17.0 Release
ANSYS Meshing Advanced Techniques PADT Lunch & Learn Series
PADT (Phoenix Analysis and Design Technologies) • Channel Partner for ANSYS
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Overview • ANSYS Meshing Overview • Advanced Tet Methods • Advanced Hex Methods • Selective Meshing Methods • Using Global Meshing Methods
• Using Local Meshing Methods • Clean Up Tools • Quality Metrics • Meshing Tips and Tricks
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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
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What are the Element Types • Tetrahedral
• Hexahedral • Polyhedral (CFD)
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Advanced Tet Methods
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Hexahedral vs. Tetrahedral Elements • Advantages of tetrahedral over hexahedral: – Easier to mesh more complex geometry: •
Mesh quality is often easier to achieve with tetrahedral (or poly) mesh 1 tet to 4 hex Tet mesh can be easily converted to hex mesh, but if the quality is bad, what’s the point?
•
Mesh transitioning with hex mesh can be problematic Small mesh size on holes need to transition to larger size elsewhere, but transitioning hex mesh can be a problem
All Hex
Choosing the proper mesh element type will improve the mesh generation efficiency
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Patch Conforming versus Independent
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Tetrahedral Methods Patch Independent
Patch Conforming • Bottom up approach: Meshing process • Edges Faces volume • All faces and their boundaries are respected (conformed to) and meshed (except with defeaturing tolerance) • 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
• 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
To access it • Insert Method • Set to Tetrahedrons • Set to Patch Conforming
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Tetrahedral Method: Algorithm comparison Patch conforming : details caputred
Patch independent : details ignored
Delaunay mesh - smooth growth rate
Octree mesh . approximate growth rate
Geometry with small details
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Advanced Hex Methods
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Hexahedral versus Tetrahedral Elements • Advantages of hexahedral over tetrahedral: – Less elements = Faster solution time with better accuracy • Naturally anisotropic: Fewer elements required as mesh is aligned with the physics – Fewer elements for given number of nodes – 3 mostly parallel sets of faces (improves solution accuracy)
However, this assumes the geometry is such that the hex mesh is more efficient and that the structured mesh aligns to the physics
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Hexahedral Mesh
Tetra mesh - 48 000 Cells
Hex Meshing • Reduced element count – Reduced run time
• Elements aligned in direction of flow – Reduced numerical error
Hexa mesh - 19 000 Cells
Initial Requirements • Clean geometry • May require geometric decomposition
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Sweep Meshing Source & Target selection 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
Define the number of intervals on the side face(s)
Sweep Path
Sweep around an axis Requires selection of both - Source & target
Note • Specifying both Source & Target accelerate meshing
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Generation of wedges & hex elements
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Multizone Meshing Mesh Method & Behavior • 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
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Automatic Method Mesh Method & Behavior • 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
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Meshing Options Global Meshing Parameters
Local Meshing Parameters
Global Settings - Physics Preference - Shape Checking
Mesh Method - Tet, Hex, Multizone, Sweep
Global Sizing - Size Functions - Relevance Center
Advanced Tools - Inflation, Pinch and match controls
Global Inflation Layers
Local Sizings - Edge, Face, Body
Advanced Settings - Number of CPUS
Mesh Edit Tools - Node move/merge - Contact Match
Mesh Statistics
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Global Meshing Methods
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Global Mesh Controls • Global mesh controls are used to make global adjustment in the meshing strategy, which includes sizing functions, inflation, smoothing, defeaturing, parameter inputs, assembly meshing… • 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 •
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level of mesh refinement Advanced Size Functions for resolving regions with curvatures and proximity of surfaces
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Smart defaults !
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Defaults •
Five options under “Physics Preference” – Mechanical, Nonlinear Mechanical, Electromagnetics, CFD and Explicit
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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”
The Nonlinear Mechanical Physics Preference Option has been added in R17.0 and may results in higher mesh quality for FEA users
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Sizing: Size Function • Controls the growth and distribution of mesh in important regions of high curvature or close proximity of surfaces
• Five Options: – – – – –
Adaptive Proximity and Curvature Curvature Proximity Uniform
• When CutCell Meshing is active with ‘Proximity’ or ‘Proximity and Curvature’, 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 Size Function (SF)
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Sizing: Sizing Function Examples
SF: Adaptive • 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
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SF: 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
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SF: 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
Sizing: Element Size Element Size (only available when the size function is set to Adaptive) • 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
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Local Meshing Methods
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Sizing Recommended 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
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
Only Element Size type is available for CutCell meshing
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Advanced Size Function in Global settings should be disabled
Requires a Coordinate system for the sphere
Sizing: Sphere of Influence On Vertex
On Bodies
– Available with or without Advanced Size Functions – Sets the average element size around the selected vertex – Inputs:
– 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
• Sphere radius and Element size • Center of the sphere is defined by a model vertex
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Sizing: Bodies of Influence Bodies 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 Line BOIs
Surface BOI
Without BOIs
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Solid BOI
The ‘Body of Influence’ itself will not be meshed
Face Meshing Control • Forces Mapped/Pave meshes on selected mappable surfaces – Face Meshing with advanced control is supported for • • • •
Sweep, Patch Conforming, Hexa Dominant Quad Dominant and Triangles MultiZone Uniform Quad/Tri and Uniform Quad
– RMB on Mesh and Show/Mappable Faces to display all mappable faces If Face Meshing fails, ( ) icon appears adjacent to corresponding object in the Tree outline. The mesh will still be created but will ignore this control.
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Face Mesh Control: Advanced Vertex Options •
Sweep/MultiZone (2D and 3D) support for Advanced Vertex options on Face meshing
S
S S
S
Denoted by S, C or E
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S
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S
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
Matching face mesh
If ‘Match Control’ fails, ( ) icon appears adjacent to corresponding object in the outline Tree, however the mesh is created ignoring it
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Symmetry Control • Define mesh periodicity on faces (3D) or edges (2D) • Faces/edges pairs should be topologically and geometrically identical • Can be assigned to several face/edge pairs at once • Not supported with – Assembly Meshing – Post Inflation Algorithm – Patch Independent tetrahedrons
– Two types of Symmetry Controls useful for CFD: • ‘Cyclic’ for rotational periodicity • ‘Linear Periodic’ for translational periodicity
Matching face mesh
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Inflation Used 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)
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Contact Match Mesh • Connect mesh at mesh level • At R17 only works with tet mesh • Could avoid problems with shared topology
Notes: • • • •
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Mesh generated as separate parts Use Contact Match controls to match mesh Create Named Selection to inspect matched mesh Node merge can be added as second step to make mesh conformal
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Nonlinear Adaptive Mesh • Nonlinear Adaptive Region • Enables you to change
•
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the mesh during the solution phase to improve precision without incurring a great deal of computational penalties. The Nonlinear Adaptive Region feature is completely automatic. Limitations: All 2D elements, Only Linear 3D elements
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Selective Meshing Methods
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Selective Mesh What is selective meshing? • Selectively picking bodies and meshing them incrementally
Why selective meshing? • Bodies can be meshed individually • Mesh seeding from meshed bodies influences neighboring bodies (user has • • • •
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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
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Selective Mesh
Meshing first the pipe then the block
Local Meshing Clear meshes on individual bodies Generate 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
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Meshing first the block then the pipe
Selective Mesh Example : Meshing cylinder first and then block
Recording Mesh Operations • 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 • We can create Named Selection to define an order
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Selective Mesh Selective Body Updating • Remeshing only bodies that have changed • Access option through RMB click on Geometry Properties • No: All geometry updated, all bodies remeshed. • Associatively: Accommodates for body topology change (add/delete) (slower) • Non-Associatively: Assumes no topology change (faster)
Example : Geometric change to block
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Clean Up Tools
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Node Merge & Node Move • Node Merge: enables to merge mesh nodes within a specified tolerance, making the mesh conformal across bodies, parts, and assemblies. Node Merge can be performed on solid, sheet, and line bodies • Node Move: Dynamically pick and drag nodes around (quality plots updated real time) • History of moves is recorded in Worksheet and allows for “Undo”
Note: If you update your mesh (Mesh object>Update), the application maintains your movements in the Worksheet until the mesh is cleared (zero nodes) using the RMB option Clear Generated Data or you refresh data from the CAD source; In these cases all the Node Move movements are lost
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Mesh Connections • Mesh Connections: enable you to join the meshes of topologically disconnected surface bodies that may reside in different parts. This is an alternative to connecting the geometry in SpaceClaim or DM
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Contact Matches • Contact Matches: enable you to match mesh nodes between topologically disconnected solids within a specified tolerance. They are an alternate option to imprinting faces in SpaceClaim or DM.
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Defeaturing • Removes small geometry features meeting the tolerances using Pinch or/and Automatic Mesh Based Defeaturing controls in order to improve the mesh quality. • Automatic Mesh Based Defeaturing (AMBD) when it is ‘On’, features smaller than or equal to the value of Defeaturing Tolerance are removed automatically. Note: Defeaturing Tolerance can be set maximum up to Global Min Size/2. Use a high value for Global Min Size and use Local Sizing controls for defining Local Min Sizes smaller than this value. Defeaturing will respect local min sizes and will clean only those faces/bodies with no local control. This approach is recommended over Virtual Topology
AMBD On
AMBD Off
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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
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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 before
after
before
after
• Not supported for – CutCell – Patch Independent – Multizone & General Sweep
Vertex-Vertex
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Edge-Edge
Diagnostics: Close vertices (New in R17.0) ● Close Vertices: Checks for vertices that are very close together: – Scale can be adjusted depending on model sizes
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Virtual Topology When to use? • To merge together a number of small (connected)
• • •
Without VT
With VT
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 automatically
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manually
Virtual Topology In some instances it may be desirable to modify topology to allow application of some desired effect (e.g., mesh control, load, support, …): • Split face at vertices • Split Edge • Add a hard vertex
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Quality Metrics
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Impact of the Mesh Quality 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 physics 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 …
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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
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Element Quality • Element Quality: • Composite quality metric that ranges between 0 and 1 • Ratio of the volume to the square root of the cube of the sum of the square of the edge lengths for 3D elements
• 2D quad/tri elements: 0 Worst
• 3D brick elements:
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1 Perfect
Aspect Ratio 2-D: • Length / height ratio: δx/δy 3-D • Area ratio • Radius ratio of circumscribed / inscribed circle
Large aspect ratio are accepted where there is no strong transverse gradient (boundary layer ...)
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δy δx
Limitation for some iterative solvers • A < 10 … 100 • (CFX: < 1000)
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∞ Worst
1 Perfect
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Jacobian Ratio • Jacobian Ratio =
𝑅𝑗𝑚𝑎𝑥 𝑅𝑗𝑚𝑖𝑛
• Where 𝑅𝑗 = det(𝐽𝐹 ) at sampling locations and 𝐽𝐹 =
• Jacobian Ratio ≥ 1 – If 𝑅𝑗𝑚𝑎𝑥 and 𝑅𝑗𝑚𝑖𝑛 have opposite signs then the Jacobian ratio = -100 and is unacceptable 1 Perfect
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∞ Worst
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𝜕𝐹1 𝜕𝐹1 𝜕𝑥 𝜕𝑦 𝜕𝐹2 𝜕𝐹2 𝜕𝑥 𝜕𝑦
in 2D
Skewness • Skewness: 0 - 1 • Two methods for determining skewness:
Optimal (equilateral) cell
– Equilateral Volume deviation: – Skewness =
optimal cell size cell size optimal cell size
– Applies only for triangles and tetrahedrons – Normalized Angle deviation:
max
– Skewness =
e min max max e , e 180 e
Actual cell Circumsphere
min
– Where 𝜃𝑒 is the equiangular face/cell (60 for tets and tri’s, and 90 for quads and hexas) – Applies to all cell and face shapes 0 1 – Used for hexa, prisms and pyramids Perfect Worst
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Orthogonal Quality
On cell
On face
A1 Orthogonal Quality (OQ)
c1
f1
Derived directly from Fluent solver discretization •
For a cell it is the minimum of: Ai ci Ai fi | Ai || ci | | Ai || f i |
A1
f3
c3
c2
e1
f2
e3
A2
e2
A3
A3
computed for each face i
Ai ei For the face it is computed as the minimum of | Ai || ei | 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
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1 Perfect
A2
Smoothness Checked in solver • Volume Change in Fluent
Recommendation: Good: 1.0 < σ < 1.5 Fair: 1.5 < σ < 2.5 Poor: σ > 5 … 20
– 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
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CFD 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
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Mechanical Shape Checking Standard Mechanical - Linear, modal, stress and thermal analysis Aggressive Mechanical - Large deformation and nonlinear analysis Nonlinear Mechanical - Nonlinear analysis
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Factors Affecting Quality Geometry problems • Small edge • Gaps • Sharp angle
Geometry cleanup in SpaceClaim or Virtual topology & pinch in Meshing
Meshing parameters • Sizing Function On / Off • Min size too large • Inflation parameters
Mesh setting change
– Total height – Maximum angle
• Hard sizing Meshing methods • Patch conformal or patch independent tetra • Sweep or Multizone • Cutcell
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Mesh setting change
Display Option: Color by quality • • •
Probe Element Values
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Find Min or Max value
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Display mesh contours
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Displays mesh color by quality metrics Options to probe quality or show min/max Contour band can be adjusted
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
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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
For more information about the different mesh metrics please consult module 05: Mesh Quality
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Grid Dependency Study • Solution run with multiple meshes
• Note : For all runs the computed Y+ is valid for wall function (first cell not in laminar zone)
x8
DP 0
DP 3
2%
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Grid Dependency Study • Option 1: Run Convergence Study on Result of interest
• Option 2: Set up convergence study by parameterizing mesh sizing
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Parameterizing Your Model and Mesh
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Named Selections
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•
Create Named Selections for all surfaces that have a load or support applied
•
Create Named Selections for all surfaces/bodies that have a mesh setting applied
•
Assign loads, boundary conditions, and mesh settings to Named Selections
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Parameterize your Model
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•
Geometry
•
Material Properties
•
Mesh
•
Loads
•
Results
•
Command Snippets
•
UDF’s
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Tips and Tricks
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Tips and Tricks •
ACT Extensions: • FE Info 17.0: Locates nodes and elements by IDs • Iterative Meshing 17.0: Iterated through mesh configurations and reports best mesh • Mesh Settings 17.0: Export mesh settings to re-use in another project • Workbench Poly Meshing for Fluent 17.0: Create Fluent meshing polyhedral meshes in the Workbench meshing interface. • Nonlinear Diagnostics: Identify and display highly distorted elements after solver failure
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Questions?
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