FINITE ELEMENT ANALYSIS I



LECTURE 3

GENERATING THE MESH

• THE ULTIMATE OBJECTIVE IN BUILDING A SOLID MODEL IS TO MESH IT WITH NODES AND ELEMENTS

• STEPS IN MESHING:

ONCE YOU HAVE THE GEOMETRY

SET THE ELEMENT ATTRIBUTES

• Element type (for example, BEAM3, SHELL61, etc.)

• Real constant set (usually comprising the element's geometric properties, such as thickness or cross-sectional area)

• Material properties set (such as Young's modulus, thermal conductivity, etc.)

• Element coordinate system

• Section ID (for BEAMS)

ASSIGN THE ELEMENT ATTRIBUTES

• Directly to solid model entities

• Assigning default attributes – you have to change the default with every solid model part that you mesh. Attributes assigned directly to the solid model entities will override the default attributes.

A further note for shell elements – these can be defined having variable thickness

SET THE MESHING CONTROLS

• The default mesh controls that the ANSYS program uses may produce a mesh that is adequate for the model you are analyzing.

• In this case, you will not need to specify any mesh controls.

• However, if you do use mesh controls, you must set them before meshing your solid model.

• Mesh controls allow you to establish such factors as the element shape, midside node placement, and element size to be used in meshing the solid model.

• This step is one of the most important of your entire analysis, for the decisions you make at this stage in your model development will profoundly affect the accuracy and economy of your analysis.

THE ANSYS MESHTOOL:

The many functions available via the MeshTool include:

• Controlling SmartSizing levels

• Setting element size controls

• Specifying element shape

• Specifying meshing type (free or mapped)

• Meshing solid model entities

• Clearing meshes

• Refining meshes

• RATHER THAN USE THE MESHTOOL WE CAN SET MESHING OPTIONS MANUALLY

• AT A MINIMUM YOU SHOULD SET THE ALLOWABLE ELEMENT SHAPE (MSHAPE)FOR THOSE ELEMENTS THAT CAN DEGENERATE TO A TETRAHEDRON OR TRIANGLE.

• A MIXTURE OF ELEMENT SHAPES IS NOT RECOMMENDED BY ANSYS – Exception is for transitional pyramid elements

• Element shape specification is closely related to the type of meshing that you request (free or mapped)

• USUALLY YOU WILL SET A DEFAULT SIZE (SMRTSIZE, DESIZE), THEN YOU USE MSHAPE TO SPECIFY THE SHAPE AND THEN AMESH OR VMESH TO CREATE THE MESH.

• FOR FREE MESHING AFTER SETTING THE DEFAULT ELEMENT SIZE (DESIZE) ANSYS RECOMMENDS THE USE OF SMRTSIZE FOR A BETTER MESH – ALSO ONCE YOU ISSUE SMRTSIZE MESH IMMEDIATELY THE WHOLE MODEL BY USING AMESH,ALL OR VMESH,ALL SO THAT ANSYS TAKES INTO CONSIDERATION GRADING THE MESH AT SMALL FEATURES.

• SMRTSIZE CAN BE USED AT THE BASIC LEVEL BY SPECIFYING A FINE MESH OR A COARSE MESH

• SMRTSIZE CAN BE USED AT A MORE ADVANCED LEVEL BY SPECIFYING ELEMENT SIZE LEVELS ON OR NEAR INDIVIDUAL ENTITIES

• SMRTSIZE CAN BE USED IN CONJUNCTION WITH COMMANDS THAT SPECIFY MESH SIZE ON INDIVIDUAL ITEMS OR LOCALLY - SUCH COMMANDS ARE LESIZE, KESIZE, AESIZE – IN THIS CASE SMRTSIZE HANDLES MESH TRANSITION ETC.

• LOCAL MESH CONTROLS ARE USED TO GET A BETTER MESH AT SAY CRACKS AND OTHER STRESS CONCENTRATIONS

• USING MOPT YOU CAN ALSO CONTROL THE MESH ON THE INTERIOR OF AN AREA OR VOLUME WHERE THERE ARE NO LINES TO GUIDE THE SIZE OF THE MESH.

• TRANSITIONAL PYRAMID ELEMENTS ARE USEFUL IF YOU ARE TO USE MAPPED MESHING FOR ONE PART OF THE MODEL AND FREE MESHING FOR ANOTHER PART

• THIS CAN HAPPEN WHEN WE HAVE COMPLEX GEOMETRIES

• HEXAHEDRAL ELEMENTS CAN BE USED FOR THE MAP-MESHABLE REGION WHILE TETRAHEDRAL ELEMENTS ARE USED FOR THE REMAINDER

• HIGH-GRADIENT REGIONS MAY REQUIRE HEXAHEDRAL ELEMENTS TO CAPTURE DETAIL, WHILE FOR OTHER, LESS CRITICAL REGIONS, TETRAHEDRAL ELEMENTS MAY BE SUFFICIENT

• USING A MIX OF HEXAHEDRAL AND TETRAHEDRAL ELEMENT SHAPES LEADS TO NONCONFORMITIES IN A MESH

• PROBLEMS IN MATHEMATICAL CONTINUITY ARE AVOIDED BY INSTRUCTING ANSYS TO AUTOMATICALLY CREATE PYRAMID ELEMENTS

• TO REDUCE COMPUTATIONAL TIME YOU CAN CONVERT DEGENERATE TETRAHEDRAL ELEMENTS TO THEIR NON-DEGENERATE FORMS (TCHG)

• IN ANSYS WE CAN ALSO DO LAYER FREE MESHING (LESIZE) FOR 2D-SOLIDS. THIS CREATES:

• Uniform (or moderately varying) element size along the line.

• Steep transitions in element size and number in the direction normal to the line.

Such meshes are suitable for simulating CFD boundary layer effects, electromagnetic skin layer effects, etc

GENERATE THE MESH

• WE HAVE A CHOICE BETWEEN A FREE MESH AND A MAPPED MESH (MSHKEY)

• IN FREE MESHING OPERATIONS ANY MODEL GEOMETRY, EVEN IF IT IS IRREGULAR, CAN BE MESHED

• FOR AREA FREE MESHING ELEMENTS CAN HAVE A TRIANGULAR, QUADRILATERAL OR A MIXTURE OF THE TWO ELEMENT SHAPES

• FOR VOLUME FREE MESHING TETRAHEDRAL ELEMENT SHAPES MUST BE USUALLY USED

• A SPECIAL TYPE OF FREE MESHING, CALLED FAN TYPE MESHING, IS AVAILABLE FOR CERTAIN CONTACT ANALYSIS

• A MAPPED MESH REQUIRES THAT AN AREA OR VOLUME BE REGULAR i.e. IT REQUIRES SOME SPECIAL CRITERIA

• FOR AREA MAPPED MESHING, ELEMENTS CAN HAVE A TRIANGULAR OR QUADRILATERAL ELEMENT SHAPES

• FOR VOLUME MAPPED MESHING BRICK ELEMENT SHAPES MUST BE USED

• If an area is bounded by more than four lines, it cannot be map meshed. However, some of the lines can be combined or "concatenated" to reduce the total number of lines to four.

• THE AMAP COMMAND OFFERS THE EASIEST WAY TO OBTAIN A MAPPED AREA MESH

• WE CAN SPECIFY LINE DIVISIONS ON OPPOSITE SIDES OF THE AREA SUCH THAT THE DIVISIONS PERMIT A TRANSITION MAPPED QUADRILATERAL MESH (See details in ‘CONTROLS FOR FREE AND MAPPED MESHING’ under ‘Generating the mesh’ in the ‘Modelling and Meshing Guide’)

• FOR VOLUME MAPPED MESHING the volume must take the shape of a brick (bounded by six areas), wedge or prism (five areas), or tetrahedron (four areas)

• As with lines, you can add AADD or concatenate ACCAT areas if you need to reduce the number of areas bounding a volume for mapped meshing. If there are also lines bounding the concatenated areas, the lines must be concatenated as well.

• YOU CAN CREATE A MAPPED VOLUME MESH BY SPECIFYING LINE DIVISIONS ON OPPOSITE EDGES OF THE VOLUME SUCH THAT THE DIVISIONS PERMIT A TRANSITION MAPPED HEXAHEDRAL MESH

• TRANSITION MAPPED HEXAHEDRAL MESHING IS ONLY APPLICABLE TO SIX-SIDED VOLUMES (WITH OR WITHOUT CONCATENATION). (See details in ‘CONTROLS FOR FREE AND MAPPED MESHING’ under ‘Generating the mesh’ in the ‘Modelling and Meshing Guide’)

Some other points:

• With MSHMID we can control the placement of Midside Nodes for some required effect.

• A PART OF A MESH CAN BE ALSO COPIED TO SOME OTHER PARTS OF THE SOLID MODEL

• IF YOU NEED TO MESH MULTIPLE AREAS or volumes AT ONE TIME, YOU SHOULD CONSIDER ISSUING THE MOPT,ORDER,ON COMMAND SO THE MESH IS CREATED IN THE SMALLEST AREA or volume FIRST. THIS HELPS ENSURE THAT YOUR MESH IS ADEQUATELY DENSE IN SMALLER AREAS or volumes AND THAT THE MESH IS OF A HIGHER QUALITY.

• YOU CAN ASSIGN ORIENTATION KEYPOINTS AS ATTRIBUTES OF A LINE FOR 3D-BEAM MESHING (LATT). With this feature you can also include twist in the beam.

• In addition to using VMESH to generate volume elements, you can generate a volume mesh from a set of detached exterior area elements (facets). For example, this capability is useful in situations where you cannot mesh a particular area.

• USING VOLUME SWEEPING, YOU CAN FILL AN EXISTING UNMESHED VOLUME WITH ELEMENTS BY SWEEPING THE MESH FROM A BOUNDING AREA THROUGHOUT THE VOLUME.

• Unlike other methods for extruding a meshed area into a meshed volume [VROTAT, VEXT, VOFFST, and VDRAG commands], volume sweeping [VSWEEP] is intended for use in existing unmeshed volumes (See details in ‘MESHING YOUR SOLID MODEL’ under ‘Generating the mesh’ in the ‘Modelling and Meshing Guide’)

• FOR SIMULATING A GASKET JOINT YOU MUST GENERATE AN INTERFACE MESH AS PART OF THE OVERALL MESHING PROCEDURE (See details in ‘MESHING YOUR SOLID MODEL’ under ‘Generating the mesh’ in the ‘Modelling and Meshing Guide’)

• "BADLY SHAPED" ELEMENTS CAN, ON OCCASION, CAUSE VERY POOR ANALYTICAL RESULTS

• FOR THIS REASON, THE ANSYS PROGRAM PERFORMS ELEMENT SHAPE CHECKING TO WARN YOU WHENEVER ANY OPERATION CREATES AN ELEMENT HAVING A POOR SHAPE

• BUT AN ELEMENT THAT GIVES POOR RESULTS IN ONE ANALYSIS MIGHT GIVE PERFECTLY ACCEPTABLE RESULTS IN ANOTHER ANALYSIS

• EVEN THOUGH ANSYS ISSUES SEVERAL WARNING MESSAGES THE ANALYSIS MAY STILL BE CORRECT

• ON THE OTHER HAND IF ANSYS DOES NOT ISSUE ANY WARNINGS IT DOES NOT MEAN THAT THE ANALYSIS IS CORRECT

• SO IT IS THE RESPONSIBILITY OF THE USER TO CHECK IF THE ELEMENTS’ SHAPES ARE O.K. FOR THE APPLICATION

• FOR FURTHER DETAILS ON HOW TO CONTROL SOME ASPECTS OF ELEMENT SHAPE CHECKING (SHPP) REFER TO ‘ELEMENT SHAPE CHECKING’ under ‘Meshing your solid model’ - ‘Generating the mesh’ in the ‘Modelling and Meshing Guide’)

• If you are not happy with the mesh you can:

• Remesh with new element size specifications.

• Use the accept/reject prompt to discard the mesh, then remesh.

• Clear the mesh, redefine mesh controls, and remesh.

• Refine the mesh locally.

• Improve the mesh (for tetrahedral element meshes only).

• FOR MORE DETAILS REFER TO ‘CHANGING THE MESH’ under ‘Generating the mesh’ in the ‘Modelling and Meshing Guide’)

• READ ALSO ‘SOME HINTS AND CAUTIONS’ under ‘Generating the mesh’ in the ‘Modelling and Meshing Guide’)

• THE (CPCYC) COMMAND CAN BE USED WHEN THERE IS CYCLIC SYMMETRY BOTH IN LOADING AND IN THE GEOMETRY. FOR MORE DETAILS REFER TO ‘USING THE CPCYC AND MSHCOPY COMMANDS’ under ‘Generating the mesh’ in the ‘Modelling and Meshing Guide’)

• LOADS CAN BE EITHER APPLIED ON THE GEOMETRY FEATURES OR ELSE DIRECTLY ON THE NODES AND ELEMENTS

• FOR MORE DETAILS REFER TO ‘GENERATING THE MESH’ IN THE ANSYS MODELLING AND MESHING GUIDE

• DO THE EXAMPLES THAT EXIST IN THE ‘GENERATING THE MESH’ CHAPTER (6 hours)

• TRY THEM OUT FIRST USING THE MENU SYSTEM AND THEN BE SURE TO PERFORM THE SAME ACTIVITY USING THE COMMAND LINE.

• AS ANOTHER EXERCISE ON MESHING TRY TO MESH THE SOLID MODEL YOU CREATED FOR YOUR FIRST PROJECT IN THIS MODULE.

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