# 2025-07-17 — Technical Lead Log

## OP_09: NXOpenSolver Implementation for Hydrotech Beam

**Status:** COMPLETED — committed as `390ffed`

### What was done
Completed `NXOpenSolver.evaluate()` and `close()` in `projects/hydrotech-beam/studies/01_doe_landscape/nx_interface.py`.

### Implementation pattern
Follows M1_Mirror/SAT3_Trajectory_V7 FEARunner pattern exactly:
1. `create_iteration_folder()` — HEEDS-style isolation, copies model + writes params.exp
2. `run_simulation()` — journal handles expression import, geometry rebuild, FEM update, solve
3. Extractor pipeline: displacement → stress → mass

### Key technical decisions

1. **Element type auto-detection**: Used `element_type=None` for stress extraction instead of hardcoding CQUAD4. The extractor checks all solid types (CTETRA, CHEXA, CPENTA, CPYRAM) then falls back to shell if needed. This is safer since we haven't confirmed the beam's mesh type.

2. **Mass extraction fallback chain**: `_temp_mass.txt` → `_temp_part_properties.json`. The solve journal writes both files. The fallback handles edge cases where the primary file is missing.

3. **OP2 naming convention**: `beam_sim1-solution_1.op2` (lowercase sim stem + lowercase solution name with spaces→underscores). Verified from `NXSolver.run_simulation()` logic.

4. **Expression unit mapping**: All 4 DVs default to `mm` in `_write_expression_file()` which is correct for beam_half_core_thickness, beam_face_thickness, holes_diameter. `hole_count` is unitless but the NX journal's ImportFromFile handles this correctly.

5. **LAC compliance**: `close()` only calls `session_manager.cleanup_stale_locks()`, never kills NX processes. This follows the LAC critical lesson.

### Files changed
- `projects/hydrotech-beam/studies/01_doe_landscape/nx_interface.py` — +254/-149 lines

### Testing
- Syntax validation: ✓
- Stub solver smoke test: ✓
- Factory pattern test: ✓
- All required methods present: ✓
- Real NX testing requires Windows/dalidou — not possible from this sandbox