ICEM Mesh for CFD Analysis

ICEM Mesh for CFD Analysis
ICEM Mesh for CFD Analysis
Index
1.
2.
3.
4.
5.
6.
7.
8.
2
Introduction to ICEM
Geometry Handling
Shell Meshing
Volume Meshing
Prism Meshing
Mesh Preparation Before Output to Solver
Output to Solver
ICEM CFD Hexa
1. Introduction to ICEM
3
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
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)
•
•
•
•
5
Advanced mesh editing
Solver Setup
Output to 100+ Solvers
Scripting … and much more…
GUI and Layout
Utility Menu
Utility
Icons
Function Tabs
Selection Toolbar
Model
Tree
Display Triad
Data Entry
Panel
6
Message
Window
Histogram
Window
File and Directory Structure
•
Use of many files
•
All files can optionally be associated within a Project
•
Primary file types:
– Not one large common database
– For faster input/output
– Establishes working directory
– Settings (*.prj) file contains associated file names
– Tetin (.tin): Geometry
•
•
•
•
Geometry entities and material points
Part associations
Global and entity mesh sizes
Created in Ansys ICEM CFD or CAD Interface
.prj
– Domain file (.uns)
• Unstructured mesh
.tin
– Blocking file (.blk)
.blk
• Blocking topology
.uns
– Attribute file (.fbc, .atr)
• Boundary conditions, local parameters & element types
– Parameter file (.par)
• solver parameters & element types
.fbc
– Journal and replay file(.jrf, .rpl)
• Record of performed operations (echo file)
7
.rpl
.par
.jrf
Mouse Usage
•
•
‘Dynamic’ viewing mode (click and drag)
– left:
rotate (about a point)
– middle:
translate
– right:
zoom (up-down)
screen Z-axis rotation (sideways)
– Wheel
zoom
Selection mode (click)
– left:
select (click and drag for box select)
– middle:
apply operation
– right:
unselect last selection
•
•
8
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
Utility Menus
File Menu
(file i/o)
9
Edit Menu
View Menu
Info Menu
Settings
Menu
(preferences)
Help Menu
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!
10
• Several common functions
are duplicated as utility
icons:
Open Project
Open/Save/Close
Geometry
Open/Save/Close
Mesh
Save Project
Open/Save/Close
Blocking
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
11
– Local Coordinate System
•
•
•
•
•
•
•
Used by:
Select location
Measuring
Node/point movement/creation
Alignment
Loads
Transformation
– Surface display
• Wireframe
• Solid
• Transparent
Help
•
•
• Bubble explanation with cursor
positioning
12
Menu Driven
– Searchable
– Includes tutorials
– Programmers guide (for ICEM
CFD/Tcl scripting procedures)
Hyper-link to specific topic
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.
Output
13
Set Boundary Conditions and Parameters
Write mesh for 100+ solvers.
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
Set constraints,
displacements, define
contacts, initial velocity,
rigid walls
Constraints
Loads
Solve
options
14
Set force, pressure and
temperature loads
Set parameters, attributes,
create subcases, write out
input file, run solver
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
Select By Part
Geometry
Polygon
Select all
Cancel
Flood fill to Angle
Only visible
Entity Filter
Flood fill to Curve
Mesh attached to Geometry
Mesh
Circle
Entire/Partial toggle
Blocks
Toggle Dynamic
Mode (F9)
15
By Subset
From
Corners
Set Flood Fill
All Shells
angle
Toggle between mesh and geometry
Faceted
Geometry
Segments
In between
segments
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
•
•
16
– 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
Model Tree: Parts
•
•
•
17
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
General Order of Workflow
18
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
• 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
19
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
20
2. Geometry Handling
21
Geometry handling
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
22
This Jet engine model was built solely with
ICEM CFD geometry tools
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!
• 3rd-party import
• Parasolid
– ACIS (.sat)
• STEP/IGES
• GEMS
– DWG/DXF
• 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 3rd 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
23
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
24
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)
25
Tri tolerance =
0.1
Tri tolerance = 0.001
Geometry Creation Tools
First 3 icons to
create geometry
•
•
•
•
•
•
•
•
•
•
26
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
Surface Parameter
Surface-Surface Intersection
Project Curve on Surface
Segment Curve
Concatenate Curves
Surface Boundary Extraction
Modify Curves
Create Midline
Create Section Curves
•
•
•
•
•
•
•
•
•
•
•
•
•
Untrim Surface
Curtain Surface
Extend Surface
Geometry
Simplification
–
–
From Curves
Curve Driven
Sweep Surface
Surface of Revolution
Loft Surface Over
Several Curves
Offset Surface
Midsurface
Segment/Trim Surface
Merge/Reapproximate
Surface
•
Convex Hull
Cartesian
Shrinkwrap
Create Std
Geometry
–
–
–
–
–
–
Sphere
Box
Cylinder
Plane
Disc
Trim normal to
curve
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
27
Faceted Geometry Handling
Create/Modify Faceted
•
•
•
•
•
•
•
•
•
•
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•
•
Coarsen Surface
Create new Surface •
Merge Edges
•
Split Edges
•
Swap Edges
•
Move Nodes
Merge Nodes
•
•
28
Create Triangles
Delete Triangles
Split Triangles
•
•
•
•
•
•
Align Edge to Curve
Close Faceted Holes
Trim by Screen Loop
Trim by Surface Loop
Repair Surface
Create Curve
Restrict Triangles
Delete Triangles
Move to new Surface
Move to new Surface
Merge Surfaces
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
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 1/2500th 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
30
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?
Build Topology
31
Yellow curves
Now you can find the indicate that the
surface is probably
hole
missing or the gap is
greater than the
tolerance
Red curves indicate that
surfaces meet within
the tolerance setting
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
Filtering
Tetra octree and patch
dependent surface
mesher enforce nodes on
the curves
Needs smaller
mesh size at
fillets
32
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
Check off to disable
segmenting
33
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 1/10th 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
34
0.1
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
35
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)
36
3. Shell Meshing
37
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
38
• 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
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
•
Mesh tab
•
Global Mesh Size
–
For entire model
–
Scale factor
• Global setting by which many local settings are
multiplied
• Good for scaling overall mesh
–
•
Parameters relative to
scale factor
– Max size
– Min size limit
– Max deviation
39
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
Global Shell Meshing Parameters
•
40
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
•
•
•
•
41
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)
Or set on
If done in the Part Mesh Setup spreadsheet you must toggle on Apply
individual
inflation parameters to curves
curves
Local Surface Mesh Setup
•
42
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
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
– Initial spacing from either curve end
– Bunching laws
Side 1 – Expansion ratios from either curve
end
– Matching of node spacing to
adjacent curves
– For a better description, refer to the
Side 2
Hexa chapter – Edge Parameters
Arrow
• Select curves first, middle mouse to
shows side
accept selection, then type in parameters/
1 and side
sizes - Apply
Display
• Right mouse select
in Model Tree,
Curves -> Curve
Tetra/Hexa Sizes
or
• Curve Node
Spacing
Tetra sizes
2
43
Node spacing
Local Curve Mesh Setup – Dynamic and Copy
•
44
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
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
45
• Set in Global
Mesh Setup
or locally
using Surface
Mesh Setup
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!
loop 3
loop 2
– 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)
loop 1
Filtered or
deleted curves
(dormant)
• 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
Build topology MUST be done first to build
surface connectivity and curves
46
Patch Dependent – Common Options
•
•
Surrounding mesh done afterwards is
conformal to existing mesh
47
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
– 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
Patch Dependent Mesher - Boundary option
•
•
48
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
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
•
•
•
•
•
•
49
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
Matches up with
previously meshed
surfaces
50
Volume
around is
first
meshed
Nearest nodes projected
to surface and only
surface mesh is left
Autoblock
•
Surface (2D) blocks are created automatically
from each surface
–
–
•
Blocks structurally connected
–
•
Internal, blocks aren’t recognized or visible
For further description of blocking, refer to Hexa
chapter
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
–
–
•
51
Ignore size
Mapped or free (unstructured as in patch dependent)
Build Topology MUST be run beforehand
Shrinkwrap
•
•
•
•
•
•
•
52
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)
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
– All Tri
– Quad w/one Tri
• Almost all quad except with one tri per surface
Global
• Single tri allows transition between uneven
settings
mesh distribution on loop edges
• Where pure quad will fail
– Quad Dominant
• Allows for several transition triangles
• Very useful in surface meshing complicated
surfaces where a pure quad mesh may have poor
Local
quality
surface
– All Quad
settings
• These mesh types will look different with the different
mesh methods
All quad,
autoblock
53
Examples
done with
patch
dependent
mesher
Compute Mesh
•
•
54
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
4. Volume Meshing
55
Introduction to Volume Meshing
• 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
– Both geometry and shell mesh
– Start from existing shell mesh
– Start from just geometry
• Delauney/T-grid
• Octree tetra
• Octree tetra
– Quick
– Portions of model
– Robust
• Advancing Front
already meshed
– Smoother gradients, size transition
– Walk over features
– Set sizes on rest
• Hex Core
• Cartesian
– Prism layers
•
Hex
Dominant
– Fastest
• “Prism”
• Have to set sizes
56
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
• Define density regions
(optional)
• Applying mesh size
within volume where
geometry doesn’t
exist
57
• 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
• Compute Prism (optional)
– As separate process
– Also option to run automatically
following tetra creation
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
58
– User selects surfaces that form a closed volume
Mesh Types
• 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
59
Pure tetra
Tetra/Prism
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
• 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
60
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
61
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
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
Mesh detail
• Delete curves to ignore hard edges
• Or filter points/curves under Build Diagnostic Topology
Sliver ignored
Geometry
62
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
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.
63
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
Material
point
•User defined
volumes kept
•ORFN region is
discarded
64
Tetra Process, Cont’d
•
65
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
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 (1/10th 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 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
66
Using Points and Curves with Tetra Octree
• Curves and points included
• Mesh size specified on curves and
surfaces
Mesh captures detail
Coarse mesh ‘walks over’ detail in
surface model
• Curves and points not included
• Mesh size specified only on surfaces
67
• 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
Tetra Octree - Options
68
• 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)
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
69
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
70
Auto subdivision at
tighter radius of
curvature
Curvature Based Refinement
•
•
71
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
– 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
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
Cells in Gap = 5
72
Min size limit
Min size limit (1/5th smaller)
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
A
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”
c
B
73
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.
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
74
Split edge
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
75
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
• 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
76
Mesh Methods – ANSYS TGrid
• 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
77
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
78
79
Octree
Adv.front
Delauney
ANSYS TGrid
Comparison
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
80
Density from 2 points
makes a line.
The width defines the
radius of the cylinder
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
81
5.
82
Prism Meshing
Prism Meshing
• Inflation layers
– To better simulate boundary layer effects
– Mesh orthogonal to surface with faces perpendicular to bou
•
83
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
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
Other global parameters
– Usually specify 3 of the above 4 parameters
explained later
• 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
Height ratio
ial height
Total height
(r)
– Individual surface/curve height/ratio/layers will override these global de
faults if set
Initial height
(h)
84
Growth Law Comparison
• The growth rate of Wb-exponential is greater than exponential
• The growth rate of exponential is greater than linear
Linear
85
Exponential
Wb-Exponential
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
86
Initial
height = 0
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
87
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)
Only one
volume part
selected
– For interior surface mesh, this defines the
–
allowable volumes for extrusion
Selecting no volume parts has the same result as
selecting all volume parts
Edge of Interior surface
Both or no volume parts
selected
88
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
Collisions occurred when the height was
0.4 on all surfaces
89
Height = 0.2
No collisions after
Setting Prism Parameters on Curve
s
• 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
90
Height = 0.003
Run Prism
91
•
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.
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
92
Prism – Quality Control Options
93
•
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:
See next slides
height = h(r)(n-1) (m)(n-1)
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 60o), may not have space for a fillet ratio
Fillet Ratio = r/h
r
Fillet Ratio = 0.0
94
Fillet Ratio = 0.5
Fillet Ratio = 1.0
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 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.
Pyramids
160 o
Original mesh
95
Max prism angle = 180
o
o
Max prism angle = 140 .
Prism Options – Max Prism Angle - Continued
•
o
A high (up to 180 ) Max Prism Angle keeps the prism layers connected
around tight bends.
– Set this at 180 to prevent pyramids where possible
Max Prism Angle = 140
96
Max Prism Angle = 180
Prism Options – Max Height Over Base
Height
(h)
– Restricts prism aspect ratio
– Prism layers stop growing in regions where prism aspect ratio would exc
eed specified value
Base (b)
• Number of prism layers would not be preserved locally
– Mesh is made conformal with pyramids at prism boundaries
h/b
– Acceptable values vary widely (typically 0.5 – 8)
Largest height over
smallest base length
Pyramids
Max Height Over Base not set
97
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
Height
(h)
– 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)
Limit factor not set
98
h/b
Largest height over
smallest base length
Limit factor = 0.5
Prism Options-Part Control
•
•
•
•
99
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
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)
100
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
101
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 1st
ts
step
– Set PENTA_6 to Smooth
– Decrease the Up to quality value so as not to distort prism elements t
oo much
2nd
step
The prisms get compromised a
bit when everything is on
smooth
102
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
103
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
104
6. Mesh Preparation Before Output to
Solver
•
105
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
106
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)
107
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
108
Mesh Checks - Possible Problems
•
•
•
•
•
•
109
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
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
110
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
111
Selecting a histogram
bar will display the
elements in that range
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
112
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
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
113
defined tolerance – absolute or relative to minimum
edge length of mesh elements
7.
114
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
115
ABAQUS
ACE-U
ADINA
AUTOCFD
AcFlux
ACUSOLVE
VULCAN
SCRYU
AIRFLO3D ALPHA-FLOW ANSYS
ATTILA
WIND
STARCD
BAGGER
CEDRE
CFD-ACE
CFDesign
WINDMASTER
TDF
CFD++
CFL3D
CFX-4
CFX-5
CFX-TASCflow ZEN
SC/Tetra
CGNS
CHAD
C-MOLD
COBALT
COMCO
USM3D
STARS
CONCERT3D
CRSOL
CRUNCH
CSP
DATEX
USMKV3V
TGRID
DSMC-SANDIA DTF
EM
EXODUS
FANSC
VECTIS
SpecElem
FASTEST-3D
FASTU
FENFLOSS
FIDAP
FIRE
VRML
STL
FLEX
FLOTRAN FLOWCART
FLUENT V6
GASP
GLS3D(ADH) GMTEC
GSMAC-DF
Trio_U
SPECTRUM-CENTRIC
GUST
HAWK
HDF
IBM-BEM
ICAT
TSAR
TEAM
ICU
IDEAS
IMPNS
INCA
iPLES
UGRID
TRANAIR
KIVA-3
KIVA-4
LAURA
LL-DYNA3D
LS-DYNA3D
UH3D
POLY3D
MACS
MAGREC
MAZe
MOUSE
MULTIBLOCK
USA
POPINDA
N3SNATUR
NASTRAN NEKTON
NOPO
NPARC
SAUNA
PRECISE
NSU3D
NS3D
PAB3D
PARC
SPLITFLOW
RADIOSS
PATRAN
PHOENICS PLOT3D
PMARC
POLYFLOW
TNO
RTT
NUMECA
ACRi
FLOW-LOGIC FLUENT V4
VSAERO/USAERO TLNS3Dmb
Mesh Formats
•
i j k
116
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
Boundary Conditions
•
Part highlights white
when selected
BC’s shown for solver Fluent_V6
117
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)
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
118
Write Input
•
119
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
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
•
120
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
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
•
121
Can submit solver run if the corresponding
environment variable is set to find the solver
executable
– For Ansys, it is ANSYS_EXEC_PATH
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
122
8.
123
ICEM CFD Hexa
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
Geometry
124
Mesh without
projection
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
– Combinations of top-down and bottom-up methods can be used
125
Geometry Requirements for Hexa
Use same geometry (tetin) as used with Tetra
– Does not necessarily need to be a completely enclosed
volume
Blocking
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
126
Associating Face to Surface
to a dummy point family or
Interpolation can effectively
mesh where geometry doesn’t
exist
Geometry/Blocking Nomenclature
• Geometry
• Blocking
– Point
– Vertex
– Curve
– Edge
– Surface
– Face
– Volume
– 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
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
129
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
130
– Can always convert blocks between free and mapped afterward:
Edit Block > Convert Block Type
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
131
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
132
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
133
Moving Vertices of Different Association
• 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
134
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
135
Edge Parameters
Spacing 2 = 1.0
Side 2
Ratio 2 = 1.5
Arrow
indicates
side 1 and
side 2
Ratio 1 = 1.5
17 meshing
laws
Spacing 1 = 1.0
Side 1
Side 1 (base of arrow)
params
Requested
Actual
Side 2 (head of
arrow) params
Spacing can be linked to
another edge
136
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
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
137
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
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
139
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
• L-grid (quarter O-grid)
•
Reduce skew where a block corner must lie on a continuous curve/surface
• Cylinders
• Complex geometries
•
Improves efficiency of node clustering near walls for CFD applications
No O-grid
140
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
Select specific blocks or
around face, edge, or
vertex
141
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
O-grid passes
through this face
142
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
143
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
144
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
Selected edge
factor = 1
145
Factor = 0.3
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
146
Right mouse click on the histogram to
access options like show, replot, or done
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
147
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
148
Resets everything to fully visible
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
149
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
1
2
2
150
1
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
151
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
Select these
vertices
152
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
All visible vertices not
sharing the reference
index will be moved
153
Move vertices
in this plane
Select this
edge
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
154
Surface mesh and
scan planes
Create Block – Wedge
Degenerate
– Select 6 vertices or locations
5
– Order is important (see picture)
– Grid lines converge at vertices 1 and 4
– Results in penta6 (prism) elements along one edge
Quarter-O-Grid
2
4
1
6
3
– Results in all Hexa “wedge” (Y configuration)
Degenerate
155
Quarter-O-Grid
Create Block – Swept Block
•
•
156
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
6
5
2
1
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
Example: meshing
around knife-edge
wings
157
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
Select
these two
vertices
158
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
21.1765o
(360/17)
Base (0 0 0)
159
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
2 Ways of Blocking the Same Geometry
• Creating a fork by Merge vertices
Delete
block
Merge 2 vertex
pairs
161
Merge 2
more
vertex
pairs
2 Ways of Blocking the Same Geometry
• Creating a fork by Extrude faces
Extrude 1
162
Extrude 2
Associate
Multiple Ways of Blocking the Same Geometry
• Creating a fork with Top-down methods
split
Two
quarter
O-grids
One
quarter
O-grid
One
quarter
O-grid
163
Delete
blocks
Move
vertices
Topology
• What is common in these?
– Their block topology
164
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
splits
Single block
with o-grid
165
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
Select Project vertices to
have it automatically
project new visible
vertices to the nearest
place on their associated
geometric entities
166
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
Linear
Original edge
Control point
• Split edge will also override the automatic interpolation of edges during mesh
computation
167
Multiple
splits
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)
168
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
169
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
170
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
171
Create Block – Hexa (Vertex Locations)
• 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
Select 8
vertices
5
1
7
3
4
Select 2
faces
172
6
2
8
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
2
3
4
5
6
7
8
Press
middle
button
173
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
2
1
1
2
174
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
Equal number of nodes
across hole
175
Delete block permanently
Number of nodes can be
unequal
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
176
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
177
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
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
179
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
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
180
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
Select face
181
Select
edge
Face split is
normal to
selected edge
VORFN
blocks are
what gets
split
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
182
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
Select diagonally across vertices
Selected faces
Merge Face
actually merges
blocks on both
sides of the faces
183
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
184
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
감사합니다.
185
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement