How to Prevent Common CAE Errors for Beginners
Overview of Beginner Errors
Teacher, please help me... I've redone the analysis 3 times, and I get different errors every time. I'm about to give up.
All CAE engineers have walked the same path. I fought errors every day during my first month. Common beginner errors actually follow patterns, and knowing them in advance will cut your analysis time in half.
| Error Category | Frequency | Severity | Detection Difficulty |
|---|---|---|---|
| Unit System Mismatch | Very High | Critical | Easy to Miss |
| Mesh Quality | High | High | Detectable by Tools |
| Boundary Condition Errors | High | Critical | Requires Physical Intuition |
| Convergence Failure | Medium | Medium | Detected by Residual Monitor |
| Misinterpretation of Results | Medium | High | Requires Experience |
Error 1: Unit System Mismatch — The Most Frequent and Most Critical
So, let me start with the most common error.
By far, it's mixing unit systems. For example, if you use a model drawn in mm in CAD but input Young's modulus in SI units (m-based), the stress will be off by a million times. You expect 1 mm of displacement but get 1000 m — that's when you go "???"
| Unit System | Length | Mass | Force | Stress | Common Software |
|---|---|---|---|---|---|
| SI | m | kg | N | Pa | OpenFOAM |
| mm-ton-s | mm | tonne | N | MPa | Abaqus, LS-DYNA |
| mm-kg-ms | mm | kg | kN | GPa | Ansys (Caution!) |
I think I've done that before... How do I prevent it?
Follow three rules:
- Document the unit system at project start
- After inputting material properties, always perform a dimensional check (Steel Young's modulus = 206 GPa = 206,000 MPa = 2.06×10¹¹ Pa)
- When viewing results, verify with common sense: "Is this displacement physically reasonable?"
Error 2: Mesh Quality Issues
What's the next most common error?
Mesh quality. Too coarse and you lose accuracy; too fine and the calculation won't finish in days. Worse, if elements are distorted and flattened, the calculation blows up. Imagine a triangle crushed flat — there's no way to get accurate answers with such elements.
Mesh quality indicators to check:
| Indicator | Ideal Value | Acceptable Limit | Problem Value |
|---|---|---|---|
| Aspect Ratio | 1.0–3.0 | < 10 | > 20 → Accuracy Loss |
| Skewness | 0–0.25 | < 0.75 | > 0.95 → Divergence Risk |
| Jacobian | > 0.5 | > 0.0 | < 0 → Inverted Elements (Critical) |
How do I judge mesh quality? Can I tell by looking at it?
Most software has a quality check function. Use it to verify the values in the table above. Another critical task is mesh convergence verification. Progressively refine the mesh from coarse to fine, and confirm results stabilize. Many beginners skip this, but one extra hour of work prevents three days of recalculation.
Error 3: Boundary Condition Setup Mistakes
I've heard boundary condition errors are scary...
This is the worst kind. The calculation completes successfully, but the results are wrong. No error messages pop up, so it's hard to notice. For example, if you fix both ends of a cantilever beam instead of just one, displacement values will be completely different, but the contour plots look fine.
Common mistakes:
- Over-constraint: Fixing more DOF than necessary → unnaturally small deformation
- Under-constraint: Allowing rigid body motion → singular matrix, calculation failure
- Wrong load direction: Confusion of coordinate systems (global/local)
- Misused symmetry: Using symmetric model with asymmetric loading
- CFD inlet/outlet: Specifying velocity at outlet (over-constraint), setting pressure = 0 at inlet
Is there a way to verify boundary conditions are correct before results are computed? It's painful to discover mistakes after the solution is done.
The most reliable method is reaction force checking. If you apply 100 N, the sum of constraint reaction forces must equal 100 N. If not, something is wrong. Also, visualize the deformation as an animation. You'll immediately spot if it moves in the opposite direction.
Error 4: Convergence Failure — "Calculation Won't End"
What about the "non-convergence" problem mentioned in the workflow section?
Memorize failure patterns to respond quickly. Beginners should suspect these five issues first:
| Root Cause | Symptom | Solution |
|---|---|---|
| Mesh Quality | Diverges early in calculation | Check quality indicators, remesh problem areas |
| Material Properties | Diverges with non-physical values | Re-check unit system, verify property magnitudes |
| Load Steps | Non-linear analysis doesn't converge | Divide load into stages, increase substeps |
| Contact Settings | Penetration | Adjust contact stiffness, verify initial gap |
| Time Steps | CFD oscillation/divergence | Set Courant number ≤ 0.5, reduce relaxation |
Error 5: Misinterpretation of Results
If the calculation finishes without errors, the results must be trustworthy, right?
That's the final trap. "Calculation completed" and "answer is correct" are completely different things. It's common to see "clean contour plots but stress values that are physically impossible." Always check these:
- Deformation shape: Does deformation direction and magnitude match intuition?
- Stress singularities: High stresses at constraint nodes are artificial (ignore them)
- Energy balance: External work ≈ internal strain energy?
- Comparison to theory: Does simplified problem match theoretical solution?
Pre-Analysis Checklist
10-Item Checklist Before Starting Analysis
- Have you decided and documented the unit system?
- Have you appropriately simplified the CAD model?
- Have you verified all mesh quality indicators?
- Are material property values and units correct?
- Are boundary conditions physically reasonable?
- Are constraints complete without excess?
- Are load direction, magnitude, and coordinate system correct?
- Is the convergence criterion appropriate?
- Have you prepared comparison targets (theory or experiments)?
- Do you have a plan for mesh convergence verification?
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