Buckingham $\pi$-Invariant Test‑Time Projection for Robust PDE Surrogate Modeling

PDE surrogate models such as FNO and PINN struggle to predict solutions across inputs with diverse physical units and scales, limiting their out-of-distribution (OOD) generalization. We propose a $\{pi}$-invariant test-time projection that aligns test inputs with the training distribution by solving a log-space least squares problem that preserves Buckingham π-invariants. For PDEs with multidimensional spatial fields, we use geometric representative $\{pi}$-values to compute distances and project inputs, overcoming degeneracy and singular points that limit prior $\{pi}$-methods. To accelerate projection, we cluster the training set into K clusters, reducing the complexity from O(MN) to O(KN) for the M training and N test samples. Across wide input scale ranges, tests on 2D thermal conduction and linear elasticity achieve an average MAE reduction up to $\approx\!91\%$ with minimal overhead. This training-free, model-agnostic method is expected to apply to more diverse PDE-based simulations.