ANSYS Help > Mechanical APDL > Modeling and Meshing Guide

Ch. 5 (skim), 6, 7.1 – 7.7, 11.1

CH 5: Solid Modeling

Bottom-Up vs. Top-Down Construction

Boolean Operations;  Drag and Rotate;  Copy or Move;  Meshing – free or mapped;

Copy or Move Meshed Bodies;  Solid model loading; Clear and Delete or Modify

Hierarchy in entities: volumes, the highest-order entities, are bounded by areas, which are bounded by lines, which in turn are bounded by keypoints.

• [Delete’ Areas Only’ – or – ‘Areas and Below’]

• Currently active coordinate system effects modeling

Hard Points:

Special purpose keypoints, not part of geometry construction, can be additional mesh control, allow loading or retrieve results at arbitrary points on the model

In 2D, fillet LINES to round a corner between areas

In 3D, fillet AREAS to create a round corner for volumes

Dragging or rotating or offsetting lines, creates areas

Dragging or rotating or offsetting or extruding areas, creates volumes

Dragging or rotating or offsetting or extruding meshed areas creates meshed volumes

Unmeshed volumes can be meshed by sweeping a mesh pattern from one face.

Primitives may be used to create a model.  (Primitives are commonly used geometric shapes)

Primitives are created with respect to the current position of the working plane

The interface between two touching primitives will create a seam of discontinuity in the finite element model, unless you take steps to "weld" that seam shut, using operations such as merge, add, glue, or overlap.

Boolean Operations: 

Like we add or subtract numbers by math operations, we can operate on geometric shapes using Boolean operations.

Save the model before each attempt at a Boolean operation.

Cannot be used on meshed entities.  May delete any loads or constraints on the entities.

The model entities ID numbers are usually changed by Boolean operations.

Boolean Operations Settings effect results. [most useful:  delete or keep the original entities]

Operations: Intersect, Add, Subtract, Overlap, Partition, Glue

The working plane may be used in many Boolean operations

Geometric entities can be moved and/or copied:

• Generate pattern, reflect, transfer from one CSYS to another

• Entities can also be scaled up or down

Solid Model Loads:

Loads may be easier to define on the solid model vertices, edges, faces, or bodies [as opposed to the finite element model: nodes and elements]

Before each solve, ANSYS automatically transfers all solid model loads to the finite elements and nodes

Solid model loads are INDEPENDENT of the finite element mesh

Solid model loads:

• Can be manually transferred to the FE model (to check/verify before solving)

• Can be displayed graphically as symbols and can be listed to check values

Mass and inertia checks: computes length of lines or area or volume as well as c.g. location, MOI

Boolean operations can fail:

• Degeneracies – usually when an edge or face is collapsed to a point (or a face to a line)

• Discontinuities – sharp change in geometry

• (Section 5.10 discusses ways to prevent or overcome Boolean operation failures)

CH 6: Importing Solid Models (from IGES or other formats)

“CONNECTION Products” (ADD-ONS, cost extra) – direct connection to Pro/E

CH 6 – focuses on IGES import, but shows many common problems for all imports:

• Short edges, thin (high aspect ratio) areas, small and sliver areas, and detect/repair open gaps

• Also shows many “fixes” such as: merge, collapse, and split for lines and areas; fill holes, remove boss, repair open boundaries, and detect “non-manifold” lines

CH 7: Meshing

FREE (unrestricted) or MAPPED (required pattern and shape) mesh choice

Setting element “Attributes:” TYPE, REAL, MAT,ESYS, SECT
   Attributes are created as tables of data. Later, assigned to elements, as created

Mesh Controls:  degenerate element shape – can be automatic, may affect accuracy

• Degenerate PLANE182 (triangles) produce warnings (as in HW-1)

• Many options – global setting, geometric entity setting, SMRTSIZE, & advanced

• MAPPED meshing – more restrictions on mesh controls, may require combining lines or “concatenation”

Meshing: Lines & Volumes – beam “orientation,” sweeping

7.5.8      Element Shape Checking (automatic)

The fact that you receive even hundreds of element shape warnings does not necessarily mean that element shapes will cause any inaccuracy of results. (Conversely, if you do not receive any warnings about element shapes, that does not guarantee accurate results.)

7.5.8.8.  Deciding Whether Element Shapes Are Acceptable:  

Never ignore an element shape warning.   Consider the effect on simulation results.  Verification of analysis results by comparison with other analyses, test data, or hand calculations is essential

7.7 MESHING HINTS:

       Avoid sharp corners, avoid extreme transition in element size, avoid excessive curvature, mesh surfaces of difficult volumes to find problems, specific sequence required for models that have both higher- and lower-order elements.

CH 11: Numbering Controls

How are numbers assigned to model entities ? 

Sequentially, starting from the lowest available number

Why do we care about numbering ?

We may need to refer to a specific entity (kp, line, area, or volume) by its number

Boolean operations change the numbers of the entities (and below) which are operated on

MERGE entities (usually KP or nodes)

COMPRESS numbers to removed unused numbers

Control the starting number for the next entities which will be created

OFFSET numbers – add a constant to shift the numbered pattern  

READING ASSIGNMENT:

ANSYS Help > Mechanical APDL > Structural Analysis Guide, Chapter 8.1 - 8.6