Finite Element Analysis I

LECTURE 2

SOLID MODELLING

· THE ULTIMATE PURPOSE OF AN F.E.A. ANALYSIS IS TO MATHEMATICALLY MODEL A PHYSICAL SYSTEM.

· THIS MODEL IS MADE UP OF ALL THE NODES, ELEMENTS, MATERIAL PROPERTIES, REAL CONSTANTS, BOUNDARY CONDITIONS THAT ARE USED TO REPRESENT THE PHYSICAL SYSTEM.

· THERE ARE 2 WAYS OF CREATING THE NODES AND ELEMENTS THAT MAKE UP THE MODEL – DIRECT GENERATION OR SOLID MODELLING

· IN DIRECT GENERATION WE CREATE THE NODES AND ELEMENTS THEMSELVES – A HARD TASK FOR A COMPLEX MODEL BUT RELATIVELY EASY FOR LINE MODELS

· IN SOLID MODELLING WE FIRST CREATE THE GEOMETRY AND THEN CREATE THE MESH AUTOMATICALLY

· YOU WILL BE USING SOLID MODELLING A LOT SO …..

· WE WILL START WITH PRODUCING THE SOLID MODEL WHICH DEFINES THE BOUNDARIES OF NODES AND ELEMENTS

· THERE ARE TWO APPROACHES – THE BOTTOM UP TECHNIQUE AND THE TOP DOWN TECHNIQUE

· IN THE BOTTOM UP TECHNIQUE WE START FROM KEYPOINTS, THEN BUILD LINES, AREAS AND VOLUMES

· IN THE TOP DOWN TECHNIQUE WE USE BOOLEAN OPERATIONS AND PRIMITIVES TO CREATE OUR SOLID MODEL.

· FOR MORE DETAILS READ THE ‘SOLID MODELLING’ PART IN THE MODELLING AND MESHING GUIDE.

· WHAT TO USE DEPENDS ON THE COMPLEXITY OF OUR MODEL.

· IT IS ALWAYS A GOOD IDEA TO THINK AND PLAN YOUR WORK BEFORE ACTUALLY STARTING ON THE COMPUTER

· IN ANSYS YOU CAN USE A NUMBER OF WORKING PLANES AND LOCAL COORDINATE SYSTEMS IN ORDER TO EASE THE WORK OF SOLID MODELLING.

· START WITH CHOOSING THE MODEL TYPE i.e. LINE (includes 2-D axisymmetric), 2-D SOLID, 3-D SHELL, 3-D SOLID.

· CHOOSE THE ELEMENT ORDER – Linear, quadratic, Linear with extra displacement shapes, Polynomial (for structural elements)

· READ ‘LIMITATIONS ON JOINING DIFFERENT ELEMENTS’ IN THE MODELLING AND MESHING GUIDE (Planning your approach).

· TAKE ADVANTAGE OF SYMMETRY WHERE POSSIBLE – AXISYMMETRY (ABOUT GLOBAL –Y AXIS), REPETITIVE SYMMETRY, REFLECTIVE SYMMETRY

· SYMMETRY MUST BE IN THE GEOMETRY, THE LOADING, THE MATERIAL PROPERTIES AND IN THE CONSTRAINTS

· A SPECIAL AXISYMMETRIC HARMONIC ELEMENT IS USED WHEN THE GEOMETRY IS AXISYMMETRIC BUT THE LOADING IS NOT – Read ‘Axisymmetric elements with non axisymmetric loads’ in the elements reference manual

· DECIDE HOW MUCH DETAIL TO INCLUDE – TO DO THIS YOU MUST HAVE AN IDEA OF HOW YOUR MODEL WILL BEHAVE.

· DECIDE ON THE MESH DENSITY – TRANSITION MESHES ARE THE BEST BET !

· IT IS A GOOD IDEA TO PERFORM A MESH DENSITY CHECK – i.e. use a finer mesh and see whether there is a large variation in results.

· GLOBAL & LOCAL COORDINATE SYSTEMS – Locates geometry items like nodes, keypoints, etc

· DISPLAY COORDINATE SYSTEM – Determines the coordinate system in which geometry items are displayed.

· NODAL COORDINATE SYSTEM – Defines the d.o.f. directions & the orientation of nodal result data in POST26. In POST1 the results are shown in the results coordinate system.

· ELEMENT COORDINATE SYSTEM – Defines the material property directions, directions of surface loads & the orientation of element result data.

· RESULTS COORDINATE SYSTEM – Rotates the nodal and elemental results data to a particular coordinate system for viewing results, etc.

· READ ‘COORDINATE SYSTEMS’ IN THE MODELLING AND MESHING GUIDE

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

PLEASE SCROLL DOWN

(HAND OUT SOLID MODELLING PROJECT)

SOLID MODELLING PROJECT

Ø DIVIDE YOURSELVES INTO GROUPS.

Ø YOU CAN START YOUR MODEL NOW.

Ø YOU CAN ASK ON ANY DIFFICULTIES THAT YOU ENCOUNTER.

Ø HAND IN THE MODEL IN 4 WEEKS TIME.

Ø TRY TO MAKE GOOD USE OF SYMMETRY, COORDINATE SYSTEMS AND WHAT WAS COVERED IN CLASS TODAY.

Ø CONSULT THE ‘MODELLING AND MESHING GUIDE’ AS HELP FOR FINISHING YOUR PROJECT.

Ø YOU GET 10% FOR THIS PROJECT

Below are examples you can go through

FEA1 – COURSEWORK TEST 1 – DECEMBER 2003

TIME ALLOWED 2 Hours

You are reminded to concentrate only on your work during the test. Failure to do so may result in some marks being deducted.

For parts (1) and (2) hand in a file on a floppy disc and a hard copy version of it.

For part (3) you can hand in a hand-written reply in which you can include sketches.

1) Using bottom up modelling and Boolean operations build a 3-D model of the nozzle-sphere intersection shown above.

(8 marks)

2) Free mesh the nozzle-sphere intersection using solid elements. Be sure to use a transition mesh wherever you think there will be a high stress gradient. The model will ultimately be loaded with internal pressure.

(6 marks)

3) Give a brief overview of how you would go about map- meshing the solid.

(6 marks)