Upload
chester-lynch
View
233
Download
6
Embed Size (px)
Citation preview
© Fluent Inc. 04/19/234-1
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Volume Meshingand the
Sizing Function
© Fluent Inc. 04/19/234-2
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Approach
To potentially reduce discretization errors, and to reduce cell count, a ‘high’ quality hex mesh is preferred.
For a hex mesh, complicated geometries (volumes) typically need to be decomposed into simpler ones so that one of the hex meshing schemes can be used.
In some instances, some geometries may be too complex and decomposition for hex meshing is impractical or impossible. In these instances use a tet/hybrid mesh.
© Fluent Inc. 04/19/234-3
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Volume Meshing Volume Meshing Form:
Upon 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 combinations Hex
Map Submap Tet-Primitive Cooper Stairstep
Hex/Wedge Cooper
Tet/Hybrid TGrid
© Fluent Inc. 04/19/234-4
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Volume Meshes - Hex Examples Hex: Map
Hex: Submap
Hex: Tet-Primitive
Hex: Cooper
Hex: Stairstep
© Fluent Inc. 04/19/234-5
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Hex/Wedge and Tet/Hybrid Examples
Hex/Wedge: Cooper
Tet/Hybrid: TGrid
© Fluent Inc. 04/19/234-6
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Hex Meshing - Map
Volumes that are mappable by default: A logical cube All faces map-able (or Submap-able) and mesh is matching
Map Scheme
mesh
mesh
© Fluent Inc. 04/19/234-7
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Hex Meshing - Submap Volumes that are Submap-able by default:
All faces map-able or submap-able Topological matching of opposite faces
Submap Scheme
mesh
mesh
© Fluent Inc. 04/19/234-8
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Hex Meshing - Tet-Primitive
All hex elements in a four-sided (tet) volume Volumes directly meshable using Tet-Primitive scheme
How the Tet Primitive Scheme works Connect center points on edges, faces and the volume Map the four sub-volumes
Tet-Primitive scheme
Tet Primitive
Mesh
© Fluent Inc. 04/19/234-9
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Hex Meshing - Cooper The Cooper Scheme, in essence, 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 do not have to be premeshed. In practice, at least one source face must not be meshed and must span across
the entire cross section. Faces that intersect the source faces are referred to as Side faces.
Side faces must be Mappable or Submappable.
Cooper direction
Source Faces Side Faces
© Fluent Inc. 04/19/234-10
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Permissible Cooper Geometries Cooper Scheme - permissible geometries
A volume with multiple source faces on multiple sides Holes or “wells” are allowed
source faces
source faces
source faces
© Fluent Inc. 04/19/234-11
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Impermissible Cooper Geometries
Cooper Scheme - impermissible geometries
(a) Cannot construct logical cylinder, the side faces of which is mappable
(b) All source faces are meshed
(c) Cannot use Cooper (readily) with multiple source faces on opposing ends.
(c)
(a)
(b)
A
B
C
© Fluent Inc. 04/19/234-12
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Cooper Application Cooper scheme - Application
When the Cooper scheme is selected, a source face list box appears in the panel.
GAMBIT will automatically select all source faces for direct Cooper-able volumes (scheme comes up as default).
If GAMBIT chooses the sources faces, you should check the source face list and visually check for an intelligent selection. Change, if necessary.
If GAMBIT fails to pick a set of source faces, you can either manually change the vertex types on the side faces or manually select the source faces.
© Fluent Inc. 04/19/234-13
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Hex Meshing - Stairstep
The Stairstep scheme creates a single-block structured mesh. The Stairstep scheme creates and meshes a faceted volume the shape of
which approximates the volume to be meshed. The original volume is not meshed. Faceted volume:
is generated as a result of the meshing process is not connected to original volume.
Assignment of continuum and boundary zonetypes must be applied to faceted volume.
The Stairstep scheme can be used for quickmesh when boundary mesh is less important.
‘Body-fitted’ boundary mesh willbe implemented in future.
© Fluent Inc. 04/19/234-14
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Tetrahedral/Hybrid Meshing Tetrahedral/Hybrid Mesh Scheme - TGrid
Automatic - most volumes can be meshed without decomposition. Use boundary layers to create hybrid grids (prism layers on boundaries to
capture important viscous effects). Using on volumes that are adjacent to volumes that have been meshed
with hex elements will automatically result in a transitional layer of pyramids.
Hex mesh first
Tet mesh second
© Fluent Inc. 04/19/234-15
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Tet/Hybrid Meshing: Troubleshooting Quality 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 appropriate sized
triangular mesh. Difficulties may arise in generation of hybrid mesh.
Cannot grow pyramids from high aspect-ratio faces. Prism and pyramid generation may not work properly between surfaces
forming very acute angles.
low quality pyramid
prism layer
acute angle
© Fluent Inc. 04/19/234-16
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Sizing Functions Sizing Function controls mesh distribution in a region of space
(Edges, Faces, and Volumes) in a manner analogous to the way grading controls mesh distribution on edges.
Sizing Function accessed through Toolbar:
Without a Size Function With a Size Function
© Fluent Inc. 04/19/234-17
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Sizing Function Types
Sizing Function requires the specification of Type, Entities, and Parameters.
Sizing Function ‘Type’ controls method by which scope of sizing function is obeyed.
Fixed Scope is defined as a fixed region about
a source. Curvature
Scope is defined as a region near highly curved surfaces.
Proximity Scope is defined as a region within a
specified distance from objects.
© Fluent Inc. 04/19/234-18
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Sizing Function Definition Each Sizing Function Type requires the
specification of: Entities
Source entity defines shape and location of the ‘origin’ of affected region.
Attachment entities host the mesh that will be affected.
Parameters Defines growth rate of cells in affected region for
Fixed and Curvature Type. Defines number of cells in gaps for Proximity
Type. Defines extent of affected region in Attachment
entities. Defines maximum cell size allowed in affected
region.
© Fluent Inc. 04/19/234-19
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Fixed Sizing Function - Source Source
Can be vertices, edges, faces, or volumes Can be internal or external to attachment entities Source entity defines shape of scope
© Fluent Inc. 04/19/234-20
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Sizing Function - Attachments The attached entities host mesh to be affected.
© Fluent Inc. 04/19/234-21
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Fixed Sizing Function - Parameters
Start size Size adjacent to the source
Growth rate Ratio of two adjacent mesh-element
edge size Distance
Determine boundary of size function Size limit
Maximum allowable size for attachment entity
© Fluent Inc. 04/19/234-22
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Curvature Sizing Function Modifies size according to geometric
curvature Sources can only be face entities Parameters
Angle - Maximum allowable angle between outward pointing normals for any two adjacent mesh elements located immediately adjacent to the surface of a source
Others are as with Fixed
© Fluent Inc. 04/19/234-23
Introductory GAMBIT Notes GAMBIT v2.0 Jan 2002
Fluent User Services Center
www.fluentusers.com
Proximity Size Function Specifies number of cells in face gap (3D) and edge
gap (2D) Parameters
Cells per gap - number of mesh layers in the gap. Distance - maximum distance from the source at
which size function applies. Size limit
Limitations Becomes slow on large models Improper use may result in abrupt change in size Solutions
Use multiple size functions Specify large value for distance Increase resolution by changing the defaults for
background grids