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Chapter 5
Prism Meshing
ICEM CFD Introductory
Course
Prism Meshing
5-2
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Training ManualPrism Meshing
• Inflation layers
– To better simulate boundary layer effects
– Mesh orthogonal to surface with faces
perpendicular to boundary layer flow direction
• Procedure
– Set Global Prism Parameters
– Select Parts to grow layers from
• Typically wall boundaries
– Set Local Parameters for each part
• Local overrides global
• Zero or blank entries will defer to global settings
– Run mesher
• From existing mesh
– Extrude into tetra/hexa mesh
– Extrude from surface tri mesh, then fill volumes
• Run automatically during Volume Mesh creation
Prism Meshing
5-3
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Training ManualPrism - Global Parameters
• Global Prism Parameters
– Growth law
• exponential: height = h(r)(n-1) [n is layer #]
• linear: height = h(1+(n-1)(r-1))
– Initial height of first layer – h in formulae above
• Auto calculated if not specified
– Based on factor of edge length of base
triangle/quad
– Height determined so that top layer volume is
slightly less than that of tetra/hex just above it
– Number of layers n
– Height ratio r
– Total height of all layers
– Usually specify 3 of the above 4 parameters
• Compute params will calculate the remaining
parameter (total height usually left blank)
– Or specify only Height ratio and Number of
layers for auto calculation of initial height
– These are the defaults. Individual surface/curve
height/ratio/layers will override these defaults if
set
Other global parameters
explained later
Initial
height (h)
Total
height
Height
ratio (r)
Prism Meshing
5-4
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Training ManualSmooth Tetra/Prism Transition
• Leave initial height as “0”
– This causes the initial height to float in order to
reduce the volume change between the last
prism and adjacent tetra.
Initial
height
specified
Initial
height = 0
Prism Meshing
5-5
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Training ManualSetting Prism Parameters on Parts
• Prism extrusion areas defined by the parts
– Mesh > Part Mesh Setup
– Toggle on Prism for parts where inflation layers
are desired
• Surface mesh (tri/quad) gets extruded into prisms
– Set Height, Height Ratio, Num Layers
• Will use global defaults if not set or zero
Applying these
settings causes
these parameters to
be applied to each
individual surface
within each part
If Apply inflation
parameters to
curves is toggled
on, they will also be
set on each curve
within each part
Prism Meshing
5-6
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Training ManualSetting Prism Parameters on Volume Parts
• Normally toggle prism on only for parts
that contain surfaces (becomes surface
mesh)
• Can also toggle on prism for parts that
contain material points (becomes
volume mesh)
– For interior surface mesh, this defines
the allowable volumes for extrusion
– Selecting no volume parts has the same
result as selecting all volume parts
Only one
volume part
selected
Interior surface
Both or no volume
parts selected
Prism Meshing
5-7
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Training ManualSetting Prism Parameters on Surfaces
• Mesh > Surface Mesh Setup
• You can specify different local height and
ratio on any selected surface without
moving the surface to a new part
• Usually set height and/or ratio smaller on
specific surfaces to avoid collision
Height = 0.2Height on part = 0.4
Collisions occurred when the
height was 0.4 on all surfaces
No collisions after
Prism Meshing
5-8
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Training Manual
• Mesh > Curve Mesh Setup
• You can get Prism to transition linearly across
a surface by not setting a height on the surface,
but instead set a different height on each curve
on the opposite sides of the prism surface
• Height ratio and Num. of layers have no affect
on prism for curve settings
Setting Prism Parameters on Curves
Height = 0.003
Height =
0.01
Height = 0 on
surface
Prism Meshing
5-9
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Training ManualRun Prism
• Separately
– Mesh > Compute Mesh > Prism Mesh
– The Select Parts for Prism Layer button pops up the
same menu as the Part Mesh Setup, except non-
prism related columns aren’t displayed
– Input
• Existing Mesh
• From File (saves memory by
not loading mesh)
• Automatically linked into volume mesh
– Toggle on Create Prism Layers when tetra meshing
– Not advisable if this is the first mesh for a particular
geometry
– Must be confident about setup parameters and
sizing
– Running prism separately allows you to smooth
and error-check the tri or tetra mesh first.
Prism Meshing
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Training Manual
Input can be a surface mesh or volume mesh
– Surface mesh
– Must be a closed boundary mesh
– Must specify a volume part
– Use tetra fill methods after:
– Delaunay
– Advancing Front
– T-Grid
– Volume mesh
– Moves and reconnects tetras
Input as Surface or Volume Mesh
Delaunay
fill
Prism extrudes
into tetras
Prism Meshing
5-11
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Training ManualPrism – Quality Control Options
• Fix marching direction
– Maintains normal from surface
– Can cause intersections with other mesh
• Min prism quality
– Either re-smooth directionally or cap/replace with
pyramids if quality not met
• Ortho weight
– Weighting factor for node movement from 0 -
improving triangle quality, to 1 - improving prism
orthogonality
• Fillet ratio
• Max prism angle
• Max height over base
• Prism height limit factor
• Ratio multiplier (m)
– For varying exponential growth:
height = h(r)(n-1) (m)(n-1)
See next slides
Prism Meshing
5-12
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Training ManualPrism Options – Fillet Ratio
– Blends prism grid lines around sharp corners
• 0 = no fillet
• 1 = fillet ratio equals last prism height
– Improves angles further away from the corner
– Orients prisms more in direction of flow
– If meshing tight spaces with tight curves (less than 60º), may not
have space for a fillet ratio
Fillet Ratio = 0.0 Fillet Ratio = 0.5 Fillet Ratio = 1.0
r h
Fillet Ratio = r/h
Prism Meshing
5-13
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Training ManualPrism options – Max Prism Angle
– Controls prism layer growth around bends or adhering to adjacent
surfaces
– If the Max (internal) Prism Angle is not met, the prism layers will
end and be capped off with pyramids in those locations
– Usually set in the 120º to 179º range
– Experience pays off here. If extruding from one part and not its
neighbor, and the angle between the two surfaces is greater than
the Max Prism Angle, the prisms will detach and be capped off
with pyramids. This prevents bending the prisms that might
create lower-quality internal angles. However, the pyramids are
usually of lower quality, too.
Original mesh Max prism angle = 180
o
Max prism angle = 140
o
.
160
o
Pyramids
Prism Meshing
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Training Manual
Max Prism Angle = 140 Max Prism Angle = 180
Prism Options – Max Prism Angle - Continued
• A high (up to 180
o
) Max Prism Angle keeps the
prism layers connected around tight bends.
– Set this at 180 to prevent pyramids
Prism Meshing
5-15
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Training ManualPrism Options – Max Height Over Base
– Restricts prism aspect ratio
– Prism layers stop growing in regions where
prism aspect ratio would exceed specified value
• Number of prism layers would not be preserved
locally
– Mesh is made conformal with pyramids at prism
boundaries
– Acceptable values vary widely (typically 0.5 – 8)
Max Height Over Base = 1.0Max Height Over Base not set
Height
(h)
Base
(b)
h/b
Largest height over
smallest base length
Pyramids
Prism Meshing
5-16
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Training ManualPrism Options – Prism Height Limit Factor
– Restricts prism aspect ratio
• Prism height will not expand once this factor is met
– Uses the same height over base factor as the
previous metric except prism layers are not
capped off with pyramids
– Preserves the specified number of prism layers
– Will fail if sizes of adjacent elements differ by
more than a factor of 2
– Acceptable values vary widely (typically 0.5 – 8)
Limit factor = 0.5Limit factor not set
Height
(h)
Base
(b)
h/b
Largest height over
smallest base length
Prism Meshing
5-17
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Training ManualPrism Options-Part Control
• New volume part
– Can specify new Part for prism
elements
• Must specify if extruding from surface-
only mesh
• If extruding into volume mesh, prism
will inherit tetra volume Part if not
specified
• Side part
– For quad faces on side boundary
• Top part
– For tri faces capping off top of last
prism layer
• Extrude into orphan region
– Extrude prisms away from existing
volume, not into it
– Must specify new volume, side and
top part, or they’ll be in ORFN
Leaving
these parts
blank will
inherit the
names from
the current
mesh
Prism Meshing
5-18
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Training ManualPrism Options - Smoothing
• Prepares tri/tetra for best prism quality
– Set surface/volume steps to 0 if only extruding
one layer
• Otherwise defaults adequate
• Value depends on model/user experience
– Set surface smoothing steps to zero for a
tri/tetra mesh that is already smoothed
– Triangle quality type
• Laplace typically best for eventual prism quality
• Other types recommended only for trouble shooting
– comes with experience
– Max directional smoothing steps
• Redefines extrusion direction based on initial prism
quality
• internally calculated for each layer
• Other Advanced Prism Meshing Parameters
– Detailed in Help menu (usually left default)
• Read a Prism Parameters File
– To set all values from a previously saved file,
perhaps from a previous model (.prism_params
file, written when prism is run)
Prism Meshing
5-19
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Training ManualSmoothing a Tetra/Prism Mesh
After generating prisms:
Edit Mesh > Smooth Mesh Globally
– Prisms are smoothed during prism generation
– If input mesh was a tetra mesh, the tetras adjacent to
the last prism layer will be messed up
– First smooth only the tetras and tris
• Set PENTA_6 to Freeze
• Don’t want to modify the prism layers at this point
– Once tetra and tri elements are as smooth as
possible, smooth all elements
– Set PENTA_6 to Smooth
– Decrease the Up to quality value so as not to distort
prism elements too much
The prisms get
compromised a bit
when everything is
on smooth
1st
step
2nd
step
Prism Meshing
5-20
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Training ManualSplitting Prism Layers
– If many prism layers are desired, it is faster,
but less robust – to create “fat” layers and
then split them with mesh editing
– Edit Mesh > Split Mesh > Split Prisms
– Fix ratio: The first layer is split such that its
resulting layers employ the given growth
ratio
– Fix initial height: The first layer is split such
that its first sub-layer is of the given height
– Specify the number of layers to result from
each existing layer
– Can split specified or all existing layers
Prism Meshing
5-21
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Training ManualRedistributing Prism Layers
– Edit Mesh > Move Nodes > Redistribute Prism
Edge
– Fix ratio: The initial height and subsequent layer
heights will be adjusted to achieve this growth
ratio
– Fix initial height: The growth ratio is the variable
that will be adjusted to achieve this initial height
Redistribute prism layers after splitting
Prism Meshing
5-22
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Training ManualDemo – 3D Pipe Junction
• Setting prism parameters
• Extrude prism into tetra mesh
• Initial height as zero
• Smoothing
• Cut plane
Prism Meshing
5-23
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Training ManualWorkshops
• Workshop 1 – Waterjacket Section
– Setting prism parameters
– Prism into tetra mesh
– Inspecting pyramids
– Setting local heights
– Remesh to get rid of pyramids
• Workshop 2 – Wingbody
– Prism from surface mesh
– Create a density region
– Delaunay fill
– Smoothing
– Hexa core fill