Geometry, selections, and components

Your model already understands parameters. Now you need to learn something just as important: recognize physical regions even if the length, mesh and internal numbers change that MAPDL assigns to nodes and elements.

Objectives

By completing M02 you will be able to demonstrate that:

• You distinguish entities from the solid model and the finite element model.
• Explain why a selection is a temporary state and a component is a persistent reference.
• You combine sets by S, A, R and U.
• You select nodes by location and elements by position of their centroid.
• You explicitly control tolerance by SELTOL.
• You use a local system as a mobile reference linked to the tip.
• Audits components with *GET,COUNT and relationships between counts.
• You retrieve the complete model before continuing through ALLSEL,ALL.

Prerequisites and files

• Have completed M01 or understand parameters, expressions and checks with *IF.
• Recognize that the beam occupies 0 ≤ X ≤ beam_l, 0 ≤ Y ≤ beam_h and 0 ≤ Z ≤ beam_b.
• Keep the system consistent SI: meters, newtons and pascals.

How to use this lesson

Recommendation: Think of each region as a whole before writing NSEL. If you can't predict how many entities will capture a selection, you still don't understand it.

Session map

Before you code — Make a prediction

Imagine that MAPDL assigns nodes to the free end 241–255. By increasing the length of 1.0 m a 1.2 m and refine the mesh, will they still represent the tip?

There is no guarantee. IDs describe how it was stored this meshing, not what a region physically means. Instead, the phrase “nodes located at the tip” can be expressed by geometry:

NSEL,S,LOC,X,beam_l

In this lesson we will go a step further: we will place the origin of a local system at the tip and we will always select it with X=0. The instruction will be relative to the part, not to the particular size of the case.

Mental model: two overlapping models

MAPDL maintains two entity families in the same database. The solid model describes geometry; the finite element model describes its discretization.

Step 1 — Build the parametric geometry

The M01 block remains:

/PREP7
ET,1,SOLID185
MP,EX,1,young
MP,PRXY,1,nu
TYPE,1
MAT,1

BLOCK,0,beam_l,0,beam_h,0,beam_b
ESIZE,mesh_h
VMESH,ALL

A BLOCK isolated creates a volume delimited by six areas, twelve lines and eight keypoints. VMESH adds elements and nodes. Your IDs may change even though the solid represent the exact same beam.

Before reducing any selection, we save the full size of the model:

ALLSEL,ALL
*GET,n_nodes,NODE,0,COUNT
*GET,n_elements,ELEM,0,COUNT

These two values are our baseline. At the end they should reappear exactly.

Step 2 — Think of selections as set algebra

The second command argument xSEL indicates how the new condition is combined with the active set:

That is why a selection sequence must be read from top to bottom. If an operation leaves the set empty, a subsequent reselection cannot retrieve entities that were no longer active.

Step 3 — Control geometric tolerance

A calculated coordinate and a stored coordinate can differ by tiny fractions. MAPDL uses a tolerance on non-integer value selections. Its automatic logic depends of VMIN and VMAX; for example, if both are equal and not null, you can take the 0.5 % requested value

A global selection at X=1.0 m could include a much wider band of the expected when the mesh is very fine. In a reproducible script it is convenient to declare the criterion:

select_tol=MIN(beam_h,beam_b)*1E-5
SELTOL,select_tol

For the base case, select_tol=5E-7 m. It is small compared to the geometry and sufficient to absorb numerical noise. Tolerance remains active until redefined, so in the end we will restore it:

SELTOL,

Step 4 — Create fixed_nodes

We start in the global Cartesian system and select the face X=0:

CSYS,0
NSEL,S,LOC,X,0
*GET,n_fixed,NODE,0,COUNT
CM,fixed_nodes,NODE
ALLSEL,ALL

The pattern contains four decisions:

1. NSEL,S starts a new nodal set.
2. LOC,X,0 expresses the region by a geometric condition.
3. *GET immediately checks that the selection exists.
4. CM save a photo before restoring everything with ALLSEL.

Order is essential. If you were to execute ALLSEL before CM, fixed_nodes would contain all nodes.

Step 5 — Define a local tolerance at the tip

LOCAL defines a coordinate system and CSYS decides which one is active. Local system numbers must be greater than 10; we will use the 11:

LOCAL,11,CART,beam_l,0,0
CSYS,11
NSEL,S,LOC,X,0
*GET,n_tip,NODE,0,COUNT
CM,tip_nodes,NODE

CSYS,0
ALLSEL,ALL

We have not rotated the shafts; we only translate the origin to X=beam_l. In the system global, the tip is in X=beam_l. In the 11 system, it is in X=0.

LOC interprets X, Y and Z in the active system. That's why CSYS,0 is not cosmetic cleanup: it prevents later selections from operating in the wrong coordinate system.

Step 6 — Create top_nodes and mid_elems

With the global system restored, the top face is defined by its height:

NSEL,S,LOC,Y,beam_h
*GET,n_top,NODE,0,COUNT
CM,top_nodes,NODE
ALLSEL,ALL

For the central band, we change the entity class. ESEL,CENT checks the coordinate of each element centroid:

ESEL,S,CENT,X,0.4*beam_l,0.6*beam_l
*GET,n_mid_elems,ELEM,0,COUNT
CM,mid_elems,ELEM
ALLSEL,ALL

The band must contain elements, but not all. That relationship is more stable than requiring a number exact count, because it may vary with the meshing strategy or version.

Step 7 — Construct tip_top_nodes via intersection

The top edge of the tip meets two simultaneous conditions: belong to tip_nodes and to top_nodes.

CMSEL,S,tip_nodes
CMSEL,R,top_nodes
*GET,n_tip_top,NODE,0,COUNT
CM,tip_top_nodes,NODE
ALLSEL,ALL

CMSEL,S retrieves the first component. CMSEL,R intersects it with the b. The result must be non-empty, less than the full face of the tip and less than all the top face:

0 < n_tip_top < n_tip
0 < n_tip_top < n_top

Step 8 — Demonstrate that the ends do not overlap

A positive-length beam cannot have nodes belonging to both ends:

CMSEL,S,fixed_nodes
CMSEL,R,tip_nodes
*GET,n_overlap,NODE,0,COUNT
ALLSEL,ALL

*GET,n_restored,NODE,0,COUNT

The expected conditions are:

n_overlap  = 0
n_restored = n_nodes

The first is a physical check. The second is a health check: it confirms that no accidental filters will continue to affect subsequent modules.

Step 9 — Turn regions into an executable contract

A component that exists by name may be empty or contain too much. The deliverable evaluates all relationships and summarizes the result in passes:

passes=1
*IF,n_fixed,LE,0,THEN
  passes=0
*ENDIF
*IF,n_overlap,NE,0,THEN
  passes=0
*ENDIF
*IF,n_restored,NE,n_nodes,THEN
  passes=0
*ENDIF

Then generate legible evidence:

*CFOPEN,m02_selection_audit,csv
*VWRITE
('case,beam_l,beam_h,mesh_h,n_nodes,n_elements,n_fixed,n_tip,n_top,n_mid_elems,n_tip_top,n_overlap,n_restored,passes')
...
*CFCLOS

Open m02_selection_audit.csv and check that the last column is valid 1. Exact counts may vary; the relationships recorded in the file 02_expected_results.csv must always be adhered to.

The pattern to keep

Translated to APDL:

NSEL,S,...
*GET,n_region,NODE,0,COUNT
CM,region_nodes,NODE
ALLSEL,ALL

CMSEL,S,region_nodes
! operation on the region
ALLSEL,ALL

This pattern separates the definition of a region from its use. In M05 we may apply constraints or loads to components whose meaning has already been proven.

Bug hunt

Download 02_bug_hunt.mac. The file runs, but contains five logical defects:

1. A physical region defined by node IDs.
2. A component created after restoring all nodes.
3. A local system that continues to operate unintentionally.
4. A tolerance of the same order as the mesh size.
5. A partial selection of elements that propagates to the end.

For each defect, write: incorrect state → probable symptom → check that reveals it → minimum correction. It is not enough to just point to the line.

Verifiable challenge

Open 02_challenge.mac. The length is now 1.2 m and the mesh size is 0.02 m.

1. Create all five components without writing IDs.
2. Use local system 11 to define tip_nodes.
3. Build tip_top_nodes by intersection.
4. Demonstrate that the ends do not overlap.
5. Generate m02_selection_audit.csv with the same schema as the solution.

Self-assessment

Evidence of learning

• 02_geometry_selections.mac executed from a clean session.
• m02_selection_audit.csv with passes=1.
• 02_challenge.mac completed for the beam of 1.2 m.
• Diagnostic table of the five defects of 02_bug_hunt.mac.
• Brief explanation of why a geometric selection is more stable than a list of IDs.

Validation checklist

• I distinguish entities from the solid model and the finite element model.
• I can anticipate the effect of S, A, R and U.
• Explicitly defined and restored SELTOL.
• I created fixed_nodes and tip_nodes without IDs.
• Restored CSYS,0 after using the 11 system.
• I built tip_top_nodes as an intersection of two components.
• I checked that fixed_nodes and tip_nodes (non-overlapping)
• I verified that ALLSEL,ALL retrieves all nodes.
• The challenge generates a CSV with passes=1.
• I did not apply loads or solve the model in this module.