scheme
– Initial block has i,j,k indices aligned with Cartesian x,y,z, global coordinates
– Subsequent blocks created by the split operation will maintain that orientation
– O-grids will not conform to this orientation, so each O-grid creates a new index direction (O3,
O4, etc…) (i,j,k correspond to dimension 0,1,2)
– Vertex Indices can be displayed by right clicking on
Vertices > Indices
in the model tree i=1 j=2 k=4 O-grid3=1
1 2 4 3:1
148
Using Index Control
Select corners
is often faster than toggling the index arrows
• Blocks can be turned off and on based on indices
– Use the index control to turn blocks off and on
– Many operations can be applied to only the visible blocks
• Split blocks
• Rescale O-grid
Resets everything to fully visible
149
VORFN Part
• A region of blocks called VORFN surrounds the blocking
– The reserved part name
VORFN
is created in the model tree when blocking is initialized
– It is used to maintain the global index scheme
– Indices begin at 0 (in
VORFN
region)
– Deleting blocks normally (non-permanently) just changes the part to
VORFN
• Blocks can be moved back out of
VORFN
to another part
– Deleting blocks
permanently
gets rid of the block and rebuilds
VORFN
as an O-grid
Original
VORFN
Selected blocks are actually removed and
VORFN
is rebuilt
(as an Ogrid)
Turn on to see
VORFN
VORFN becomes an O-grid after deleting blocks permanently
Removing Splits and Ogrids
• Splits and O-grids can be deleted by using
Merge vertices > propagate
– Select two vertices at the ends of an edge running in the direction you want to delete
– Middle click, then
Confirm
– The second vertex merges to the first vertex when
Merge to average
is off, and the merge will be propagated
– If deleting an O-grid, only select the vertices of one radial edge
Deleting a split
Deleting an O-grid
2
1
150
2
1
151
Extrude Face
• Select block face (select face or use two corner method)
– Interactive
• Drag with the pointer
– Extrude a Fixed distance
• Enter distance
2 corner method
– Extrude along curve
• Select curve
Extrude along curve
Select face
Set Location
Set X, Y, and/or Z of selected vertices
– Select
Ref. Vertex
, then select vertices to move or use the index control and select all visible vertices (“v”)
– Select directions to move (Modify X, Y, Z)
• Can use a local coordinate system (cylindrical, Cartesian)
• If cylindrical coor. System, (x, y, z) becomes (r,
θ, z)
– Enter values or use the values from the
Ref. vertex
or screen location
– Method can be
Set Position
or
Increment
–
Apply
152
Select these vertices
Z-dir z
Set z coordinate of vertices
Align Vertices
Align multiple vertices in one plane with vertices in another plane (or split dimension)
– Select
Along edge direction
, then select the edge that connects the two planes, or runs approximately normal to the split plane that you want to move inside
– Select
Reference vertex
, then select any vertex in the plane you want to align to. These vertices will remain fixed
– Select the plane to allow vertex movement within (XY, YZ, XZ, or User Defined). The
User Defined
plane must be specified with a normal vector, such as (1 1 0)
–
Apply
Select any of these vertices as reference
153
All visible vertices not sharing the reference index will be moved
Move vertices in this plane
Select this edge
154
Create Blocks – 2D to 3D
A 2D blocking can be extruded into a 3D blocking by three different methods
–
Multizone Fill
• Auto creation of 3D blocking from enclosed 2D surface blocks
–
Translate
–
Rotate
Rotate
Number of nodes in circumferential direction
Extrudes points into curves and curves into surfaces where
3D geometry doesn’t exist
Surface mesh and scan planes
155
Create Block – Wedge
Degenerate
– Select 6 vertices or locations
– Order is important (see picture)
– Grid lines converge at vertices 1 and 4
– Results in penta6 (prism) elements along one edge
Quarter-O-Grid
– Results in all Hexa “wedge” (Y configuration)
4
1
6
5
3
2
Quarter-O-Grid
Degenerate
156
Create Block – Swept Block
•
Swept
(Unstructured)
– Select 6 vertices or locat ions
– Different order than deg enerate, quarter o-grid
– Results in unstructured mesh
– Some tris/prisms
• Can also
Convert Block Type
– To
Structured
– To
Unstructured
(2D)
– To
Swept
(3D)
– Hex blocks as well as de generate wedges
2
1
6
5
4
3
Collapse Blocks
Collapsing blocks
– Select edge to define collapse direction
– Select blocks to collapse
– Results in a degenerate block (converges to penta6
(prism) elements)
– Degenerate block is often deleted
Select edge
157
Example: meshing around knife-edge wings
158
Select these two vertices
Split Vertex
Separates or undoes merged vertices, including vertices merged due to a collapsed block
– Select any numbers of vertices and middle click
–
Apply
– If you have deleted any blocks permanently after vertices were merged, this operation may not work to undo the merge. This is because a new index scheme is configured when blocks are permanently deleted
159
Periodicity in Geometry
First must be specified in tetin file
•
Global Mesh Size -> Set up Periodicity
–
Translational
• Enter vector which specifies magnitude and direction
–
Rotational
• Enter
Axis
vector – only specifies direction
• Enter
Base
point that axis goes through
• Enter
Angle
in degrees
21.1765
o
(360/17)
Base (0 0 0)
Axis (0 0 1)
Periodicity in Blocking
Vertices then made periodic in blocking
– Select
Edit Block -> Periodic Vertices -> Create
– Select pairs of vertices at a time
– The second vertex of each pair will move to the periodic position of the first vertex
– When you move one vertex, its pair will move with it
– Subsequent splits will also be periodic
– Visually verify with
RMB
on
Vertices -> Periodic
, and
Faces -> Periodic Faces
in model tree
– A face becomes periodic only if its 4 corner vertices are periodic
160
Subsequent splits are also periodic
161
2 Ways of Blocking the Same Geometry
• Creating a fork by Merge vertices
Delete block
Merge 2 vertex pairs
Merge 2 more vertex pairs
2 Ways of Blocking the Same Geometry
• Creating a fork by Extrude faces
162
Extrude 1
Extrude 2
Associate
163
Multiple Ways of Blocking the Same Geometry
• Creating a fork with Top-down methods
split
Two quarter
O-grids
Delete blocks
Move vertices
One quarter
O-grid
One quarter
O-grid
164
Topology
• What is common in these?
– Their block topology
Topology
• All these parts have the same basic topology
– Blocking strategy for all pipes are similar.
– Single block with O-grid
– The only difference is the number of splits
added to help control the blocking
– Create the one block, then split, and add O
-grid last
This helical blocking is quickly created with extrude along curve
165
Single block with o-grid splits
Single block with 5 splits and o-grid
Single block with multiple splits and ogrid
Extend Split
•
Think of a split as a plane (even though it does not have to be planar)
•
Select an edge at the outside of this “plane” and it will extend in all directions
– Split only goes through the displayed blocks
– Use the index control to limit the displayed blocks
Select edge
Select edge
166
Select
Project vertices
to have it automatically project new visible vertices to the nearest place on their associated geometric entities
Shaping Edges – Split Edge
•
Edges are by default linear before projection
•
Edges can be shaped using split edge
–
Spline
–
Linear
–
Control point
•
Use
Move vertex
to move splits after splitting
Spline
Multiple splits
167
Linear
Original edge
Control point
•
Split edge will also override the automatic interpolation of edges during mesh computation
168
Shaping Edges – Split Edge Example
•
Using split edge to shape block faces to make a hole where it doesn’t exist in the geometry
Geometry
(gear)
169
Shaping Edges – Link Edge
•
Use one edge to control the shape of another
– Select source edge then target edge(s)
– Enter factor (higher number = greater curvature)
Source edge
Target edge
Factor = 0.9
Factor = 1.3
Factor = 0.5
170
Bottom-Up Meshing Methods
• Top-down is, generally, more
robust
• Bottom-up methods improve
flexibility
–
Transform Blocks
•
Translate
•
Rotate
•
Mirror
•
Scale
–
2D to 3D
extrusion
•
Translate
•
Rotate
– Block independently and merge
–
Extrude face
–
Create block
171
Transforming Blocks
•
Simply transform selected blocks or make a copy and merge the transformed copy with the previous blocking
–
Select blocks to transform
–
Select
Method
•
Translate
•
Rotate
•
Mirror
•
Scale
–
Enter parameters necessary for method
Mirror example
172
Create Block – Hexa (Vertex Locations)
Select 8 vertices
•
Create block types
–
Hexa
• 8 vertices or locations (selection order is important)
(Choose a vector direction and do the same order on opposite faces)
• 2 faces
–
Quarter O-grid
(Y-grid)
–
Degenerate
5
1
2
6
3 7
4
8
Select 2 faces
173
Create Block – Hexa (Geometry Locations)
•
What if I don’t have 8 vertices to select?
–
Select the vertices that you do have
–
Press middle mouse button
–
Select the rest of the locations on the screen
–
The same order must be maintained as before
1
3
4
2
7
5
6
8
Press middle button
174
Merge Vertices
•
One by one
•
Multiple within tolerance
– When merging individually, select two vertices at a time
– With
Merge to average
off, the second vertex will merge to the first
– With
Merge to average
on, both vertices merge to the middle of the two
– Use to join separate topologies together
– Use to make degenerate blocks
1
2
2
1
Delete Blocks – Permanently
•
Delete blocks will, by default, move blocks to the part
VORFN, which is more stable than permanently deleting blocks (doesn’t recompute indices)
•
However, there are some situations where deleting permanently is useful
–
Deleting all of VORFN can serve as a repair tool in complex topologies (indices get reconfigured)
–
Deleting individual blocks will free up node connectivity across VORFN blocks
175
Equal number of nodes across hole
Delete block permanently
Number of nodes can be unequal
176
Three Basic uses of O-Grids
•
Three basic uses of O-grids
–
Capture the shape of the geometry
•
Usually done early in the blocking process
–
Improve element quality in block corners where surfaces do not make a corner
(smooth transition at block corner)
–
Improve efficiency of node clustering near walls
•
Boundary layer resolution
–
These last two are usually accomplished with the same O-grid, and done late in the blocking process
Quarter O-grid
Example of using an O-grid to capture the basic shape as the first step in the process
177
Refinement
•
Defines integer multipliers of elements across block interfaces
–
Can only be used with certain solvers
–
Refine: Factor > 1 (enter integer)
–
Coarsen: Factor < 1 (enter fraction – 1/2, 1/3, etc.)
Select refinement edge direction or select “All”
Select block(s) to refine within
Factor
= 1/3
Resolve Refinements
•
Creates 1-to-1 node connections for refinements done on the blocking
–
Only works on refinement ratio multiples of 3
–
Operates on the unstructured mesh only
Multiple steps of refinement and resolving refinements can be done (3, 9, 27,
1/3, 1/9, 1/27, etc…)
178
179
Edge Parameters – Linked Bunching
•
Link node counts and distribution law to another edge
–
Can link to one master edge or a series of edges with the same end-bounds
–
Select the large edge first
–
Select the small edge on side 1 when selecting the
Link
Edge
Side 2
Side 1
The long edge gets linked to all these shorter edges
Match Edges
•
Matches the end spacing to another edge
–
Reference edge and target edge must meet at the same vertex
–
Does not link spacing
–
The effect is usually only noticed when the target edge has an end spacing larger than the reference edge.
Target edge
180
In this example, the side 1 node spacing of the target edge is set to the same node spacing as side 2 of the reference edge
Reference edge
Split Face
•
Split Face
is actually a split block operation
•
Split Face
splits the adjacent blocks in a non-active part
(usually VORFN), and what is left visible is the end of the split on the visible faces
–
Select Face to split
–
Left click on edge and drag split
–
Split will be normal to the selected edge
181
Select face
Select edge
Face split is normal to selected edge
VORFN blocks are what gets split
182
Merge Blocks
•
Merge blocks
–
Select blocks to merge, then middle click
–
Apply
–
You cannot merge blocks of different parts unless you first change them to the same part
Select these blocks
183
Merge Faces
•
Merge Faces
–
Select 2 corners diagonally across faces to merge
–
Apply
–
You cannot select across O-grids because this is a different index direction
–
This actually merges blocks on both sides of the selected faces
Selected faces
Select diagonally across vertices
Merge Face
actually merges blocks on both sides of the faces
184
Output Blocks
•
Reduces number of blocks in a multiblock mesh, which reduces solver time
–
Three steps (in order)
1.
Initialize
output bocks
2.
Turn on
Output blocks
3.
Merge blocks (automatic or manual)
Before
29 blocks
After
8 blocks
Initializing output blocks to the full blocking
BEFORE merging blocks will prevent the mesh from being altered when merging blocks
Output blocks
can be toggled on and off between the merged blocking and the full blocking
185
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