Atomizer/atomizer-field/UNIT_CONVERSION_REPORT.md

Unit Conversion Issue - Analysis and Fix

Date: November 24, 2025 Issue: Stresses displaying 1000× too large

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Root Cause Identified

BDF File Unit System

The BDF file contains: PARAM UNITSYS MN-MM

This defines the Nastran unit system as:

• Length: mm (millimeter)
• Force: MN (MegaNewton) = 1,000,000 N
• Mass: tonne (1000 kg)
• Stress: Pa (Pascal) = N/m² [NOT MPa!]
• Energy: MN-mm = 1,000 N-m = 1 kJ

Material Properties Confirm This

Young's modulus from BDF: E = 200,000,000

• If units were MPa: E = 200 GPa (way too high for steel ~200 GPa)
• If units are Pa: E = 200 MPa (way too low!)
• Actual: E = 200,000,000 Pa = 200 GPa ✓ (correct for steel)

What pyNastran Returns

pyNastran reads the OP2 file and returns data in the same units as the BDF:

• Displacement: mm ✓
• Force/Reactions: MN (not N!)
• Stress: Pa (not MPa!)

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Current vs Actual Values

Stress Values

What we claimed	Actual value	Correct interpretation
117,000 MPa	117,000 Pa	117 kPa = 0.117 MPa ✓
46,000 MPa (mean)	46,000 Pa	46 kPa = 0.046 MPa ✓

Correct stress values are 1000× smaller!

Force Values

What we claimed	Actual value	Correct interpretation
2.73 MN (applied)	2.73 MN	2.73 MN = 2,730,000 N ✓
150 MN (reaction)	150 MN	150 MN = 150,000,000 N ✓

Force values are correctly stored, but labeled as N instead of MN

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Impact

What's Wrong:

1. Stress units incorrectly labeled as "MPa" - should be "Pa"
2. Force/reaction units incorrectly labeled as "N" - should be "MN"
3. Visualization shows stress 1000× too high
4. Reports show unrealistic values (117 GPa stress would destroy steel!)

What's Correct:

1. ✅ Displacement values (19.5 mm)
2. ✅ Material properties (E = 200 GPa)
3. ✅ Geometry (mm)
4. ✅ Actual numerical values from pyNastran

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Solution

Option 1: Convert to Standard Units (Recommended)

Convert all data to consistent engineering units:

• Length: mm → mm ✓
• Force: MN → N (divide by 1e6)
• Stress: Pa → MPa (divide by 1e6)
• Mass: tonne → kg (multiply by 1000)

Benefits:

• Standard engineering units (mm, N, MPa, kg)
• Matches what users expect
• No confusion in reports/visualization

Changes Required:

• Parser: Convert forces (divide by 1e6)
• Parser: Convert stress (divide by 1e6)
• Update metadata to reflect actual units

Option 2: Use Native Units (Not Recommended)

Keep MN-MM-tonne-Pa system throughout

Issues:

• Non-standard units confuse users
• Harder to interpret values
• Requires careful labeling everywhere

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Implementation Plan

1. Fix Parser (neural_field_parser.py)

Lines to modify:

Stress Extraction (~line 602-648)

# CURRENT (wrong):
stress_data = stress.data[0, :, :]
stress_results[f"{elem_type}_stress"] = {
    "data": stress_data.tolist(),
    "units": "MPa"  # WRONG!
}

# FIX:
stress_data = stress.data[0, :, :] / 1e6  # Convert Pa → MPa
stress_results[f"{elem_type}_stress"] = {
    "data": stress_data.tolist(),
    "units": "MPa"  # Now correct!
}

Force Extraction (~line 464-507)

# CURRENT (partially wrong):
"magnitude": float(load.mag),  # This is in MN, not N!

# FIX:
"magnitude": float(load.mag) * 1e6,  # Convert MN → N

Reaction Forces (~line 538-568)

# CURRENT (wrong):
reactions = grid_point_force.data[0]  # In MN!

# FIX:
reactions = grid_point_force.data[0] * 1e6  # Convert MN → N

2. Update Unit Detection

Add UNITSYS parameter detection:

def detect_units(self):
    """Detect Nastran unit system from PARAM cards"""
    if hasattr(self.bdf, 'params') and 'UNITSYS' in self.bdf.params:
        unitsys = str(self.bdf.params['UNITSYS'].values[0])
        if 'MN' in unitsys:
            return {
                'length': 'mm',
                'force': 'MN',
                'stress': 'Pa',
                'mass': 'tonne',
                'needs_conversion': True
            }
    # Default units
    return {
        'length': 'mm',
        'force': 'N',
        'stress': 'MPa',
        'mass': 'kg',
        'needs_conversion': False
    }

3. Add Unit Conversion Function

def convert_to_standard_units(self, data, unit_system):
    """Convert from Nastran units to standard engineering units"""
    if not unit_system['needs_conversion']:
        return data

    # Convert forces: MN → N (multiply by 1e6)
    if 'loads' in data:
        for force in data['loads']['point_forces']:
            force['magnitude'] *= 1e6

    # Convert stress: Pa → MPa (divide by 1e6)
    if 'results' in data and 'stress' in data['results']:
        for stress_type, stress_data in data['results']['stress'].items():
            if isinstance(stress_data, dict) and 'data' in stress_data:
                stress_data['data'] = np.array(stress_data['data']) / 1e6
                stress_data['units'] = 'MPa'

    # Convert reactions: MN → N (multiply by 1e6)
    # (Handle in HDF5 write)

    return data

4. Update HDF5 Writing

Apply conversions when writing to HDF5:

# Reactions
if 'reactions' in self.neural_field_data['results']:
    reactions_data = np.array(self.neural_field_data['results']['reactions']['data'])
    if unit_system['force'] == 'MN':
        reactions_data *= 1e6  # MN → N
    hf.create_dataset('results/reactions', data=reactions_data)

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Testing Plan

1. Create Unit Conversion Test

def test_unit_conversion():
    """Verify units are correctly converted"""
    parser = NastranToNeuralFieldParser('test_case_beam')
    data = parser.parse_all()

    # Check stress units
    stress = data['results']['stress']['cquad4_stress']
    assert stress['units'] == 'MPa'
    max_stress = np.max(stress['data'][:, -1])  # Von Mises
    assert max_stress < 500, f"Stress {max_stress} MPa too high!"

    # Check force units
    force = data['loads']['point_forces'][0]
    assert force['magnitude'] < 1e7, "Force should be in N"

    print("[OK] Units correctly converted")

2. Expected Values After Fix

Property	Before (wrong)	After (correct)
Max stress	117,000 MPa	117 MPa ✓
Mean stress	46,000 MPa	46 MPa ✓
Applied force	2.73 MN	2,730,000 N
Max reaction	150 MN	150,000,000 N

3. Validation Checks

• ✓ Stress < 500 MPa (reasonable for steel)
• ✓ Force magnitude matches applied loads
• ✓ Material E = 200 GPa (correct for steel)
• ✓ Displacement still 19.5 mm

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Risk Assessment

Low Risk:

• ✅ Only affects numerical scaling
• ✅ No changes to data structure
• ✅ Easy to verify with test
• ✅ Can be fixed with multiplication/division

What Could Go Wrong:

• ⚠ Other BDF files might use different UNITSYS
• ⚠ Some files might already be in correct units
• ⚠ Need to handle multiple unit systems

Mitigation:

• Always check PARAM UNITSYS first
• Add unit system detection
• Log conversions clearly
• Add validation checks

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Recommendations

Immediate Actions:

1. ✅ Update parser to detect UNITSYS
2. ✅ Add unit conversion for stress (Pa → MPa)
3. ✅ Add unit conversion for forces (MN → N)
4. ✅ Update metadata to reflect conversions
5. ✅ Add validation checks

Long-term:

• Support multiple Nastran unit systems
• Add unit conversion utilities
• Document unit assumptions clearly
• Add warnings for unusual values

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Conclusion

The system is working correctly - pyNastran is reading the data accurately.

The issue is labeling - we incorrectly assumed MPa when Nastran uses Pa.

The fix is simple - divide stress by 1e6, multiply forces by 1e6, update labels.

After fix:

• Stress: 117 MPa (reasonable for steel) ✓
• Force: 2.73 MN = 2,730 kN (reasonable for large beam) ✓
• All other values unchanged ✓

System will be production-ready after this fix! 🚀

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Unit Conversion Analysis v1.0 Issue: 1000× stress error Root cause: MN-MM unit system misinterpretation Fix: Scale factors + label corrections