cfd pro 14.5 l04 mesh methods

© 2013 ANSYS, Inc. December 12, 2013 1 Release 14.5

14. 5 Release

Introduction to ANSYS CFD Professional

Lecture 04 Meshing Methods


Why Multiple Methods?

• Choice can depend on;

– Physics

– Geometry

– Resources

• Mesh could require just one or a combination of methods.

• Example – Typical mesh design based on geometric, physics and resource considerations.

Methods

High aspect ratio cells (Inflation) near wall to capture boundary layer

gradients

Tet cells used here to mesh complex region

Hex cells used to mesh simple

regions

Cells refined around small geometric

details and complex flow


• In the Outline, right click Mesh, Insert > Method

– Select body in Details View

• Or, in the Graphics Window, Select body(s) , right click, Insert > Method

– Body automatically selected in Details View

• Method is selectable using the drop-down box

– Automatic, Tetrahedrons, Hex Dominant, Sweep or Multizone

Inserting Methods


Tetrahedrons Method


Method Behavior

• Generates tetrahedral elements - two algorithms are available:

• Patch Conforming

• Patch Independent

Tetrahedrons Method


Method & Algorithm Behavior

• Bottom up approach: Meshing process starts from edges, faces and then volume

• All faces and their boundaries are respected (conformed to) and meshed

• Good for high quality (clean) CAD geometries

– CAD cleanup required for dirty geometry

• Sizing is defined by global and/or local controls

• Compatible with inflation

Access

• Insert Method and set to Tetrahedrons

– Additional drop down box for algorithm choice appears - Set to Patch Conforming

Tetrahedrons Method: Patch Conforming


Method & Algorithm Behavior

• Top down approach: Volume mesh generated first and projected on to faces and edges

• Faces, edges and vertices not necessarily conformed to

– Controlled by tolerance and scoping of Named Selection, load or other object

• Good for gross de-featuring of poor quality (dirty) CAD geometries

• Method Details contain sizing controls


Access

• Insert Method and set to Tetrahedrons

– Additional drop down box for algorithm choice appears - Set Patch Independent

Tetrahedrons Method: Patch Independent


Tetrahedrons Method: Algorithm Comparison (Surface Mesh)

Geometry containing

small details

Patch Conforming: All

geometric detail is

captured

Patch Independent: Can

ignore and defeature

geometry


Tetrahedrons Method: Algorithm Comparison (Volume Mesh)

Geometry containing

small details

Patch Conforming:

Delaunay mesh –

smooth growth rate

Patch Independent: Default

Octree Mesh – approximate

growth rate

Smooth Transition option

creates Delaunay mesh


Patch Conforming

• Clean CAD, accurate surface mesh

Patch Independent

• Dirty CAD, defeatured surface mesh

Tetrahedrons Method: Application Examples


Sizing for Patch Conforming

• Mesh sizing for the Patch Conforming algorithm is defined by Global & Local Controls

• Automatic refinement based on curvature and/or proximity accessible in Global Controls

– Details of Global & Local Controls covered in separate lectures

• Choice of surface mesher algorithm in global controls

Tetrahedrons Method: Control

Proximity

Curvature


Sizing for Patch Independent

• Defined in Patch Independent Details

• Automatic curvature & proximity refinement option

Defeaturing Control for Patch Independent

• Set Mesh Based Defeaturing On & Defeaturing Tolerance

• Assign Named Selections to preserve geometry

Tetrahedrons Method: Control

Defeaturing Tolerance Off Named Selection assigned and Defeaturing Tolerance set.


Hex Meshing


Hex Meshing

• Reduced element count

– Can reduce run time

• Elements aligned in direction of flow

– Reduced numerical error

Methods Available

• Sweep

• MultiZone

• Hex Dominant (not recommended for CFD)

Initial Requirements

• Clean geometry

• May require geometric decomposition

Introduction

Tet Mesh

Elements: 48K

Sweep Mesh

Elements: 19K


Sweep Meshing

ANSYS Meshing can identify sweepable bodies automatically

• Rotational Sweep bodies are not identified

To make bodies sweepable

• Decompose into a number of topologically simpler, sweepable bodies

Unsweepable body Decomposed in CAD

Sweepable!


Method Behavior

• Meshes source surface, sweeps through to the target

– Body must have topologically identical faces on two ends, (which act as source and target faces)

• Generates hex/wedge elements

• Side faces must be mappable

• Only one source and one target face is allowed

– Alternative ‘Thin’ sweep algorithm can have multiple source and target faces

Access

• Insert Method and set to Sweep

Sweep Meshing

Source Face Target Face

Side Face(s)

Sweep Path


• Inflation

– Must specify at least Source manually

– 2D inflation scoped to source face and boundary defined by edge(s)

Sweep Meshing

Sweep Mesh with inflation

Manual Selection

Source/Target (Src/Trg Selection)

• Automatic selection requires that the application find the Source and Target.

– Specifying both Source & Target will accelerate meshing

– Rotational sweeping requires both Source & Target to be selected


Target

Src/Trg Selection: Automatic Thin & Manual Thin

• Selects an alternate sweep algorithm

• Advantages

– Capable of sweeping multiple Source & Targets

– Can perform some automatic defeaturing

• Disadvantages

– For Multibody Parts only one division across the sweep is allowed

– Inflation & Sweep Bias not allowed

Sweep Meshing

Source

Faces

Source Faces

Imprinted on Target


Method Behavior

• Automatically decomposes geometry into blocks

• Generates structured mesh, where block topology permits

– Remaining region (Free Mesh) filled with unstructured Hexa Core or Tetra or Hexa Dominant mesh.

• Can select source & target faces automatically or manually

– Can have multiple source faces

• Compatible with 3D inflation

Access

• Insert Method and set to Multizone

MultiZone Meshing

MultiZone

Mesh

Target faces should also be selected as “Source” for Multizone Method as mesh is swept from both directions


• Mapped Mesh Type - determines the shape of the elements used to fill structured regions

• Hexa

• Hexa/Prism - For swept regions, the surface mesh can allow triangles for quality and transitioning

• Prism

• Surface Mesh Method – specifies method to create the surface mesh.

• Program Controlled - automatically uses a combination of Uniform and Pave mesh methods depending on the mesh sizes set and face properties

• Uniform - creates a highly uniform mesh

• Pave - creates a good quality mesh on faces with high curvature and when neighbouring edges have a high aspect ratio

MultiZone Meshing

Surface mesh method = Uniform

Surface mesh method = Pave


Automatic Method


Method Behavior

• Combination of Tetrahedron Patch Conforming and Sweep Method

– Automatically identifies sweepable bodies and creates sweep mesh

– All non-sweepable bodies meshed using tetrahedron Patch Conformal method


Access

• Default Method where not specified

• Can specify by inserting Method and setting to Automatic

Automatic Method


Meshing Multiple Bodies


• Mesh or clear meshes on individual bodies

• Select body(s) right click for context menu

• Subsequent bodies will use the attached face mesh

• The meshing results (cell types) will depend on the meshing order

• Adjust/add mesh controls – able to remesh only affected body

• Selective body updating if geometry modified

Selective Meshing

Meshing first the block then the pipe Meshing first the pipe then the block


Recording Mesh Operations

• When using selective meshing the order of meshing can be recorded for automated future use

• Right click Mesh in the Outline for Context Menu

• Worksheet is generated recording mesh operations as ordered steps

• Named Selections are automatically created for each meshed body for reference in the Worksheet

– Example: Meshing cylinder then block

Selective Meshing


Workshop 1 – ANSYS Meshing Basics

cfd pro 14.5 l04 mesh methods

Documents

conforming tetrahedrons

patch conforming mesh

method behavior

method body

method algorithm behavior

small details patch

release introduction

tetrahedrons additional