Syntax and parameters

In M00 you ran a prepared simulation. Now you will make the model stop being a rigid sequence: will receive inputs, check if they make sense, and make their first engineering decision.

Objectives

By completing M01 you will be able to demonstrate that:

• Replace scattered numbers with parameters with physical meaning and known units.
• You distinguish inputs, derived quantities, analytical references and FEA results.
• You build APDL expressions and predict their trend before executing.
• You stop invalid entries by *IF.
• You define an array with *DIM and you walk through it using *DO.
• You generate a table CSV and select the first admissible alternative.

Prerequisites and files

• Have completed M00 or recognize flow /PREP7 → /SOLU → /POST1.
• Know the meaning of elastic modulus, moment of inertia, and deflection.
• Work with the coherent system SI: meters, newtons and pascals.

How to use this lesson

Recommendation: Calculate uy_ref and sigma_ref on paper before running. Prediction converts simulation in a falsifiable test, not in an outline to look at passively.

Session map

Before you code — Make a prediction

In M00 the height was 0.10 m. Imagine that we duplicate it:

beam_h=0.20

Will the deflection be one-half, one-quarter, or one-eighth?

For a rectangular section:

I = B·H³/12
uy = P·L³/(3·E·I)

Since I ∝ H³ and uy ∝ 1/I, we obtain uy ∝ 1/H³. Doubling the height reduces the theoretical deflection to 1/8. Save this prediction: will be our first test.

Mental model — The script contract

We will organize the program as if it were an engineering function:

This separates four categories that should not be confused:

An analytical reference is not “the correct result” by decree. It's an independent comparison, based on different hypotheses, which helps detect unit errors, loads or stiffness.

Step 1 — Find the magic numbers

Open 01_start.mac. You'll find expressions like:

MP,EX,1,210E9
BLOCK,0,1.0,0,0.10,0,0.05
ESIZE,0.025
F,ALL,FY,-1000/n_tip

The values are not incorrect, but their meaning is hidden and they appear within the instructions. Changing the length requires finding each occurrence of 1.0 and deciding whether it represents length, coordinate or anything else.

Gather the entries at the beginning:

! --- INPUTS: SI units ---
beam_l=1.0
beam_h=0.10
beam_b=0.05
young=210E9
nu=0.30
tip_force=-1000
mesh_h=0.025

Then replace the physical numbers:

MP,EX,1,young
MP,PRXY,1,nu
BLOCK,0,beam_l,0,beam_h,0,beam_b
ESIZE,mesh_h
NSEL,S,LOC,X,beam_l
F,ALL,FY,tip_force/n_tip

Step 2 — Build verifiable expressions

Calculate the inertia first:

inertia=beam_b*beam_h**3/12

In APDL, ** represents power. beam_h*3 is not an alternative form: is another operation.

Add references:

uy_ref=tip_force*beam_l**3/(3*young*inertia)
uy_ref_abs=ABS(uy_ref)
sigma_ref=ABS(tip_force)*beam_l*(beam_h/2)/inertia

For the base case, approximately:

Step 3 — Reject invalid inputs

Wait until VMESH to discover that a dimension is zero produces messages far from the cause. It is better to check the contract immediately:

*IF,beam_h,LE,0,THEN
  /COM,ERROR: beam_h must be positive
  /EOF
*ENDIF

*IF,mesh_h,GT,beam_h/2,THEN
  /COM,WARNING: mesh_h is larger than beam_h/2
*ENDIF

The first block represents an error that prevents you from continuing. The second is a warning: the model can be executed, but cross discretization deserves attention.

Controlled Experiment

1. Temporarily change beam_h practically zero.
2. Run the script.
3. Check that no geometry is created.
4. Restaura. beam_h=0.10.

Don't celebrate that MAPDL “failed correctly.” Celebrate that your program spotted the issue before handing over absurd data to the modeler.

Step 4 — Store data in arrays

We want to test four heights without creating four independent parameters:

*DIM,heights,ARRAY,4
heights(1)=0.08
heights(2)=0.10
heights(3)=0.12
heights(4)=0.14

*DIM reserve an array. Each position contains an alternative. The array describes data; still doesn't indicate what to do with them.

Step 5 — Iterate through alternatives with *DO

*DO,i,1,4
  trial_h=heights(i)
  trial_i=beam_b*trial_h**3/12
  trial_uy=ABS(tip_force)*beam_l**3/(3*young*trial_i)
  trial_stress=ABS(tip_force)*beam_l*(trial_h/2)/trial_i
*ENDDO

The Index i successively takes the values 1, 2, 3 and 4. On every lap, trial_h represents a different height. Analytical calculation costs practically nothing; that's why it's a good place to learn loops. Running a full simulation on each lap will arrive at M09.

Step 6 — Convert limits to a decision

passes=0
*IF,trial_uy,LE,uy_limit,THEN
  *IF,trial_stress,LE,stress_limit,THEN
    passes=1
    *IF,selected_h,EQ,0,THEN
      selected_h=trial_h
    *ENDIF
  *ENDIF
*ENDIF

selected_h starts at zero. It's only updated for the first valid alternative, so at the end it contains the lowest admissible height of an ordered array.

The first valid candidate is 120 mm.

Step 7 — Write reusable evidence

A table is more useful than values buried in the output:

*CFOPEN,m01_design_table,csv
*VWRITE
('height_m,uy_ref_m,sigma_ref_pa,passes')

! Inside the loop:
*VWRITE,trial_h,trial_uy,trial_stress,passes
(E16.8,',',E16.8,',',E16.8,',',F2.0)

*CFCLOS

When executing the deliverable, it will appear m01_design_table.csv on the Working Directory. Open it in a text editor before taking it to a spreadsheet: you must be able to understand its structure without relying on another application.

Bug hunt

Download 01_bug_hunt.mac. Don't run it yet. Find five flaws:

1. A height expressed in incompatible units.
2. An impossible mesh size.
3. A power written as multiplication.
4. An incorrectly signed stress magnitude to compare with a limit.
5. A geometry that ignores the parameter block.

For each defect write: probable symptom, cause and minimal correction.

Verifiable challenge

Open 01_challenge.mac and complete ALL four of THEM.

1. Add a fifth height of your choice.
2. Walk through the five alternatives without duplicating the expressions.
3. Identify the first alternative that satisfies both deflection and stress limits.
4. Generate a CSV with inputs, references, and status.

Self-assessment

Evidence of learning

• 01_parametric_beam.mac executed from a clean session.
• m01_design_table.csv with four alternatives.
• Archive the challenge with a fifth alternative.
• Table "defect → symptom → causes → correction" of the bug hunt.
• Prediction and explanation of the 1/8 ratio obtained by doubling the height.

Validation checklist

• I gathered all the physical entries into a documented block.
• I eliminated the magic numbers of geometry, material, mesh, and charge.
• I distinguish entries, derivatives, references and results FEA.
• The script rejects non-positive dimensions and mesh.
• The CSV matches the expected results within tolerance.
• The first valid height of the base assembly is 120 mm.
• I completed the bug hunt and challenge without using the GUI.