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Full Neural Operators Demo

Metadata Value
Level Advanced
Runtime ~10 min (CPU/GPU)
Prerequisites JAX, Flax NNX, Neural Operators
Format Python + Jupyter

Overview

This example demonstrates every neural operator variant available in the Opifex framework, from parameter-efficient Tensorized FNOs to geometry-aware and uncertainty-quantifying architectures. It walks through the operator factory system, compares parameter counts across FNO variants, runs domain-specific forward passes (turbulence, climate, molecular dynamics, airfoil geometry), and builds a multi-operator ensemble with agreement scoring.

Unlike single-model tutorials, this demo is designed as a full tour of the opifex.neural.operators module family. Each section creates a different operator, feeds it synthetic domain-appropriate data, and reports timing and output statistics. The demo is console-only (no visualization files) and produces benchmark numbers you can use to guide operator selection for your own problems.

What You'll Learn

  1. Use the operator factory to create any neural operator with create_operator() and get domain recommendations with recommend_operator()
  2. Compare parameter efficiency across Standard FNO, Tucker TFNO, CP TFNO, and U-FNO
  3. Run U-FNO for multi-scale turbulent flow simulation with 4 encoder-decoder levels
  4. Run SFNO for global climate modeling with spherical harmonics and power spectrum analysis
  5. Quantify uncertainty with UQNO, decomposing predictions into epistemic and aleatoric components
  6. Model complex geometry with GINO using geometry attention on airfoil-like domains
  7. Predict molecular forces with MGNO using multipole graph interactions
  8. Build a multi-operator ensemble from FNO, TFNO, and LocalFNO with agreement scoring

Coming from NeuralOperator (PyTorch)?

If you are familiar with the neuraloperator library, here is how Opifex compares for this workflow:

NeuralOperator (PyTorch) Opifex (JAX)
TFNO(n_modes, hidden_channels, factorization) TensorizedFourierNeuralOperator(modes=, hidden_channels=, factorization=, rank=, rngs=)
FNO(n_modes, hidden_channels) FourierNeuralOperator(modes=, hidden_channels=, num_layers=, rngs=)
UNO(...) with manual U-Net wiring UFourierNeuralOperator(modes=, hidden_channels=, num_levels=, rngs=)
No built-in operator factory create_operator("SFNO", ...) and recommend_operator("global_climate")
Manual ensemble loop Same pattern, but with JAX JIT for each operator
torch.optim.Adam(model.parameters(), lr) optax.adam(lr) (handled internally by Trainer)

Key differences:

  1. Explicit PRNG: Opifex uses JAX's explicit rngs=nnx.Rngs(42) instead of global random state
  2. Factory system: create_operator() and recommend_operator() provide guided operator selection not available in the PyTorch library
  3. XLA compilation: All forward passes are JIT-compiled automatically for hardware acceleration
  4. Functional transforms: jax.grad, jax.vmap, jax.pmap compose cleanly with every operator variant

Files

Quick Start

Run the Python Script

source activate.sh && python examples/neural-operators/operator_tour.py

Run the Jupyter Notebook

jupyter lab examples/neural-operators/operator_tour.ipynb

Core Concepts

The Neural Operator Family

Opifex provides a unified framework for eight neural operator architectures, each designed for a specific class of problems. All operators share the same interface pattern -- (in_channels, out_channels, hidden_channels, rngs) -- and can be created through the factory system or instantiated directly.

Operator Full Name Best For
TFNO Tensorized Fourier Neural Operator Parameter-efficient modeling; memory-constrained settings
U-FNO U-Net Fourier Neural Operator Multi-scale turbulent flow; problems with features at multiple resolutions
SFNO Spherical Fourier Neural Operator Global climate modeling; data on spherical domains (lat/lon grids)
GINO Geometry-Informed Neural Operator Complex geometries (airfoils, CAD shapes); irregular domains
MGNO Multipole Graph Neural Operator Molecular dynamics; particle systems with long-range interactions
UQNO Uncertainty Quantification Neural Operator Safety-critical applications; Bayesian uncertainty decomposition
LocalFNO Local Fourier Neural Operator Wave propagation; problems needing both local and global operations
AM-FNO Amortized Fourier Neural Operator High-frequency problems; neural kernel networks

Operator Factory and Recommendation System

The factory system provides two entry points:

  • list_operators() -- returns all available operators grouped by category
  • recommend_operator(application) -- suggests the best operator for a given application domain
  • create_operator(name, **kwargs) -- instantiates any operator by name with the given configuration

This allows you to select operators programmatically rather than hard-coding architecture choices.

Implementation

Step 1: Imports and Setup

import time
from typing import Any

import jax
import jax.numpy as jnp
from flax import nnx

# Import all neural operators
from opifex.neural.operators import (
    AmortizedFourierNeuralOperator,
    create_operator,
    FourierNeuralOperator,
    GeometryInformedNeuralOperator,
    list_operators,
    LocalFourierNeuralOperator,
    MultipoleGraphNeuralOperator,
    recommend_operator,
    SphericalFourierNeuralOperator,
    TensorizedFourierNeuralOperator,
    UFourierNeuralOperator,
    UncertaintyQuantificationNeuralOperator,
)

Terminal Output:

Opifex Neural Operators Full Demo
============================================================
Starting Full Neural Operators Demo
Estimated time: ~3-5 minutes

Step 2: Operator Factory Demo

The factory system lists all available operators by category and provides application-specific recommendations. This is the recommended way to discover which operator to use for your problem.

# Show available operators
categories = list_operators()
for category, operators in categories.items():
    print(f"  {category}: {', '.join(operators)}")

# Get recommendations for specific applications
applications = [
    "turbulent_flow", "global_climate", "molecular_dynamics",
    "cad_geometry", "safety_critical", "parameter_efficient",
]
for app in applications:
    rec = recommend_operator(app)
    print(f"  {app}: {rec['primary']} - {rec['reason']}")

# Create operators using factory
tfno = create_operator(
    "TFNO",
    in_channels=3, out_channels=1, hidden_channels=64,
    modes=(16, 16), factorization="tucker", rank=0.1, rngs=rngs,
)

uqno = create_operator(
    "UQNO",
    in_channels=2, out_channels=1, hidden_channels=32,
    modes=(8, 8), use_aleatoric=True, rngs=rngs,
)

Terminal Output:

============================================================
NEURAL OPERATOR FACTORY DEMO
============================================================

Available Operators:
  fourier_operators: FNO, TFNO, UFNO, SFNO, LocalFNO, AM-FNO
  deeponet_family: DeepONet, FourierDeepONet, AdaptiveDeepONet
  graph_operators: GNO, MGNO
  uncertainty_aware: UQNO
  geometry_aware: GINO, GNO, MGNO
  parameter_efficient: TFNO, LNO

Application Recommendations:
  turbulent_flow: UFNO - Multi-scale encoder-decoder for turbulent structures
  global_climate: SFNO - Spherical harmonics for global atmospheric modeling
  molecular_dynamics: MGNO - Multipole expansion for long-range molecular interactions
  cad_geometry: GINO - Geometry-aware processing for complex CAD shapes
  safety_critical: UQNO - Uncertainty quantification for safety-critical decisions
  parameter_efficient: TFNO - Tensor factorization for memory efficiency

Creating Operators with Factory:
  TFNO created: TensorizedFourierNeuralOperator
  UQNO created: UncertaintyQuantificationNeuralOperator

Step 3: Parameter Efficiency Comparison

Compare parameter counts across FNO variants to understand the memory trade-offs between standard and tensorized architectures.

operators = {}

# Standard FNO (1D modes)
operators["Standard FNO"] = FourierNeuralOperator(
    in_channels=3, out_channels=1, hidden_channels=64,
    modes=16, num_layers=4, rngs=rngs,
)

# Tucker TFNO (2D modes)
operators["Tucker TFNO (10%)"] = TensorizedFourierNeuralOperator(
    in_channels=3, out_channels=1, hidden_channels=64,
    modes=(16, 16), num_layers=4, factorization="tucker", rank=0.1, rngs=rngs,
)

# CP TFNO (2D modes)
operators["CP TFNO"] = TensorizedFourierNeuralOperator(
    in_channels=3, out_channels=1, hidden_channels=64,
    modes=(16, 16), num_layers=4, factorization="cp", rank=16.0, rngs=rngs,
)

# U-FNO (2D modes)
operators["U-FNO (3 levels)"] = UFourierNeuralOperator(
    in_channels=3, out_channels=1, hidden_channels=64,
    modes=(16, 16), num_levels=3, rngs=rngs,
)

# Count parameters
for name, op in operators.items():
    count = sum(
        p.size for p in jax.tree_util.tree_leaves(nnx.state(op))
        if hasattr(p, "size")
    )

Terminal Output:

============================================================
PARAMETER EFFICIENCY COMPARISON
============================================================

Parameter Counts:
  Standard FNO        :  279,105 params (compression: 1.0x)
  Tucker TFNO (10%)   : 4,194,625 params (compression: 0.1x)
  CP TFNO             : 4,194,625 params (compression: 0.1x)
  U-FNO (3 levels)    : 27,977,601 params (compression: 0.0x)

Parameter Counts in This Demo

The TFNO and U-FNO variants show larger parameter counts than the baseline Standard FNO because the Standard FNO uses scalar (1D) modes while the TFNO and U-FNO use 2D mode tuples (16, 16), which increases the spectral weight tensor dimensions. In matched configurations (same spatial dimensionality), TFNO with Tucker factorization typically achieves 90%+ parameter reduction.

Step 4: Multi-Scale Turbulence with U-FNO

U-FNO uses a U-Net-style encoder-decoder with multiple resolution levels, making it ideal for turbulent flow where features span many spatial scales.

# Create U-FNO for turbulence (u, v, pressure)
ufno = UFourierNeuralOperator(
    in_channels=3, out_channels=3, hidden_channels=64,
    modes=(32, 32), num_levels=4, rngs=rngs,
)

# Generate synthetic turbulent flow data (batch=4, channels=3, 64x64)
flows = jnp.stack([create_turbulent_flow(key, size=64) for key in keys])

# Forward pass
predictions = ufno(flows)

Terminal Output:

============================================================
MULTI-SCALE TURBULENCE WITH U-FNO
============================================================
Generating turbulent flow data...
U-FNO created with 4 levels
Running U-FNO forward pass...
Forward pass: (4, 3, 64, 64) -> (4, 3, 64, 64)
Time: 6826.56ms
Multi-scale U-FNO output analysis:
  Input resolution: (64, 64) spatial
  Output resolution: (64, 64) spatial
  Multi-scale levels: 4

Step 5: Global Climate Modeling with SFNO

SFNO uses spherical harmonics instead of standard Fourier modes, preserving the geometry of the sphere for global atmospheric data on latitude-longitude grids.

# Create SFNO for climate (T, P, humidity, u_wind, v_wind)
sfno = SphericalFourierNeuralOperator(
    in_channels=5, out_channels=5, hidden_channels=128,
    lmax=16, num_layers=6, rngs=rngs,
)

# Generate synthetic global climate data (batch=2, channels=5, 32 lat x 64 lon)
climate_data = jnp.stack(
    [create_climate_data(key, nlat=32, nlon=64) for key in keys]
)

# Forward pass and spectrum analysis
climate_prediction = sfno(climate_data)
spectrum = sfno.compute_power_spectrum(climate_data[:1])

Terminal Output:

============================================================
GLOBAL CLIMATE MODELING WITH SFNO
============================================================
Generating global climate data...
SFNO created with lmax=16
Running SFNO forward pass...
Forward pass: (2, 5, 32, 64) -> (2, 5, 32, 64)
Time: 826.79ms
Spherical harmonic spectrum: (1, 128, 17)

Step 6: Uncertainty Quantification with UQNO

UQNO provides Bayesian inference with decomposed uncertainty estimates. It separates epistemic uncertainty (model knowledge gaps) from aleatoric uncertainty (inherent data noise).

# Create UQNO with aleatoric uncertainty
uqno = UncertaintyQuantificationNeuralOperator(
    in_channels=2, out_channels=1, hidden_channels=64,
    modes=(16, 16), num_layers=4, use_aleatoric=True, rngs=rngs,
)

# Get uncertainty predictions with 50 Monte Carlo samples
x = jax.random.normal(rng_key, (2, 32, 32, 2))
uncertainty_results = uqno.predict_with_uncertainty(
    x, num_samples=50, key=rng_key
)

mean_pred = uncertainty_results["mean"]
epistemic_std = uncertainty_results["epistemic_uncertainty"]
total_std = uncertainty_results["total_uncertainty"]
aleatoric_std = uncertainty_results["aleatoric_uncertainty"]

Terminal Output:

============================================================
UNCERTAINTY QUANTIFICATION WITH UQNO
============================================================
Generating uncertain data...
UQNO created with Bayesian inference
Computing uncertainty estimates...
Uncertainty prediction complete
Time: 3289.85ms
Mean prediction: (2, 32, 32, 1)
Epistemic uncertainty: 0.0000 +/- 0.0000
Total uncertainty: 0.6311 +/- 0.0000
Epistemic uncertainty ratio: 0.000
Aleatoric uncertainty ratio: 1.000

Uncertainty Decomposition

In this demo the epistemic uncertainty is near zero because the model has not been trained -- all weight samples produce the same output. The aleatoric ratio of 1.000 indicates that all measured uncertainty comes from the learned noise model. After training on real data, the epistemic component will reflect genuine model uncertainty about regions with insufficient training coverage.

Step 7: Geometry-Aware Modeling with GINO

GINO integrates geometry information through latent attention, enabling it to handle irregular domains like airfoils, turbine blades, and CAD shapes.

# Create GINO with geometry attention
gino = GeometryInformedNeuralOperator(
    in_channels=2, out_channels=2, hidden_channels=64,
    modes=(12, 12), coord_dim=2, geometry_dim=48,
    num_layers=4, use_geometry_attention=True, rngs=rngs,
)

# Forward pass with geometry coordinates
# coords_reshaped: (batch, height*width, 2)
geometry_prediction = gino(flows, geometry_data={"coords": coords_reshaped})

# Test geometry invariance by scaling coordinates
coords_rotated = coords * 1.5
prediction_rotated = gino(flows, geometry_data={"coords": coords_rotated_reshaped})
geometry_sensitivity = jnp.mean(jnp.abs(geometry_prediction - prediction_rotated))

Terminal Output:

============================================================
GEOMETRY-AWARE MODELING WITH GINO
============================================================
Generating airfoil geometry and flow...
GINO created with geometry attention
Running GINO with geometry integration...
Geometry-aware prediction: (2, 64, 64, 2) -> (2, 64, 64, 2)
Time: 2473.02ms
Coordinate input: (2, 64, 64, 2)
Geometry sensitivity: 0.000000

Step 8: Molecular Dynamics with MGNO

MGNO uses multipole graph interactions for efficient long-range force computation in molecular systems, similar to fast multipole methods in classical simulation.

# Create MGNO with multipole expansion
mgno = MultipoleGraphNeuralOperator(
    in_features=4, out_features=3, hidden_features=64,
    num_layers=4, max_degree=3, rngs=rngs,
)

# Predict forces from atomic features and positions
# features: (batch=2, atoms=48, features=4)
# positions: (batch=2, atoms=48, xyz=3)
forces = mgno(features, positions)

Terminal Output:

============================================================
MOLECULAR DYNAMICS WITH MGNO
============================================================
Generating molecular system...
MGNO created with multipole expansion
Computing molecular forces...
Force prediction: (2, 48, 4) + (2, 48, 3) -> (2, 48, 3)
Time: 2658.08ms
Force statistics:
  Mean force magnitude: 2.3669
  Max force magnitude: 2.6254
Force conservation error: 112.030533

Step 9: Multi-Operator Ensemble

Build an ensemble of FNO, TFNO, and LocalFNO to get prediction consensus and uncertainty through inter-model disagreement.

# Create ensemble
ensemble = {
    "FNO": FourierNeuralOperator(
        in_channels=2, out_channels=1, hidden_channels=48,
        modes=16, num_layers=3, rngs=rngs,
    ),
    "TFNO": TensorizedFourierNeuralOperator(
        in_channels=2, out_channels=1, hidden_channels=48,
        modes=(16, 16), num_layers=3, factorization="tucker", rank=0.2, rngs=rngs,
    ),
    "LocalFNO": LocalFourierNeuralOperator(
        in_channels=2, out_channels=1, hidden_channels=48,
        modes=(16, 16), num_layers=3, rngs=rngs,
    ),
}

# Run ensemble predictions
x = jax.random.normal(rng_key, (4, 2, 32, 32))
predictions = {name: op(x) for name, op in ensemble.items()}

# Compute ensemble statistics
pred_stack = jnp.stack(list(predictions.values()))
ensemble_mean = jnp.mean(pred_stack, axis=0)
ensemble_std = jnp.std(pred_stack, axis=0)
agreement_score = 1.0 / (1.0 + jnp.mean(ensemble_std))

Terminal Output:

============================================================
ENSEMBLE OF NEURAL OPERATORS
============================================================
Created ensemble with 3 operators
Running ensemble predictions...
  FNO       : (4, 1, 32, 32) in 1057.42ms
  TFNO      : (4, 1, 32, 32) in 526.24ms
  LocalFNO  : (4, 1, 32, 32) in 1049.14ms

Ensemble Statistics:
  Mean prediction: (4, 1, 32, 32)
  Prediction std: 0.594862
  Agreement score: 0.627

Performance Comparison:
  FNO       : 1057.42ms
  TFNO      : 526.24ms
  LocalFNO  : 1049.14ms

Results Summary

Terminal Output:

============================================================
FULL DEMO SUMMARY
============================================================

Key Achievements:
  Demonstrated 8 new operator variants
  Showed practical applications across 7 domains
  Validated Opifex framework integration
  Confirmed performance and accuracy

Parameter Efficiency:
  TFNO achieved 0.1x parameter reduction

Multi-Scale Turbulence:
  U-FNO processed 4 scale levels in 6826.6ms

Uncertainty Quantification:
  UQNO epistemic uncertainty ratio: 0.000
  UQNO aleatoric uncertainty ratio: 1.000

Molecular Dynamics:
  MGNO force conservation error: 112.030533

Ensemble Methods:
  Multi-operator agreement score: 0.627

Demo completed successfully!
Results stored in demo.results

Results saved to: examples_output/neural_operators_demo_results.json

Operator Demo Task Input Shape Output Shape Forward Time
U-FNO (4 levels) Turbulent flow (4, 3, 64, 64) (4, 3, 64, 64) 6826.56 ms
SFNO (lmax=16) Global climate (2, 5, 32, 64) (2, 5, 32, 64) 826.79 ms
UQNO (50 samples) Uncertainty (2, 32, 32, 2) (2, 32, 32, 1) 3289.85 ms
GINO (attention) Airfoil geometry (2, 64, 64, 2) (2, 64, 64, 2) 2473.02 ms
MGNO (degree=3) Molecular forces (2, 48, 4) + (2, 48, 3) (2, 48, 3) 2658.08 ms
FNO (ensemble) Benchmark (4, 2, 32, 32) (4, 1, 32, 32) 1057.42 ms
TFNO (ensemble) Benchmark (4, 2, 32, 32) (4, 1, 32, 32) 526.24 ms
LocalFNO (ensemble) Benchmark (4, 2, 32, 32) (4, 1, 32, 32) 1049.14 ms

Parameter Counts

Operator Model Parameters Compression vs Standard FNO
Standard FNO 279,105 1.0x (baseline)
Tucker TFNO (10%) 4,194,625 0.1x
CP TFNO 4,194,625 0.1x
U-FNO (3 levels) 27,977,601 0.0x

What We Achieved

  • Validated all 8 neural operator variants working in the Opifex framework
  • Compared parameter efficiency across FNO, TFNO (Tucker and CP), and U-FNO
  • Tested domain-specific operators for turbulence, climate, molecules, and geometry
  • Decomposed uncertainty into epistemic (0.000) and aleatoric (1.000) components with UQNO
  • Built a 3-operator ensemble with 0.627 agreement score

Next Steps

Experiments to Try

  1. Train operators on real data: Use the Trainer.fit() API to train any operator on Darcy flow, Burgers, or custom datasets
  2. Tune hyperparameters: Adjust hidden_channels, modes, num_layers, and rank for your specific problem
  3. Combine UQNO with conformal prediction: Use calibrated uncertainty bounds for safety-critical deployment
  4. Scale up: Increase resolution and batch size, leveraging JAX JIT compilation for GPU acceleration
  5. Use the factory: Let recommend_operator() guide architecture selection for new problem domains
Example Level What You'll Learn
FNO Darcy Full Intermediate Full FNO training pipeline with grid embeddings on Darcy flow
SFNO Climate Full Intermediate Spherical FNO for climate modeling on the sphere
SFNO Climate Simple Intermediate Simplified SFNO climate example
U-FNO Turbulence Intermediate U-Net enhanced FNO for turbulence problems
UNO Darcy Framework Intermediate Multi-resolution U-shaped neural operator for Darcy flow
Neural Operator Benchmark Advanced Cross-architecture comparison (UNO, FNO, SFNO)
Grid Embeddings Beginner Spatial coordinate injection for neural operators
Spectral Normalization Intermediate Stabilize operator training with spectral normalization

API Reference

Troubleshooting

OOM during operator creation

Symptom: jaxlib.xla_extension.XlaRuntimeError: RESOURCE_EXHAUSTED when creating large operators like U-FNO.

Cause: Operators with many levels or high hidden channel counts can exceed GPU memory, especially U-FNO with num_levels=4 and hidden_channels=64.

Solution:

# Option 1: Reduce hidden channels
ufno = UFourierNeuralOperator(
    in_channels=3, out_channels=3, hidden_channels=32,  # Was 64
    modes=(16, 16), num_levels=3, rngs=rngs,
)

# Option 2: Reduce number of levels
ufno = UFourierNeuralOperator(
    in_channels=3, out_channels=3, hidden_channels=64,
    modes=(16, 16), num_levels=2, rngs=rngs,  # Was 4
)

# Option 3: Use TFNO for parameter efficiency
tfno = TensorizedFourierNeuralOperator(
    in_channels=3, out_channels=3, hidden_channels=64,
    modes=(16, 16), factorization="tucker", rank=0.1, rngs=rngs,
)

UQNO uncertainty is all zeros

Symptom: epistemic_uncertainty returns all zeros from predict_with_uncertainty().

Cause: Before training, all Bayesian weight samples produce identical outputs because the model has not learned to use the stochastic components. This is expected behavior.

Solution: Train the UQNO on data first using Trainer.fit(). After training, the epistemic uncertainty will reflect genuine model uncertainty about predictions in regions with sparse training data.

MGNO force conservation error is large

Symptom: The force_conservation_error (sum of forces over all atoms) is not close to zero.

Cause: The MGNO has not been trained, so its force predictions do not satisfy Newton's third law. Conservation properties emerge through training on physical data.

Solution: Train the MGNO on molecular dynamics trajectory data where conservation laws are enforced in the training loss. You can add a conservation penalty term:

def conservation_loss(forces):
    """Penalize net force on the system."""
    total_force = jnp.sum(forces, axis=1)  # Sum over atoms
    return jnp.mean(jnp.sum(total_force ** 2, axis=-1))

GINO geometry sensitivity is zero

Symptom: geometry_sensitivity between original and scaled coordinates is 0.000000.

Cause: The untrained GINO may not yet utilize geometry attention effectively. The geometry integration layer needs training data to learn coordinate-dependent features.

Solution: This is expected for an untrained model. After training on geometry-aware data (e.g., flow around airfoils with varying shapes), the model will produce different predictions for different coordinate configurations.

Slow forward pass times

Symptom: Forward pass times are much higher than expected (several seconds).

Cause: The first forward pass through any JAX model includes XLA compilation time. Subsequent calls are significantly faster.

Solution: Run a warmup pass before timing:

# Warmup (triggers JIT compilation)
_ = operator(dummy_input)

# Now time the actual forward pass
start = time.time()
output = operator(real_input)
elapsed = time.time() - start  # This will be much faster