AM 120 App A - Fluent-mesh quality PDF

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Appendix A Mesh Quality

Introduction to ANSYS Meshing

ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved.

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Release 12.0 March 2010

Appendix A: Mesh Quality

Overview

Training Manual

• Mesh Quality Metrics in ANSYS Meshing – Skewness – Aspect Ratio – Worst Element

• Mesh Quality Considerations for the FLUENT Solver – General Considerations – Impact of Mesh Quality on the Solution

• Mesh Quality Considerations for the CFX Solver • Factors Affecting Mesh Quality – – – –

CAD Issues Mesh Resolution and Distribution Meshing Method I fl ti Inflation

• Strategies to Improve Mesh Quality – – – – –

CAD Cleanup Virtual Topology Pi h C Pinch Controls t l Sensible Mesh Sizings and Inflation Settings General Recommendations

• Workshop A.1 Virtual Topology for an Auto Manifold • Workshop A A.2 2 FLUENT and CFX Mesh Q Quality alit Metrics ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved.

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Appendix A: Mesh Quality

Mesh Quality Metrics in ANSYS Meshing •

Mesh Metrics are available under Mesh Options to set and review mesh metric information and and to to evaluate mesh quality

•

Different physics and different solvers have different requirements for mesh quality

Training Manual

• Mesh metrics available in ANSYS Meshing include: – – – – – – –

Element Quality Aspect Ratio Jacobian Ration Warping Factor Parallel Deviation Maximum Corner Angle Skewness

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Appendix A: Mesh Quality

Mesh Quality Metrics

Training Manual

Skewness

optimal (equilateral) cell

Two methods for determining skewness: 1. Based on the Equilateral Volume deviation: •

Skewness = optimal cell size − cell size optimal cell size

• •

Applies only to triangles and tetrahedra Default method for tris and tets actual cell

2. Based on the deviation from a Norma lized Angle deviation:

θ θ Skewness = max⎡θmax − θe , e − min ⎤ ⎢ θ min θe ⎥⎦ ⎣ 180 −θe Where θe is the equiangular face/cell (60 for tets and tris, and 90 for quads and hexas) • Applies to all cell and face shapes 0 • Used for prisms and pyramids Perfect

circumsphere

θ max

•

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Appendix A: Mesh Quality

Mesh Quality Metrics

Training Manual

Aspect Ratio •

•

Aspect for generic triangles and quads quads is is aa function of the ratio of longest side to the shortest side of the reconstructed quadrangles (see User Guide for details) E Equal l to t 1 (id (ideal) l) for f an equ ililateral t l ttriangle i l or a square

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aspect ratio = 1

high-aspect-ratio quad

aspect ratio = 1

high-aspect-ratio triangle

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Appendix A: Mesh Quality

Mesh Quality Statistics in ANSYS Meshing

Training Manual

• The min, max, averaged and standard deviation for the selected mesh metric are shown for the surface mesh (after Preview Surface Mesh generation) and for the volume mesh (after Preview Inflation layer or Generate Mesh generation) • The worst elements can be highlighted using the Show Worst Elements under the Mesh object in the Tree Outline

ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved.

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Appendix A: Mesh Quality

Mesh Quality Considerations for FLUENT

Training Manual

• FLUENT requires high quality mesh to avoid numerical diffusion • Several Mesh Quality Metrics are involved in order to quantify the quality, however the skewness is the primary metric • The aspect ratio and cell size change mesh metrics are also very important • In worst scenarios and depending on the solver used (density based or pressure based) FLUENT can tolerate poor mesh quality. However some applications may require higher mesh quality, resolution and good mesh distribution • The location of poor quality elements helps determine their effect • Some overall mesh quality metrics may be obtained in Ansys Meshing under d th the Statistics St ti ti object bj t • Additional mesh quality metrics may be retrieved in FLUENT GUI under Mesh/Info/Quality from the menu, or using the TUI commands ‘ ‘mesh/quality’ h/ lit ’ ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved.

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Appendix A: Mesh Quality

Mesh Quality Requirements for FLUENT

Training Manual

• The most important mesh metrics for Fluent are: – Skewness – Aspect Ratio – Cell Size Change (not implemented in Ansys Meshing)

For all/most applications: • For Skewness: – For Hexa, Tri and Quad: it should be less than 0.8 – For tetrahedra: it should be less than 0 0.9 9

• For Aspect Ratio: – It should be less than 40, but this depends on the flow characteristics – More than 50 may be tolerated at the inflation layers

• Poor mesh quality may lead to inaccurate solution and/or slow convergence • Some applications may require even lower skewness than the suggested value

• For Cell Size Change: – It should be between 1 and 2. ANSYS, Inc. Proprietary © 2010 ANSYS, Inc. All rights reserved.

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Appendix A: Mesh Quality

Skewness and the Fluent Solver

Training Manual

• High skewness values are not recommended • 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. • Classification of the mesh quality metrics based on skewness: 0 0 25 0-0.25 Excellent

0 0.25-0.50 25 0 50 very good

0.50-0.80 0 50 0 80

0.80-0.95 0 80 0 95

0.95-0.98 0 95 0 98

0.98-1.00* 0 98 1 00*

good

acceptable

bad

Inacceptable*

• * In some circumstances the pressure based solver in Fluent can handle meshes containing a small percentage of cells with skewness ~0.98.

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Appendix A: Mesh Quality

Impact of the Mesh Quality on the Solution

Training Manual

(max,avg)CSKEW=(0.912,0.291) (max,avg)CAR=(62.731,7.402)

Mesh 1

Example

VzMIN≈-90ft/min VzMAX≈600ft/min 600ft/min

Large cell size change

Mesh 2

(max,avg)CSKEW=(0.801,0.287) (max,avg)CAR=(8.153,1.298)

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VzMIN≈-100ft/min VzMAX≈400ft/min

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Appendix A: Mesh Quality

Mesh Quality Considerations for CFX

Training Manual

• Mesh quality requirements are somewhat different for the CFX solver than for the FLUENT solver due to the difference in the solver structure for the two codes – Fluent uses a a cell-centered scheme, in which the fluid flow variables are allocated at the center of the computational cell, and the mesh-element is the same as the solver-element – CFX employs a vertex-centered scheme for which the fluid flow variables are stored at the cell vertex, and the solver-element or control volume is a “dual” of the mesh-element. This means that the vertex of the meshelement is the center of the solver solver-element element

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Appendix A: Mesh Quality

Mesh Quality Considerations for CFX

Training Manual

• The CFX solver calculates 3 important measures of mesh quality at the start of a run and updates them each time the mesh is deformed – Mesh Orthogonality – Aspect Ratio – Expansion Factor +--------------------------------------------------------------------+ | Mesh Statistics | +--------------------------------------------------------------------+ Domain Name: Air Duct Mi nimum i O Orthogonalit th lit y AA ngle l [d [degrees] ] = 20 20.4 4 ok k Maximum Aspect Ratio = 13.5 OK Maximum Mesh Expansion Factor = 700.4 ! Domain Name: Water Pipe Minimum Orthogonality Angle [degrees] = 32.8 ok Maximum Aspect Ratio = 6.4 OK Maximum Mesh Expansion Factor = 73.5 ! Global Mesh Quality Statistics : Minimum Orthogonality Angle [degrees] = 20.4 ok Maximum Aspect Ratio = 13.5 OK Maximum Mesh Expansion Factor = 700.4 !

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Good (OK)

Acceptable (ok)

Questionable (!)

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Appendix A: Mesh Quality

Mesh Orthogonality in CFX

Training Manual

•Orthogonality measures alignment of: • ip-face normal vector, n, & • node-to-node vector, s.

• Orthogonality Factor = n·s, >1/3 desirable • Orthogonality Angle = 90 90º-acos(n·s) acos(n s), >20 >20º desirable • Are these different than Max/Min Face Angles in CFD Post? YES! – Face angles correspond to angles between edges – One can have an acceptable Face Angle and an unacceptable Orthogonality Angle if an element is skewed in two directions…

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Appendix A: Mesh Quality

Mesh Expansion Factor in CFX

Training Manual

Expansion factor measures how poorly the nodal position corresponds to the control volume centroid

•

Mesh Expansion Factor ≈ ratio of largest to smallest element volumes surrounding a node, node...