lecture 4: volume fill methods -...

© 2011 ANSYS, Inc. December 21, 2012

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14.5 Release

Lecture 4: Volume Fill Methods

Introduction to ANSYS

Fluent Meshing


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Agenda

• Overview of Volume Mesh Generation

• Auto Mesh Panel

• Tet Mesh Creation

• Hexcore Mesh Creation

• Prism Generation

• Hex-Tet Transition Methods

• Volume Mesh Improvement

• Clearing the Mesh

• Appendix


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Agenda


• Auto Mesh Panel







• Appendix


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Overview of Volume Mesh Generation

Typical Volume mesh generation steps 1. Read (or import) boundary mesh(es)

2. Check quality/connectivity of boundary mesh

3. Improve & repair boundary mesh

4. Via Auto Mesh Panel, set meshing parameters and define local refinement regions

5. Generate volume mesh

6. Inspect quality of volume mesh

7. Use Auto-Node-Move to improve quality

8. Set boundary/fluid types and names

9. Save volume mesh

Surface mesh for a grid containing only triangular elements. Fluent Meshing will fill such a supplied surface mesh with high quality tetrahedra or hexcore with optional prism layers.


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Domains to Define Closed Volumes

• Fluent Meshing uses the concept of “Domains” to allow closed volumes to be defined allowing

• Closed volumes to be meshed separately with different parameters

• i.e. A “body-by-body” approach to volume meshing

• Domains contain a list of zones making up the closed region

• Only one domain can be active

• Domain creation/manipulation is mostly done automatically today by Fluent Meshing during volume mesh generation

Select the boundary

zones, give Name

and click New to

create a new

domain. Activate is

used to activate the

selected domain.


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Agenda


• Auto Mesh Panel







• Appendix


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Auto Mesh Panel • Ability to define all hybrid mesh parameters in one panel

• Choose whether to grow Prisms

• Choose how any exposed quad faces will be treated

• Pyramids to transition from quad to tet

• Non Conformal transition from quad to tet

• Choose Volume fill options

– Tetrahedra

– Hexcore

– No fill (after creating prism/pyramid domain created only)

• Additional Options:

• To merge the resulting cell zones

• To identify baffles and auto-orient normals

• Once the settings have been chosen, a single click on Mesh will generate prisms, pyramids, non-conformals, tet or hexcore

• Apply will save the settings and user can then save the mesh prior to volume fill with all their meshing settings intact


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Auto Mesh Process 1. Set Prisms

2. Set Quad/Tet Transition

3. Set Volume Fill

4. Mesh!


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Agenda


• Auto Mesh Panel






• Appendix


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Prisms Fluent Meshing has the capability to extrude 3D prisms starting from either:

• Quadrilateral boundary faces

• Triangular boundary faces

• CutCell Surfaces (see separate lecture)

Used to:

• Resolve the boundary layer region forviscous/thermal CFD calculations

• Extend portion of domain for which volume mesh exists

– e.g. increase length of outlet pipe by sweeping cells in a given direction

• Create volume mesh (hex cells) from quadrilateral boundary face region

Zonal hybrid mesh example

Tri surface swept downwards to

create cylinder from prism cells

Viscous hybrid mesh example


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Normal Orientation

• Before Prism elements are created, it is critical that face normals are facing in the correct direction for growth

• Visualise using the Normal Attribute and altering the Normal Scale

• If incorrect, normals can be manually flipped in the Boundary Manage Panel or from within the Orient option in the Prism panel (see later)

• Face normals will also dictate the growth direction of pyramid elements


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Generating Prism Layers - Workflow

1. Display the face normals using

• Display ->Grid ->Attributes

• Flip direction in Boundary -> Manage if required

2. In Mesh -> Prism, select boundary zones to grow prisms from

3. Set the prism layers Growth parameters in the Growth tab and in the Growth Options panel

4. Change any global prism parameters if required in the

• Direction/Improve/Project tabs

5. Click on Create to generate the prism cells

Mesh Prisms


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Generating Prisms Layers

• New surface zones created by the extrusion

– Prism-cap: Cap of the prism cells

– Prism-side: Side of the prism cells

– No “prism-side” zone is generated if all adjacent surfaces are projected onto and retriangulated

• Adjacent boundary zones are automatically retriangulated, incorporating new quad faces

• Irreversible process! Save mesh recommended prior to prism generation!

• Zones are then separated in two

– Tri part

– Quad part

– Merged together if “Merge Cell Zones” selected in Auto Mesh Panel

Prism-side Prism-cap

Symmetry Symmetry-quad

Prism extrusion with retriangulation of the symmetry


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Selecting Offset Methods

There are four methods available to grow the prism layers:

• Uniform: all cells in each layer have the same height

• Aspect ratio: all cells in each particular prism layer have the same aspect ratio

– Useful for large variations in the mesh size on the boundary zone

• Last-ratio: the aspect ratio of the last layer of cells and the height of the first layer is specified

• Minimum-height: all cells grown in each layer will be at a minimal perpendicular height with respect to the previous layer

– If the boundary zone from which the prism layer is grown has a sharp corner, the prism layer will also have a sharp corner

Minimum

height Uniform


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Selecting Growth Method

There are four options for Growth Method:

• Constant: each layer has a constant height or aspect ratio

– hn=h1

• Linear: the height/aspect ratio grows linearly with the growth rate given by a slope parameter

– hn = h1+s(n-1)

• Geometric: the height/ aspect ratio grows in geometry progression with a specified rate

– hn = rn-1h1

• Exponential: the height/aspect ratio grows exponentially with the rate given by a specified exponent

– hn = h1e(n-1)p

Note: the last-ratio offset method, which is automatic, does not require a Growth method – it is calculated internally and user must provide

enough layers to enable slow growth


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Prisms Growth Options

Prisms extrusion near a car wheel

Ignored area

Shrinking

• Fluent Meshing has intelligent options to allow high quality prism layers to be grown even on very complex geometries

• Shrinking algorithms are on by default and these will ensure prisms do not collide/intersect

• Optionally, the user can choose to allow to completely ignore areas of proximity where prism growth will result in poor quality elements. Non conformal interfaces are then used from the exposed quads to transition to tet elements

• See Appendix on Advanced Prism Controls for further details


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Direction Controls

• The settings in the Direct tab control the direction of growth of the prism layers

– Method: Grows prisms in Normal direction to each face on the Boundary Zone or in a specified Uniform direction

• Orient Normals: Orients the normal vectors of adjacent tris/quads on each selected Boundary Zone so that they are consistently pointing on one side of the zone by selecting material point

Normal Method Uniform Method


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Direction Controls

• Max Angle Change specifies the maximum angle by which the normal at a node is allowed to change during smoothing at each advancing layer. Increasing to 60 degrees recommended to improve robustness.

• Orthogonal layer specifies the number of layers that should remain orthogonal – Orthogonal layers will not be smoothed to improve quality

– Only available with Normal method

– Set to 0 by default – much less robust growth with orthogonal layers

Max Angle Change = 45° (2 layers : Max skew = 1) Max Angle Change = 70° (3 layers : Max skew = 0.92)


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Project Controls “Project” controls how adjacent zones are

considered in the prism growth:

• Retriangulate Adjacent Zones option causes auto-quad-imprint and retriangulation of the adjacent zones – It can be turned off when you want to experiment

with various controls without changing the surface mesh

– Once quads are imprinted to adjacent zones and they are re-triangulated there is no going back to the purely triangular mesh.

• Max Adjacent Zone Angle sets the angle of the adjacent zone with the direction of the prism growth, below which the zone is retriangulated. Advise to increase to 60 degrees for robustness. – Once you are satisfied with the controls, you can

either regenerate the prisms with retriangulation or you can remesh the adjacent face zones

Max. Adjacent Zone Angle = 40°

Angle > 40° Angle < 40°

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It is advisable to save the mesh before prism

generation with your settings Applied as your

surface mesh will be changed afterwards.


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Agenda


• Auto Mesh Panel







• Appendix


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Tri/Tet Grid Generation Process in Fluent Meshing

Two phases: • Initial mesh generation: Create initial

mesh of the volume. Coarse, highly skewed elements used as a starting point for the final volume mesh

• Refinement on initial mesh: Add nodes and cells to initial mesh trying to improve quality

Initial mesh

Boundary refinement Cell zone refinement

Initialisation and refinement can be done automatically or

manually. Default (auto) settings will be used here.


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Mesh Initialization Troubleshooting

Main reasons for failed tet initialization

• Intersected surface mesh

– Fluent Meshing directs user to the problem faces by face ID -> Copy and paste the face ID into the Bounds tool to manually zoom on the area which can then be fixed

– Resolve by clearing volume Mesh -> Clear and then using boundary modify or

– TUI/boundary/resolve-self-intersection

– Alternatively, display problem and resolve manually

• Use /boundary/mark-face-intersection and display

Boundary problems can be addressed using tools such as

• Boundary Modify

– Repair high skewness meshes

– Remove multiply connected nodes

– Delete and reconstruct faces

– Move nodes of intersecting faces

• Boundary Nodes

– Check and Merge free nodes

– Check and remove incorrect multi faces

Intersecting faces

Disconnected “free” faces


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Dead/Fluid/Solid Regions

• By default, during tet and hexcore meshing the largest enclosed volume is taken as the Main Fluid Region and any smaller enclosed volumes are labelled as “dead” and not refined

• Any volumes with a fluid BC applied (e.g. Inlet or outlet type) to a boundary will also be labelled Fluid type and refined

• The user has the options to:

1. Automatically Delete Dead Zones if only a single volume is of interest. E.g. In-cylinder combustion where the valve is not to be meshed (dead void)

2. Assign all “Dead Zones” automatically as fluid regions and refine. E.g. For separate porous regions in the CFD calculation

3. Assign all “dead volumes” as solid regions and refine. E.g. For conjugate heat transfer simulations

• Mesh Manage can be used to reassign and activate cell zone types later

dead zone

fluid zone


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Tri/Tet Mesh Creation

• There are two Tet Mesh Refinement Methods:

– Advancing front (Default and recommended)

– Skewness (Legacy method)

• Automatic mesh refinement

– Init & Refine (one-step tet mesh)

• Performs initialization and refinement

– Refine

• Performs refinement only if pre-initialised

• Max Cell Volume should be prescribed. Compute will calculate based on max edge length in the mesh using perfect tet formula.

• A Geometric Growth Rate (e.g. 1.2 for 20% growth) can be used to achieve coarser or finer volume meshes from the same surface mesh starting point.

• Additionally to global refinement, Local Regions can be created

• Apply will save your settings so you can close and use Auto Mesh to mesh. Saving the mesh after this will save these settings along with the mesh information.

Volumetet=


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Advancing Front Mesher – Growth Rates

Geometric Growth 1.6

0.8 million tet cells

Avg skew 0.345



Avg skew 0.251



Avg skew 0.214

Lower growth rates result in smoother volumetric growth, lower average skewness and larger meshes


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Local Mesh Refinement Mesh Tri/Tet Local Regions… allows a local rectangular region to be defined in which the

cells are refined to a specified size. Useful in refining local regions, e.g. wake behind bluff body

• Init : Creates a default region that encompasses the entire domain

• Input the coordinates of the Center of the box

• Specify Length of extents in X, Y and Z

• Optionally Rotate (Orient) the domain in X, Y and Z

• Define an Outside Range and Growth Rate to grade the mesh size outside the box

• Specify a Max Cell Volume

– TUI: /mesh/tritet/local-regions/ideal-vol to calculate ideal cell size from edge length

• Multiple regions can be created and Activated/Deactivated


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Agenda


• Auto Mesh Panel







• Appendix


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• Hexcore is a hybrid meshing scheme that generates tet cells adjacent to triangular boundary meshes and a Cartesian mesh in the core flow regions.

• Combines automation and flexibility of tet/hybrid meshes with greatly reduced cell counts and increased accuracy, particularly in swirling flows

• Hanging node refinement allows efficient transition from boundaries to interior.

• Fully compatible with boundary layer prism meshes

Introduction

2. Fill with Hex

3. Peel back hex near prism cap and fill gap with Tets

1. First Create Prisms


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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 both on the interior surface and in the volume

• The volumetric change at the transition locations is 8

• The rate of change is based on the buffer layer value

Quad-split transition

• Between the surface of the Hexcore and the tetrahedral region, a predefined quad-split transitioning is being utilized to avoid pyramid growth

Cell Topology and Transitions

Buffer layer of 0

Buffer layer of 1 Buffer layer of 0 Buffer layer of 2


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Standard Hexcore Meshing Controls

• Max Cell length is the largest edge of the largest cell in the domain. The default is calculated based on the largest triangle edge in the boundary mesh. Compute will re-calculate this value

• Buffer layers specify the number of additional layers of hex cells to subdivide before growing to the next level. By default set to 1 but we recommend using 2 or 3 for a slower growth rate. • Peel layers control the thickness of the tetrahedral gap between hexahedra core and the geometry. Default is 1, but a value of zero can be used in many cases to increase hex/tet ratio. Increasing to 2 or 3 can help improve tet layer initialisation robustness for very complex models. • Set Non-Fluid Type and if to Delete Dead Zones (void regions) • Add Local Regions (refinement boxes) if needed • Modify Tet Controls if needed (growth rate for tet layer, etc) • Apply will save your settings so you can close and use Auto Mesh to mesh. Saving the mesh after this will save these settings along with the mesh information.


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Hexcore Meshing Controls – External Flow • If you have planar external boundaries, Define

Hexcore Extents allows you to extend the hexcore mesh to specified domain coordinate extents and/or selected planar boundaries.

• Specify... opens the Outer Domain Parameters panel where you can choose whether to

1. Specify Coordinate Extents for the farfield or

2. Specify tri Boundary Extents to replace with quad boundaries

• Alignment tolerances can be used to snap the grid to Cartesian XYZ axes


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Hexcore Refinement • Local Region Controls allow local hexahedral regions to be defined, where a maximum

mesh size can be specified. This is particularly useful for refining the volume mesh in areas of high gradients such as wake regions, vortex regions or in shear layers.

• Click on Local Regions… in the Hexcore panel to access the Hexcore Refinement Region panel

• Assign a Name of the region

• Using the Level, assign a Max Length of the cells within the region volume

• Assign a Center point

• Assign the Length of the box in each direction

• Optionally assign an rotation of the box in each global coordinate system using the Orient option

• Define the region

• Init will initialize the centre and length

based on the bounding box of active

domain

• Ensure the region is Activated! – Ability to Activate and Deactivate regions

– Ability to overlap regions – smallest size wins

– Draw shows region extents and max size

as red cube in graphics viewer.


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Hexcore Meshing Examples

Example of HexCore meshes with and without Local refinement

Deactivated Local Refinement Activated Local Refinement


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Mesh Export

• Files can be written as .msh.gz OR .cas.gz with Write as Polyhedra option

• Option converts parent-child hanging node cells to poly cells and is recommended

• Fluent Meshing operations on return to meshing mode are limited in R14.5 for polyhedral cells

Transfer to Fluent Solver

Mode will automatically

convert to polyhedral


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Agenda


• Auto Mesh Panel







• Appendix


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Quad-Tet Transition

There are two ways to transition from quad to tet cells • Pyramids

– Conformal mesh

• Non-conformal interfaces

– Multiple algorithms depending on usage

Non-conformal interface at prism-side. Use when quads are very high aspect

ratio and pyramids would be bad quality as a result

Pyramid layer on quad surface. Pyramid Caps and Pyramid Side zones can be generated


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Agenda


• Auto Mesh Panel







• Appendix


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Checking the Mesh

• As in the Fluent Solver, use Mesh Check to check the mesh:

– Connectivity of faces and cells

– Number of nodes per cell

– Number of faces per cell

– Face-handedness

• Should be right-handed

– Cell volume

• Should be positive

• Check Quality will give identical quality checks to the Fluent Solver


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• There are many different options for Quality Measure in the Fluent Meshing software

• Standard Fluent/Gambit measures of equivolume and equiangle skew exist – good for tet meshes

• Mesh smoothness can be evaluated by the Size Change and Edge Ratio measures

• Warp applies to quad faces and indicates twisting of quad sides of prism cells

• Recommended measure to use for most meshes is Ortho Skew

– varies between 0->2 and should be kept below 0.95 for optimal solver robustness, preferably < 0.9

– Cells above Ortho Skew of 1 are degenerate

– Equal to (1 – Fluent Solver Orthogonal Quality)

• All skewness types in Fluent Meshing follow the rule that high is bad and low is good

Volume Mesh Quality


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Measuring Mesh Quality

Plot cell quality distribution

• Display Plot Cell Distribution...

• Use the Compute button to report Maximum and Minimum quality

• Average value can be computed from :

– Report Cell Limits...


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Displaying Poor Quality Cells

• Cells with high skewness, aspect ratio etc can be viewed in the graphics window.

• Problems in the boundary mesh can often be diagnosed in this way and rectified.

• In this example the sharp angle where the wheel and ground meet is causing skewed cells.


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Skewness - How Bad is Bad?

High skewness values lead to inaccurate solutions & slow convergence.

Generally try to keep maximum skewness of volume mesh < 0.95.

• However this value is strongly related to type of physics and the location of the cell

FLUENT reports negative cell volumes if volume mesh contains degenerate cells. There are new options in FLUENT to deal even with negative elements in Release 14 – please refer to Poor Mesh Numerics Options in the FLUENT User Guide.

Possible classification based on skewness:

*Note that often the pressure based coupled solver available from FLUENT 6.3 onwards can handle meshes containing a small % of cells with skewness ~0.99.


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Judging and Improving Cell Quality

A volume mesh is considered to be “bad” if it contains

• Very highly skewed cells (skewness > 0.98)

• Degenerate cells (skewness ≥ 1)

• Very high aspect ratio cells (unless wall resolving mesh with good growth away from high aspect ratio cells)

• Negative volumes

Cell Quality can be improved by:

• Clearin the Mesh using Mesh -> Clear and improving surface mesh quality

• Use CAD or other upstream preprocessors to fix geometric problems such as sharp angles or slither volumes

• Auto-Node-Move Panel

• Conversion to Polyhedral cells (for tet mesh) in Fluent Solver Mode

• Mesh Smoothing in Fluent Solver Mode


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Auto Node Move

• Available through Mesh-> Tools -> Auto Node Move

• A Quality Measure is provided (e.g. Skewness)

• A Quality Limit is provided

• Nodes on cells above the quality limit will only be moved if the angle between faces is greater than the Dihedral angle

– Reducing the angle will respect less features

– Subsequent quality improvement is more likely to succeed

• Nodes within selected Cell Zones and on selected Boundary Zones will be moved laterally

– Deselect Restriction Option to allow normal movement

• Prism quad face Warp can also be improved and Negative Volume Cells fixed from this panel#

• Click Apply to execute

• After a volume mesh has been generated, Auto Node Move is a powerful tool to allow

automated local node movement to improve cell quality

For more careful usage, Semi-Auto

Correction will show a proposed change and

the user can choose to accept or refuse the

change on a step-by-step basis.


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Useful GUI and TUI Commands Additional GUI commands

• Mesh Manage can be used to

– manipulate (translate, rotate, scale) volume meshes

– Merge/Rename/Change type of Cell Zones

– List Cell Zones to see number and type of elements per zone

Additional TUI commands:

• /mesh/separate

– Commands to separate cell zones on a number of criteria – can be used to separate and delete skewed cells or separate by shape etc.

• /mesh/tri-tet/preserve-cell-zone

– can be used in cases where prisms are generated in advance or if you have premeshed volume regions (e.g. Hex volumes from GAMBIT/ICEM). Then the Hexcore/tet mesh can be generated without the need of mesh domains.

• /mesh/domain/create-by-point

– Allows automatic domain creation by providing a material point – useful for very complex domains with large numbers of boundaries


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Agenda


• Auto Mesh Panel







• Appendix


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Clearing the Mesh

• For Tet or Hexcore type meshes where boundaries are not modified by the volume meshing process the simple Mesh Clear command will suffice to return the user to the starting point

• Mesh Clear deletes all volume mesh and “interior” type Face Zones. Switch “interior” to “internal” type to keep!

• However, if prisms, pyramids or CutCell meshes are created then the likelihood is that clearing the mesh will not return the mesh to the original state

• In the case of pyramids or prisms, imprinting onto neighbouring boundaries usually occurs, changing the surface mesh

• New “Prism-cap” and “Pyramid-cap” boundaries are created which are not removed by the Mesh Clear command

• For these reasons we reiterate here that it is highly recommended to use “Apply” buttons within Fluent Meshing to save the settings and then write a mesh out prior to creating the mesh. This is the cleanest way to return to the starting point again.


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Summary The Auto Mesh Panel offers a single area within Fluent Meshing to generate high

quality hybrid meshes containing prism, pyramid, tet and hex elements.

All volume fill techniques allow the user a high degree of control to specify growth rates and mesh refinement within the cell zones created.

Local Refinement boxes can be used to reduce cell count in critical flow areas.

Sub-volumes can be automatically deleted if unrequired or they may be meshed as fluid or solid type by setting “non-fluid type” options.

Meshes may be created in a body-by-body approach using domains which can be used to describe and activate individual enclosed volumes.

Tools are available to visualize poor quality elements in order to allow users to clear the mesh and fix the boundaries at problematic locations before re-filling the volume.

Quantitative tools allow the user to diagnose meshing issues before moving to the solver.

Auto Node Move can be used to improve cell quality by performing primitive operations locally


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14.5 Release

Appendix A

Tet Meshing Additional Info


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Tri/Tet Mesh Initialization Controls You can set general initialization parameters which apply to both auto and manual initialization, through:

Mesh Tri/Tet Initialization Controls

Cells smaller than the sliver size will

be assigned a size of zero

Nodes within the node tolerance of

each other will be considered

duplicate.

Will perform swapping based on Delaunay violations to improve surface

skewness prior to initialization


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Improving Tri/Tet Cell Quality

After tet meshing the user has the option to further improve cell quality.

Mesh Tools Tri/Tet Improve General allows operations including node smoothing and face swapping.

Improve will only move a node if the operation results in a local improvement in cell quality (for Laplace or variational methods).

Tet Improve is generally

superceded by Auto Node Move


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Removing Boundary Slivers

Highly skewed sliver cells still present at the end of Tri/Tet meshing can be improved in the Tri/Tet Improve panel

• Options include Smoothing of boundary and interior nodes

• Swap boundary : externalizes and then removes sliver cells from the volume mesh by swapping edges (see next slide)

• Refine Boundary : splits edges and performs node smoothing on the new node to improve skewness

• Refine Interior : places a new node near the cell centroid and performs swapping and smoothing to improve skewness

• Collapse – attempts to collapse the edge of a sliver cell on one of it’s neighbors

• Boundary Freeze will avoid any changes to the surface mesh. Boundary options are greyed out

Mesh Tools Tri/Tet Improve

Tet Improve is generally

superceded by Auto Node Move


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Removing Boundary Silvers – Swap Boundary

Fluent Meshing swaps the interior faces for the boundary faces when removing sliver cells

Warning: Features in the geometry may be altered during the “swap boundary” operation, especially when angle between normals is large

Boundary mesh after sliver removal Original boundary mesh


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Modify Cells

• Mesh Tools Cell Modify

• Hit Skew to zoom highlight cell with worst quality criteria

• Use Operations to move nodes etc. to locally improve cell

• Next Skew will move to the new worst cell, etc.

• Hotkey Note:

– Moves to next skew

– Reset Skew

– Increases display bounds

– Decreases display bounds

• Modify Cells is analogous to Boundary Modify panel but for volume rather than surface elements – it has been generally superceded by Auto Node Move


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Manage Cell Zones Panel

1. It is possible to specify in the Manage Cell Zones panel which zone(s) will (not) be refined.

Mesh Manage Activate

2. When you apply, only highlighted zones

will be refined.

3. Also when you display, only active zones

can be displayed.

Notice other cell zone manipulations you can do in this panel (copy, rotate, delete, etc)

Scaling by a factor of -1 in a direction and 1 in other directions will reflect the mesh about the direction chosen.

Copy Zone(s) can be used to along with negative scale factor to create a full model from half a model when there is a plane of symmetry.


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Mesh Goes Inside Geometry

• My wrapped surface or otherwise seems good but after I tet mesh I find tetra cells inside my geometry where it should be a void. Is there a way to find leak holes?

• Yes there is an option as follows:

• Display a cut plane of the final tet mesh and use control-c to find the cell ID of a cell in the zone which should not be meshed and also one that should be meshed. Then use these IDs with this TUI to trace the path between them. (You may need to use overlays in scene description to show where the hole is)

– /mesh/tet/trace-path-between-cells


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Activating Zones Using Prescribed Points

1. Create a file describing each active region:

• Prescribed points (coordinates of a point within a zone)

• Type:

– Solid

– Fluid

• Name of zone

2. Use text-interface command to activate appropriate zones:

((1550.50 -466.58 896.41) fluid heater-#)

((1535.83 -643.14 874.71) fluid below-htr-#)

((1538.73 -444.28 952.69) fluid above-htr-#)

((1389.18 -775.51 825.97) fluid plenum-#)

/mesh/zone> auto-set-active

Active zones file ["active-zones.inp"] "active-

regions.inp"


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Failure of Check Mesh

• If mesh check fails, possible cause is presence of ‘left-handed’ faces

• and non positive volumes (Fluent Meshing will report this).

• Face handedness is a parameter which can take value of 0 or 1. • 0- face is left handed,

• 1- face is right handed.

• This value relates to direction of cell face normal vectors.

• All face vectors have to point outwards (right handedness) as finite volume method requires.

• TUI command mesh/repair-face-handedness can sometimes help

• Finite volume method also requires that each cell has positive volume.

• If the volume check fails for these reasons, check the quality of the surface mesh and remesh.


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Boundary Slivers in Tetrahedral Meshes

Sliver cells = flat tetrahedra, two faces on boundary mesh.

Sliver cells have small circumspheres:

• Difficult to refine

To check for sliver cells:

• Report Boundary Cell Limits...

Slivers should be dealt with automatically during meshing but to deal with sliver cells manually use Tri/Tet improve Slivers

tetrahedron degenerates to a

sliver if the top node moves

from here

to here


60

Creating Domains for Volume Fill A domain is the set of boundary zones which enclose the volume region where Tets or

Hexcore meshes have to be generated

The global domain always exists and includes all zones

A new domain may need to be defined…

• After import of volume and surface meshes

• After generation of prisms or pyramids

• To define specific parameters for different sub-volumes (tet growth rates, buffer layers for hexcore etc)

Select the zones and click New to create the domain. Activate is used to activate the domain

Mesh Domains


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Creating Domains for Volume Fill • Only one domain can be activated at a time

• Auto Mesh controls, i.e. prism/ pyramid/ nonconformal/ tet/ hexcore controls), reports, and displays apply only to the active domain. Only boundaries in the active domain will be listed.

– Users can display free faces to check missing zones in a domain

• Important TUI commands

– /mesh/domains/create-from: create a domain with all the surface zones adjacent to the specified cell zone (as well as the cell zone itself) and activate it – useful if you wish to delete only one volume from a multi-volume mesh and recreate it

– /mesh/tritet/preserve-cell-zone: selected cell zones will not be cleared during volume fill, even though they are in the active domain

• Reduces the need for domain creation

– /mesh/domains/create-by-point: allows user to give a coordinate point and Fluent Meshing will create a domain based on the surrounding boundaries.

• Domains created in a Fluent Meshing Session can be written and read back into another Session later via the “File” menus in GUI and TUI


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Fluent Meshing Beta functionality

Beta Functionalities are accessible through script commands

– Note that these are not formally tested and documented

• Face zone specific Tet mesh growth rate – (enable-zone-growth).

Which will enable TUI;

– /mesh/tritet/control/set-zone-growth-rate

– /mesh/tritet/control/clear-zone-growth-rate

Wheel 1.1

Mirror 1.05

Car

1.3

FarField

1.6


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14.5 Release

Appendix B

Prism Meshing Additional Info


64

Hybrid Mesh Limitations

Prism and pyramid generation can fail or yield high skewness meshes due to:

• Disconnected surface meshes with holes which need closing/sewing

• Acute angles between the zones from which they are grown and adjacent zones, or narrow gaps – For prism generation, the shrink and ignore algorithms can help

• Pyramids may grow from high skewness or high aspect ratio quad faces – Difficult to grow a pyramid from highly sheared or long and thin quads

– Better to use non-conformal


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Improve Controls

“Improve” controls how the faces (triangles or quadrilaterals) are smoothed before growing the next layer:

• Smoothing and Swapping

– Swap Edges, Smooth Edges and Smooth Nodes: Enables smoothing and swapping of edges and nodes

– Skewness: Any cap face with skewness greater than the specified value is improved

• Quality check

– Check Quality: Checks the quality of each prism layer, as it is created

– Max. Allowable Skewness sets the maximum limit on skewness (cap face). Fluent Meshing will stop growing prisms if it is reached

• Warp

– Ability to iteratively Improve warp for quad faces (make them more planar)

– Will not deteriorate skewness


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Zone Specific Growth - Baffles

• Grow on Two Sided Wall option to grow prisms on two sided walls, i.e., zero thickness walls such as baffles

– For T-junction configurations, facets are ignored to avoid the creation of skewed cells

Prisms extruded on both sides of baffle

Prisms grown on both sides of a two-sided wall or “baffle”

Ignored zones at non-manifold nodes


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Prism Growth Options • The Prism Growth Options are intelligent controls for dealing with sharp corners, thin

gaps and complex geometry

– Automatic adjustment of prisms height or faces ignored for extrusion

– Detect prisms-prisms and prisms-wall collision

– General Options • Remove Invalid Layer: option to keep or remove the layer for which Fluent Meshing reported a quality

problem on the prism-cap and at which the extrusion stopped

• Grow Individually from Multiple Zones: toggle to create a separate cell zone for prisms grown from each boundary zone

• Ignore Invalid Normals: ignore faces for extrusion in places with invalid normals

Ignore invalid Normals OFF

Max. skewness = 1

Ignore invalid Normals ON

Max. skewness = 0.78


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Prisms Growth Options • Proximity Options:

– Allow Shrinkage: avoids collision/contact between boundary layers growing in narrow gaps by decreasing the height of the prism layers

– Keep First Layer Offset: to be used when specified first height defined to get correct Y+

– Gap Factor: controls the gap size

• The smaller the factor, the smaller the gap between opposing prisms

• Increase if problem to initialize the volume fill

– Allow Ignore: removes or “peels off” the boundary layer at sharp corners and at proximity

– Max Shrink Factor: controls the shrinkage of the prisms layers

• Only available when Allow Ignore is ON

• Value must be between 0 and 1

• The smaller the value the more shrinkage, so the less faces are ignored

– Max Aspect Ratio: approximate maximum allowable aspect ratio for prisms

• Only available when Allow Ignore is OFF

• If value is too low, intersection might occur

Prisms extrusion near a car wheel

Ignored area

Shrinking


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Allow Shrinkage OFF

Allow Shrinkage ON

Prisms Growth Options

Gap Factor = 0.3

Keep First Layer ON

Gap Factor = 0.8

Allow Ignore ON


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Prism Growth Options

Allow Shrinkage OFF Allow Ignore OFF

(one layer one, bad skewness)

Allow Shrinkage ON Allow Ignore OFF

Default parameters

Allow Shrinkage ON Allow Ignore ON

Default parameters

Allow Shrinkage and Ignore ON Default Gap Factor = 0.5 Max Shrink Factor = 0.8

Allow Shrinkage and Ignore ON Default Gap Factor = 0.5 Max Shrink Factor = 0.2

Allow Shrinkage and Ignore ON Gap Factor = 0.3

Max Shrink Factor = 0.2


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• Three new Prism quality improvement tools were added – Improve (Previously only in TUI )

• Smooth/Improve Cells

– Post Ignore

• Remove bad cells using non-conformals to transition from prism side quads to tet

– Tet improve Cavity

• Tets can be squashed between prism layers – this allows the user to remove tets and surrounding prisms of bad quality and use non-conformals to transition from prism side quads to tet

Fluent Meshing 14.5 Prism Quality Tools


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• Post-ignore allow us to better dictate final quality of the prism and tet mesh

• Post ignore inflation are based on these principals – Grow prisms (inflation layers) everywhere without any quality based

halting of growth.

– Improve quality as much as possible using smoothing routines

– Remove any stack of prism cells containing any bad prism cells, based on

• Quality

• Collision/Self intersection

• Edge of acute angle (defined by user-provided feature loop on cap)

– Remove additional stacks of neighbouring Prisms to form a proper cavity

– Fill the cavity with tets, using non-conformal transition

Fluent Meshing 14.5 Post ignore Prism


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• We start by creating Prisms but we switch OFF – Check Quality

– Pre ignore options

– Setup


Poor quality prisms will be created in certain regions


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• Purpose – Use post ignore to remove bad prism

cells in the sharp gap

– Strategy

• Adjust Max Cell Quality

• Use Mark Prism Cap to get a preview of cells to be removed

• Hit Ignore Prism when the preview is deemed correct

• Prisms will be removed based on the faces marked

• Quad sides will be exposed and user must create non-conformals and domain to finish mesh


Marked cells highlighted in graphics window

Final Mesh


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• Tet improve Cavity – Prism quality was good, but tet quality was bad

– Identify bad quality tets

• Remove tet+ surrounding tets

• Optionally remove prisms adjacent to tet

• Fill cavity with tet

Fluent Meshing 14.5 Post Cavity re-mesh


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Prism Improve

•Smooth/Improve Prism Cells above Max Quality Limit •May require /mesh/separate/separate-cells-by-shape first


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Prism Post Ignore


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79


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Improve Prisms

• Skewness criteria during extrusion is calculated on the prism-cap so cell skewness can be higher than Max. Allowable Skewness

• Post-extrusion operations available to improve prism cell quality

– /mesh/prism/improve/improve-prism-cells: smoothes bad cells and ring of neighboring cells to improve quality

– /mesh/prism/improve/smooth-prism-cells: optimization based smoothing of prism cells

– More controls available in /mesh/prism/improve/


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Prism Layers are failing. What can I do to stop this? (1)

**Always save the mesh with the latest settings you are trying prior to testing prism growth. Once grown the adjacent zones will have quads imprinted so clearing the volume mesh does not return you to the original state!!

BEFORE GROWING PRISMS ALWAYS CHECK:

• Are normals pointing in the right direction?

– Use beta scheme command (enable-color-by-normal #t)

– Switch to “color by normal”

– Display all boundaries prisms should be grown from

– Grey indicates direction of prism growth (yellow on opposite side)

– You could need to

• flip normals on certain boundaries using Boundary -> Manage -> Flip Normals

• Align normals within a boundary using Boundary -> Manage -> Orient

• Use TUI command

– /boundary/orient-faces-by-point


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2nd THINGS TO ALWAYS CHECK

• Do you have free faces or multi faces where you should not?

– Display free (blue) and multi (yellow) faces only

– Zoom on any highlighted faces (use color by ID)

– Bring back other faces to understand what part of geometry is problem

– Fix by patching in Fluent Meshing or back in preprocessor (see How to Fill Patches)



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What is the max skew on the boundaries you are growing from? It should be below 0.8.

• Use Report->Face Quality limits to check max skew

• If Max Skew > 0.8 use global improve commands under Boundary -> Mesh –> Improve (smooth and swap work best for non-wrapper meshes)

• If there still exist some triangles > 0.8 skew use boundary modify to improve the skewness (See Tutorial 1 for Fluent Meshing). Hint: Use right arrow key in graphics window to move to next max skew, fix and repeat.



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Everything looks good in the surface mesh and steps 1-3 are ok but prisms are still failing!

• Try changing the two parameters – Mesh -> Prisms -> Direction -> Max Angle Change increase to 60 or 70

degrees to give more freedom to smoother as layers are grown

– Mesh -> Prisms -> Project -> Max Adjacent Zone Angle to a higher value if you have prisms you want to project the prism quad sides onto neighbouring boundaries but the angle change between boundaries is less than 45 degrees.

– Refer to these angle definitions on slides 16 and 18 and in the User Guide



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If steps 1-4 have not solved the problem you need to see the bad layer to diagnose the issue.

• Change the default and untick the Prism -> Growth Options “remove invalid layer” to OFF

• Now when it fails you will be able to see the failed grid to diagnose problems

• Display all boundaries with the string “quad” and the string “prism-side”

• Use wildcards in the Zone Selection Helper to aid you to select these

• Look for areas where you have prism sides first and understand if you should have these. Bear in mind you will need non-conformals or pyramids grown off these quads if you do have prism sides

• Do prism sides or quads stop in strange places? – If they do the angle specified for projection on adjacent zones may need to be

increased to ensure quads are projected properly

– If there are highly complex areas where several planes meet and node normals are not defined the “allow ignore” option will perhaps be the only answer unless a CAD change can be made. • Allow ignore will mean prism side quads are exposed and best to use the non-conformal

method after that to grow tets rather than very skewed pyramids off these quad zones.



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Prism Layers are Freezing at first Layer

This can happen when the first cell height is set inappropriately for all the mesh or part of the mesh

• E.g. Prisms could be set in mm whereas mesh is scaled in metres

• Or locally somewhere you could have elements which are e.g. 1mm across and your first height is 5mm which will cause very small aspect ratios to occur (0.2 in this case meaning tall thin prisms)

A quick check is to switch to aspect ratio based prisms and specify a sensible aspect ratio such as 10 to see if they are growing now.

Check the min edge length on each boundary by highlighting the boundary in Fluent Meshing prisms panel and clicking “edge size limits”. If the minimum is much larger than your first cell height you could have the problem described above.

Workarounds are to either

• Coarsen the surface mesh so you don’t have the problem of very low aspect ratio cells (tall & thin)

• Limit the min aspect ratio using TUI: /mesh/prism/controls/offset/min-aspect-ratio. This is 0 (and therefore not used) by default but setting to 1-10 will shrink in the cells to avoid very tall thin cells in these areas.

• Use an aspect ratio based offset specification instead of a height based specification.


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Fixing Sharp Angles

The region where the wheel meets the ground is a problem area. One would like to be able to grow overlapping meshes in the way just described, but limitations in Fluent Meshing's calculation of normals prevents this. This leaves two options.

1. Alter the geometry such that prisms can be grown from it by blunting the sharp angle or

2. Use the Ignore Proximity option in the Prism Growth Options subpanel.


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Fixing Sharp Angles – Solution 1 – Ignore Zones

This option causes the prisms not to be grown where surfaces come into close proximity, see image.

In this example case, prisms are grown from the ground (using the aspect-ratio offset method so that prisms further from the car will be thicker since the triangles are larger) and from the tyre (using the minimum-height offset method).

Near the junction of the tyre and the ground no prisms grow and prism sides are exposed.

Prisms will be shrunk until they become smaller than the max Shrink Factor after which point the area will be ignored from prism growth instead.


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Fixing Sharp Angles – Solution 2 – Smoothing Nodes

Smoothing the nodes at the apex requires all the nodes at the apex to be selected so that they can be smoothed en masse. The easiest way to select all these nodes is as follows:

1. Create a feature edge at the apex.

2. Use the text command /boundary/mark-duplicate-nodes. This will mark the nodes at the apex and those on the feature edge.

3. Display the marked nodes by deselecting all the Options toggles and Zones under Nodes, Faces and Cells in the Display Grid panel, selecting only the Node Zone containing the nodes on the apex and clicking display.

4. Use the polygon select to select all the nodes.

These nodes can now be smoothed. In this particular case the results are not satisfactory as skewed triangles are created, which would lead to a poor quality volume mesh, see image.


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Fixing Sharp Angles – Solution 3 – Chamfer Geometry

This approach has a few simple steps:

1. Copy and translate one of the surfaces (in this example, the ground) so that it intersects the other.

2. Delete the original surface.

3. Intersect the surfaces using the Boundary/Intersect panel. When doing this select the “Separate” option so that overlap zones can be easily deleted.

4. Delete the overlap zones.

5. Translate the surface from step 1. back to the original location.

6. Create two separate triangles connecting the two zones, see Figure 2. This is easiest done using the graphics hot keys: Ctrl-n to set to node filter; pick three nodes with the mouse; F5 to create.

7. The gap between these triangles can now be filled using another simple hot key sequence: Ctrl-e to set edge filter; pick a triangle edge on the gap; F-5 to create. This works by Fluent Meshing tracing a feature edge loop around the gap starting from the selected edge, then triangulating within the loop, see Figure 3.

You should now have two unconnected surfaces with a gap between them, as shown in Figure 3.

8. Repeat this process to fill the entire gap. You may need to

create more individual triangles as the tracing of the feature

edge loop can only encompass a limited number of triangle

edges (though many more than shown in the above

picture).

Now we can grow prisms in this region, as illustrated in Figure

4.

1 2

3 4


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Overlapping Boundary Layers

Here we are going to grow overlapping boundary layers. This is not a facility which Fluent Meshing provides directly but can be achieved where surface meet at close to 90 degrees. This can be done as follows:

1. Grow the prism layers from the flatter of the two surfaces so that they project to the adjacent surface, which is retriangulated; then delete these prisms and their cap to leave the new surface mesh, see Figure 1

2. Grow prisms from the other surface including the new quad portion, see Figure 2

3. Grow prisms from the triangular part of the first surface, completing the process, illustrated in Figure 3

4. Note prism settings must be kept consistent for steps 1-3 above, e.g. Method, growth rate, # of layers, etc.

Figure 1 Figure 2 Figure 3


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Revolve Face Zone

TUI command to generate prisms by revolving a face zone

• /mesh/manage/revolve-face-zone

Input needed • Axis

– Negative coordinate might be needed to get the right direction

• Origin

• Angle

– One angle value needed for each layer

15 layers with 5° angle


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Inserting Prisms into Volume Mesh

From Fluent Meshing 13 we can insert prisms into any volume mesh Fluent Meshing can read in (that excludes polyhedral or adapted grids)

• Read in Volume mesh

• Apply prism settings

• Ensure normals are oriented correctly (pointing into volume mesh)

• Click create and say yes to question OR

• Use /mesh/cutcell/create-prism (this will orient normals for you based on supplied cell zone)

• Fluent Meshing will grow prisms then morph volume mesh to match prism cap

• This method can be used to insert prisms on “hexcore to boundaries” zones


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14.5 Release

Appendix C

Hexcore Meshing Additional Info


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Non-Standard Hexcore Meshing Controls • Interface Smoothing specifies the

parameters controlling the smoothing of the interface between tet and hex regions.

• Invoking “Smooth Interface” may improve quality at the hexcore interface, but will increase mesh generation time

• Number of Iterations sets the number of smoothing iterations on the parent face.

– Increasing the number of iterations will help to initialize the tets and obtain lower maximum skewness in the tet region.

– This is at the expense of an increase the value of maximum skewness of the hex cells near the hex-tet interface.

• Relaxation Factor sets the value of under relaxation for smoothing the parent face.

• The default values for interface smoothing work very well in most cases!

This should be considered as an advanced option!


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Hexcore Meshing Controls

• Only Hexcore allows the user to prevent the automatic creation of the tetrahedral layer after hexcore generation.

• When the Only Hexcore option is enabled, the hexcore is created and the tetrahedral domain is activated. User can manually create the tet mesh in a separate step.


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14.5 Release

Appendix D

Prism Meshing Additional Info


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• Select the quad Boundary Zone

from which the pyramids are grown

• Set the Options used to grow

pyramids:

– Skewness will attempt to create

pyramids with lowest possible

skewness using either “neighbor” or

“centroid” method

– Neighbor will use nodes from

neighboring triangles as the apex of

the pyramid

– Centroid will always place the apex

above the centroid of the quad

“Centroid” method “Neighbor” method

Pyramid Options Mesh Pyramids

*pyramid-side

*pyramid-cap


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Generating a Pyramid Mesh Layer Set Offset Scaling and Fill Cap:

• Offset Scaling will set the scaling factor for the height of the pyramid.

– The height = Offset Scaling x characteristic length of the quadrilateral face.

• Offset factor specifies the fraction of the computed pyramid height (offset) by which the pyramid heights will be randomly adjusted.

– 0.1 will adjust the height by 10%.


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14.5 Release

Appendix E

Non Conformals Additional Info


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Generating a Non-conformal Interface

• Quad-Tet interface to be used when pyramid growth yields problematic cells, e.g. at prism sides where quads have very high aspect ratio

• Select the quad Boundary Zone from which the non-conformal interfaces are generated

• Set the Retriangulation Method:

– Prism: remeshes all quad zones named prism-side* with specific algorithm

• Other quad surfaces are meshed with remesh algorithm

– Quad Split: split the quad faces diagonally into tri faces

– Remesh: remeshes all the zones comprising quad faces based on the edge and surface feature angles specified

Mesh Non-conformal


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14.5 Release

Appendix F

Quality Metrics Additional Info


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Volume Mesh Quality: Skewness

Two methods for determining skewness:

1. Based on the equilateral volume:

– Skewness =

– Applies only to triangles and tetrahedra

– Default method for tris and tets

2. Based on the deviation from a normalized equilateral angle:

– Skewness (for a quad) =

– Applies to all cell and face shapes

– Always used for prisms and pyramids

max max min

90

90

90

90,

min

max

optimal cell size cell size

optimal cell size

optimal (equilateral) cell

actual cell

circumsphere


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Volume Mesh Quality: Smoothness and Aspect Ratio

Change in size should be gradual (smooth).

smooth change large jump in

in cell size cell size

• Aspect ratio is ratio of longest

edge length to shortest edge

length.

• Equal to 1 (ideal) for an

equilateral triangle or a

square.

aspect ratio = 1 high-aspect-ratio quad

aspect ratio = 1 high-aspect-ratio triangle


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14.5 Release

Appendix G

Component Changes with Cavity Remeshing


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Cavity Remeshing Components can be removed, inserted or swapped

without remeshing the entire domain with the cavity remesh tool

• Mesh Tools Cavity Remesh

To replace a component,

• Highlight the old component in the LHS and the new in the RHS list

To remove a component

• Highlight only a component in the LHS list

To add a component

• Highlight only a component in the RHS list

To improve an area of mesh

• Do not highlight any component!

• Define bounding box and hit Create.


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Cavity Remeshing

Cavity Remeshing panel • Bounding box centre and dimensions

for a component highlighted can be calculated via the Compute button

• The Scale factor is will allow this box to be scaled up in relation to than the component bounding box

• Orient allows the box to be rotated about X, Y and Z axes

• Create Face Group toggle will create a face group from the resulting cavity boundary zones for easy export into a different Fluent Meshing session (Useful for large cases, see User Guide.)

• Use Draw to display

the bounding box


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Cavity Remeshing Example - Sedan

• Cavity Re-meshing for design changes

• Sedan case to remove wing mirror shape and replace with new design

– Ability to remove all tet cells and prism layers in any tet region

– Will also work for Hexcore but can currently only remove tet regions. May need to use more peel layers to increase size of tet region in order to utilize cavity remeshing later

• Compatible with prism layers – see tutorial

Original Mesh Cavity + new Design Cavity re-meshed

lecture 4: volume fill methods -...

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