REVIEW 3 major objections 5 minor 98 references
An approximate Riemann solver approach in Physics-Informed Neural Networks for hyperbolic conservation laws
T0 review · 3 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Roe-averaged Jacobians put shock jump conditions into PINN training
desk verdict A genuinely useful PINN tweak for shocked flows, with an overclaimed guarantee that should be softened before publication. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The Roe-averaged Jacobian matrix $\tilde A$, defined by density-weighted averages $\tilde u$, $\tilde v$, $\tilde H$ (and the Eulderink–Mellema averaged state for relativistic hydrodynamics), satisfies the jump identity $F(U_R)-F(U_L)=\tilde A(U_R-U_L)$. The network's PDE residual $\partial_t U + \tilde A\,\partial_x U=0$ is computed with this modified matrix only inside the compression set $\Omega_s$ defined by a negative velocity gradient or negative divergence, so the shock-detection criterion decides where the modification acts. This machinery transfers the approximate Riemann solver's conservation property into the loss function.
What would settle it
On the Sod or Lax shock tube, run the LRPINN with the settings of Algorithm 2 and track $\int (U_{\mathrm{pred}}(x,t)-U_{\mathrm{pred}}(x,0))\,dx$ over time; if this conservation residual grows like the LLPINN curve in Figure 7 instead of staying near $10^{-3}$, the claim that the Roe replacement enforces jump conditions fails. Alternatively, evaluate the trained network on a fine uniform grid across the shock and check whether $F(U_R)-F(U_L) \approx s(U_R-U_L)$ with the predicted $s$; a clear violation indicates the property did not survive training.
Extended reading notes
Core claim
The central discovery is that enforcing the Roe property $F(U_R)-F(U_L)=\tilde A(U_R-U_L)$ locally, through the Roe-averaged Jacobian, is enough to turn the quasi-linear residual into a device that carries the jump conditions into the network training. Standard PINNs, even with viscous pretraining, mislocate shocks and lose contact waves; LLPINNs correctly predict shock speeds but violate conservation because their eigenvalue-substituted Jacobian does not satisfy the Roe property. LRPINNs replace the Jacobian only in regions where $\partial u/\partial x \le -M$ (or, in two dimensions, where $\nabla\cdot u \le -M$), with the Roe matrix built from neighbor states, and leave the true Jacobian elsewhere. The authors show that this yields accurate shock locations, sharp transitions, and conserved mass, momentum, and energy over time, and extends to two dimensions by applying the modification direction-by-direction with dimensional splitting.
Load-bearing premise
The load-bearing premise is that training on the quasi-linear residual with the Roe Jacobian in compression regions makes the network output satisfy the weak conservation law at the shock; the paper proves the Roe matrix has the jump property, but does not prove that residual minimization transfers this property to the network output.
Editorial extensions
If this is right
- LRPINNs solve one-dimensional Riemann problems for Euler and relativistic Euler equations without any prior knowledge of shock speeds, matching or exceeding WENO-Z accuracy up to about 1600 grid points in the tested double-shock case.
- Global conservation of mass, momentum, and energy stays approximately constant over time, unlike LLPINNs where conservation residuals grow with time (Figure 7).
- The method correctly handles transonic rarefactions in a moving-frame Sod tube because the Roe replacement is limited to compression regions, avoiding the entropy-violating behavior of pure Roe solvers.
- In two-dimensional Riemann problems, divergence-based detection plus dimensional splitting yields sharp shock and contact transitions while maintaining cylindrical symmetry (Problem 6).
- The approach is not tied to Roe specifically: the authors state that other approximate solvers or the exact Riemann solver could be substituted by replacing eigenvalues and reconstructing the modified Jacobian.
Reading between the lines
- The same selectively Roe-averaged residual principle could be transferred to other PINN formulations for conservation laws with available approximate Riemann solvers, such as shallow-water or magnetohydrodynamics, where Roe-type matrices exist; the authors gesture at this but do not test it.
- Because the shock speed emerges from the Roe property rather than being an input, LRPINNs may be useful for inverse problems where shock trajectory or wave speed is the quantity to recover; that use is not explored here.
- The sharper-shock-but-smoother-vortex trade-off observed in two dimensions suggests that a hybrid loss, mixing the Roe-modified residual with a spectral or multiscale term, could recover small-scale features; this is a testable design not addressed by the paper.
- A direct comparison of conservation residual against a fully conservative PINN formulation, such as integral-form residuals, would clarify whether the local Roe replacement or the quasi-linear structure is the active ingredient.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes two modifications of locally linearized PINNs (LLPINNs) for hyperbolic conservation laws with shocks. First, a generalized LLPINN computes shock speeds from neighboring network states via Rankine-Hugoniot formulas and uses Lax entropy conditions to select which eigenvalue to replace, removing the need to prescribe shock velocities in advance. Second, LRPINNs replace the Jacobian in detected compression regions by a Roe-averaged Jacobian, aiming to enforce jump conditions and improve conservation. Both methods are extended to the 2D Euler equations using a divergence-based shock detector and direction-wise Jacobian modification. The authors test the methods on 1D Euler shock-tube problems, 1D relativistic hydrodynamics, and 2D Riemann problems, comparing against exact solutions, WENO-Z, and HLLC solvers. They report sharper shock resolution and better conservation than classical PINNs and LLPINNs, with LRPINN accuracy comparable to or better than WENO at moderate resolution.
Significance. If the central claims were fully established, the paper would make a useful contribution: it would provide a practical way to embed approximate Riemann solver information into PINNs without a priori shock speeds, and the numerical comparisons show consistent gains over standard PINNs and the original LLPINN. The detailed hyperparameter tables and algorithm listings are a strength, and the inclusion of relativistic and 2D problems broadens applicability. However, the central theoretical claim that jump conditions are 'ensured' is not proven; the manuscript's own conservation diagnostic shows only approximate conservation. The contribution is therefore better characterized as an empirical method with heuristic motivation, whose claims need to be scaled back or supplemented with a weak-form consistency argument.
major comments (3)
- [Section III.C, Eq. (50), and Algorithm 2] The argument that LRPINNs 'robustly ensure' jump conditions is not established. Eq. (50) is an algebraic identity for constant states U_L and U_R, but Algorithm 2 only uses the Roe-averaged Jacobian inside the pointwise residual D[U] = ∂U/∂t + A_roe ∂U/∂x evaluated at collocation points. Minimizing the sum of squared pointwise residuals does not minimize any weak-form residual, and a smooth network can satisfy the modified quasi-linear system without satisfying the integral conservation law. The paper's own conservation check in Fig. 7 shows a residual of order 1e-3, not machine precision, so the 'ensures' language is not supported. Please either add a proof or argument connecting residual minimization to Rankine-Hugoniot satisfaction, augment the loss with an explicit weak-form or integral conservation term, or soften the claims to 'encourages' or 'approximately enforces'.
- [Algorithm 2, steps 7-8] The left and right states are obtained by evaluating the network at x_s ± Δx, where Δx is a hyperparameter. Since the network is smooth and Δx is small (Table I, values around 0.001 to 0.02), U_L and U_R are not the two branches of a discontinuous Riemann state; in the limit Δx→0 their Roe average tends to A(U(x_s)), so the modification degenerates to the local Jacobian. The paper should justify that these neighboring values are meaningful Riemann states for the shock and report sensitivity to Δx; otherwise the mechanism is not demonstrably acting as a Riemann solver at an interface.
- [Section V, Conclusions] The statement that the generalized LLPINN 'ensures that the Rankine–Hugoniot relations (22) and entropy conditions (23) are satisfied by construction' is an overstatement. Algorithm 1 replaces an eigenvalue by s_i in the quasi-linear residual, which encodes the shock speed in the differential operator but does not by itself enforce the jump condition in the network output. The conclusion should be rephrased to describe the intended design and the observed empirical behavior rather than a guaranteed property.
minor comments (5)
- [Section IV.A, Problem numbering] The problem numbering is inconsistent: 'Problem 4' appears twice (Sod shock tube in a moving reference frame, and relativistic Sod shock tube), and 'Problem 5' appears twice (relativistic double shock wave and, implicitly, the next problem). Please renumber the 1D and 2D problems sequentially.
- [Eqs. (31)-(32)] The shock-speed formulas appear to be missing a square-root symbol and closing parentheses: the expressions should read s_- = u_L - a_L * sqrt( (1/(2γ)) * ((γ+1) p_R/p_L + γ-1) ) and similarly for s_+. Please check the typesetting.
- [Algorithm 3 and Algorithm 4] There are small typographical errors in the residual definitions: Algorithm 3 line 12 and Algorithm 4 line 11 contain a double plus sign (∂U/∂t + Ã ∂U/∂x + + B̃ ∂U/∂y), and Algorithm 3 step 5 writes '∂_y v' without the ∂ symbol. These should be corrected.
- [Abstract and Section V] The abstract says the method 'applies jump conditions through entropy constraints,' but the algorithms use Lax entropy conditions only to select which eigenvalue to modify; the jump conditions enter through the Rankine-Hugoniot shock-speed formulas. Please clarify this distinction.
- [Figure 7] The caption says 'conserved quantities' but does not specify which component or combination is plotted, nor whether the difference is normalized. Please state whether the plotted quantity is mass, momentum, energy, or a composite, and clarify the normalization.
Circularity Check
No significant circularity: LRPINN and generalized LLPINN results are validated against exact and reference solvers, nothing is fitted to the target solution, and the only self-citations are non-load-bearing optimizer references.
full rationale
The derivation is self-contained against external benchmarks, so no circular step is identified. Generalized LLPINNs compute shock speeds from the closed-form Rankine-Hugoniot expressions (31)-(32) evaluated at the network's own neighboring states; these speeds are inputs derived from the physics, not parameters fitted to the exact solution, and the exact Riemann solution, WENO-Z solver, and 1D HLLC schemes are used only for post-hoc error comparison (Tables II-III), never as training data. The Roe modification is imported from the standard defining property of the Roe average, F(U_R)-F(U_L)=A_roe(U_R-U_L) (Eq. 50, attributed to Roe 1981 and Eulderink & Mellema 1995), which is stated as a required identity rather than derived as a prediction; hence there is no renaming of a known result, and no ansatz is smuggled in through self-citation. The conclusions' claim that RH relations are satisfied 'by construction' and that LRPINNs 'robustly ensure' jump conditions overstates what pointwise residual minimization guarantees, and the paper's own Figure 7 shows conservation residuals of order 1e-3 rather than machine precision; however, this is an inferential gap or correctness risk, not circularity, because the stated identity (50) is not defined in terms of the claimed conclusion and the central results (accurate shock locations and jumps matching exact solutions) are independently demonstrated. The paper itself flags its main limitations, admitting that LLPINNs fail to satisfy property (50) and that small-scale vortex structures are smoothed in Problems 7-8, which further weighs against a charge of disguised circularity. Self-citations [59,60] (Urbán, Stefanou, Pons; Kiyani, Shukla, Urbán, Darbon, Karniadakis) support only the quasi-Newton optimizer implementation, and [94] (Aloy, Pons, Ibáñez) is cited only as an example of classical flux reformulation; none is load-bearing for the physics claim. No fitted parameter is renamed as a prediction, and no uniqueness theorem is imported from the authors' prior work. Verdict: no significant circularity (score 1).
Assumptions & free parameters
free parameters (6)
- Shock detection threshold M (and Mx, My in 2D) =
0.05 to 0.1 in 1D; Mx and My from 1e-5 to 0.7
- Neighbor spacing Delta x (and Delta y) =
0.001 to 0.7 across problems
- Viscous pretraining coefficient nu =
1e-3 to 3e-3 per problem
- Viscous pretraining iterations =
1500, 2000, or 3000 depending on problem
- Gradient-annihilation constant a =
not reported
- Network architecture (layers x neurons) =
7L20N, 6L20N, 6L30N
assumptions (5)
- domain assumption The solution satisfies the ideal-gas equation of state p=(gamma-1) rho epsilon with fixed gamma.
- standard math Weak solutions satisfy the Rankine-Hugoniot condition (22) and Lax entropy inequalities (23) select physical shocks.
- ad hoc to paper Replacing the true Jacobian by the Roe-averaged Jacobian only in the detected compression region makes the PINN residual an adequate surrogate for the weak conservation law.
- ad hoc to paper The network's values at x_s +/- Delta x during training are valid left and right states across the shock, so the computed shock speeds and Roe averages are meaningful.
- ad hoc to paper Pretraining with viscosity nu Laplacian U and then setting nu=0 leads to convergence to the inviscid solution.
Cite this review
Pith. "Pith review of An approximate Riemann solver approach in Physics-Informed Neural Networks for hyperbolic conservation laws." pith.science (2026). https://pith.science/paper/WBVJBQYM
@misc{pith2026250611959,
author = {Pith},
title = {Pith review of: An approximate Riemann solver approach in Physics-Informed Neural Networks for hyperbolic conservation laws},
year = {2026},
howpublished = {\url{https://pith.science/paper/WBVJBQYM}},
note = {Machine review of arXiv:2506.11959}
}
read the original abstract
This study enhances the application of Physics-Informed Neural Networks (PINNs) for modeling discontinuous solutions in both hydrodynamics and relativistic hydrodynamics. Conventional PINNs, trained with partial differential equation residuals, frequently face convergence issues and lower accuracy near discontinuities. To address these issues, we build on the recently proposed locally linearized PINNs (LLPINNs), which improve shock detection by modifying the Jacobian matrix resulting from the linearization of the equations, only in regions where the velocity field exhibits strong compression. However, the original LLPINN framework required a priori knowledge of shock velocities, limiting its practical utility. We present a generalized LLPINN method that dynamically computes shock speeds using neighboring states and applies jump conditions through entropy constraints. Additionally, we introduce locally Roe PINNs (LRPINNs), which incorporate an approximate Roe Riemann solver to improve shock resolution and conservation properties across discontinuities. These methods are adapted to two-dimensional Riemann problems by using a divergence-based shock detection combined with dimensional splitting, delivering precise solutions. Compared to a high-order weighted essentially non-oscillatory solver, our method produces sharper shock transitions but smoother solutions in areas with small-scale vortex structures. Future research will aim to improve the resolution of these small-scale features without compromising the model's ability to accurately capture shocks.
Figures
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When this happens, we change the training set and continue the training process
note We say ``at most'' because the optimization process for a given set of points may be interrupted before completing 500 iterations if the optimizer does not find a proper value of step-length _k . When this happens, we change the training set and continue the training proc...
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merlin.mbs apsrev4-1.bst 2010-07-25 4.21a (PWD, AO, DPC) hacked
FUNCTION id.bst "merlin.mbs apsrev4-1.bst 2010-07-25 4.21a (PWD, AO, DPC) hacked" ENTRY address archive archivePrefix author bookaddress booktitle chapter collaboration doi edition editor eid eprint howpublished institution isbn issn journal key language month note number orga...
2010
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merlin.mbs apsrmp4-1.bst 2010-07-25 4.21a (PWD, AO, DPC) hacked
FUNCTION id.bst "merlin.mbs apsrmp4-1.bst 2010-07-25 4.21a (PWD, AO, DPC) hacked" ENTRY address archive archivePrefix author bookaddress booktitle chapter collaboration doi edition editor eid eprint howpublished institution isbn issn journal key language month note number orga...
2010
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