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Active Learning with Selective Time-Step Acquisition for PDEs
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Proposes STAP, an active learning add-on for PDE trajectory surrogates that acquires only selected time steps from the numerical solver, filling skipped steps with the surrogate, and selects them with a variance-reduction acquisition function.
Problem
Reducing the cost of generating numerical-solver training data for autoregressive neural surrogate models of PDE trajectories, where existing active learning acquires entire trajectories.
Approach
Instead of acquiring full trajectories, select a sampling pattern of time steps per initial condition; skipped steps are filled by the surrogate, and the pattern is chosen by maximizing a committee-based variance-reduction acquisition function, greedily optimized with bit-flip search and used as an add-on to full-trajectory AL methods.
Key results
- SBAL+STAP achieves the best averaged log RMSE on all five PDEs, e.g., Burgers -3.674 vs. -3.388 for SBAL and KS -0.349 vs. -0.275 for SBAL.
- Its variant STAP 10 reduces batch-selection time to 15.4 s versus 62.5 s for QbC/SBAL in INS while still improving over the base method.
- In the multistep FNO setting, SBAL+STAP improves Burgers 8L/8 log RMSE to -2.058 versus -1.893 for SBAL.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Add STAP selective time-step acquisition to SBAL | FNO PDE trajectory surrogate · Burgers, KdV, KS, INS, CNS | log RMSE: Burgers: -3.388 -> -3.674; KdV: 0.030 -> -0.088; KS: -0.275 -> -0.349; INS: -0.461 -> -0.525; CNS: 2.422 -> 2.363 | Helped |
| Replace optimized STAP time-step selection with Bernoulli sampling at p=1/16 (ablation) | SBAL+STAP · Burgers, KdV, KS, INS, CNS | log RMSE: Burgers: -3.674 -> -3.231; KdV: -0.088 -> 0.053; KS: -0.349 -> -0.365; INS: -0.525 -> -0.529; CNS: 2.363 -> 2.375 | Mixed |
| Replace optimized STAP time-step selection with Bernoulli sampling at p=1/8 (ablation) | SBAL+STAP · Burgers, KdV, KS, INS, CNS | log RMSE: Burgers: -3.674 -> -3.152; KdV: -0.088 -> 0.049; KS: -0.349 -> -0.359; INS: -0.525 -> -0.525; CNS: 2.363 -> 2.370 | Mixed |
| Replace optimized STAP time-step selection with Bernoulli sampling at p=1/4 (ablation) | SBAL+STAP · Burgers, KdV, KS, INS, CNS | log RMSE: Burgers: -3.674 -> -3.102; KdV: -0.088 -> 0.018; KS: -0.349 -> -0.346; INS: -0.525 -> -0.515; CNS: 2.363 -> 2.372 | Mixed |
| Replace optimized STAP time-step selection with Bernoulli sampling at p=1/2 (ablation) | SBAL+STAP · Burgers, KdV, KS, INS, CNS | log RMSE: Burgers: -3.674 -> -3.458; KdV: -0.088 -> -0.064; KS: -0.349 -> -0.324; INS: -0.525 -> -0.500; CNS: 2.363 -> 2.392 | Mixed |
| Replace pairwise committee acquisition with mean-field STAP acquisition (ablation) | SBAL+STAP · Burgers, KdV, KS, INS, CNS | log RMSE: Burgers: -3.674 -> -3.608; KdV: -0.088 -> -0.065; KS: -0.349 -> -0.326; INS: -0.525 -> -0.529; CNS: 2.363 -> 2.360 | Mixed |
| Reduce STAP greedy optimization steps from 100 to 10 (ablation) | SBAL+STAP · Burgers, KdV, KS, INS, CNS | log RMSE: Burgers: -3.674 -> -3.524; KdV: -0.088 -> -0.118; KS: -0.349 -> -0.316; INS: -0.525 -> -0.502; CNS: 2.363 -> 2.401 | Mixed |
Surprising findings
- Random Bernoulli sampling of time steps alone often improves over full-trajectory SBAL, suggesting sparse time-step sampling is inherently advantageous.
- STAP still outperforms baselines when the initial training set has only one trajectory, despite erroneous surrogate-generated inputs.
Limitations
- Skipped time steps are filled with surrogate predictions, which can produce out-of-distribution or unstable synthetic inputs (e.g., KdV simulation crashes).
- Acquisition function is a heuristic approximation to expected error reduction under committee assumptions, not a provably exact criterion.
- Experiments are limited to a fixed set of benchmark PDEs with a single FNO architecture and committee size M=2.
Experimental setup
Datasets: Burgers (simulated, FDM) KdV (simulated, pseudospectral) Kuramoto-Sivashinsky (simulated, ETD-RK4) incompressible Navier-Stokes (simulated, Crank-Nicolson) compressible Navier-Stokes (simulated)
Benchmarks: Burgers KdV Kuramoto-Sivashinsky incompressible Navier-Stokes compressible Navier-Stokes
Metrics: log RMSE log NRMSE log MAE log quantile RMSE (99%, 95%, 50%) wall-clock time
Baselines: Random QbC LCMD SBAL Bernoulli sampling Ber(p)
Builds on: Query-by-Committee Expected error reduction Fourier Neural Operator SBAL LCMD
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