Lecture 3: Geometry

Description and Volume

Meshing
ANSYS Fluent Getting Started

Release 2019 R1

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Geometry to Mesh in Watertight Geometry Workflow

• Main steps in the watertight geometry workflow

Describe
Import Switch to
Surface Mesh Geometry / Volume Mesh
Geometry Solution
Create Regions

This module

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Part 1: Geometry Description

• Describe Geometry
‐ Capping surfaces
‐ Fluid-Fluid boundary types
‐ Share Topology task
• Regions
‐ Fluid/Solid/Dead
▪ Fluid region identification criteria
‐ Create Regions task
‐ Update Regions task
• Boundary-related Tasks
‐ Add Boundary Type
‐ Update Boundaries
‐ Setup Rotational Periodic Boundaries

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Geometry Description and Region Creation

• Three options available for Geometry Type

‐ These options help Fluent Meshing determine what enclosed
volumetric regions should be created from the surface mesh
‐ Remaining inputs, such as capping, are displayed based on the
selected option
• If necessary, openings can be capped for fluid region
extraction
• A region is a volume formed by a watertight 1.
2.
enclosure of meshed surfaces 3.
‐ Models can contain multiple regions
‐ Regions can be fluid, solid or dead (voids)
‐ Any combination of different region types is allowed
• Region type can be changed if the correct type is not
automatically detected

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Geometry and Regions: Instructor Demo

• Geometry Type: Solids Only

‐ Capping
‐ Create Regions
‐ Update Regions
‐ Fluid-Fluid Boundary Type
‐ Solids only without capping
• Geometry Type: Fluids Only
• Geometry Type: Fluids and Solids and/or Voids

Following slides are included for future reference, instructor will not present all slides

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Only Solid Regions with Capping Surfaces

Solid geometry of cooling jacket

Two distinct fluid regions will be extracted

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Capping Surfaces by Label

Labels can be easily selected from the list

(selection in graphics window also possible)

Name and Zone Type used for boundary

condition in solution mode

Select from familiar Fluent boundary types

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Labels for Capping Surfaces

• Named selections used to create labels for capping surfaces should not include "inlet" or
"outlet"
‐ Will result in conversion of solid regions to fluid
Incorrect

Correct

Use "in" or "out" instead

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Capping Surfaces by Zone

For various reasons, labels might Selection by zone is possible

not be defined for capping surfaces Selection in graphics window (right click) often easier
due to confusing zone names

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Capping Surfaces Must be Closed

Solid geometry with symmetry plane

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Additional Considerations for Capping Surfaces

• Multiple faces can be used to create a single cap

• Select two faces (inner and outer) to define an annular cap

• Tilted faces are not supported

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Surface Caps

• Meshed surfaces are created to cap openings in the solid

• Surfaces are colored by boundary condition type when first created
‐ Inlet types:green, outlet types:red, symmetry:yellow, wall:gray
▪ Color changes when the next cap is created

Inlet Outlet

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Create Regions Task

• Enter the estimated number of fluid regions

‐ This value is used for diagnostic purposes only
‐ If the number of detected fluids regions is different than
estimated number, a warning will be reported and you can
investigate
‐ Region type can be changed in Update Region task as needed

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Update Regions Task

• After regions are created, the Update Regions

task allows region name and type to be
changed
‐ Change type if assignment was wrong
Regions can be
displayed by type

‐ Change name for display in solution mode

Hover mouse over region name to

highlight zones in graphics window
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Fluid Regions Criteria

• By default, all bodies are considered solid regions and all voids
are considered dead regions
• A region will be converted to a fluid if any of the following
criteria are satisfied
‐ Any meshed surface (including capping surfaces) forming a boundary of an
enclosed region is assigned a boundary type of inlet or outlet
‐ A body is named "*fluid*", "air*" or “*enclosure*"
▪ Or a named selection with these strings is assigned to the body
▪ Any regions sharing "internal" boundaries will change to fluid regions

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Solid Region No Capping Surface

Solenoid Geometry

All regions created as solid

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Apply Shared Topology Task
• It is recommended to perform shared topology in
SpaceClaim on all models before importing into Fluent
• However, in certain cases, that might not be possible
‐ For instance share topology fails, or SpaceClaim is not available
• In some cases, the Apply Share Topology workflow
task in Fluent can be used instead
‐ The share topology task is not a geometry repair tool – the CAD
geometry still has to be clean even though topology has not been
shared
• If a multibody part is imported without shared
topology, it will be detected by Fluent and an Apply
Shared Topology Task will be added in the workflow
after the surface mesh has been created

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Apply Shared Topology Task
• Click Mark Gaps to visualize the areas that
will be shared
• Confirm that all shared surfaces have been
properly marked
‐ If necessary increase or Max Gap Distance and click
Mark Gaps again
‐ The Max Gap Distance should be no larger than
one-half of the value specified for the Minimum Size
in the Create Surface Mesh task
• Click Apply Share Topology when satisfied
• Successful topology share produces a
successful surface mesh
‐ Shared topology surfaces must be remeshed to
share a common mesh

Topology marked for sharing is displayed in

the graphics window

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Only Fluid Regions

Blood Vessel Geometry

Boundary types can be changed in

Update Boundaries task.
Change Selection Type to "zone" if
boundaries have not been pre-defined.
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Update Boundaries Task

• Assigned names and boundary types can be changed

‐ Typical naming conventions on boundaries, such as "inlet",
"outlet", "symmetry",… are recognized and used to assign initial
boundary type
‐ Multiple boundaries can be assigned a specific type all at once by
right clicking and selecting Set Boundary Type in the context menu
‐ Boundary names used in Solution Mode

Additional Update Boundaries tasks can

be added downstream in the workflow

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Fluid and Solid Regions

1.

2.

Cooling jacket geometry with symmetry plane

3.

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Fluid-Fluid Boundary Types

• Two possibilities at a surface shared by two fluid regions

Fluid Zone Porous Zone Fluid Zone

Fluid Zone Porous Zone Fluid Zone

Upper: Fluid-fluid boundaries are walls. Flow cannot pass. Undesirable in this
example. Boundary layers reveal presence of walls
Lower: Fluid-fluid boundaries are internal. Flow can pass.
No BL mesh on internals
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Labels at Fluid-Fluid Boundaries

• Images on previous slide apply when no named selection or label has been
assigned to the fluid-fluid boundary
‐ Type is automatically assigned by Fluent
• Care must be taken if named selections are to be applied to these boundaries
‐ For instance maybe it is desired to name these for later use in postprocessing
• Rules
‐ Use "internal" for beginning of names of any fluid-fluid interfaces that are not walls
or
‐ Set wall-to-internal option to "yes" in Describe Geometry task

Do not do both – errors could result

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Add Boundary Type

• Add Boundary Type tasks can be added to create

additional boundaries which would then appear in the
boundary conditions panel in solution mode

Example: replace automatically

generated zone names under
boundary conditions in solution mode

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Rotational Periodic Boundaries

• Set Up Rotational Periodic

Boundaries task can be
inserted after surface mesh
creation
‐ Only one periodic set
▪ Multiple faces can be supported but
must be placed in just 2 named
selections
‐ Translational periodicity not
supported
• Options
‐ Automatically detect angle and axis
of rotation (vector and origin), or
‐ Manually specify inputs

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Part 2: Create Volume Mesh Task

• Boundary Layer Settings

‐ Offset method and inputs
• Volume Fill Methods
• Volume Mesh Quality
‐ Definition of quality and acceptable range
‐ Reporting
‐ Improve Volume Mesh
• Parallel Meshing
• Switch to Solution
‐ GUI and TUI

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Boundary Layer Settings
• Select Offset Method Type and enter Number of Layers
‐ Default Number Of Layers is 3
▪ Ok to ensure a few cells are aligned with surfaces or for models with
small gaps
▪ For cases with high accuracy requirements for boundary layer effects,
consider using 10-20 layers for high fidelity resolution
‐ Other inputs in the panel depend on choice of offset method
• Added to walls of all fluid regions**
‐ Imprinted on the faces of inlet, outlet, symmetry and internal
boundaries
• The same boundary layer settings are applied to all
fluid regions
‐ Advanced custom journal task can overcome this

• Boundary layers are not available for solid regions

‐ If necessary, set the zone to fluid in meshing mode to get boundary
layers and change it back to solid in solution mode
‐ Also could be possible with an advanced custom journal task
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Boundary Layer Offset Method
• Ratio of the last prism cell height to • Ratio of the first prism cell height
the size of first non b.l. cell to the size of the base prism
• Pros: smooth growth rate and prism • Pros: useful for variable mesh
cell to volume change at outer b.l. size on boundaries, consistent
edge growth rate
• Cons: no control of 1st layer height • Cons: no control of 1st layer
and inconsistent total height height and inconsistent total
smooth-transition aspect-ratio
height

• All cells have same first layer • All cells have same first layer
height height and same ratio of last
• Pros: control of 1st layer height prism
and consistent total height • Pros: control of 1st layer height
• Cons: can lead to problems with and smooth transition from prism
high or low aspect ratio cells to cell at b.l. edge
uniform last-ratio • Cons: growth rate varies to satisfy
other inputs

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Volume Meshing: Review

• Four options for volume fill method

• Review meshing of Arcjet model with all
methods

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Volume Meshing: Polyhedra

3.7 million cells

Min. orthogonal quality: 0.16

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Volume Meshing: Poly-hexcore

6.1 million cells

Min. orthogonal quality: 0.16

In most cases, poly-hexcore uses less RAM during solve and achieves faster time to solution than
comparable standard hexcore or polyhedral meshes
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Volume Meshing: Hexcore

16.9 million cells

Min. orthogonal quality: 0.13

*Additional Improve Volume Mesh task was required, initial quality was 0.01
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Hexcore Parameters and Shape Transitions

• Non-conformal cell structure

‐ A Hexcore mesh is always non-conformal at the hex-tet transition
and the mesh can only be used solvers that support non-conformal
cells (i.e. Fluent)
• Hanging-node transition
‐ Hexcore uses a hanging-node cell structure Buffer layer of 0
‐ The volumetric change at the transition locations is 8
‐ Use Buffer layers to control the growth rate
A buffer-layer of 2 is normally recommended
Buffer layer of 1
• Peel Layer
‐ Ability to make core region in hexa closer (low peel layer) or
further (high peel layer) to the boundary prisms
‐ A peel layer of 1 is normally recommended
• Buffer Layers and Peel Layers work the same way in
poly-hexcore as they do in standard hexcore

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Hexcore Growth in Volume

Hexcore mesh (Buffer layers = 1) Hexcore mesh (Buffer layers = 3)

Hexcore mesh (Peel layers = 1) Hexcore mesh (Peel layers = 3)

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Volume Meshing: Tetrahedral

14.8 million cells

Min. orthogonal quality: 0.12*

*Additional Improve Volume Mesh task was required, initial quality was 0.01
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Summary: Volume Meshing of Arcjet

• All volume meshes created from same starting surface mesh

• All volume meshes achieved good quality using minimal inputs for the global
sizing function in the Create Surface Mesh task
‐ Hexcore and Tetrahedral meshes required additional Improve Volume Mesh task
• Poly-hexcore and Polyhedra achieved significantly lower cell counts
• In many test cases, poly-hexcore has been found to have lower cell count than
standard hexcore, with lower memory requirements during solve and faster
solution times
‐ Poly-hexcore also exhibits better quality distribution than standard hexcore

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Volume Mesh Quality

• Orthogonal Quality reported in console on

completion of volume meshing
‐ Or any time later using Mesh > Check Quality
• Orthogonal quality is a measure of alignment
between normal vectors of the cell faces and
vectors connecting cell centroids with face
centroids and with centroids of neighboring
cells
‐ Quality ranges from 1 (perfect) to 0 (poor)
‐ ANSYS Fluent documentation recommends the following
range
▪ Minimum value > 0.01
▪ Average value much higher
‐ If possible try for minimum value > 0.1
▪ Improve Volume Mesh task can help

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Improve Volume Mesh Task

• Example: Arcjet with Tetrahedral Mesh

Initial minimum orthogonal quality too low (0.01)

Although final criteria of 0.15 was not reached, 0.13 is a

good value for minimum Orthogonal Quality

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Parallel Meshing
• Parallel Meshing is supported in Fluent
for the following volume fill methods >11x

‐ Poly-Hexcore
‐ Tetrahedral
▪ Only prism layers are meshed in parallel with
tetrahedral
• The meshing effort is shared by multiple
processes operating simultaneously, e.g. >7x

in parallel
‐ Reduces the time needed to complete the mesh
compared to meshing with just one process
‐ Performance gains are higher for larger meshes
(several millions of cells or more)
▪ 5X to 11X speedup on industrial test cases >6x

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Launching Parallel Meshing

• Enter value > 1 for meshing processes in

Launcher panel
• If starting from the command line,
specify –tmx (where x > 1 is the number
of meshing processes)
‐ Or –tx (where x > 1 is the number of processors
for both meshing and solver)
• HPC licenses are not r

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Meshing in Parallel
• If volume fill method is poly-hexcore, or
tetrahedral, and Fluent was launched with
number of meshing processes > 1, Enable Parallel
Meshing will be activated
‐ Remember with the tetrahedral method, only the boundary
layer prisms are meshed in parallel
• It is recommended to add a BOI or body sizing
when using parallel meshing for poly-hexcore in
the watertight geometry workflow in 2019
Release 1
‐ This will ensure consistency between serial and parallel
meshing
or
‐ In later releases this will not be necessary
• For local sizing controls, try to maintain 2^N ratio
for the sizes with respect to the global min size,
especially for BOIs
‐ Standard practice for octree approaches
‐ Helps to ensure close match between size control value and
resulting mesh sizes
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Switch to Solution

• Use GUI

• Or TUI
‐ switch-to-solution-mode

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Summary: Describe Geometry and Related Tasks

• Inputs provided in the Describe Geometry panel help Fluent to identify

regions as solid, fluid or void prior to region creation
• Share topology task for multibody parts
‐ It is recommended to perform share topology in SpaceClaim whenever possible
• Capping surfaces are used to assist with extracting flow volumes from solid
CAD models
• Fluid regions are identified by having boundaries with inlets or outlets
‐ Or certain specific names of bodies or body based named selections
• Region types and names can be changed in Update Regions
• Boundary names and types can be set in Update Boundaries or Add Boundary
type tasks

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Summary: Create Volume Mesh Task

• Create Volume Mesh task performs two separate operations

‐ Creating the boundary layer
▪ Various offset methods available
‐ Creating the volume mesh
▪ Various volume fill methods available
• Parallel meshing available for poly-hexcore
‐ And for tetrahedral, but limited to boundary layer prisms only
• Use Improve Volume Mesh task if initial quality is unsatisfactory
• Using the Watertight Geometry workflow allows everyone to take advantage of the
end-to-end, single window experience in ANSYS Fluent
‐ No need to be an expert user of Fluent Meshing
‐ Watertight Geometry workflow is a powerful meshing tool in its own right
▪ Quickly create high quality CFD meshes with minimal user input
o Using behind-the-scenes, inbuilt intelligence and automation

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Watertight Geometry Workflow Limitations**

• Can’t group multiple bodies in one NS in SCDM or DM to merge

corresponding regions
• Capping by NS or Face Zone: no edge loops
• One global BL control on all walls; no individual BL assignment
‐ Possible with custom journal task
• Not possible to use different volume mesh types in different regions

** Limitations of Workflow Tab compared to

standard Fluent Meshing.
These limitations do not apply to standard
Fluent Meshing.

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