CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-1
Mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-2
Introduction
? CFX-5 mesh generator provides,
Automatic mesh generation
> Unstructured triangular surface mesh generation
> Tetrahedral/prismatic volume mesh generation
There are three modes of volume meshing in CFX-5
> Advancing Front and Inflation (AFI)
> Paving and Isomeshing
> Volume Mesh Import (VMI)
Mesh adaption
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-4
Surface Meshing
? Surface meshing
Delaunay surface meshing
Advancing front (AF) surface
meshing
? Both methods are well recognized
and have established track records
on industrial problems
? Both surface meshers mesh
individual surfaces
? Default surface mesher is Delaunay
surface mesher
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-5
Surface Meshing
? AF is traditionally slower than Delaunay
? AF produces higher quality elements at boundaries
? AF mesher cannot mesh closed surfaces
? AF is less robust on
surface meshes with large variation in edge length scale around a
surface mesh vertex
surfaces with small gaps
? Delaunay mesh quality is not good on poorly parameterized surfaces
? Delaunay works on closed surfaces
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-6
AF,vs,Delaunay
Surface Meshing
DelaunayAF Surface Mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-7
AFI Mesher
? Domain based - surfaces can only be meshed if a fluid domain has
been defined
? Can view only surface mesh in CFX-Build
? Surface and volume mesh automatically generated when definition file
is written
? The process of setting an element length scale can be viewed as a 3
step process
Assign suitable background length scale (global length scale)
Override this locally on surfaces and regions close to them
> This can be done semi-automatically using curvature
sensitivity
Override the above two (when necessary) using mesh controls
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-9
Mesh Parameters
? Setting mesh parameters
Volume mesh spacing
Maximum edge length - background scale -
used everywhere unless
overridden
Surface mesh spacing
> Use volume spacing - use background
length from volume meshing
> Angular resolution - curvature sensitive
> Relative error - curvature sensitive
> Constant - constant length scale on all
surfaces
Target number of nodes to adjust grid spacing
Stretch factors for stretching grids
Apply proximity
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-10
? Surface mesh parameters
Value (degrees) - code chooses edge
length such that the set angle is subtended
at the centre of circle with radius equal to
smallest radius of curvature,Default is 18
degrees (recommended 5 to 60 degrees)
Value (fraction) - deviation of mesh from
surface as a fraction of local minimum
radius of curvature
Minimum edge length - lower bound on
length scale (default 1% of volume
background scale)
Maximum edge length - upper limit on
length scale (default same as volume
background scale)
Volume expansion factor - rate of
expansion of mesh scale from surface to
interior
Curvature Sensitive Mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-11
Without surface curvature sensitive
meshing
With surface curvature sensitive
meshing.
Curvature Sensitive Mesh Examples
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-12
Proximity Sensitivity in Surface Mesh
With proximity featureWithout proximity feature
? Edge Proximity is used to automatically modify mesh elements in
regions where curved and flat surfaces are in close proximity
> Bounding curves of surface,sense” nearby curves and increase mesh
density locally
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-13
Proximity Sensitivity in Surface Mesh
? Surface Proximity is used to automatically reduces the mesh size in
regions where surfaces are in close proximity and the original mesh
does not resolve the gap sufficiently,
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-14
Mesh Controls
? Mesh Controls
Act as sources which reduce the
local element edge length scale
Location defined using any
valid CFX-Build point/surface
description
Can be of the following types:
> Point
> Line
> Triangle
> Surface
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-15
Mesh Controls
Point
Line
Triangle
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-16
Mesh Controls
? Mesh Controls:
Influence controlled by mesh
control values
Local element length scale
Radius - region of fixed local
element length scale from mesh
control
Expansion factor - geometric rate
of increase of local element
length scale beyond radius
Affects both surface and volume
meshes
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-17
Mesh Controls
? Surface Mesh Control:
Constant length scale (cell edge
length) with radius of control
and expansion factor
Angular resolution with min and
max edge length and expansion
factor
Relative error with min/max
edge length and expansion factor
Effect on Volume mesh is
minimal
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-18
Verify Surface Mesh
? Allow to look at the mesh on a
particular surface before actually
create the entire surface mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-19
Examples (I)
X
Y
Z
X
Y
Z
X
Y
Z
x
y
z
unit cube Global Element Edge Length - 0.2
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-20
Examples (II)
X
Y
Z
x
y
z
mc1
mc1
Point
Edge Length - 0.05
Radius - 0.2
Expansion Factor - 1.2
Line
Edge Length - 0.05
Radius - 0.2
Expansion Factor - 1.2
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-21
Examples (III)
X
Y
Z
x
y
z
Triangle
Edge Length - 0.05
Radius - 0.2
Expansion Factor - 1.2
Surface
Edge Length - 0.02
Expansion Factor - 1.2
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-22
Examples (IV)
X
Y
Z
x
y
z
Stretch factor - 0.5 3.0 1.0Stretch factor - 1.0 1.0 2.0
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-23
X
Y
Z
x
y
z
Combined Point Mesh Control and Stretching
Examples (V)
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-24
Inflation
? Inflation
Generation of prismatic element layers from triangular surface
elements
Computationally efficient mesh in boundary layer region
Can control the number,thickness and expansion rate of inflation
layers
Can inflate from any surface or boundary condition
Inflation is a volume mesh generation process - Inflated elements
are not visible within CFX-Build
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-25
Inflation
? Set Inflation Parameters:
Specify number of layers
Specify geometric expansion
factors
Specify inflation thickness
multiplier
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-26
Inflation
? Create inflation boundary:
Set maximum thickness for
inflation layer (limited by local
surface edge length)
Select surfaces for inflation
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-27
Inflation
? Inflation
Where inflation layers meet or overlap,tetrahedral elements are
generated (i.e,a mesh generated with AFI will always contain
tetrahedra)
Inflated mesh region
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-28
Summary
? AFI - tetrahedral/prismatic elements
Mesh controls for volume and surface mesh
Inflation parameters for prismatic elements
? Import volume mesh (mixed elements,hexahedra,tetrahedra,
triangular prism and pyramid)
? 2001AEA Technology
7-1
Mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-2
Introduction
? CFX-5 mesh generator provides,
Automatic mesh generation
> Unstructured triangular surface mesh generation
> Tetrahedral/prismatic volume mesh generation
There are three modes of volume meshing in CFX-5
> Advancing Front and Inflation (AFI)
> Paving and Isomeshing
> Volume Mesh Import (VMI)
Mesh adaption
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-4
Surface Meshing
? Surface meshing
Delaunay surface meshing
Advancing front (AF) surface
meshing
? Both methods are well recognized
and have established track records
on industrial problems
? Both surface meshers mesh
individual surfaces
? Default surface mesher is Delaunay
surface mesher
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-5
Surface Meshing
? AF is traditionally slower than Delaunay
? AF produces higher quality elements at boundaries
? AF mesher cannot mesh closed surfaces
? AF is less robust on
surface meshes with large variation in edge length scale around a
surface mesh vertex
surfaces with small gaps
? Delaunay mesh quality is not good on poorly parameterized surfaces
? Delaunay works on closed surfaces
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-6
AF,vs,Delaunay
Surface Meshing
DelaunayAF Surface Mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-7
AFI Mesher
? Domain based - surfaces can only be meshed if a fluid domain has
been defined
? Can view only surface mesh in CFX-Build
? Surface and volume mesh automatically generated when definition file
is written
? The process of setting an element length scale can be viewed as a 3
step process
Assign suitable background length scale (global length scale)
Override this locally on surfaces and regions close to them
> This can be done semi-automatically using curvature
sensitivity
Override the above two (when necessary) using mesh controls
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-9
Mesh Parameters
? Setting mesh parameters
Volume mesh spacing
Maximum edge length - background scale -
used everywhere unless
overridden
Surface mesh spacing
> Use volume spacing - use background
length from volume meshing
> Angular resolution - curvature sensitive
> Relative error - curvature sensitive
> Constant - constant length scale on all
surfaces
Target number of nodes to adjust grid spacing
Stretch factors for stretching grids
Apply proximity
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-10
? Surface mesh parameters
Value (degrees) - code chooses edge
length such that the set angle is subtended
at the centre of circle with radius equal to
smallest radius of curvature,Default is 18
degrees (recommended 5 to 60 degrees)
Value (fraction) - deviation of mesh from
surface as a fraction of local minimum
radius of curvature
Minimum edge length - lower bound on
length scale (default 1% of volume
background scale)
Maximum edge length - upper limit on
length scale (default same as volume
background scale)
Volume expansion factor - rate of
expansion of mesh scale from surface to
interior
Curvature Sensitive Mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-11
Without surface curvature sensitive
meshing
With surface curvature sensitive
meshing.
Curvature Sensitive Mesh Examples
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-12
Proximity Sensitivity in Surface Mesh
With proximity featureWithout proximity feature
? Edge Proximity is used to automatically modify mesh elements in
regions where curved and flat surfaces are in close proximity
> Bounding curves of surface,sense” nearby curves and increase mesh
density locally
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-13
Proximity Sensitivity in Surface Mesh
? Surface Proximity is used to automatically reduces the mesh size in
regions where surfaces are in close proximity and the original mesh
does not resolve the gap sufficiently,
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-14
Mesh Controls
? Mesh Controls
Act as sources which reduce the
local element edge length scale
Location defined using any
valid CFX-Build point/surface
description
Can be of the following types:
> Point
> Line
> Triangle
> Surface
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-15
Mesh Controls
Point
Line
Triangle
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-16
Mesh Controls
? Mesh Controls:
Influence controlled by mesh
control values
Local element length scale
Radius - region of fixed local
element length scale from mesh
control
Expansion factor - geometric rate
of increase of local element
length scale beyond radius
Affects both surface and volume
meshes
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-17
Mesh Controls
? Surface Mesh Control:
Constant length scale (cell edge
length) with radius of control
and expansion factor
Angular resolution with min and
max edge length and expansion
factor
Relative error with min/max
edge length and expansion factor
Effect on Volume mesh is
minimal
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-18
Verify Surface Mesh
? Allow to look at the mesh on a
particular surface before actually
create the entire surface mesh
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-19
Examples (I)
X
Y
Z
X
Y
Z
X
Y
Z
x
y
z
unit cube Global Element Edge Length - 0.2
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-20
Examples (II)
X
Y
Z
x
y
z
mc1
mc1
Point
Edge Length - 0.05
Radius - 0.2
Expansion Factor - 1.2
Line
Edge Length - 0.05
Radius - 0.2
Expansion Factor - 1.2
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-21
Examples (III)
X
Y
Z
x
y
z
Triangle
Edge Length - 0.05
Radius - 0.2
Expansion Factor - 1.2
Surface
Edge Length - 0.02
Expansion Factor - 1.2
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-22
Examples (IV)
X
Y
Z
x
y
z
Stretch factor - 0.5 3.0 1.0Stretch factor - 1.0 1.0 2.0
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-23
X
Y
Z
x
y
z
Combined Point Mesh Control and Stretching
Examples (V)
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-24
Inflation
? Inflation
Generation of prismatic element layers from triangular surface
elements
Computationally efficient mesh in boundary layer region
Can control the number,thickness and expansion rate of inflation
layers
Can inflate from any surface or boundary condition
Inflation is a volume mesh generation process - Inflated elements
are not visible within CFX-Build
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-25
Inflation
? Set Inflation Parameters:
Specify number of layers
Specify geometric expansion
factors
Specify inflation thickness
multiplier
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-26
Inflation
? Create inflation boundary:
Set maximum thickness for
inflation layer (limited by local
surface edge length)
Select surfaces for inflation
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-27
Inflation
? Inflation
Where inflation layers meet or overlap,tetrahedral elements are
generated (i.e,a mesh generated with AFI will always contain
tetrahedra)
Inflated mesh region
CFX 5.5 V 5.5.1H
? 2001AEA Technology
7-28
Summary
? AFI - tetrahedral/prismatic elements
Mesh controls for volume and surface mesh
Inflation parameters for prismatic elements
? Import volume mesh (mixed elements,hexahedra,tetrahedra,
triangular prism and pyramid)
