Differentiation Schemes

Goal

Provide a reliable workflow to select a differentiation scheme, generate stencils, and assess accuracy for simulation discretization.

Requirements

• Python 3.8+
• NumPy (for stencil computations)
• No heavy dependencies

Inputs to Gather

Decision Guidance

Scheme Selection Flowchart

Is the field smooth?
├── YES → Is domain periodic?
│   ├── YES → Use central differences or spectral
│   └── NO → Use central interior + one-sided at boundaries
└── NO → Are there shocks/discontinuities?
    ├── YES → Use upwind, TVD, or WENO
    └── NO → Use central with limiters

Quick Reference

Script Outputs (JSON Fields)

Workflow

1. Identify requirements - derivative order, accuracy, smoothness
2. Select scheme - Run scripts/scheme_selector.py
3. Generate stencils - Run scripts/stencil_generator.py
4. Estimate error - Run scripts/truncation_error.py
5. Validate - Test with manufactured solutions or grid refinement

Conversational Workflow Example

User: I need to discretize a second derivative for a diffusion equation on a uniform grid. I want 4th-order accuracy.

Agent workflow:

1. Select appropriate scheme:
   bashCopy

   python3 scripts/scheme_selector.py --smooth --periodic --order 2 --accuracy 4 --json
   

2. Generate the stencil:
   bashCopy

   python3 scripts/stencil_generator.py --order 2 --accuracy 4 --scheme central --json
   

3. Result: 5-point stencil with coefficients [-1/12, 4/3, -5/2, 4/3, -1/12] / dx².

Pre-Discretization Checklist

• Confirm derivative order and target accuracy
• Choose scheme appropriate to smoothness and boundaries
• Generate and inspect stencils at boundaries
• Estimate truncation error vs physics scales
• Verify with grid refinement study

CLI Examples

# Select scheme for smooth periodic problem
python3 scripts/scheme_selector.py --smooth --periodic --order 1 --accuracy 4 --json

# Generate central difference stencil for first derivative
python3 scripts/stencil_generator.py --order 1 --accuracy 2 --scheme central --json

# Generate 4th-order second derivative stencil
python3 scripts/stencil_generator.py --order 2 --accuracy 4 --scheme central --json

# Estimate truncation error
python3 scripts/truncation_error.py --dx 0.01 --order 2 --accuracy 2 --scale 1.0 --json

Error Handling

Interpretation Guidance

Stencil Properties

Truncation Error Scaling

Common Stencils

Security

Input Validation

• --order (derivative order) is validated as a positive integer with an upper bound
• --accuracy is validated as a positive even integer for central schemes
• --scheme is validated against a fixed allowlist (central, upwind, compact)
• --dx and --scale are validated as finite positive numbers
• No user-supplied strings are interpolated into code paths or shell commands

File Access

• Scripts read no external files; all inputs are provided via CLI arguments
• Scripts write only to stdout (JSON output); no files are created unless the agent explicitly uses the Write tool

Tool Restrictions

• Read: Used to inspect script source, references, and user configuration files
• Bash: Used to execute the three Python scripts (stencil_generator.py, scheme_selector.py, truncation_error.py) with explicit argument lists
• Write: Used to save generated stencil coefficients or scheme recommendations; writes are scoped to the user's working directory
• Grep/Glob: Used to locate relevant files and search references

Safety Measures

• No eval(), exec(), or dynamic code generation
• All subprocess calls use explicit argument lists (no shell=True)
• Stencil computation uses only NumPy linear algebra on small, bounded matrices (stencil width limited by accuracy order)
• All output is deterministic JSON with no shell-interpretable content

Limitations

• Boundary handling: Stencil generator provides interior stencils; boundaries need special treatment
• Nonuniform grids: Standard stencils assume uniform spacing
• Spectral: Not covered by stencil generator

References

• references/stencil_catalog.md - Common stencils
• references/boundary_handling.md - One-sided schemes
• references/scheme_selection.md - FD/FV/spectral comparison
• references/error_guidance.md - Truncation error scaling

Version History

• v1.1.0 (2024-12-24): Enhanced documentation, decision guidance, examples
• v1.0.0: Initial release with 3 differentiation scripts