OZEN ENGINEERING, INC.

MESHING WORKSHOP Thursday, November 13 th, 2014 Metin Ozen, Ph.D., ASME Fellow OZEN ENGINEERING, INC. www.ozeninc.com

WHAT DO WE DO? Ozen Engineering, Inc. helps solve challenging and multidisciplinary engineering problems with industry leading computational simulation technologies We provide advanced Multi-Physics FEA Computational Fluid Dynamics (CFD) simulations

INTRODUCTION TO ANSYS MESHING In this lecture we will learn: Process for pre-processing using ANSYS tools What is the ANSYS Meshing? Meshing Fundamentals How to launch ANSYS Meshing? ANSYS Meshing interface Geometry concepts Meshing methods

PREPROCESSING WORKFLOW Import/ Geometry Creation Geometry Modifications Meshing Solver Sketches and Planes 3D Operations Meshing Methods 3D Operations Booleans, Decompose, etc. Hybrid Mesh: Tet, Prisms, Pyramids Extrude, Revolve, Sweep, etc Geometry Import Options Bi-Directional CAD/ Neutral Geometry Cleanup and Repair Automatic Cleanup Simplification, Mid-surface, Fluid Extraction Hexa Dominant, Sweep meshing Assembly Meshing Global Mesh Settings Local Mesh Settings Sizing, Controls, etc.

WHAT IS ANSYS MESHING ANSYS Meshing is a component of ANSYS Workbench Meshing platform Combines and builds on strengths of preprocessing offerings from ANSYS: ICEM CFD, TGRID (Fluent Meshing), CFX-Mesh, Gambit Able to adapt and create Meshes for different Physics and Solvers CFD: Fluent, CFX and POLYFLOW Mechanical: Explicit dynamics, Implicit Electromagnetic Integrates directly with other WB systems

MESHING FUNDAMENTALS Purpose of the Mesh Equations are solved at cell/nodal locations Domain is required to be divided into discrete cells (meshed) Mesh Requirements Efficiency & Accuracy Quality Refine (smaller cells) for high solution gradients and fine geometric detail. Coarse mesh (larger cells) elsewhere. Solution accuracy & stability deteriorates as mesh cells deviate from ideal shape

MESHING PROCESS IN ANSYS MESHING Physics, Sizing, Inflation, Pinch, Sizing, Refine, Pinch, Inflation, Preview surface mesh, Inflation Mesh metrics, Charts

LAUNCHING ANSYS MESHING ANSYS Meshing is launched within Workbench 2 ways : From Analysis Systems Fluid Flow (Fluent), Fluid Flow (CFX), From Component Systems Mesh Double click Mesh in the System or right click and select Edit

GRAPHICS USER INTERFACE Toolbars Outline Graphics window Worksheet Details view Manage views Mesh Metrics Section Planes Message window Entity Details Bar Units Bar

OUTLINE Three default sections Geometry Bodies Coordinate Systems Default global & user defined systems Mesh Meshing operations (controls & methods) displayed in the order in which they are inserted In the tree Right clicking on any object launches a context sensitive menu Example: contains commands to generate, preview, clear mesh etc.

DETAILS VIEW Accessing Object Details Select an object (in the Outline) Related information to that object are displayed in the Details View below Ex: Select a body ( Fluid ) in the Outline Details of Fluid : contains graphical and geometric details To access meshing details Click the Mesh object or any of the inserted objects The Details View provides options to review, edit or values for every object in the Tree input

GEOMETRY CONFIGURATION MULTIPLE PARTS Geometry composed of Multiple parts No connection between parts (no face sharing) Contact Region is automatically created between 2 faces Each part meshed independently Results in Non-conformal interface. Meshes do not match. No nodes connection. Grid interface - Fluent GGI - CFX Independent faces

GEOMETRY CONFIGURATION MULTI-BODY PARTS Geometry composed of multiple bodies in a part Depend on Shared Topology method (in DM) None» Results in a none connection between the bodies (similar to multiple parts) Automatic Faces in contact imprinted & fused Form a single face shared between the 2 bodies Results in Conformal mesh Common face acts as Interior

GEOMETRY CONFIGURATION MULTIPLE BODY PARTS Geometry composed of multiple bodies in a part Imprints Faces are imprinted on each other like faces Contact Region is automatically created For identical mesh on these faces, use Match Control Results in unconnected mesh non conformal interface Grid interface - Fluent GGI - CFX

MESHING 3D GEOMETRY 3D cell Types First Meshing Approach Part/Body based Meshing occurs at part or body level. Meshing Methods are scoped to individual bodies. Method assignment can be automatic or manual. Bodies contained in one part are conformally meshed. Part/Body Methods Tetrahedrons. Tetras only Sweep. Prisms & hexahedrons MultiZone. Mainly hexahedron Hex Dominant Not for CFD Automatic. Combines any types

MESHING 3D GEOMETRY Second Meshing Approach Assembly Meshing Meshes an entire model in one process. Assembly of parts Performs boolean operations. Volume filling, intersection & combination Does not require prior fluid body definition or shared topology. Conformal mesh created across parts. Assembly Meshing Methods Generate mainly Hexahedrons Tetrahedrons Part/Body Meshing & Assembly Meshing not interoperable Cut Cell Meshing

MESHING METHODS In this lecture we will learn: Meshing Methods for Part/Body Meshing Assembly Meshing covered separately Methods & Algorithms for; Tetrahedral Meshing Hex Meshing 2D Meshing Meshing Multiple Bodies Selective Meshing Recording Meshing Order

WHICH METHOD TO CHOOSE? Why Multiple Methods? Choice depends on : Physics Geometry Resources High aspect ratio cells (Inflation) near wall to capture boundary layer gradients Cells refined around small geometric details and complex flow Mesh could require just one or a combination of methods. Hex (3d) or Quad (2d) cells used to mesh simple regions Tet (3d) or Tri (2d) cells used here to mesh complex region

PATCH CONFORMING VERSUS INDEPENDENT Patch Conforming Clean CAD, Accurate surface mesh Patch Independent Dirty Geometry, defeatured surface mesh

TETRAHEDRONS METHODS Patch Conforming Bottom up approach: Meshing process Edges Faces 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 To access it Insert Method Set to Tetrahedrons Set to Patch Conforming Patch Independent Top down approach: Meshing process Volume meshed first projected on to faces & edges Faces, edges & vertices not necessarily conformed 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 Compatible with inflation To access it Insert Method Set to Tetrahedrons Set to Patch Independent

TETRAHEDRONS METHOD : CONTROL Patch Conforming - Sizing 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 Patch Independent - Sizing Sizing for the Patch Independent algorithm defined in Patch Independent Details Automatic curvature & proximity refinement option Name Selec. assigned & defeaturing Tol = 0.02 Features > 0.02m respected Defeaturing Control Set Mesh Based Defeaturing On Set Defeaturing Tolerance Assign Named Selections to selectively preserve geometry Defeaturing Tolerance off

TETRAHEDRONS METHOD : ALGORITHM COMPARISON Patch conforming : details caputred Patch independent : details ignored Delaunay mesh - smooth growth rate Octree mesh. approximate growth rate Geometry with small details

HEXA MESH - INTRODUCTION Hex Meshing Reduced element count Reduced run time Tetra mesh - 48 000 Cells Elements aligned in direction of flow Reduced numerical error Hexa mesh - 19 000 Cells Initial Requirements Clean geometry May require geometric decomposition

SWEEP MESHING Mesh Method & Behavior Generates hex/wedge elements Meshes source surfaces Sweeps through to the target Body must have topologically identical source and target faces Side faces must be mappable A sweep path must be identified Only one source and one target face is allowed Alternative thin sweep algorithm can have multiple source & target faces Sweep Direction Source face Target face Sweep Path To access it Insert Method Set to Sweep Target Face Side Face(s) Source Face

SWEEP MESHING Source & Target selection Automatic Source & Target faces identified automatically Requires that the mesher find the sweeping direction Manual source & Manual source and target User selection Source face colored in red Target face colored in blue Rotational Sweeping Sweep around an axis Requires selection of both - Source & target Note Specifying both Source & Target accelerate meshing Define the nbr of intervals on the side face(s) Generation of wedges & hex elements Sweep Path

SWEEP MESHING Source & Target selection Automatic Thin & Manual Thin Alternate sweep algorithm Advantages Sweep multiple Source & Target faces Can perform some automatic defeaturing Source Faces Target Limitations X For multibody parts only one division allowed across the sweep X Inflation not allowed X Sweep bias not allowed Source Faces imprinted on Target

SWEEP MESHING Sweep and Inflation Compatibility with Src/Trg Selection X X X Sweep Mesh - No Inflation Use of Inflation Defined on source face ( NOT on target one) From boundary edges (2D) Swept through volume Sweep Mesh with Inflation

SWEEP MESHING Geometry Identifying sweepable bodies Automatic detection of sweepable bodies Rotational ones are not identified Right mouse button Identification method Right click on mesh object Outline tree Select : Sweepable Bodies Making bodies sweepable Decompose bodies into multi-simple topological shapes Perform decomposition in CAD/DM Unsweepable Decompose Sweepable bodies in green color Sweep Mesh

MULTIZONE MESHING Mesh Method & Behavior Based on blocking approach (ANSYS ICEM CFD Hexa) Automatically decomposes geometry into blocks Generates structured hexa mesh where block topology permits Remaining region filled with unstructured Hexa Core or Tetra or Hexa dominant mesh Src/Trg Selection Automatic or Manual source selection Multiple source faces Select Target faces as Source Compatible with 3D Inflation To access it Insert Method Set to Multizone

MULTIZONE MESHING Mapped Mesh Type Determines which elements to use Hexa Default Only Hexahedral elements are generated Hexa/prism For quality and transition, triangles will be inserted on the surface mesh (sources) Prism Only prisms will be generated Useful when the adjacent volume is filled in with tet mesh Hexa Geometry Hexa - Prism

MULTIZONE MESHING Surface Mesh Method Specify a method to create the surface mesh Uniform Uses a recursive loop-splitting method which creates a highly uniform mesh Pave Creates a good quality mesh on faces with high curvature, and also when neighboring edges have a high aspect ratio Program controlled Combination of Uniform and Pave methods depends on the mesh sizes set and face properties Geometry Pave Uniform

AUTOMATIC METHOD Mesh 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 Compatible with inflation To access it Default method Insert method Set to Automatic

2D MESHING Automatic Triangles Mesh Method & Behavior Quadrilateral Dominant & Triangles Patch conforming methods MultiZone Quad/tri Patch Independent Methods Associated with face mesh type All Tri Quad/tri All Quad MultiZone Quad/Tri MultiZone Quad Advanced size function & local size controls are supported

2D MESHING 2D Mapped mesh Control Mapped Surface Meshes Local mesh controls Fully Mapped surface meshes Specified edge sizing/intervals Inflation Boundary edges are inflated Global & local inflation controls are supported

2D MESH SOLVER GUIDELINES ANSYS Fluent For a 2D analysis in Fluent generate the mesh in the XY plane Z = 0 For axisymmetric applications y 0 and make sure that the domain is axisymmetric about x axis In ANSYS Meshing, by default, a thickness is defined for a surface body and is visible when the view is not normal to the XY Plane. This is purely graphical no thickness will be present when the mesh is exported into the Fluent 2D solver ANSYS CFX For 2D analysis in CFX, create a volume mesh (using Sweep) 1 element thick in the symmetry direction, i.e., Thin Block for Planar 2D Thin Wedge (< 5 ) for 2D Axis-symmetric

SELECTIVE MESHING What is? Selectively picking bodies and meshing them incrementally Why? Bodies can be meshed individually Mesh seeding from meshed bodies influences neighboring bodies (user has control) Automated meshing can be used at any time to mesh all remaining bodies When controls are added, only affected body meshes require remeshing Selective body updating Extensive mesh method interoperability

SELECTIVE MESHING Meshing first the pipe then the block Local Meshing Clear meshes on individual bodies Generate meshes on individual bodies Subsequent bodies will use the attached face mesh The meshing results (cell types) will depend on the meshing order Adjust/add controls able to remesh only affected body Meshing first the block then the pipe Select body(s) Right click

SELECTIVE MESHING Recording Mesh Operations Example : Meshing cylinder first and then block Use it to record the order of meshing to automate future use Right click Mesh in the Outline to access it A Worksheet is generated Record mesh operations as ordered steps Named Selections are automatically created for each meshed body for reference in the Worksheet

SELECTIVE MESHING Selective Body Updating Remeshing only bodies that have changed Access option through Tools > Options No: All geometry updated, all bodies remeshed. Associatively: Accommodates for body topology change (add/delete) (slower) Non-Associatively: Assumes no topology change (faster) Example : Geometric change to block

GLOBAL MESH CONTROLS In this section, we will learn about: Introduction to Global Mesh Controls Defaults General Sizing Controls & Advanced Size Functions Global Inflation Assembly Meshing Controls Statistics

MESHING PROCESS IN ANSYS MESHING

GLOBAL MESH CONTROLS (1) Global mesh controls are used to make global adjustment in the meshing strategy, which includes sizing functions, inflation, smoothing, defeaturing, parameter inputs, assembly meshing inputs, etc. Minimal inputs Automatically calculates global element sizes based on the smallest geometric entity Smart defaults are chosen based on physics preference Makes global adjustments for required level of mesh refinement Advanced Size Functions for resolving regions with curvatures and proximity of surfaces Smart defaults!

GLOBAL MESH CONTROLS (2) Physics Based Settings Physics and Solver Preferences Global Mesh Sizing Controls Relevance and Relevance Center Advanced Size Functions Smoothing and Transition Span Angle Center Curvature Normal Angle Proximity Accuracy and Cells Across Gap Inflation Inflation Option, Inflation Algorithm Collision Avoidance Maximum Angle, Fillet Ratio, Smoothing Assembly Meshing Activation of CutCell/Tetrahedrons Meshing Patch Confirming Options Activation of Advancing Front Method Advanced Numer of CPUs for Parallel Part Meshing Shape Checking Element midside nodes Defeaturing Pinch based Automatic Mesh Based Statistics Mesh statistics, Quality criteria

GLOBAL MESH CONTROLS (3)

DEFAULTS Four options under Physics Preference CFD, Mechanical, Explicit and Electromagnetic Three options under Solver Preference when CFD is selected: Fluent, CFX and POLYFLOW Mesh setting defaults are automatically adjusted to suit the Physics Preference and Solver Preference Assembly Meshing is active only when Physics Preference is CFD and Solver Preference is Fluent

SIZING : ADVANCED SIZING FUNCTIONS Controls the growth and distribution of mesh in important regions of high curvature or close proximity of surfaces Five Options: Off. Unavailable for Assembly Meshing Proximity and Curvature Curvature Proximity Fixed When CutCell Meshing is active with Proximity or Proximity and Curvature Advanced Size Function (ASF), Proximity Size Function Sources control is displayed to specify the regions of proximity between Edges, Faces or Faces and Edges in the computation of Proximity ASF

SIZING : ADVANCED SIZING FUNCTION EXAMPLES ASF: Off The edges are meshed with global Element Size Then the edges are refined for curvature and 2D proximity At the end, corresponding face and volume mesh is generated Transition of cell size is defined by Transition ASF: Curvature Determines the Edge and Face sizes based on Curvature Normal Angle Finer Curvature Normal Angle creates finer surface mesh Transition of cell size is defined by Growth Rate ASF: Proximity Controls the mesh resolution on proximity regions in the model Fits in specified number of elements in the narrow gaps Higher Number of Cells Across Gap creates more refined surface mesh Transition of cell size is defined by Growth Rate

SIZING : ELEMENT SIZE Element Size Element size used for the entire model This size will be used for meshing all edges, faces and bodies Default value based on Relevance and Initial Size Seed User can input required value as per geometry dimensions Element size option available when Advanced Size Function is not used

SIZING : MIN AND MAX SIZE Min Size Minimum element size that the size function will generate Some element sizes may be smaller than this size depending on the edge length Max Face Size Maximum face size that the size function will generate Not supported by CutCell meshing Max Size Maximum element size that can be grown in the interior of volume mesh Mouse Pointer serves to estimate mesh sizes Min Size Max Face Size Min Size Max Face Size Max Size Max Size

SIZING : GROWTH RATE Define the ratio between sizes of adjacent cells On surfaces and inside the volumes Growth Rate = 1.1 Growth Rate = 1.2 (Default) Mesh size: GR = 1.1 : 1,263,297 cells GR = 1.2 : 587,026 cells GR = 1.3 : 392,061 cells Growth Rate = 1.3

SIZING : TRANSITION Controls the rate at which elements grow Two level control for transition Slow (Default for CFD, Explicit), produces smooth transitions Fast (Default for Mechanical and Electromagnetic), produces more abrupt transitions Not available for Cutcell meshing Hidden for sheet models, ignored for assemblies containing sheets, when ASF is On Fast Slow

SIZING : SPAN ANGLE CENTER Controls curvature based refinement for Edges Three options and corresponding span angle ranges are Coarse: 91 to 60 Medium: 75 to 24 Fine: 36 to 12 Not available for Cutcell meshing Coarse Medium Fine

INFLATION Inflation Used to generate thin cells adjacent to boundaries Required for capture of wall adjacent boundary layers Resolve viscous boundary layer in CFD Resolve thin air gaps in Electromagnetic analysis Resolve high stress concentration regions in Structures Cells are created by inflating from the surface mesh into the volume (3d) or inflating from the boundary edge onto the face (2d) Options to control growth

INFLATION : AUTOMATIC INFLATION Three options None Select this for manual inflation settings using local mesh controls Program Controlled All the faces are selected for inflation except: Faces scoped to a Named Selection Faces with manual inflation defined Faces in contact regions Faces in symmetry Faces that belong to a part or body that has a mesh method defined that does not support 3D inflation, such as sweep or hex-dominant Faces in sheet bodies All Faces in chosen Named Selection: can grow inflation layers from faces grouped in one named selection

INFLATION : INFLATION OPTIONS Five options: Smooth Transition All available for Patch Conformal (PC ) tets and Assembly meshing Smooth Transition Maintains smooth volumetric growth between each adjacent layer. Total thickness depends on the variation of base surface mesh sizes (Default) First Layer Thickness Maintains constant first cell height throughout Total Thickness Maintains constant total height of inflation layer throughout First Aspect Ratio Controls the heights of the inflation layers by defining the aspect ratio of the inflations that are extruded from the inflation base Last Aspect Ratio Creates inflation layers using the values of the first layer height, maximum layers, and aspect ratio controls

INFLATION : INFLATION ALGORITHMS Two Algorithms Post Pre Patch independent meshes (including Assembly) use Post Post First Tet grows then Inflation process starts Tet mesh is undisturbed, if the inflation options are altered Default option for Patch Independent Tetrahedrons Preview Inflation is available only with Pre Algorithm Pre Surface mesh is inflated first, then rest of the volume mesh grows Default method for Patch Conforming Tetrahedrons

INFLATION: AUTOMATIC INFLATION EXAMPLE Patch Conforming Tets MultiZone Cutcell

INFLATION : ADVANCED OPTIONS Collision Avoidance: Control to detect proximity regions and adjust the cells in the inflation layer. None Does not check for proximity regions Layer Compression (Default for Fluent) Compresses inflation layers in the proximity regions Maintains the given number of layers in the proximity regions May stair-step if needed (will give a warning) Stair Stepping (Default for CFX) Inflation layers are stair stepped in the proximity regions Removing layers locally in steps to avoid collisions as well as bad quality at sharp corners When Cutcell meshing is used, both Layer Compression and Stair Steeping algorithms are used depending on the geometry complexity. Generates combination of Pyramids and Tets to fill the stair step

INFLATION : COLLISION AVOIDANCE EXAMPLE Example Layer Compression Stair Stepping

DEFEATURING Removes small geometry features meeting the tolerances using Pinch or/and Automatic Mesh Based Defeaturing controls in order to improve the mesh quality. Not all meshing methods can take advantage of these controls. Pinch Tolerance control removes small features at the mesh level depending on the Pinch Tolerance value provided. ANSYS Meshing offers global and manual pinch controls. Automatic Mesh Based Defeaturing (AMBD) when it is On, features smaller than or equal to the value of Defeaturing Tolerance are removed automatically. AMBD Off AMBD On With Pinch

STATISTICS Option to view the mesh quality metric Exhaustive list of quality metrics Orthogonal Quality mesh quality metrics Option to view the Mesh Metric chart Intuitive controls available under Mesh Metric Chart Various options to explore under the Controls See lecture 7 for details

LOCAL MESH CONTROLS In this section, we will learn about: Local mesh controls (Mesh sizing, Refinement, Match control, Inflation, etc) How to apply local controls? Effect of local controls on mesh

LOCAL MESH CONTROLS Control the mesh locally Depends on the Mesh Method used Local Mesh Controls are: Sizing- For Vertex, Edge, Face and Body Contact Sizing - For Edge and face Refinement- For Vertex, Edge and Face Mapped Face Meshing - For Face Match Control - For Edge and Face Pinch - For Vertex and Edge Inflation - For Edge and Face Only Sizing and Inflation local controls are available for CutCell meshing Non-CutCell meshing local controls CutCell meshing local controls The latest control added on a particular entity overrides any prior controls

SIZING Recommended for locally defining the mesh sizes You can only scope sizing to one geometry entity type at a time For example: you can apply sizing to a number of edges or a number of faces, but not a mix of edges and faces. Four Types of Sizing option Element Size specifies average element edge length on bodies, faces or edges Number of Divisions specifies number of elements on edge(s) Body of Influence specifies average element size within a body Sphere of Influence specifies average element size within the sphere Sizing options vary depending on Vertices the entity type chosen Entity/Option Element Size Number of Divisions Body of Influence Sphere of Influence x Edges x x x Faces x x Bodies x x x Only Element Size type is available for CutCell meshing Advanced Size Function in Global settings should be disabled Requires a Coordinate system for the sphere

SIZING : EDGES Sizing Type: Element Size Sizing Type: Number of Divisions Edge meshed with constant element size of 60mm Edge meshed with 10 elements The Curvature Normal Angle and/or the Growth Rate maybe not displayed depending on the ASF used

SIZING : EDGES Bias Type and Bias Factor Specify the grading scheme and factor Bias Type: grading of elements towards one end, both ends, or the center Bias Option: Bias Factor: is the ratio of the largest element to the smallest element Smooth Transition: defined by Growth Rate which is ratio of size of an element with size of previous element. (Growth Rate = Bias Factor^(1(n-1))

SIZING : EDGES Behavior Soft: Sizing will be influenced by global sizing functions such as those based on proximity and/or curvature as well as local mesh controls Hard: Size control is strictly adhered to Transition between hard edges (or any edge with bias) and adjacent edge and face meshes may be abrupt Hard edges or edges with bias will override Max Face Size and Max Size properties Influenced by global Proximity advanced size function. No influence from other global settings Soft Hard Number of Division = 4 Number of Division = 4

SIZING : FACES & BODY (VOLUME) Element Size Defines the maximum element size on the face Element Size Defines the maximum cell size on the Body

SIZING : SPHERE OF INFLUENCE On Vertex Available with or without Advanced Size Functions Sets the average element size around the selected vertex Inputs: Sphere radius and Element size Center of the sphere is defined by a model vertex On Bodies Available with or without Advanced Size Functions Constant element size is applied within the confines of a sphere Use coordinate system to define the center of the Sphere

SIZING : BODIES OF INFLUENCE Bodies of influence (BOI) Lines, surfaces and solid bodies can be used to refine the mesh Accessible when ASF is On Not available for CutCell meshing Line BOIs Surface BOI Solid BOI The Body of Influence itself will not be meshed Without BOIs

MAPPED FACE MESHING Creates structured meshes on selected mappable surfaces Mapped Face Meshing with advanced control is supported for Sweep, Patch Conforming, Hexa Dominant Quad Dominant and Triangles Mapped Face Meshing with basic control is supported for MultiZone Uniform Quad/Tri and Uniform Quad RMB on Mesh and Show/Mappable Faces to display all mappable faces If Mapped Face Meshing fails, ( ) icon appears adjacent to corresponding object in the Tree outline. The mesh will still be created but will ignore this control.

MAPPED FACE MESHING: INTERNAL NO. OF DIVISIONS If face is defined by two loops, then the Internal Number of Divisions field is activated User can specify the number of divisions across the annular region Also useful for defining number of divisions along sweeping direction for Multizone when there are no side edges Mapped face is swept to create pure hex mesh

MATCH CONTROL Define periodicity on faces (3D) or edges (2D) The two faces or edges should be topologically and geometrically the same A match control can only be assigned to one unique face/edge pair Match controls are not supported with Post Inflation Algorithm Match Control with Patch Independent tetrahedrons not supported yet Two types of match controls available: Cyclic and Arbitrary Not available for CutCell meshing Matching face mesh If Match Control fails, ( ) icon appears adjacent to corresponding object in the outline Tree, however the mesh is created ignoring it

MATCH CONTROL: CYCLIC Define Rotational periodic Full Model Periodic Model Model is symmetrical at 90 Selected Faces for Match control Matching face mesh

PINCH To improve quality Pinch control removes small features (edges or narrow regions) at the mesh level The Pinch feature is supported for the following mesh methods: Patch Conforming Tetrahedrons Thin Solid Sweeps Hex Dominant meshing Quad Dominant Surface Meshing Triangles Surface meshing Not supported for CutCell meshing

INFLATION Used to generate prism layers (as explained in Global settings chapter) Inflation layer can be applied to faces or bodies using respectively edges or faces as the boundary Inflation layer grown on edge boundary (red) Inflation layer grown on face boundary (red)

MESH QUALITY In this section, we will learn: Impact of the Mesh Quality on the Solution Quality criteria Methods for checking the mesh quality Tools to improve quality in Meshing Concept of Assembly Meshing Assembly Meshing Methods & Controls

PREPROCESSING WORKFLOW Import/ Geometry Creation Geometry Modifications Meshing Solver Sketches and Planes 3D Operations Meshing Methods 3D Operations Booleans, Decompose, etc. Hybrid Mesh: Tet, Prisms, Pyramids Extrude, Revolve, Sweep, etc Geometry Import Options Bi-Directional CAD/ Neutral Geometry Cleanup and Repair Automatic Cleanup Simplification, Mid-surface, Fluid Extraction Hexa Dominant, Sweep meshing Assembly Meshing Global Mesh Settings Local Mesh Settings Sizing, Controls, etc. Check Mesh Quality

IMPACT OF THE MESH QUALITY Good quality mesh means that Mesh quality criteria are within correct range Orthogonal quality Mesh is valid for studied physics Boundary layer Solution is grid independent Important geometric details are well captured Bad quality mesh can cause; Convergence difficulties Bad physic description Diffuse solution User must Check quality criteria and improve grid if needed Think about model and solver settings before generating the grid Perform mesh parametric study, mesh adaption

IMPACT OF THE MESH QUALITY ON THE SOLUTION Example showing difference between a mesh with cells failing the quality criteria and a good mesh Unphysical values in vicinity of poor quality cells

IMPACT OF THE MESH QUALITY ON THE SOLUTION Diffusion example (max,avg) CSKEW =(0.912,0.291) (max,avg) CAR =(62.731,7.402) Mesh 2 Mesh 1 Vz MIN -90ft/min Vz MAX 600ft/min Large cell size change (max,avg) CSKEW =(0.801,0.287) (max,avg) CAR =(8.153,1.298) Vz MIN -100ft/min Vz MAX 400ft/min

GRID DEPENDENCY Solution run with multiple meshes Note : For all runs the computed Y+ is valid for wall function (first cell not in laminar zone) x8 DP 0 DP 3 2%

GRID DEPENDENCY Hexa cells can be stretched in stream direction to reduce number of cells Bias defined on inlet and outlet walls Bias defined on inlet edges 16 000 cells (~DP2) Delta P = 310 Pa (~DP3)

HEXA VS. TETRA Hexa Hexa: Concentration in one direction Angles unchanged Tetra: Concentration in one direction Angles change Prism: Concentration in one direction Angles unchanged Solution for boundary layer resolution Hybrid prism/tetra meshes Prism in near-wall region, tetra in volume Automated Reduced CPU-time for good boundary layer resolution Tetra Prism Tetra (in volume) Prisms (near wall)

MESH STATISTICS AND MESH METRICS Displays mesh information for Nodes and Elements List of quality criteria for the Mesh Metric Select the required criteria to get details for quality It shows minimum, maximum, average and standard deviation Different physics and different solvers have different requirements for mesh quality Mesh metrics available in ANSYS Meshing include: Element Quality Aspect Ratio Jacobean Ration Warping Factor Parallel Deviation Maximum Corner Angle Skewness Orthogonal Quality For Multi-Body Parts, go to corresponding body in Tree Outline to get its separate mesh statistics per part/body

MESH QUALITY METRICS Orthogonal Quality (OQ) Derived directly from Fluent solver discretization For a cell it is the minimum of: Ai f i Ai ci Ai f i Ai ci computed for each face i For the face it is computed as the minimum of On cell computed for each edge I Where Ai is the face normal vector and fi is a vector from the centroid of the cell to the centroid of that face, and ci is a vector from the centroid of the cell to the centroid of the adjacent cell, where ei is the vector from the centroid of the face to the centroid of the edge At boundaries and internal walls ci is ignored in the computations of OQ c 1 A 1 c 3 A 3 Ai ei Ai ei f 3f 1 f 2 c 2 A 2 A 1 A 3 On face e 3 e 1 e 2 A 2 0 1 Worst Perfect

MESH QUALITY METRICS Skewness Optimal (equilateral) cell Two methods for determining skewness: 1. Equilateral Volume deviation: Skewness = optimal cell size cell size optimal cell size Applies only for triangles and tetrahedrons 2. Normalized Angle deviation: Skewness = max e 180 e max, Where e is the equiangular face/cell (60 for tets and tris, and 90 for quads and hexas) Applies to all cell and face shapes Used for hexa, prisms and pyramids e e min min max Circumsphere Actual cell 0 1 Perfect Worst

MESH QUALITY Mesh quality recommendations Low Orthogonal Quality or high skewness values are not recommended Generally try to keep minimum orthogonal quality > 0.1, or maximum skewness < 0.95. However these values may be different depending on the physics and the location of the cell Fluent reports negative cell volumes if the mesh contains degenerate cells Skewness mesh metrics spectrum Orthogonal Quality mesh metrics spectrum

ASPECT RATIO 2-D: Length / height ratio: δx/δy 3-D Area ratio Radius ratio of circumscribed / inscribed circle δy δx Limitation for some iterative solvers A < 10 100 (CFX: < 1000) Large aspect ratio are accepted where there is no strong transverse gradient (boundary layer...)

SMOOTHNESS Checked in solver Volume Change in Fluent Available in Adapt/Volume 3D : σ i = V i / V nb Recommendation: Good: 1.0 < σ < 1.5 Fair: 1.5 < σ < 2.5 Poor: σ > 5 20 Expansion Factor in CFX Checked during mesh import Ratio of largest to smallest element volumes surrounding a node

SECTION PLANES Displays internal elements of the mesh Elements on either side of plane can be displayed Toggle between cut or whole elements display Elements on the plane Edit Section Plane button can be used to drag section plane to new location Clicking on Edit Section Plane button will make section plane s anchor to appear Multiple section planes are allowed For large meshes, it is advisable to switch to geometry mode (click on geometry in the Tree Outline), create the section plane and then go back to mesh model

MESH METRIC GRAPH Displays Mesh Metrics graph for the element quality distribution Different element types are plotted with different color bars Can be accessed through menu bar using Metric Graph button Axis range can be adjusted using controls button (details next slide) Click on bars to view corresponding elements in the graphics window Use to help locate poor quality elements

MESH METRIC GRAPH CONTROLS Elements on Y-Axis can be plotted with two methods; Number of Elements Percentage of Volume/Area Options to change the range on either axis Specify which element types to include in graph Tet4 = 4 Node Linear Tetrahedron Hex8 = 8 Node Linear Hexahedron Wed6 = 6 Node Linear Wedge (Prism) Pyr5 = 5 Node Linear Pyramid Quad4 = 4 Node Linear Quadrilateral Tri3 = 3 Node Linear Triangle Te10, Hex20, Wed15, Pyr13, Quad8 & Tri6 non-linear elements

MESH QUALITY CHECK FOR CFX 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 Minimum Orthogonality Angle [degrees] = 20.4 ok 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! Good (OK) Acceptable (ok) Questionable (!)

MESH QUALITY CHECK FOR FLUENT Grid check tools available Check : Perform various mesh consistency checks Report Quality : lists worse values of orthogonal quality and aspect ratio TUI command mesh/check-verbosity sets the level of details in the report

FACTORS AFFECTING QUALITY Geometry problems Small edge Gaps Sharp angle Meshing parameters Sizing Function On / Off Min size too large Inflation parameters Total height Maximum angle Hard sizing Meshing methods Patch conformal or patch independent tetra Sweep or Multizone Cutcell Geometry cleanup in Design Modeler or Virtual topology & pinch in Meshing Mesh setting change Mesh setting change

VIRTUAL TOPOLOGY When to use? To merge together a number of small (connected) faces/edges To simplify small features in the model To simplify load abstraction for mechanical analysis To create edge splits for better control of the surface mesh control Virtual cells modify topology Original CAD model remains unchanged New faceted geometry is created with virtual topology Restrictions Limited to developable surfaces Virtual Faces cannot form a closed region Without VT automatically With VT manually

AUTOMATIC VIRTUAL TOPOLOGY Automatically creating Virtual Faces Left Click Virtual Topology in Model Tree Set Behaviour in Details Controls aggressiveness of automatic VT algorithm Low: merges only the worst faces (and edges) Medium & High: try to merge more faces Select if Face Edges shall be merged Right Click Virtual Topology and click Generate Virtual Cells Manually creating a Virtual Face RMB on Model tree and select Insert Virtual Topology Select Virtual Topology from the Tree Outline Pick faces or edges, RMB and Insert Virtual Cell All VT entities created can be seen in different colors if Virtual Topology is selected in Tree Outline

PINCH Pinch control removes small features automatically or manually at the mesh level Slivers Short Edges Sharp Angles The Pinch feature works on vertices and edges only The Pinch feature is supported for the following mesh methods: Patch Conforming Tetrahedrons Thin Solid Sweeps Hex Dominant meshing Quad Dominant Surface meshing Triangles Surface meshing Not supported for» CutCell before after before after Vertex-Vertex Edge-Edge

ASSEMBLY MESHING

ASSEMBLY MESHING Behavior Meshes an entire model as single process Mesh Methods covered so far are part or body based methods Not compatible with part/body methods Two Algorithms available CutCell & Tetrahedrons Access Assembly Meshing is accessible only when Physics and Solver Preferences are set to CFD and Fluent respectively To activate, replace None by Cutcell or Tetrahedrons CutCell Note that some global and local controls are not available for Assembly Meshing (eg. Match Control) Tetrahedrons

ASSEMBLY MESHING - CUTCELL CutCell Behavior Cartesian meshing method designed for the ANSYS FLUENT solver Generates a majority of hex cells Some wedges, tets and pyramids at boundaries to capture geometry During transfer to Fluent hexa cells at size transition are converted into Polyhedra Supports Inflation Post-inflation (TGrid algorithm) Baffles not supported High inflation may fail Cutcell mesh generated first, inflation generated second (Post)

ASSEMBLY MESHING - TETRAHEDRONS Tetrahedrons Behavior Generates a Patch Independent tetra mesh with automatic defeaturing Following steps occur in background Generate CutCell Delete volume mesh Triangulate surface mesh and improve Fill with tetra mesh Compatible with inflation Pre-inflation Algorithm similar to Tetra Patch Conformal

ASSEMBLY MESHING - CONTROLS Controls Set Advanced Size Functions Proximity SF Sources : 'edges', faces or edges and faces Define correct Min Size (details next slide) Inflation defined by Global or Local controls Combined Global & Local not supported Program Control acts on Fluid bodies only Bodies can be set as Fluid in Body properties For Virtual Bodies, only automatic Program Controlled inflation can be used Define Feature and Tesselation controls (see next slide) Apply any required local size controls Statistics

ASSEMBLY MESHING - CONTROLS Example 1. Min Size too large compared to the size of the geometric detail Min Size definition Assembly Meshing is Patch Independent, geometry recovery and leakage depend on local sizes Local sizes are driven by global min sizes and local hard sizing Min Size and Prox Min Size must be set with care Local mesh size recommendation to capture 3D features Local size < ½ feature size Local mesh size recommendation to close gaps 1 / 10 local size < gap size < ¼ local size : contact sizing can be defined to close gap Gap size < 1 / 10 local size : gap closed Prior to meshing the user is advised to resolve geometry features properly in CAD/DM Avoid unnecessary geometry details Features aligned with Coord. Syst. will be more easily recovered Example2. Doubling the Min Size closes the gap

ASSEMBLY MESHING - CONTROLS Feature Capture Program Controlled : default which sets feature angle = 40 Feature Angle : user angle to define features to recover 0 to capture all Tessellation (faceting) refinement Program Controlled - default which sets tessellation refinement to 10% of the value of smallest global min size Absolute Tolerance user defined tolerance Must be set to 5-10% of smallest size (global min sizes or local hard sizing) None - Sets tessellation refinement to the CAD program or DesignModeler default setting Incorrect tessellation may lead to leakage

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