RiemannONets: Interpretable Neural Operators for Riemann Problems

TL;DR

This paper introduces RiemannONets, a neural operator framework that accurately and efficiently solves high-pressure ratio Riemann problems in compressible flows, with interpretability and real-time capabilities.

Contribution

The paper proposes a modified DeepONet with a hierarchical basis for improved accuracy, efficiency, and interpretability in solving Riemann problems, outperforming vanilla models and U-Net variants.

Findings

Modified DeepONet achieves higher accuracy and robustness.

Hierarchical basis reflects physical flow features.

Neural operators enable real-time Riemann problem solutions.

Abstract

Developing the proper representations for simulating high-speed flows with strong shock waves, rarefactions, and contact discontinuities has been a long-standing question in numerical analysis. Herein, we employ neural operators to solve Riemann problems encountered in compressible flows for extreme pressure jumps (up to $10^{10}$ pressure ratio). In particular, we first consider the DeepONet that we train in a two-stage process, following the recent work of \cite{lee2023training}, wherein the first stage, a basis is extracted from the trunk net, which is orthonormalized and subsequently is used in the second stage in training the branch net. This simple modification of DeepONet has a profound effect on its accuracy, efficiency, and robustness and leads to very accurate solutions to Riemann problems compared to the vanilla version. It also enables us to interpret the results physically as the hierarchical data-driven produced basis reflects all the flow features that would otherwise be introduced using ad hoc feature expansion layers. We also compare the results with another neural operator based on the U-Net for low, intermediate, and very high-pressure ratios that are very accurate for Riemann problems, especially for large pressure ratios, due to their multiscale nature but computationally more expensive. Overall, our study demonstrates that simple neural network architectures, if properly pre-trained, can achieve very accurate solutions of Riemann problems for real-time forecasting. The source code, along with its corresponding data, can be found at the following URL: https://github.com/apey236/RiemannONet/tree/main