This paper discusses a non-intrusive data-driven model order reduction method that learns low-dimensional dynamical models for a parametrized shallow water equation. We consider the shallow water equation in non-traditional form (NTSWE). We focus on learning low-dimensional models in a non-intrusive way. That means, we assume not to have access to a discretized form of the NTSWE in any form. Instead, we have snapshots that can be obtained using a black-box solver. Consequently, we aim at learning reduced-order models only from the snapshots. Precisely, a reduced-order model is learnt by solving an appropriate least-squares optimization problem in a low-dimensional subspace. Furthermore, we discuss computational challenges that particularly arise from the optimization problem being ill-conditioned. Moreover, we extend the non-intrusive model order reduction framework to a parametric case, where we make use of the parameter dependency at the level of the partial differential equation. We illustrate the efficiency of the proposed non-intrusive method to construct reduced-order models for NTSWE and compare it with an intrusive method (proper orthogonal decomposition). We furthermore discuss the predictive capabilities of both models outside the range of the training data.
K E Y W O R D Sdata-driven modeling, model order reduction, operator inference, scientific machine learning, shallow water equation
INTRODUCTIONShallow water equations (SWE) are a popular set of hyperbolic partial differential equations (PDEs) with the capability of describing geophysical wave phenomena, for example, the Kelvin and Rossby waves in the atmosphere and the oceans. They are frequently used in geophysical flow prediction, 1 investigation of baroclinic instability, 2,3 and planetary flows. 4 In this paper, we study a model order reduction (MOR) technique for SWE. MOR techniques allow us to construct low-dimensional models or reduced-order models (ROMs) for a large-scale dynamical system. We refer to the books 5,6 for an overview of the available techniques. These ROMs are computationally efficient and accurate, and are worthy when a full order model (FOM) needs to be simulated multiple-times for different parameter settings. Additionally, ROMs are even more valuable in the case of SWE, when the interest lies in simulating the model for a very long time horizon. MOR problems for SWE have been intensively studied in the literature, see, for example, References [7-12].