best practices: volume meshing kynan maley - siemens€“generalized cylinder mesher ... cartesian...

Best Practices: Volume Meshing

Kynan Maley

Volume meshing is the basic tool that allows the creation of the space

discretization needed to solve most of the CAE equations for:

– CFD

– Stress Analysis

– Heat transfer

– Electro-Chemistry

– Magneto Hydro Dynamics

– ...

Volume Meshing

Pipeline Meshing

Pipeline meshing allows you to:

– Change geometry, mesh type, refinements, location/number of prism layers,

etc.

– Automatically update the mesh and map the old solution

– Rapidly evaluate multiple designs

Solution mapping

– Make changes to the geometry and mesh while retaining your solution

– Physics is independent of mesh

3

Wrapper Remesher

Polyhedral

Mesher

Surface Preparation / Meshing Volume Mesh Generation

Trim Cell

Mesher

Prism

Layer

General Purpose 3D Meshers:

– Polyhedral

– Trimmer

– Tet Mesher

General purpose meshers on special

geometries tend to produce non optimal

meshes in terms of:

– Cell count

– Quality

Volume Meshing in STAR-CCM+

Examples of special geometries:

– Thin objects/parts

– Extruded parts

– Long or curved pipes/ducts

Specialized (2.5D) Meshers:

– Prism Layer Mesher

– Extruder

– Thin Mesher

– Generalized Cylinder Mesher

– Advancing Layer Mesher


2.5D meshing is a synthetic definition of those meshing techniques that

exploit the fact that certain special geometries have a general mesh in 2

dimensions while in the 3rd dimension the mesh has some form of

simplification:

– Extruded in a predetermined direction

– Extruded along the local normal direction

– Swept along a 3d curve or axis


Full volume meshing pipeline is

parallel poly mesher

Reducing memory and wall time


Golden rule of volume meshing in STAR-CCM+

Volume meshers have requirements for the input surface:

– Closed

– Manifold

– Non-intersecting

Often cell quality issues in the volume mesh can be tracked down to

face quality issues in the surface mesh

Recommendation is to use the Surface Remesher always prior to volume

meshing (with same size settings)

Volume Meshing

Quality of CAD determines path to closed, manifold, non intersecting

surface

– The surface wrapper is used for the worst quality CAD

• Also useful for de-featuring your model

– Other methods exist to fix minor CAD issues

Volume Meshing

Surface Mesh

Polyhedral mesher

– General purpose, reliable, robust

– Capable of multi-region conformal

meshing

– Suitable for Conjugate Heat

Transfer simulations

Trimmer

– Fast and high quality

– Anisotropic refinement

– Perfect for large domains such as:

• Cars in wind tunnels

• Airplanes

• Ships

• Trains

General Purpose Mesher

Polyhedral - Trimmer

Rule of thumb here is to use a trimmed mesh for cases that have large

cartesian aligned flow directions

Also useful when Trimmer Wake Refinement is needed (refinement

follows shape of boundary)

– Can be done in a local coordinate system, allowing alignment with flow

direction


Trimmer Mesher

In many situations it is desirable

to have the possibility to

accurately control the mesh size

– High gradient zones

– Shocks

– High error zones

This can be accomplished by

placing appropriate Volumetric

Controls


Refinement using Volumetric Controls

Volumetric Controls allow a

number of refinement types:

– Surface Mesh

– Volume Mesh

• Isotropic

• Anisotropic (Trimmer)

– Prism Layer Mesh


Refinement using Volumetric Controls

Surface mesh size can be set at

individual

– Boundaries

– Feature Curves

The volume mesh size is related to

the surface size and growth rate


Refinement using Boundaries and Feature Curves

Refinement levels provide a quick way to globally refine a polyhedral

mesh

– Activated within the Polyhedral Mesher model settings

– One of two refinement levels selected in Reference Values

• Level 1: Splits each polyhedral cell into 6 or 7 new cells

• Level 2: Splits each polyhedral cell into 40 to 50 new cells

• Prism Layer unchanged


Polyhedral Refinement Level

Avoid huge jumps in volume ratio,

it will cause issues

Keep the ratio as small as

possible

Prism layers can help improve

blending from near wall to far

field

Volume Mesh

Volume Ratio

The volume ratio for trimmed cells is influenced through different Growth

Rate values

On continuum level

– Trimmer > Properties: Template mesh growth rate

– Reference Values > Template Growth rate > Properties: Default Growth rate

Volume Mesh

Volume Ratio - Trimmer

On continuum, boundary and interface level

– Boundary growth rate

It controls the rate of size changes between

cells adjacent to surfaces and cells in the

core

Volume Mesh


Example Template Growth Rate options

– Boundary Growth rate

– Default Growth rate

Volume Mesh


None Very Slow Fast

What are prism cells?

– A polyhedral base, a copy of it at top and rectangular sides connecting both

Where are prism cells used?

– Wall Prism Layer (turbulence, heat transfer)

– Extruder

– Thin Mesher

– Advancing Layer Mesher

Volume Mesh

Prism Layer Mesher

The Prism Layer thickness is subtracted

from the boundary

– Offset surface

A core mesh is created

The Prism Mesh is extruded to the

boundary

Wall Prism Layers

Generation of Prism Layer Mesh

Where are Prism Layers generated?

– Only at boundaries of type Wall

Why is no Prism Layer created at my fluid-solid interface?

– Although the boundaries forming an interface are often of type Wall, being an

interface overrules this setting: At an interface no prism layers will be

generated as default

Wall Prism Layers

Locations of Prism Mesh

With the recent releases of STAR-CCM+ the creation of boundary layers

has been further improved

Today I will show you some of the Model Properties with which to

influence the prism mesh in narrow passages

– Gap Fill Percentage

– Minimum Thickness Percentage

– Layer Reduction Percentage

Wall Prism Layers

Introduction to Properties Options

Wall Prism Layers

Default

25%

10%

Default

10%

25%

Default

50%

85%

Wall Prism Layers

Expert Settings

The extruder meshing model performs an additional volume meshing

step once the core mesh has been generated

The model can be activated for any of the core mesh types and enabled

for any boundary

Care should be taken however that the extrusion volume will not

interfere with the existing mesh by intersecting it in any way

Generates prism cells which extends the confines of the starting surface

Can use any coordinate system:

– Cartesian

– Cylindrical

– Spherical

Additional Mesher

Extruder

Extruder Mesher Options

– Frozen Boundaries

Additional Mesher

Extruder

Frozen

Boundaries

On

Frozen

Boundaries

Off

Inflate the computational domain

in all directions

– One possibility is to change the Part

on the Geometry level

– Another is to extrude the outer

boundaries without the Frozen

Boundaries option

Additional Mesher

Extruder Example 1

Additional Mesher

Extruder Example 2

3L-8L

L

Generates an Extruded mesh

along lengths of a part considered

a cylinder

– Automatic cylinder detection

Additional Mesher

Generalized Cylinder

The thin meshing model allows thin regions in the geometry to have a

prismatic type volume mesh

Reason is to improve the overall cell quality and reduce the cell count

when compared to an equivalent tetrahedral or polyhedral type core

mesh

When very thin structures cannot be modeled using baffles – their

thickness must be modeled with a minimum of 3 cells through the

thickness

Advanced Mesher

Thin Mesher

Advanced Mesher

Example - Thin Mesher

The advancing layer produces prismatic cell

layers near wall boundaries

Extruding the cells from the surface into

the region volume allows for a thicker layer

with a more uniform distribution than the

prism layer mesher

Advanced Mesher

Advancing Layer

Advancing Layer Options

– Two options for Stretching Function

– Several options for Stretching Mode

To influence the advancing layer mesh at convex corners, refine the

Feature Curve at this edge

Advanced Mesher

Advancing Layer

Advanced Mesher

Advancing Layer Example - Shuttle

Directed meshing is a method for creating swept meshes from a 2D

starting surface mesh

The starting surface mesh may either be created by:

– Patching the surface and creating quadrahedral elements

– Using the surface of an existing volume mesh

The surface is then swept along a path described by the CAD geometry

This results in a high quality structured mesh

Directed Mesher

Electric Machines

Engine Powertrain

Overset Meshing:

– A background mesh enclosing the whole solution domain

– Separate meshes enclosing each body

– The regions overlap, and flow-field information is passed between them

No need to remesh during motion or after moving geometry!

Overset Mesher

Overset Mesher

Any Questions ?

Questions

best practices: volume meshing kynan maley - siemens€“generalized cylinder mesher ... cartesian...

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