Short Term : POLYFLOW 3.6 - vscht.cz · PDF file6-1 Fluent User Services Center ... Available...
Transcript of Short Term : POLYFLOW 3.6 - vscht.cz · PDF file6-1 Fluent User Services Center ... Available...
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© 2006 Fluent Inc.
Introductory GAMBIT TrainingGAMBIT 2.3 June 2006
Volume Meshing
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ApproachA high-quality hex mesh is generally preferred over a tet mesh.
Reduced discretization error and false numerical diffusion for a given mesh size.Significantly lower cell count
Example:Compare the cell count for a 10×10×10 cube using hex and tet with a cell size of 1.
Hex mesh generates 1,000 cells.Tet mesh generates 7,726 cells!
For a hex mesh, geometries typically need to be decomposed into simpler ones so that one of the hex meshing schemes can be used.In some cases, the geometry can be very complex.
Hex meshing can be expensive or impractical.In these cases, a tet or hybrid mesh is preferred in order to reduce meshing effort.
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Volume MeshingUpon picking a volume
GAMBIT will automatically choose a type based on the solver selected and the combination of the face Types of the volume.In ambiguous cases, GAMBIT chooses the Tet/Hybrid: TGrid combination
Available element/scheme type combinationsHex
Map, Submap, Tet Primitive, Cooper, StairstepHex/Wedge
CooperTet/Hybrid
TGrid, HexCore
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Volume Meshes - Hex ExamplesHex – Map
Hex -- Submap
Hex – Tet Primitive
Hex – Cooper
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Hex/Wedge and Tet/Hybrid ExamplesHex/Wedge: Cooper
Tet/Hybrid: TGrid
Tet/Hybrid: HexCore
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Hex Meshing – MapA mappable volume:
Is a logical cubeHas all faces either mappable or submappableHas topologically matching mesh on all faces.
submap face
Mesh
Mesh
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Hex Meshing – SubmapA submappable volume:
Has all faces either mappable or submappable.Has topologically matching opposite faces.
Mesh
Mesh
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Hex Meshing – Tet Primitive Tet-Primitive scheme
All hex elements in a four-sided (tetrahedral) volumeVolumes directly meshable using Tet Primitive scheme
How the tet primitive scheme worksConnect center points on edges, faces and the volumeMesh the four subvolumes using the map scheme.
Mesh
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Hex Meshing – CooperThe Cooper scheme projects or extrudes a face mesh (or a set of face meshes) from one end of a volume to the other and then divides up the extruded mesh to form the volume mesh.
The projection direction is referred to as the Cooper direction.Faces topologically perpendicular to this direction are called source faces.
Source faces need not be premeshed.At least one source face must not be meshed and must span the entire cross section.
Faces that intersect the source faces are referred to as side faces.Side faces must be either mappable or submappable
Cooperdirection
Source Faces Side Faces (two hidden)
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Cooper Examples
Volume Containing
Multiple Holes
source faces
source faces
source faces
source faces
Multiple Source Faces and Multiple Interior
LoopsSource Faces Not
Parallel
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Cooper Tool MethodologyWhen the Cooper scheme is selected, a source face list box appears in the panel. If GAMBIT chooses the sources faces
Check the source face list and verify that GAMBIT has chosen the correct faces.If necessary, change the source faces selection.
GAMBIT may not be able to resolve the source faces
Manually select the source facesIf necessary, manually change the vertex types (discussed in lecture 3) on some of the side faces
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Troubleshooting the Cooper ToolA
B
C
Problem:Source faces A, B, and C are premeshed. The Cooper tool fails. Why? How can this volume be meshed?
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Troubleshooting the Cooper ToolA
B
C
Solution:The mesh on source faces A and B cannot be projected onto face C (the source faces are overconstrained. Delete the mesh on face C in order to generate the volume mesh.
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Troubleshooting the Cooper Tool
A
B
C
Problem:A brick is split as shown. The Cooper tool fails. Why? What can be done to generate a volume mesh?
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Troubleshooting the Cooper Tool
Solution:Cooper tool fails because no logical axis exists. If faces A and B are source faces, then face C must be either mappable or submapple. Face C contains a void and can only be paved. Split the volume with a face as shown. Use Face A1 as one source face for volume 1 and use face C2 as one source face for Volume 2.
A1
Volume 1
Volume 2
C1
A
B
C
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Troubleshooting the Cooper Tool
A
B
Interior loops
Problem:The Cooper tool fails because the interior loops on source faces A and B either overlap or are close.
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Troubleshooting the Cooper Tool
A
B
A1 A2
Interior loops
Solution:Split source face A as shown. Neither face A1 nor A2 contain closed interior loops.
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How to Make a Volume CooperableThree options to use the Cooper Tool:
Manually change vertex types on the side faces to make them mappable or submappable.Manually select the source faces. GAMBIT will attempt to make side faces mappable or submappable.Enforce the map or submap scheme on the side faces.
Example: manually change the vertex types
3 Source FacesS
E
S
E
C C
E
E
E
E
E
E
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Tet/Hybrid MeshingTetrahedral/Hybrid Mesh Scheme - TGrid
Most volumes can be meshed without decomposition, regardless of complexity.Use boundary layers to create hybrid grids (prism layers on boundaries to capture important viscous effects).Use on volumes that are adjacent to volumes that have been meshed with hex elements will automatically result in a transition layer of pyramids.
Tet: TGrid
3 Hex/WedgeCooper
21 Hex Cooper
Pyramidlayer
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Tet/Hybrid Meshing – TroubleshootingQuality of the tetrahedral mesh is highly dependent on the quality of the triangular mesh on the boundaries.
Initialization process may fail or highly skewed tetrahedral cells may result if there exists:
highly skewed triangles on the boundaries.large cell size variation between adjacent boundary triangles.small gaps that are not properly resolved with appropriately sized triangular mesh.
Difficulties may arise in generating hybrid mesh.Cannot grow pyramids from high aspect-ratio faces.Prism and pyramid generation may not work properly between surfaces forming very small angles.
Low-quality pyramid
Prism layer
small angle
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HexCore MeshingCombines Tet/Hybrid mesh with Cartesian mesh in the core.Fewer cells with full automation and geometric flexibility.Important HexCore defaults:
Hexcore_Offset_LayersThe number of offset layers (cell layers between wall and hexahedral core); default value is 3.Hexcore_Quad_Surface_SplitControls quad/tri splitting and eliminates pyramid cells when turned on; see AppendixHexcore_MethodControls the method used to create HexCore –Standard or TGrid HexCore.
TGrid HexCore requires specification of buffer layers.
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HexCore Meshing
Flow Volume Around a Boat Hull Flow Volume Inside anAutomobile Manifold
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Assigning Boundary and Continuum TypesBoundary Type Form
Enter entities to be grouped into single zone in entity list box.
First choose entity type as face or edge.Select boundary type for zone (entity group).
Available types depend on SolverName zone if desired.Apply defines zone and boundary type.
Can also modify and delete zone/boundary.
By default,External faces/edges are wallsInternal faces/edges are interior
Continuum Type formContinuum types are defined in a similar way as boundary types.Multiple fluid/solid zones can be defined.Unspecified continuum zones are always assigned the fluid type.
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Example: Flow over a Heated Obstacle
BoundaryName = inlet
Type = VELOCITY_INLET
ContinuumName = obstacle
Type = SOLID
BoundaryName = outlet
Type = PRESSURE_OUTLET
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Defaults: Example: Flow over a Heated Obstacle
By default, the 4 remaining external faces have the Name and Type:
Boundary: Name = wall
Type = WALL
By default, the one remaining volume has the Name and Type
Continuum: Name = fluid
Type = FLUID
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Appendix
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Meshed Size Function from Boundary Layer Cap
Meshed Size Function starting from boundary layer cap improves size transition between the boundary layer and volume mesh.
Useful for external aerodynamics applications.Specify the Growth Rate and Max. Size for the mesh growing from the last prism layer into the volume.Example: 3D wing profile with 12 boundary layers; the meshed size function is used for smooth transition to the tetvolume mesh.
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Hex-Core Meshing – Surface Split Options
Geometry: CylinderEdit Default: Mesh.Cartesian.Hexcore_Quad_Surface_Split
1 (default)Split boundary quad into 2 triangleshanging edges created (NOT allowed in FIDAP)Smooth boundary hexes with larger hexcore
0Boundary quads are NOT splitPyramid (transition) elements createdBoundary hexes not smoothed
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FIDAP 8 Example: Flow over a Heated Obstacle
Continuum: Name = step
Type = SOLID
Boundary: Name = outlet
Type = PLOT
Boundary: Name = outlet
Type = PLOT
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Linear/Quadratic Elements(FIDAP/POLYFLOW USERS ONLY)
General toolsHigher-order elements
For FEM codes (FIDAP and POLYFLOW), the element order can be changed at all three meshing levelsOnly linear and quadratic elements are directly availableA change to quadratic element type at one level will automatically change the element type in other levels The following table presents the most commonly used and recommended quadratic element types for FEM solvers
POLYFLOW FIDAP
Edge 3-node 3-node
Face 8-node quad 9-node quad
Volume 21-node brick 27-node brick