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MESHING WORKSHOP
Thursday, November 13th, 2014
Metin Ozen, Ph.D., ASME Fellow
OZEN ENGINEERING, INC.
www.ozeninc.com
WHAT DO WE DO?
• Ozen Engineering, Inc. helps solve challenging and multidisciplinary engineering problems with
industry leading computational simulation technologies
• We provide advanced
• Multi-Physics FEA
• Computational Fluid Dynamics (CFD) simulations
INTRODUCTION TO ANSYS MESHING
• In this lecture we will learn:
– Process for pre-processing using ANSYS tools
– What is the ANSYS Meshing?
– Meshing Fundamentals
– How to launch ANSYS Meshing?
– ANSYS Meshing interface
– Geometry concepts
– Meshing methods
PREPROCESSING WORKFLOW
Sketches and
Planes
Geometry Import
Options
3D Operations
Bi-Directional
CAD/ Neutral
Geometry
Cleanup and
Repair
Automatic
Cleanup
Simplification,
Mid-surface,
Fluid Extraction
Extrude, Revolve,
Sweep, etc
3D Operations
Booleans,
Decompose, etc.
Import/ Geometry Creation
Geometry Modifications
Meshing Solver
Meshing Methods
Hybrid Mesh: Tet,
Prisms, Pyramids
Hexa Dominant,
Sweep meshing
Global Mesh
Settings
Local Mesh
Settings
Sizing, Controls,
etc.
Assembly
Meshing
WHAT IS ANSYS MESHING
• ANSYS Meshing is a component of ANSYS Workbench
– Meshing platform
– Combines and builds on strengths of preprocessing offerings from ANSYS:
• ICEM CFD, TGRID (Fluent Meshing), CFX-Mesh, Gambit
• Able to adapt and create Meshes for different Physics and Solvers
– CFD: Fluent, CFX and POLYFLOW
– Mechanical: Explicit dynamics, Implicit
– Electromagnetic
• Integrates directly with other WB systems
MESHING FUNDAMENTALS
• Purpose of the Mesh
– Equations are solved at cell/nodal locations
• Domain is required to be divided into discrete cells (meshed)
• Mesh Requirements
– Efficiency & Accuracy
• Refine (smaller cells) for high solution gradients and fine geometric detail.
• Coarse mesh (larger cells) elsewhere.
– Quality
• Solution accuracy & stability deteriorates as mesh cells deviate from ideal shape
MESHING PROCESS IN ANSYS MESHING
• Physics, Sizing, Inflation, Pinch, …
• Sizing, Refine, Pinch, Inflation, …
• Preview surface mesh, Inflation
• Mesh metrics, Charts
LAUNCHING ANSYS MESHING
– ANSYS Meshing is launched within Workbench
• 2 ways :
Fluid Flow (Fluent), Fluid Flow (CFX),
From Analysis SystemsMesh
From Component Systems
Double click
Mesh in the
System
or right click
and select
Edit
GRAPHICS USER INTERFACE
Mesh Metrics
Graphics window
Section Planes
Manage views
Details view
Outline
Toolbars
Units BarEntity Details BarMessage window
Worksheet
OUTLINE
• Three default sections
– Geometry
• Bodies
– Coordinate Systems
• Default global & user defined systems
– Mesh
• Meshing operations (controls & methods)
– displayed in the order in which they are inserted
In the tree
• Right clicking on any object
• launches a context sensitive menu
• Example: contains commands to generate, preview, clear mesh etc.
DETAILS VIEW
• Accessing Object Details
– Select an object (in the Outline)
• Related information to that object are displayed in the Details View below
• Ex: Select a body (“Fluid”) in the Outline
– Details of “Fluid” : contains graphical and geometric details
• To access meshing details
– Click the Mesh object or any of the inserted objects
– The Details View provides options to
• review,
• edit or
• input values for every object in the Tree
GEOMETRY CONFIGURATION – MULTIPLE PARTS
• Geometry composed of Multiple parts
• No connection between parts (no face sharing)
‘Contact Region’ is automatically created between 2 faces
Each part meshed independently
Results in Non-conformal interface.Meshes do not match. No nodes connection.
Independent faces
Grid interface - Fluent
GGI - CFX
GEOMETRY CONFIGURATION – MULTI-BODY PARTS
– Geometry composed of multiple bodies in a part
• Depend on ‘Shared Topology method’ (in DM)
– None » Results in a none connection between the bodies (similar to multiple parts)
– Automatic
Faces in contact imprinted & fused Form a single face shared between the 2 bodies
Results in Conformal mesh
Common face acts as ‘Interior’
GEOMETRY CONFIGURATION – MULTIPLE – BODY PARTS
– Geometry composed of multiple bodies in a part– Imprints
Faces are imprinted on each other ‘like’ faces
Contact Region is automatically created
non conformal interface
For identical mesh on these faces, use ‘Match Control’Results in unconnected mesh
Grid interface - Fluent
GGI - CFX
MESHING – 3D GEOMETRY
• 3D cell Types
• First Meshing Approach
Part/Body based• Meshing occurs at part or
body level.
• Meshing Methods are
scoped to individual bodies.
• Method assignment can be
automatic or manual.
• Bodies contained in one
part are conformally
meshed.
Part/Body Methods• Tetrahedrons.
− Tetras only
• Sweep.− Prisms &
hexahedrons
• MultiZone. − Mainly hexahedron
• Hex Dominant− Not for CFD
• Automatic.− Combines any types
MESHING – 3D GEOMETRY
• Second Meshing Approach
Cut Cell MeshingAssembly Meshing• Meshes an entire model in
one process.− Assembly of parts
• Performs boolean operations.− Volume filling, intersection &
combination
− Does not require prior fluid
body definition or shared
topology.
• Conformal mesh created
across parts.
Assembly Meshing
Methods• Generate mainly
− Hexahedrons
− Tetrahedrons
Part/Body Meshing &
Assembly Meshing
not interoperable
MESHING METHODS
• In this lecture we will learn:
– Meshing Methods for Part/Body Meshing
• Assembly Meshing covered separately
– Methods & Algorithms for;
• Tetrahedral Meshing
• Hex Meshing
• 2D Meshing
– Meshing Multiple Bodies
• Selective Meshing
• Recording Meshing Order
PREPROCESSING WORKFLOW
Sketches and
Planes
Geometry Import
Options
3D Operations
Bi-Directional
CAD/ Neutral
Geometry
Cleanup and
Repair
Automatic
Cleanup
Simplification,
Mid-surface,
Fluid Extraction
Extrude, Revolve,
Sweep, etc
3D Operations
Booleans,
Decompose, etc.
Import/ Geometry Creation
Geometry Modifications
Meshing Solver
Meshing Methods
Hybrid Mesh: Tet,
Prisms, Pyramids
Hexa Dominant,
Sweep meshing
Global Mesh
Settings
Local Mesh
Settings
Sizing, Controls,
etc.
Assembly
Meshing
WHICH METHOD TO CHOOSE?
• Why Multiple Methods?• Choice depends on :
– Physics
– Geometry
– Resources
• Mesh could require just one or a combination of methods.
High aspect ratio cells (Inflation) near wall to capture
boundary layer gradients
Tet (3d) or Tri (2d) cells used here to mesh complex region
Hex (3d) or Quad (2d) cells used to mesh
simple regions
Cells refined around small geometric details
and complex flow
PATCH CONFORMING VERSUS INDEPENDENT
Patch Conforming• Clean CAD, Accurate surface mesh
Patch Independent• Dirty Geometry, defeatured surface mesh
TETRAHEDRONS METHODS
• Bottom up approach: Meshing process • Edges Faces volume
• All faces and their boundaries are respected(conformed to) and meshed
• Good for high quality (clean) CAD geometries• CAD cleanup required for dirty geometry
• Sizing is defined by global and/or local controls• Compatible with inflation
To access it• Insert Method
• Set to Tetrahedrons• Set to Patch Conforming
Patch Conforming
• Top down approach: Meshing process • Volume meshed first projected on to faces
& edges• Faces, edges & vertices not necessarily conformed
• Controlled by tolerance and scoping of Named Selection, load or other object
• Good for gross de-featuring of poor quality (dirty) CAD geometries
• Method Details contain sizing controls• Compatible with inflation
To access it• Insert Method
• Set to Tetrahedrons• Set to Patch Independent
Patch Independent
TETRAHEDRONS METHOD : CONTROL
• Mesh sizing for the Patch Conforming algorithm is defined by Global & Local Controls
• Automatic refinement based on curvature and/or proximity accessible in Global Controls• Details of Global & Local Controls covered in
separate lectures
• Choice of surfacemesher algorithmin global controls
Patch Conforming - Sizing
TETRAHEDRONS METHOD : CONTROL
• Sizing for the Patch Independent algorithm defined in Patch Independent Details
• Automatic curvature & proximity refinement option
Defeaturing Control• Set Mesh Based Defeaturing On• Set Defeaturing Tolerance• Assign Named Selections to selectively preserve
geometry
Patch Independent - Sizing
Defeaturing Tolerance off
Name Selec. assigned & defeaturing Tol = 0.02
Features > 0.02m respected
TETRAHEDRONS METHOD : ALGORITHM COMPARISON
Geometry with small details
Patch conforming : details caputred Patch independent : details ignored
Delaunay mesh - smooth growth rate Octree mesh . approximate growth rate
HEXA MESH - INTRODUCTION
Tetra mesh - 48 000 Cells
Hexa mesh - 19 000 Cells
• Hex Meshing
– Reduced element count
• Reduced run time
– Elements aligned in direction of flow
• Reduced numerical error
• Initial Requirements
– Clean geometry
– May require geometric decomposition
SWEEP MESHING
• Generates hex/wedge elements• Meshes source surfaces Sweeps through to the
target• Body must have topologically identical source
and target faces• Side faces must be mappable
• A sweep path must be identified• Only one source and one target face is allowed
• Alternative ‘thin’ sweep algorithm can have multiple source & target faces
To access it• Insert Method
• Set to Sweep
Mesh Method & Behavior
Source FaceTarget Face
Side Face(s)
Sweep Path
Sweep Direction Source face Target face
SWEEP MESHING
Automatic• Source & Target faces identified automatically
• Requires that the mesher find the sweeping direction
• Manual source & Manual source and target • User selection• Source face colored in red• Target face colored in blue• Rotational Sweeping
Sweep around an axis Requires selection of both - Source & target
Note • Specifying both Source & Target accelerate
meshing
Source & Target selection
Define the nbr of intervals on the
side face(s)
Sweep Path
Generation of wedges& hex elements
SWEEP MESHING
Automatic Thin & Manual Thin• Alternate sweep algorithm• Advantages Sweep multiple Source & Target faces Can perform some automatic defeaturing
• Limitations X For multibody parts only one division allowed
across the sweepX Inflation not allowedX Sweep bias not allowed
Source & Target selection
Target
Source
Faces
Source Faces imprintedon Target
SWEEP MESHING
Compatibility with Src/Trg Selection
X
XX
Use of Inflation• Defined on source face ( NOT on target one)• From boundary edges (2D)• Swept through volume
Sweep and Inflation
Sweep Mesh - No Inflation
Sweep Mesh with Inflation
SWEEP MESHING
• Automatic detection of sweepable bodies• Rotational ones are not identified
• Identification method• Right click on mesh object
• Outline tree• Select : Sweepable Bodies
Identifying sweepable bodies
Geometry
Right mouse button
Sweepable bodies in green color
• Decompose bodies into multi-simple topologicalshapes
• Perform decomposition in CAD/DM
Making bodies sweepable
Unsweepable
Decompose
Sweep Mesh
MULTIZONE MESHING
• Based on blocking approach (ANSYS ICEM CFD Hexa)
• Automatically decomposes geometry into blocks• Generates structured hexa mesh where block
topology permits• Remaining region filled with unstructured
Hexa Core or Tetra or Hexa dominant mesh• Src/Trg Selection
• Automatic or Manual source selection• Multiple source faces • Select Target faces as “Source”
• Compatible with 3D Inflation
To access it• Insert Method Set to Multizone
Mesh Method & Behavior
MULTIZONE MESHING
Determines which elements to use• Hexa
• Default • Only Hexahedral elements are generated
• Hexa/prism• For quality and transition, triangles will be
inserted on the surface mesh (sources)• Prism
• Only prisms will be generated• Useful when the adjacent volume is filled in
with tet mesh
Mapped Mesh Type
Geometry
Hexa
Hexa - Prism
MULTIZONE MESHING
Specify a method to create the surface mesh• Uniform
• Uses a recursive loop-splitting method which creates a highly uniform mesh
• Pave• Creates a good quality mesh on faces with high
curvature, and also when neighboring edges have a high aspect ratio
• Program controlled• Combination of Uniform and Pave methods• depends on the mesh sizes set and face
properties
Surface Mesh Method
Geometry
Uniform
Pave
AUTOMATIC METHOD
• Combination of Tetrahedron Patch Conforming and Sweep Method• Automatically identifies sweepable bodies and
creates sweep mesh• All non-sweepable bodies meshed using
tetrahedron Patch Conformal method
• Compatible with inflation
To access it• Default method • Insert method Set to Automatic
Mesh Method & Behavior
2D MESHING
• Quadrilateral Dominant & Triangles• Patch conforming methods
• MultiZone Quad/tri• Patch Independent Methods• Associated with face mesh type
• All Tri• Quad/tri• All Quad
• Advanced size function & local size controls are supported
Mesh Method & Behavior
Automatic Triangles
MultiZoneQuad/Tri
MultiZoneQuad
2D MESHING
• Mapped Surface Meshes• Local mesh controls
• Fully Mapped surface meshes• Specified edge sizing/intervals
Control
• Boundary edges are inflated• Global & local inflation controls are supported
Inflation
2D Mappedmesh
2D MESH SOLVER GUIDELINES
• For a 2D analysis in Fluent generate the mesh in the XY plane • Z = 0
• For axisymmetric applications y 0 and make sure that the domain is axisymmetric about x axis
• In ANSYS Meshing, by default, a thickness is defined for a surface body and is visible when the view is not normal to the XY Plane. • This is purely graphical – no thickness will be
present when the mesh is exported into the Fluent 2D solver
ANSYS Fluent
• For 2D analysis in CFX, create a volume mesh (using Sweep) • 1 element thick in the symmetry direction, i.e.,
• Thin Block for Planar 2D
• Thin Wedge (< 5°) for 2D Axis-symmetric
ANSYS CFX
SELECTIVE MESHING
• What is ?– Selectively picking bodies and meshing them incrementally
• Why ?– Bodies can be meshed individually
– Mesh seeding from meshed bodies influences neighboring bodies (user has control)
– Automated meshing can be used at any time to mesh all remaining bodies
– When controls are added, only affected body meshes require remeshing
– Selective body updating
– Extensive mesh method interoperability
SELECTIVE MESHING
Clear meshes on individual bodiesGenerate meshes on individual bodies• Subsequent bodies will use the attached face
mesh• The meshing results (cell types) will depend on
the meshing order• Adjust/add controls – able to remesh only
affected body
• Select body(s) • Right click
Local Meshing
Meshing first the pipe then the block
Meshing first the block then the pipe
SELECTIVE MESHING
• Use it to record the order of meshing to automate future use
• Right click Mesh in the Outline to access it
• A Worksheet is generated • Record mesh operations as ordered steps
• Named Selections are automatically created for each meshed body for reference in the Worksheet
Recording Mesh OperationsExample : Meshing cylinder first and then block
SELECTIVE MESHING
• Remeshing only bodies that have changed
• Access option through Tools > Options• No: All geometry updated, all bodies remeshed.• Associatively: Accommodates for body
topology change (add/delete) (slower)• Non-Associatively: Assumes no topology
change (faster)
Selective Body Updating
Example :Geometricchange to block
GLOBAL MESH CONTROLS• In this section, we will learn about:
– Introduction to Global Mesh Controls
– Defaults
– General Sizing Controls & Advanced Size Functions
– Global Inflation
– Assembly Meshing Controls
– Statistics
PREPROCESSING WORKFLOW
Sketches and
Planes
Geometry Import
Options
3D Operations
Bi-Directional
CAD/ Neutral
Geometry
Cleanup and
Repair
Automatic
Cleanup
Simplification,
Mid-surface,
Fluid Extraction
Extrude, Revolve,
Sweep, etc
3D Operations
Booleans,
Decompose, etc.
Import/ Geometry Creation
Geometry Modifications
Meshing Solver
Meshing Methods
Hybrid Mesh: Tet,
Prisms, Pyramids
Hexa Dominant,
Sweep meshing
Global Mesh
Settings
Local Mesh
Settings
Sizing, Controls,
etc.
Assembly
Meshing
MESHING PROCESS IN ANSYS MESHING
GLOBAL MESH CONTROLS (1)
– Global mesh controls are used to make global adjustment in the meshing strategy, which includes sizing functions, inflation, smoothing, defeaturing, parameter inputs, assembly meshing inputs, etc.
– Minimal inputs
• Automatically calculates global element sizes based on the smallest geometric entity
• Smart defaults are chosen based on physics preference
– Makes global adjustments for required level of mesh refinement
– Advanced Size Functions for resolving regions with curvatures and proximity of surfaces
Smart defaults !
GLOBAL MESH CONTROLS (2)• Physics Based Settings
– Physics and Solver Preferences• Global Mesh Sizing Controls
• Relevance and Relevance Center• Advanced Size Functions• Smoothing and Transition• Span Angle Center• Curvature Normal Angle• Proximity Accuracy and Cells Across Gap
• Inflation– Inflation Option, Inflation Algorithm– Collision Avoidance– Maximum Angle, Fillet Ratio, Smoothing
• Assembly Meshing– Activation of CutCell/Tetrahedrons Meshing
• Patch Confirming Options– Activation of Advancing Front Method
• Advanced– Numer of CPUs for Parallel Part Meshing– Shape Checking– Element midside nodes
• Defeaturing– Pinch based– Automatic Mesh Based
• Statistics– Mesh statistics, Quality criteria
GLOBAL MESH CONTROLS (3)
DEFAULTS• Four options under “Physics Preference”
– CFD, Mechanical, Explicit and Electromagnetic
• Three options under “Solver Preference” when CFD is selected:
– Fluent, CFX and POLYFLOW
• Mesh setting defaults are automatically adjusted to suit the “Physics Preference” and “Solver Preference”
• Assembly Meshing is active only when Physics Preference is CFD and Solver Preference is Fluent
• Controls the growth and distribution of mesh in important regions of high curvature or close proximity of surfaces
• Five Options:
– Off. Unavailable for Assembly Meshing
– Proximity and Curvature
– Curvature
– Proximity
– Fixed
• When CutCell Meshing is active with ‘Proximity’ or ‘Proximity and Curvature’ Advanced Size Function (ASF), Proximity Size Function Sources control is displayed to specify the regions of proximity between “Edges”, “Faces” or “Faces and Edges” in the computation of Proximity ASF
SIZING : ADVANCED SIZING FUNCTIONS
SIZING : ADVANCED SIZING FUNCTION EXAMPLES
ASF: Curvature
• Determines the Edge and Face
sizes based on Curvature Normal
Angle
• Finer Curvature Normal Angle
creates finer surface mesh
• Transition of cell size is defined by
Growth Rate
ASF: Off
• The edges are meshed with global
Element Size
• Then the edges are refined for
curvature and 2D proximity
• At the end, corresponding face and
volume mesh is generated
• Transition of cell size is defined by
Transition
ASF: Proximity
• Controls the mesh resolution on
proximity regions in the model
• Fits in specified number of elements in
the narrow gaps
• Higher Number of Cells Across Gap
creates more refined surface mesh
• Transition of cell size is defined by
Growth Rate
Element Size
• Element size used for the entire model
– This size will be used for meshing all edges, faces and bodies
• Default value based on Relevance and Initial Size Seed
– User can input required value as per geometry dimensions
SIZING : ELEMENT SIZE
Element size option
available when Advanced
Size Function is not used
SIZING : MIN AND MAX SIZE• Min Size
– Minimum element size that the size function will generate
– Some element sizes may be smaller than this size depending on the edge length
• Max Face Size
– Maximum face size that the size function will generate
– Not supported by CutCell meshing
• Max Size
– Maximum element size that can be grown in the interior of volume mesh
Min Size ≤ Max Face Size ≤ Max Size Max Size
Mouse Pointer serves to estimate
mesh sizes
Max Face Size
Min Size
• Define the ratio between sizes of adjacent cells
• On surfaces and inside the volumes
SIZING : GROWTH RATE
Mesh size:
GR = 1.1 : 1,263,297 cells
GR = 1.2 : 587,026 cells
GR = 1.3 : 392,061 cells
Growth Rate =
1.1
Growth Rate = 1.2
(Default)
Growth Rate =
1.3
SIZING : TRANSITION• Controls the rate at which elements grow
• Two level control for transition
• Slow (Default for CFD, Explicit), produces smooth transitions
• Fast (Default for Mechanical and Electromagnetic), produces more abrupt transitions
• Not available for Cutcell meshing
• Hidden for sheet models, ignored for assemblies containing sheets, when ASF is On
Fast Slow
SIZING : SPAN ANGLE CENTER
• Controls curvature based refinement for Edges
• Three options and corresponding span angle ranges are
– Coarse: 91° to 60°
– Medium: 75° to 24°
– Fine: 36° to 12°
• Not available for Cutcell meshing
Coarse Medium Fine
INFLATION• Inflation
– Used to generate thin cells adjacent to boundaries
– Required for capture of wall adjacent boundary layers
– Resolve viscous boundary layer in CFD
– Resolve thin air gaps in Electromagnetic analysis
– Resolve high stress concentration regions in Structures
– Cells are created by ‘inflating’ from the surface mesh into the volume (3d) or inflating from the boundary edge onto the face (2d)
– Options to control growth
INFLATION : AUTOMATIC INFLATION• Three options
• None
– Select this for manual inflation settings using local mesh controls
• Program Controlled
All the faces are selected for inflation except:
– Faces scoped to a Named Selection
– Faces with manual inflation defined
– Faces in contact regions
– Faces in symmetry
– Faces that belong to a part or body that has a mesh method defined that does not support 3D inflation, such as sweep or hex-dominant
– Faces in sheet bodies
• All Faces in chosen Named Selection: can grow inflation layers from faces grouped in one named selection
INFLATION : INFLATION OPTIONS• Five options:
Total Thickness
Maintains constant total height of inflation layer
throughout
First Layer Thickness
Maintains constant first cell height
throughout
Smooth Transition
Maintains smooth volumetric growth between each
adjacent layer. Total thickness depends on the
variation of base surface mesh sizes (Default)
First Aspect Ratio
Controls the heights of the inflation layers by
defining the aspect ratio of the inflations that are
extruded from the inflation base
Last Aspect Ratio
Creates inflation layers using the values of the first
layer height, maximum layers, and aspect ratio
controls
All available for Patch Conformal (PC ) tets and Assembly
meshing
Smooth Transition
INFLATION : INFLATION ALGORITHMS• Two Algorithms
– Post
– Pre
• Patch independent meshes (including Assembly) use Post
Preview Inflation
is available only
with Pre Algorithm
Post
Pre
• Surface mesh is inflated first, then rest of
the volume mesh grows
• Default method for Patch Conforming
Tetrahedrons
• First Tet grows then Inflation process starts
• Tet mesh is undisturbed, if the inflation options are altered
• Default option for Patch Independent Tetrahedrons
INFLATION: AUTOMATIC INFLATION EXAMPLE
MultiZone
Patch Conforming Tets
Cutcell
INFLATION : ADVANCED OPTIONSCollision Avoidance: Control to detect proximity regions and adjust the cells in the inflation layer.
• None
• Does not check for proximity regions
• Layer Compression (Default for Fluent)
• Compresses inflation layers in the proximity regions
• Maintains the given number of layers in the proximity regions
• May stair-step if needed (will give a warning)
• Stair Stepping (Default for CFX)
• Inflation layers are stair stepped in the proximity regions
• Removing layers locally in steps to avoid collisions as well as bad quality at sharp corners
When Cutcell meshing is used, both Layer Compression and Stair Steeping algorithms are used depending on the geometry complexity.
Generates combination of
Pyramids and Tets to fill
the stair step
INFLATION : COLLISION AVOIDANCE EXAMPLE
Example Stair SteppingLayer Compression
DEFEATURING• Removes small geometry features meeting the tolerances using
Pinch or/and Automatic Mesh Based Defeaturing controls in order to improve the mesh quality. Not all meshing methods can take advantage of these controls.
• Pinch Tolerance control removes small features at the mesh level depending on the Pinch Tolerance value provided. ANSYS Meshing offers global and manual pinch controls.
• Automatic Mesh Based Defeaturing (AMBD) when it is ‘On’, features smaller than or equal to the value of Defeaturing Tolerance are removed automatically.
AMBD Off AMBD
OnWith Pinch
STATISTICS
• Option to view the mesh quality metric
• Exhaustive list of quality metrics
• Orthogonal Quality mesh quality metrics
• Option to view the Mesh Metric chart
– Intuitive controls available under Mesh Metric Chart
– Various options to explore under the ‘Controls’
• See lecture 7 for details
LOCAL MESH CONTROLS
• In this section, we will learn about:– Local mesh controls (Mesh sizing, Refinement, Match
control, Inflation, etc)
– How to apply local controls?
– Effect of local controls on mesh
PREPROCESSING WORKFLOW
Sketches and
Planes
Geometry Import
Options
3D Operations
Bi-Directional
CAD/ Neutral
Geometry
Cleanup and
Repair
Automatic
Cleanup
Simplification,
Mid-surface,
Fluid Extraction
Extrude, Revolve,
Sweep, etc
3D Operations
Booleans,
Decompose, etc.
Import/ Geometry Creation
Geometry Modifications
Meshing Solver
Meshing Methods
Hybrid Mesh: Tet,
Prisms, Pyramids
Hexa Dominant,
Sweep meshing
Global Mesh
Settings
Local Mesh
Settings
Sizing, Controls,
etc.
Assembly
Meshing
MESHING PROCESS IN ANSYS MESHING
LOCAL MESH CONTROLSControl the mesh locally
• Depends on the “Mesh Method” used
Local Mesh Controls are:
• Sizing- For Vertex, Edge, Face and Body
• Contact Sizing - For Edge and face
• Refinement- For Vertex, Edge and Face
• Mapped Face Meshing - For Face
• Match Control - For Edge and Face
• Pinch - For Vertex and Edge
• Inflation - For Edge and Face
Only Sizing and Inflation local controls are available for CutCell meshing
The latest control added on a particular entity overrides
any prior controls
Non-CutCell meshing local controls
CutCell meshing local controls
SIZINGRecommended for locally defining the mesh sizes
You can only scope sizing to one geometry entity type at a time
• For example: you can apply sizing to a number of edges or a number of faces, but not a mix of edges and faces.
Four Types of Sizing option
• Element Size specifies average element edge length on bodies, faces or edges
• Number of Divisions specifies number of elements on edge(s)
• Body of Influence specifies average element size within a body
• Sphere of Influence specifies average element size within the sphere
Sizing options vary depending on the entity type chosen
Only Element Size type is available for CutCell
meshing
Entity/Option Element Size Number of Divisions Body of Influence Sphere of Influence
Vertices x
Edges x x x
Faces x x
Bodies x x x
Advanced Size
Function in
Global settings
should be
disabled
Requires a
Coordinate
system for
the sphere
SIZING : EDGES
Sizing Type: Element Size
Sizing Type: Number of Divisions Edge meshed with
constant element size of
60mm
Edge meshed with 10
elements
The Curvature Normal Angle and/or the Growth Rate maybe
not displayed depending on the ASF used
SIZING : EDGESBias Type and Bias Factor
Specify the grading scheme and factor
• Bias Type: grading of elements towards one end, both ends, or the center
• Bias Option:
– Bias Factor: is the ratio of the largest element to the smallest element
– Smooth Transition: defined by Growth Rate which is ratio of size of an element with size of previous element. (Growth Rate = Bias Factor^(1(n-1))
SIZING : EDGESBehavior
Soft: Sizing will be influenced by global sizing functions such as those based on proximity and/or curvature as well as local mesh controls
Hard: Size control is strictly adhered to
Influenced by global
Proximity advanced
size function.
Soft Hard
No influence from other
global settings
• Transition between hard edges (or any edge with bias) and adjacent edge and face
meshes may be abrupt
• Hard edges or edges with bias will override Max Face Size and Max Size properties
Number of Division = 4 Number of Division = 4
• Element Size
•Defines the maximum element size on the face
• Element Size
•Defines the maximum cell size on the Body
SIZING : FACES & BODY (VOLUME)
• On Vertex
• Available with or without Advanced Size Functions
• Sets the average element size around the selected vertex
• Inputs: – Sphere radius and Element size
– Center of the sphere is defined by a model vertex
• On Bodies
• Available with or without Advanced Size Functions
• Constant element size is applied within the confines of a sphere
• Use coordinate system to define the center of the Sphere
SIZING : SPHERE OF INFLUENCE
SIZING : BODIES OF INFLUENCEBodies of influence (BOI)
– Lines, surfaces and solid bodies can be used to refine the mesh
– Accessible when ASF is On
– Not available for CutCell meshing
The ‘Body of Influence’ itself
will not be meshed
Line BOIs
Surface BOI Solid BOI
Without BOIs
MAPPED FACE MESHING• Creates structured meshes on selected mappable surfaces
– Mapped Face Meshing with advanced control is supported for
• Sweep, Patch Conforming, Hexa Dominant
• Quad Dominant and Triangles
– Mapped Face Meshing with basic control is supported for
• MultiZone
• Uniform Quad/Tri and Uniform Quad
– RMB on Mesh and Show/Mappable Faces to display all
mappable faces
If Mapped Face Meshing fails, ( ) icon
appears adjacent to corresponding object in
the Tree outline. The mesh will still be created
but will ignore this control.
• If face is defined by two loops, then the “Internal Number of Divisions” field is activated
– User can specify the number of divisions across the annular region
– Also useful for defining number of divisions along sweeping direction for Multizone when there are no side edges
MAPPED FACE MESHING: INTERNAL NO. OF DIVISIONS
Mapped face is swept to create
pure hex mesh
MATCH CONTROL• Define periodicity on faces (3D) or edges (2D)
• The two faces or edges should be topologically and geometrically the same
• A match control can only be assigned to one unique face/edge pair
• Match controls are not supported with Post Inflation Algorithm
• Match Control with Patch Independent tetrahedrons not supported yet
– Two types of match controls available:
• Cyclic and
• Arbitrary
– Not available for CutCell meshing
If ‘Match Control’ fails, ( ) icon appears adjacent to corresponding
object in the outline Tree, however the mesh is created ignoring it
Matching face
mesh
MATCH CONTROL: CYCLIC
• Define Rotational periodic
Model is symmetrical at 90°
Full Model Periodic Model
Selected Faces for
Match control
Matching face mesh
PINCH
• To improve quality Pinch control removes small features (edges or narrow regions) at the mesh level
• The Pinch feature is supported for the following mesh methods:
• Patch Conforming Tetrahedrons
• Thin Solid Sweeps
• Hex Dominant meshing
• Quad Dominant Surface Meshing
• Triangles Surface meshing
– Not supported for CutCell meshing
INFLATIONUsed to generate prism layers (as explained in Global settings chapter)
Inflation layer can be applied to faces or bodies using respectively edges or faces as the boundary
Inflation layer grown on edge boundary (red)
Inflation layer grown on face boundary (red)
MESH QUALITY
• In this section, we will learn:
– Impact of the Mesh Quality on the Solution
– Quality criteria
– Methods for checking the mesh quality
– Tools to improve quality in Meshing
– Concept of Assembly Meshing
– Assembly Meshing Methods & Controls
PREPROCESSING WORKFLOW
Sketches and
Planes
Geometry Import
Options
3D Operations
Bi-Directional
CAD/ Neutral
Geometry
Cleanup and
Repair
Automatic
Cleanup
Simplification,
Mid-surface,
Fluid Extraction
Extrude, Revolve,
Sweep, etc
3D Operations
Booleans,
Decompose, etc.
Import/ Geometry Creation
Geometry Modifications
Meshing Solver
Meshing Methods
Hybrid Mesh: Tet,
Prisms, Pyramids
Hexa Dominant,
Sweep meshing
Global Mesh
Settings
Local Mesh
Settings
Sizing, Controls,
etc.
Assembly
Meshing
Check Mesh Quality
MESHING PROCESS IN ANSYS MESHING
• Good quality mesh means that…
– Mesh quality criteria are within correct range
• Orthogonal quality …
– Mesh is valid for studied physics
• Boundary layer …
– Solution is grid independent
– Important geometric details are well captured
• Bad quality mesh can cause;
– Convergence difficulties
– Bad physic description
– Diffuse solution
• User must…
– Check quality criteria and improve grid if needed
– Think about model and solver settings before generating the grid
– Perform mesh parametric study, mesh adaption …
IMPACT OF THE MESH QUALITY
IMPACT OF THE MESH QUALITY ON THE SOLUTION
• Example showing difference between a mesh with cells failing the quality criteria and a good mesh
• Unphysical values in vicinity of poor quality cells
• Diffusion example
IMPACT OF THE MESH QUALITY ON THE SOLUTION
(max,avg)CSKEW=(0.912,0.291)
(max,avg)CAR=(62.731,7.402)
(max,avg)CSKEW=(0.801,0.287)
(max,avg)CAR=(8.153,1.298)
VzMIN≈-100ft/min
VzMAX≈400ft/min
VzMIN≈-90ft/min
VzMAX≈600ft/min
Large cell size
change
Mesh
2M
esh
1
GRID DEPENDENCY
• Solution run with multiple meshes
• Note : For all runs the computed Y+ is valid for wall function (first cell not in laminar zone)
DP 0 DP 3
x8
2%
GRID DEPENDENCY
• Hexa cells can be stretched in stream direction to reduce number of cells
• Bias defined on inlet and outlet walls
• Bias defined on inlet edges
– 16 000 cells (~DP2)
– Delta P = 310 Pa (~DP3)
HEXA VS. TETRA
• Hexa: Concentration in one direction
– Angles unchanged
• Tetra: Concentration in one direction
– Angles change
• Prism: Concentration in one direction
– Angles unchanged
• Solution for boundary layer resolution
– Hybrid prism/tetra meshes
– Prism in near-wall region, tetra in volume
– Automated
– Reduced CPU-time for good boundary layer resolution
Hexa
Tetra
Prism
Prisms (near wall)
Tetra (in volume)
MESH STATISTICS AND MESH METRICS
Displays mesh information for Nodes and Elements
List of quality criteria for the Mesh Metric
• Select the required criteria to get details for quality
• It shows minimum, maximum, average and standard deviation
Different physics and different solvers have different requirements for mesh quality
Mesh metrics available in ANSYS Meshing include:
– Element Quality
– Aspect Ratio
– Jacobean Ration
– Warping Factor
– Parallel Deviation
– Maximum Corner Angle
– Skewness
– Orthogonal Quality
For Multi-Body Parts, go to corresponding body in Tree Outline
to get its separate mesh statistics per part/body
MESH QUALITY METRICS
Orthogonal Quality (OQ)
Derived directly from
Fluent solver discretization
• For a cell it is the minimum of:
computed for each face i
For the face it is computed as the minimum of computed for each edge I
Where Ai is the face normal vector and fi is a vector from the centroid of the cell to the centroid of that face, and ci is a vector from the centroid of the cell to the centroid of the adjacent cell, where ei is the vector from the centroid of the face to the centroid of the edge
At boundaries and internal walls ci is ignored in the computations of OQ
A1
A2
A3
f1
f2f3
c2c1
c3
A1
A2
A3
e1
e2e3
On cell
|||| ii
ii
fA
fA
|||| ii
ii
cA
cA
|||| ii
ii
eA
eA
On face
0 1
Worst Perfect
MESH QUALITY METRICS
Skewness
Two methods for determining skewness:
1. Equilateral Volume deviation:
Skewness =
Applies only for triangles and tetrahedrons
2. Normalized Angle deviation:
Skewness =
Where is the equiangular face/cell (60 for tets and tris, and 90 for quads and hexas)
– Applies to all cell and face shapes
– Used for hexa, prisms and pyramids
e
mine
e
emax ,180
max
min
max
optimal cell size cell size
optimal cell size
Optimal (equilateral) cell
Circumsphere
e
0 1
Perfect Worst
Actual cell
MESH QUALITY
Mesh quality recommendations
Low Orthogonal Quality or high skewness values are not recommended
Generally try to keep minimum orthogonal quality > 0.1, or maximum skewness < 0.95. However these values may be different depending on the physics and the location of the cell
Fluent reports negative cell volumes if the mesh contains degenerate cells
Skewness mesh metrics spectrum
Orthogonal Quality mesh metrics spectrum
ASPECT RATIO
2-D:
• Length / height ratio: δx/δy
3-D
• Area ratio
• Radius ratio of circumscribed / inscribed circle
Limitation for some iterative solvers
• A < 10 … 100
• (CFX: < 1000)
Large aspect ratio are accepted where there is no strong transverse gradient (boundary layer ...)
δy
δx
SMOOTHNESS
Checked in solver
• Volume Change in Fluent
– Available in Adapt/Volume
– 3D : σi = Vi / Vnb
• Expansion Factor in CFX
– Checked during mesh import
– Ratio of largest to smallest element volumes surrounding a node
Recommendation:Good: 1.0 < σ < 1.5
Fair: 1.5 < σ < 2.5
Poor: σ > 5 … 20
SECTION PLANES
Displays internal elements of the mesh
• Elements on either side of plane can be displayed
• Toggle between cut or whole elements display
• Elements on the plane
Edit Section Plane button can be used to drag section plane to new location
• Clicking on “Edit Section Plane” button will make section plane’s anchor to appear
Multiple section planes are allowed
For large meshes, it is advisable to switch to
geometry mode (click on geometry in the Tree
Outline), create the section plane and then go
back to mesh model
MESH METRIC GRAPH
• Displays Mesh Metrics graph for the element quality distribution
• Different element types are plotted with different color bars
• Can be accessed through menu bar using Metric Graph button
• Axis range can be adjusted using controls button (details next slide)
• Click on bars to view corresponding elements in the graphics window
– Use to help locate poor quality elements
MESH METRIC GRAPH CONTROLS
• Elements on Y-Axis can be plotted with two methods;
– Number of Elements
– Percentage of Volume/Area
• Options to change the range on either axis
• Specify which element types to include in graph
– Tet4 = 4 Node Linear Tetrahedron
– Hex8 = 8 Node Linear Hexahedron
– Wed6 = 6 Node Linear Wedge (Prism)
– Pyr5 = 5 Node Linear Pyramid
– Quad4 = 4 Node Linear Quadrilateral
– Tri3 = 3 Node Linear Triangle
• Te10, Hex20, Wed15, Pyr13, Quad8 & Tri6 non-linear elements
– The CFX solver calculates 3 important measures of meshquality at the start of a run and updates them each time themesh is deformed
– Mesh Orthogonality
– Aspect Ratio
– Expansion Factor
MESH QUALITY CHECK FOR CFX
+--------------------------------------------------------------------+
| Mesh Statistics |
+--------------------------------------------------------------------+
Domain Name: Air Duct
Minimum Orthogonality Angle [degrees] = 20.4 ok
Maximum Aspect Ratio = 13.5 OK
Maximum Mesh Expansion Factor = 700.4 !
Domain Name: Water Pipe
Minimum Orthogonality Angle [degrees] = 32.8 ok
Maximum Aspect Ratio = 6.4 OK
Maximum Mesh Expansion Factor = 73.5 !
Global Mesh Quality Statistics :
Minimum Orthogonality Angle [degrees] = 20.4 ok
Maximum Aspect Ratio = 13.5 OK
Maximum Mesh Expansion Factor = 700.4 !
Good(OK)
Acceptable(ok)
Questionable(!)
MESH QUALITY CHECK FOR FLUENT
Grid check tools available
• Check : Perform various mesh consistency checks
• Report Quality : lists worse values of orthogonal quality and aspect ratio
• TUI command mesh/check-verbosity sets the level of details in the report
FACTORS AFFECTING QUALITY
Geometry problems
• Small edge
• Gaps
• Sharp angle
Meshing parameters
• Sizing Function On / Off
• Min size too large
• Inflation parameters
– Total height
– Maximum angle
• Hard sizing
Meshing methods
• Patch conformal or patch independent tetra
• Sweep or Multizone
• Cutcell
Geometry cleanup in Design Modeler
or
Virtual topology & pinch in Meshing
Mesh setting change
Mesh setting change
VIRTUAL TOPOLOGY
When to use?
• To merge together a number of small (connected) faces/edges
• To simplify small features in the model
• To simplify load abstraction for mechanical analysis
• To create edge splits for better control of the surface mesh control
Virtual cells modify topology
• Original CAD model remains unchanged
• New faceted geometry is created with virtual topology
Restrictions
• Limited to “developable” surfaces
• Virtual Faces cannot form a closed region
Without VT With VT
automatically manually
AUTOMATIC VIRTUAL TOPOLOGY
• Automatically creating Virtual Faces
– Left Click Virtual Topology in Model Tree
– Set Behaviour in Details
• Controls aggressiveness of automatic VT algorithm
• Low: merges only the worst faces (and edges)
• Medium & High: try to merge more faces
– Select if Face Edges shall be merged
– Right Click Virtual Topology and click Generate Virtual Cells
• Manually creating a Virtual Face
– RMB on Model tree and select Insert Virtual Topology
– Select Virtual Topology from the Tree Outline
– Pick faces or edges, RMB and Insert Virtual Cell
•All VT entities created can be seen in different colors if Virtual Topology is selected in Tree Outline
PINCH
• Pinch control removes small features automatically or manually at the mesh level
• Slivers
• Short Edges
• Sharp Angles
• The Pinch feature works on vertices and edges only
• The Pinch feature is supported for the following mesh methods:
– Patch Conforming Tetrahedrons
– Thin Solid Sweeps
– Hex Dominant meshing
– Quad Dominant Surface meshing
– Triangles Surface meshing
• Not supported for » CutCell
» Patch Independent
Vertex-Vertex Edge-Edge
before after before after
ASSEMBLY MESHING
MESHING PROCESS IN ANSYS MESHING
ASSEMBLY MESHING
Behavior
• Meshes an entire model as single process
– Mesh Methods covered so far are part or body based methods
– Not compatible with part/body methods
• Two Algorithms available
– CutCell & Tetrahedrons
Access
• Assembly Meshing is accessible only when Physics and Solver Preferences are set to CFD and Fluent respectively
• To activate, replace None by Cutcell or Tetrahedrons
CutCell
Tetrahedrons
Note that some
global and local
controls are not
available for
Assembly
Meshing (eg.
Match Control)
• CutCell Behavior
– Cartesian meshing method designed for the ANSYS FLUENT solver
– Generates a majority of hex cells
• Some wedges, tets and pyramids at boundaries to capture geometry
• During transfer to Fluent hexa cells at size transition are converted into Polyhedra
– Supports Inflation
• Post-inflation (TGrid algorithm)
– Baffles not supported
– High inflation may fail
• Cutcell mesh generated first, inflation generated second (Post)
ASSEMBLY MESHING - CUTCELL
• Tetrahedrons Behavior
– Generates a Patch Independent tetra mesh with automatic defeaturing
– Following steps occur in background
• Generate CutCell
• Delete volume mesh
• Triangulate surface mesh and improve
• Fill with tetra mesh
– Compatible with inflation
• Pre-inflation
– Algorithm similar to Tetra Patch Conformal
ASSEMBLY MESHING - TETRAHEDRONS
• Controls
– Set Advanced Size Functions
• Proximity SF Sources : 'edges', ‘faces’ or ‘edges and faces’
• Define correct Min Size (details next slide)
– Inflation defined by Global or Local controls
• Combined Global & Local not supported
• Program Control acts on Fluid bodies only
– Bodies can be set as Fluid in Body properties
• For Virtual Bodies, only automatic Program Controlled inflation can be used
– Define Feature and Tesselation controls (see next slide)
– Apply any required local size controls
– Statistics
• Use Orthogonal Quality for Cutcell
ASSEMBLY MESHING - CONTROLS
ASSEMBLY MESHING - CONTROLS
Min Size definition– Assembly Meshing is Patch Independent, geometry recovery and leakage
depend on local sizes
– Local sizes are driven by global min sizes and local hard sizing
• ‘Min Size’ and ‘Prox Min Size’ must be set with care
– Local mesh size recommendation to capture 3D features
• Local size < ½ feature size
– Local mesh size recommendation to close gaps
• 1/10 local size < gap size < ¼ local size : contact sizing can be defined to close gap
• Gap size < 1/10 local size : gap closed
– Prior to meshing the user is advised to resolve geometry features properly in CAD/DM
• Avoid unnecessary geometry details
• Features aligned with Coord. Syst. will be more easily recovered
Example2 . Doubling the
Min Size closes the gap
Example 1. Min Size too
large compared to the size
of the geometric detail
– Feature Capture
• Program Controlled : default which sets feature angle = 40
• Feature Angle : user angle to define features to recover
– 0 to capture all
– Tessellation (faceting) refinement
• Program Controlled - default which sets tessellation refinement to 10% of the value of smallest global min size
• Absolute Tolerance – user defined tolerance
– Must be set to 5-10% of smallest size (global min sizes or local hard sizing)
• None - Sets tessellation refinement to the CAD program or DesignModeler default setting
ASSEMBLY MESHING - CONTROLS
Incorrect tessellation may lead to
leakage
STAY AHEAD DURING CHALLENGING TIMES
• To purchase software or for consulting
please contact us: [email protected](408) 732-4665
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