fluent meshing 14.5 l03 boundary fix

8/12/2019 Fluent Meshing 14.5 L03 Boundary Fix

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2011 ANSYS, Inc. December 21,

2012

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14.5 Release

Lecture 3: Boundary

Mesh Tools

Introduction to ANSYS

Fluent Meshing


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Introduction

To grow a high quality Tet or Hexcore volume mesh,Fluent Meshing needs as input a high quality, water-

tight boundary mesh

Boundary modification tools exist to allow the user to

arrive at such an input boundary mesh.

Boundary Tools allow you to:-

Diagnose and locate problems with the boundary mesh

Fix problems with the boundary mesh Improve and refine the boundary mesh


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Agenda

Boundary Mesh Definitions

Diagnosing problems with a boundary mesh

Visual diagnostics

Quantitative diagnostics

Advanced diagnostics

Tools for macro repair i.e. at boundary level

Tools for micro repair i.e. at node/facet level

Refinement of surface meshes

Manipulation of surface meshes


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Agenda



Visual diagnostics







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Fluent Meshing has a terminology todescribe surface mesh, mesh quality and

boundary operations

These terms include:

Face and Boundary zones

Edge zones

Free and multiply connected edges

Mesh quality measures such as skewness

Delaunay violations and Swapping


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All of these refer to the boundaries of the fluid region

Boundary

Boundary zone

Face

Face zone

An edge zone refers to a group of edges of the mesh

elements which form the defining features of the face zones

Can be based upon a sharp change in angle or the boundary between

Face Zones

Also called edge loops or feature lines

Used for remeshing or imprinting operations among others

Feature lines

extracted from

exhaust manifold

geometry


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Visual Diagnostics 1Face Quality

The Display Grid panel allows the visualization of nodes, faces or cells

Multiple options available for the display of all, free or multiple or high

skewness faces

High skewness faces in red, displayed with the full mesh, by selectingtheAlland Qualityoptions and setting the Minimum skewnessto0.9



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Visual Diagnostics 2Free Faces

Free edges are mesh edges which form the boundary of only one mesh face.

For a water tight boundary, the edges should be connected to two triangles or quadrilaterals. Can be due to holes in the mesh or zero thickness walls (baffles or fins)

Free nodes are the nodes bounding the free edges.

Free edges due to a hole in the

mesh (needs to be repaired)

Free edges on a fin in a duct

(no repair required)

Faces with free edges (blue) will show thepresence of holes or zero thickness geometry



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Visual Diagnostics 3Multi Faces

Multiply connected edges are mesh edges which form the

boundary of more than two mesh faces.

Most edges should be connected to two quads/tris

Can be due to intersection of multiple face zones (t-junctions) or

meshing errors

Multiply connected faces can be visualised to see if they are valid or a

problemFaces with multiply connected edges (yellow) areconnected to more than two boundaries

Valid T-Junction Invalid collapsed faces


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Quantitative DiagnosticsSurface Mesh

Surface Mesh Quality

The chief measure of triangular surface mesh quality isskewness

0 is a perfect equilateral triangle, 1 is completely

degenerate

Max tri skewness should be ~< 0.9 to get a valid tet mesh

Max tri skewness should aim to be < 0.8 to get a good

quality tet meshwith tet skewness < 0.9

In general, the further you drop tri max skew the better the

quality of the tet mesh. E.g. Max skew < 0.5 would give

excellent tet quality ~ < 0.8 skew and generally very good

solver convergence

Skewness = 0 Skewness = 0.98


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Tools for Mesh Inspection

Report Face Limits: Calculate the

maximum skewness of selected facezones

Report Face Limits

Plot Histogram of Face quality

Distribution: Display Plot Face Distribution


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Agenda



Visual diagnostics







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Merge Boundary Nodes Boundary mesh requires connecting up into a

water-tight region before volume meshingsome

surfaces may not be connectede.g. meshes couldhave been appended therefore are disconnected

Eliminate free edges by merging free nodes.

Set atolerancedistance using either an absolute

value or as a percentage of shortest connectededge length.

Free nodes with distance

less than the tolerance will be merged.

Percentage of shortest edge length is useful for CFD

surface meshes

User can choose to connect up non-free nodes also

by deselecting only free nodes

When nodes are poorly matched the aggressive

Boundary -> Mesh -> Faceted Stitch command can

be used

Merge

Nodes

Disconnected CAD surfaces could lead to

free faces at the join


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Effect of Merging Free Nodes

Before free nodes were

merged

After free nodes are merged

with tolerance of 0.011

Left and right faces of the cube

are disconnected. There is a gap

of 0.01 here so a node merge

with a very small tolerance

would not succeed.

All faces of the cube

are now connected

no free edges

MergeNodes


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Faceted Stitch

Repair surfaces having internal cracks or very

poorly mismatched facets using FacetedStitch

Different from node merge in that triangles can be

split to add nodes

You can specify an appropriate Tolerancevalue

within which the free edges will be stitched.

The faceted stitching operation is available only for

triangular boundaries.

Warning: Large tolerances can cause faceted stitch

to collapse geometry, especially for STL where lotsof triangles are very high aspect ratio. Use

conservative tolerances (~0.01) and gradually

increase to remove free faces. Always inspect as

you go to ensure geometry remains valid. Before and after faceted stitch operation


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Improving Boundary Mesh

Improving Boundary mesh involves:

Eliminate Delaunay violations by swapping

Eliminate high skewness faces by smoothing

Removing distortions in surface mesh by projectingface zones

Remeshingor retriangulatingface zones

Generally, boundary mesh improvement is done in two steps:

1. Smooth/improve boundaries Globally(or at a boundary-level)

to reduce [possibly large numbers of] poor skewness cells to a minimum

automatically

2. Smooth/improve boundaries Locally(or at a nodal-level)

to reduce [small numbers of] poor skewness cells to a minimum manually


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Automatic Boundary Improve Operations

A number of automatic boundary-level commands can be

utilized to improve surface mesh quality

Boundary Mesh Improve...

Choose boundaries and then

Improve Smooth

Swap

to try to drop max quality below specified Quality Limit

Geometric features are protected by the parameter Angleincreasing Angle allows more freedom for the improvement

algorithms to change surface mesh


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Boundary Improve Panel


Improveallows you to improve the selected zones

based on the Quality Measure.

Quality Limitspecifies the quality limit for the

improvement operation. All elements above the

specified quality limit will be acted upon.

Anglespecifies the maximum allowable

angle between two adjacent face normals above

which smoothing cannot occur

Iterationsspecifies the number ofimproving attempts.

Preserve Boundary allows you to

disallow nodal movement for nodes on edge

loopsi.e. preserve feature edges

Boundary Improve (1)

Angle above which

smooth/swap/ improve cannot

occu r. This w i l l preserve cr i t ica l

features and disal low mo vement

f rom geometry!


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Boundary Improve (2)

Very small triangles can be collapsed en

masseusing Improve by Area

When using Min Absolute Size users should

preview their selection using the Size option

in the Display Grid panel to view the faces

with area below the selected threshold

Once satisfied with threshold Collapse can be

done successfully

Recommend to save mesh before Apply


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Boundary Improve


Smoothallows you to improve the selected zones by smoothing.

Max Angle specifies the maximum allowable angle between two

adjacent face normals. Iterations specifies the

number of smoothing attempts.

Boundary Smooth


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Delaunay Violations and Swapping

A Delaunay violation occurs if a node of a

triangle lies inside an adjacent triangles

circumcircle.

E.g. Long and thin triangles which have high skewness

Edge Swapping is typically used to remove the

Delaunay violations.

Replace or Swap the

diagonal of the quadrilateral

formed by the two triangles

with the other diagonal

Many tools in Fluent Meshing automatically

detect and remove Delaunay violations using

edge swapping while preserving features

SWAP

Poor

Skewness


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Boundary Improve


Swap allows you to improve the selected zones by edge swapping.

Skewreports the face with the

maximum skewness and the

corresponding skewness valuein the Fluent Meshing console.

Limitsreports the minimum

and maximum face area for the

zone(s) selected in the

Tri Boundary Zones selection list.

Boundary Swap


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Effect of Swap Face Edges

Before edge

swapping

After edge

swapping


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Demo of Boundary Improve Tools

Open boundary mesh Display histogram of quality and show max

skewness with compute

Display poor quality faces

Use Boundary Improve tools swap, smooth,

improve

Show new quality histogram

Update display of poor quality faces

Save surface mesh


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Agenda



Visual diagnostics







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Repairing Boundary Mesh

Repairing a boundary mesh involves: Eliminating free (disconnected) edges by

merging free nodes

Closing holes in the surface mesh

Deleting incorrect, multiply connected

faces/edges

Connecting up intersecting or overlapping face

zones which pass through one another


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Modify Boundary (1)

This panel allows you to do primitive

operations on the boundary mesh with

individual nodes, edges, faces, cells or zones.

Filterselect what type of entity you wish to

pick with RMB click

Any entities selected appear in the

Selections list

Entities can be added manually by typing

their name into the Enter Selection field

and pressing

Click on appropriate button in theOperation section to modify boundary

mesh

When working with Boundary Modify

primitive commands, the user has full

power to modify the geometry

feature edges are not protected. Merge

Nodes cannot be undone.


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Modify Boundary: Operations

Create: create nodes, faces, zones

Merge: merge pairs of nodes, faces

Collapse: Collapse nodes, edge or face into a single node.

Move To: move nodes to selected positions

Rezone: move faces to new boundary zone

Smooth: smooth nodes and zones

Split: split edges and faces Swap: swap edges

Clear: Remove the selected object(s) from the Selections section.

Delete: Delete the selected object(s) from the model.

Undo: Undo the previous operation. Move By: Move selected object by a specified distance.

Centroid: Find the centroid of the selected object.

Distance: Find distance between the selected objects.

Set: Set a line or a plane for use with the Project operation.

Project: Project all selected nodes onto a set line or plane.


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Shortcuts for Operations and Filters

Esc :Deselect last

F1: Auto-scale, scale the image so that it fits graphics window

F2: Deselect ALL

F3: Right mouse button, toggle mouse probe or mouse dolly

F4: Right mouse button, mouse probe select or polygon

F5: Create

F6: Smooth node

F7: Split (face / edge)

F8: Swap edge

F9: Merge nodes

F10: Show next skewed face

F12: Undo

: Increase Display Bounds : Decrease Display Bounds

: Zoom to next skewed face

: Reset Skewed Faces

Ctrl+A: Auto Scale

Ctrl+ C: Cell Filter

Ctrl+ D: Compute Distance

Ctrl+ E: Edge Filter

Ctrl+ F: Face Filter

Ctrl+ H: Print This Help

Ctrl+ L: Centroid/Node Coordinates

Ctrl+ N: Node Filter

Ctrl+ O: Rezone

Ctrl+ P: Project

Ctrl + R : Repair selected zones*

Ctrl+ S: Set Plane or Line

Ctrl+ V: Move

Ctrl+ W: Delete Without Confirm

Ctrl+X: Position Filter

Ctrl+Z: Zone Filter

Ctrl+ ~: Collapse

*Advanced usage

(tgsetvar! hole/separate-thread? #t) puts each patched surface into separate face zone

(tgsetvar! hole/remesh? #t) remeshes new surfaces with CFD mesh


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Demo of Boundary Modification Tools

*Can watch in slideshow mode


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Agenda



Visual diagnostics







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User Defined Groups

User Defined Groups can be created to ease boundary selections when

working on large models with long lists of boundary zones

Boundary -> Zone -> Group

Ability to create Face (or Edge) Boundary Groups

The groups will be added to the standard Face Zone Groups in all Boundary

Panels

Ability to Activategroups Only the Face Zones in the groups will appear in all Boundary Panels


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Various options existto separate boundary

zones

Boundary Separate

Two boxes in a single Boundary Zone

After Separate by Region

After Box1 separated by angle of 90 deg

After Box2 separated by selected

face seed and angle of 90 deg

Box1Box2


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Manage Face ZonesThe Boundary Manage panel allows operations on selected face zones such as:

Change (Boundary Condition) type of zone (wall, inlet, outlet etc.)

Copyzones

Deletezones

Mergezones, where master decided by

Alphabetical order

Largest zone

Rename

Rename individual zones

Change Prefix for multiple zones

FlipNormals:

Reverses direction for growing

prisms/pyramids from the face zones

Orientnormals within zones

Rotate

Scale

Note that scaling by -1, 1, 1 would reflect about X etc

Translate

List will print to the console information about the current selection including zone type, ID

number, # of elements, face types present (quad, tri or mixture)


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Zone Creation

Fluent Meshing has some

useful tools to createsimple geometries

Bounding box

Plane surface

Cylinder (includes frustrum

and annular cap)

Swept surface (from feature

edges)

Periodic

See appendix for further

details

Traditional Workflow

New Workflow


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Triangulate

The Triangulate Zones panel enables

triangulation by splitting of quadzones

Quad zones not supported by wrapper

options

Option to replace existing zone or to

create new zone


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Intersect Boundary Zones

Intersect Boundary Zones is used to connect overlapping or disconnected tri zones.

Tolerance sets the maximum distance between nodes for the operation.

Three operations are available:

Intersect: Intersect the boundaries that are touching each other but are non-conformal

with distance between them less than the Tolerance.

Join: Connect overlapping surfaces.

Stitch: Connect two surfaces, at their free edges, by specifying required Tolerance value.

Other options:

Absolute tolerance

Default tolerance is relative

based on local cell size Refinethe resulting zones

When size difference is large (robust)

Separatethe overlappingarea into another zone

Should essentially always be used


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Intersection ExampleTwo Cylinders

Intersect

Intersect and

Refine

Two inter-penetrating or

intersecting cylinders which are

not nodally conformal

Connects cylinders

Connects cylinders and refines growth


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Intersect

Before After

Boundaries

are now

connected


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Intersect and Separate

Before After

Boundaries

are

separated


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Intersect and Separate with Refine

Boundaries

are refined

Before After


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Typical Usage of Intersection Tools

Intersection tools are useful to connect meshes together at the

points of intersection

E.g. insert plane with symmetry into a hemisphereattach body to

symmetry plane or cut body in half with a symmetry plane


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Typical Usage of Intersection Tools

Intersection tools are useful to connect meshes together at the

points of intersection

E.g. insert car into a wind tunnelattach wheels to the ground


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Summary The boundary mesh generally determines success of volume mesh

generation and resulting quality of volume mesh

Boundary meshes may need to be repaired and improved before

volume mesh generation

Fix Free nodes, large size changes, high skewness, etc

Fluent Meshing includes boundary modification tools which allow

Checking of boundary mesh for problems such as

Holes which shouldnt be there -> visualise free faces

Multiply connected edges which shouldnt be there -> visualise multiple

faces

Poor quality/small faces -> Visualise poor/small quality faces

Repair and improve the boundary mesh on a macro and/or micro level

Create simple primitives and planar surfaces

Manipulate boundary zones


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14.5 Release

AppendixBoundary Modification

Further Detail & Advanced Usage


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Mesh Quality Measures

The chief measure of mesh quality for tet

elements is the Tri/Tet Skewness Method

Equilateral Volume DeviationMeasures the degree of

deviation of the area of the triangle from that of an

ideal (equilateral) face or cell which would fit into its

circumcircle.

Skewness = Ideal face areaActual face area

Ideal face area

Normalized Equiangle DeviationMeasures the

degree of deviation of the internal angles of the mesh

face from an ideal face with equal internal angles.

Skewness is plotted on a scale of 0 (good) to 1(bad or degenerate element)

optimal (equilateral) face

actual face

circumcircle

0 1

best worst


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Insert Edge ZoneConnecting Meshes (1)

The Intersect, Stitch, and Join

operations will remesh both surfaces The insert-edge-zone TUI command

will connect meshes while remeshing

only one of the surfaces

This tool is needed to connect surfacemeshes to volume meshes or other

surface meshes. E.g. we have the

ability to connect a structured hex

mesh into a triangular surface meshbefore filling the tri shell with

unstructured mesh

Wedge Volume Mesh

Wrapper surface


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Insert Edge ZoneConnecting Meshes (2)

1. BoundaryFeatures

Create Edge loop you wish to use

to use to connect> rad-out__9

in this case (edge zone ID is 9)

2. To perform the connection betweenthe yellow zone (rad-out) and the red

zone (wrapper) we use the following

TUI command:

This forces the zone wrapper to

conform to the specified edge loop.

Note that sometimes edge loops may

require separation to extract the

portion you wish to imprint.

/boundary/remesh/insert-edge-zone 9 wrapper ,


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Faces in close proximity can be marked and displayed using the command:

/boundary/mark-face-proximity

A TUI command is also available to remesh overlapping zones:

/boundary/remesh/remesh-overlapping-zones

Note: If there is gap between them, you can project one zone onto the other first.

Proximity Marking and Overlapping Zones


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Create Bounding Box

The bounding box is useful to create

an external box around a complex

model.

Computewill calculate the size of the

box based on selected face zones

Specify the dimensions as Absoluteor

Relativeto the previously calculatedvalues

For Example:

Compute first, then change to relative and

add 100 in each dimension, will create a

box with a 100 unit cushion in each dirn

Specify the Mesh Size using Edge Length

Drawwill allow you to preview the

dimensions of the box graphically

Click on Createto create the box.


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Create Cylinder (1)

Boundary Create Cylinder...

In some cases, you may want to create a cylinderwithin the existing geometry (e.g., creating an MRF

zone for problems involving moving parts such as

rotating blades or impellers). Fluent Meshing allows

you to create a cylindrical surface and mesh it using

triangular cells.

2 Axis Locations allows you to specify the locations ( X Pos,

Y Pos, Z Pos) of the points P1 and P2, defining the cylinder

axis or using Select Points to pick them from the geometry

2 Axis Nodes allows you to specify the nodes corresponding

to the points defining the cylinder axis.

Radius specifies the radius of the cylinder to be created.

Edge Length specifies the size of the cells to be created forthe cylindrical surface mesh.

Capsallows user to choose to create the circular end parts

or not. For gap filling sometimes only curved surface is

required.

Preview allows you to preview graphically the cylinder to

be created.


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Closing an annular narrow gap using Cylinder Panel

Easer to close annular region

New option

3 arc +1 Height node

Click on Select nodes

Pick three nodes on one circle

Pick one node on the other

Adjust Edge length

Un-tick caps

Include Objects

Preview

Create

Create Cylinder


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Swept surface (available only in 3D)

Boundary Create Swept surface...

Swept surface uses an edge zone

to sweep it into a face zone

Select Edge Zones from the pull-down list

Parameters to be defined:

Total Distance specifies the total distance through

which the edge is to be projected.

No. of Offsets specifies the number of interval counts

of the total distance.

Vectorspecifies the direction in which the selected

edge is to be swept.

Split Quad Faces toggles the creation of tri/quad faces

for the swept surface.


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Create Plane Surface

The Plane Surface tool is useful for

Plugging holes or gaps in the model before wrapping or as a tool to cut symmetry plane

Options using Axis Direction

Specify the axis normal to the plane

Specify the Location and on the Axis.

Compute will calculate the size of the

plane based on selected face zones.

Options using Point

Click on Select Points

The panel opens and three points can be

selected using right-mouse button

Common options

Edge length dictates the mesh size

Draw will create a preview of the face

Scale +/-can grow or shrink the face

Click on Createto create the surface.

f d


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Refine Boundary ZonesThe Refine Boundary Zones panel is used to refine

the boundary mesh based on Proximity or on

Region. Region: Defined using the Local Refinement Mesh

panel.

Max Face Size is based on the Max Cell Size set in the panel.

Proximity: refinement based on proximity.

Example of refinement using proximity

F M dif


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Feature Modify

Feature Modify is used to extract and

modify geometric features, i.e. the

edge zones or edge loops, boundingthe surface mesh.

This is critical for meshing algorithms

which create surfaces which must

adhere to features i.e. surface

remeshing, cutcell and surfacewrapper

Key Option:

Create: Extract the edge zones from

face zones based on fixed angle (default

40 degree feature angle) or semi-intelligent adaptive angle criteria.

Select Boundaries

Choose option Create

Choose Angle Criterion

Hit Apply

Apply

f l


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Surface Retriangulation Surface Retriangulation allows the user to remesh face zones using edge

zones as a starting point.

1. Create feature lines or edge zones from

selected face zones using Fixed or

Adaptiveangle.

2. Choose whether to invoke Advanced Size

Functionsto apply smoother growth ratesand allow refinement by e.g. curvature and

proximity.

3. Click Remesh to remesh a selected face

zone

1.

2.

3.

Note that remeshed faces are

new, disconnected face zones

and nodes must be merged prior

to volume meshing

f l


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Sizing Functions for Retriangulation

Global Controls

Define the overallMin, Max size as well as the global

growth rate.

Size Function Types

Curvature: Specify Min, Max size, Max Angle betweennormals and Growth Rate. (Can be applied to edge or facezones.)

Proximity: As above except you specify the number ofcells per gap instead of the max angle between normals.(Can be applied to edge or face zones.)

Meshed: The size is determined from the mesh size on thegeometry (not suitable for CAD STL). (Can be applied toedge or face zones.)

Soft: Specify the Max size and the growth rate. The localsize is allowed get smaller based on other size functions(e.g. curvature and proximity). (Can be applied to edge orface zones.)

Hard: Specify a fixed min size for the zone. The local sizefixed and is not allowed to get smaller or larger. (Can beapplied to edge or face zones.)

BOI (Body of Influence): Select a zone from the list ofzones to serve as a body of influence. Defines the max sizeof cells within that volumetric region and acts as anarbitrarily shaped refinement region for capture of flowfeatures such as turbulent wakes. (Can be applied to facezones, generally forming a closed volume).

i i f i l i


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Curvature Size Function for Retriangulation

Curvature size function

examines curvature on

edges and faces andcomputes sizes such that

size obeys min, max,

normal angle andgrowth rate settings.


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Si i F i f S f R i l i


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Sizing Functions for Surface RetriangulationFace Proximity

Face Boundary: allows you to compute

the shell proximity (edge-edge proximity

within each face)

FaceFace: allows you to compute the

proximity between two faces in the facezone(s) selected. This will lead to varying

size within a face. This is CPU-intensive.

Ignore Self: allows you to ignore face-

face proximity between faces in same

zone. This option is available only whenthe Face - Face option is enabled.

Ignore Orientation: allows you to ignorethe orientation of face normals when the

proximity size function is processed. This

option is available only when the Face -Face option is enabled.

P i i Si F i I


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Proximity Size FunctionIgnoreOrientation

In this example normals point inwards

With ignore orientation OFF the

remeshed surface is not refined along

the groove length

With ignore orientation ON the

remeshed surface is refined along the

groove length

S ft d H d Si F ti f


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Soft and Hard Size Functions forRetriangulation

Hard size function sets fixedsize and soft size function

sets a max size on an object

as shown in this simple

example

Curvature and proximity can

cause refinement

M h d Si F ti f R t i l ti


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Meshed Size Function for Retriangulation

Meshed size function allows us to take the

sizes based on sizes given on zones alreadypresent

In the below example a Meshed SF is applied to

the grey zone and the adjacent green zone is

remeshed to grow smoothly from that sizing.


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Proximity based Refinement


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Proximity-based Refinement

Zones which are close together can be

refined by the proximity-based refinement

using either the Refine Boundary Zones

panel or the TUI command:

/boundary/refine/auto-refine.

This operation refines all adjacent faces in

the same domain close to the refinedzones.

The TUI command provides more control of

the refinement process:

/boundary/refine/auto-refine

zone id/name [ ] 1

Iterations [5]

Relative Distance [1]

Project Face Zone


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Project Face Zone

A selected face zone can be

projected onto a target boundaryzone using the Project Face Zones

panel.

This is useful for reducing gaps

between nonconformal interfaces

and for boundary mesh

manipulations (project and

retriangulate).

Methods:

Normal(Normal to a face zone)

Direction(x,y,z)

Closest(Closest point on face zone)


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BOI Size Function for Retriangulation


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BOI Size Function for Retriangulation

BOI size functions can be used to limit the boundary mesh size

within a volumetric region

In the example below the surface mesh is remeshed with a BOI

restricting the growth at the rear of the car on the symmetry

plane

Separate Boundary Zones


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Often we wish to separate boundaries for setting boundary

conditions in the solver or specifying mesh properties

The Separate Face Zones panel is used to separate a single face

zone into multiple zones of the same type.

Six different options available

for separation:

Angle: separates faces with

normal vectors that differ by

an angle equal to or greater

than the specified significant

angle into different zones

Separate by Angle of 90



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Neighbour: separates faces whose adjacent cells belong to different cell zones

Region: separates non-contiguous boundary face zones (i.e. zones that are split into two

or more isolated groups) into multiple zones

Fluid zone 1 Fluid zone 2

Zone containing faces

lying between two fluid

zones

Because of free

nodes


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Seed: Separate faces using on a

selected face (or Face Seed) and

an angle or edge loop.

Seed triangle

fluent meshing 14.5 l03 boundary fix

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