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Open AccessArticle

Lagrangian Reduced Order Modeling Using Finite Time Lyapunov Exponents

1
Courant Institute of Mathematical Sciences, New York University, New York, NY 10012, USA
2
Engineering Mechanics Program, Virginia Tech, Blacksburg, VA 24061, USA
3
Department of Aerospace and Ocean Engineering, Virginia Tech, Blacksburg, VA 24061, USA
4
Department of Mathematics, Virginia Tech, Blacksburg, VA 24061, USA
*
Author to whom correspondence should be addressed.
Fluids 2020, 5(4), 189; https://doi.org/10.3390/fluids5040189
Received: 21 September 2020 / Revised: 11 October 2020 / Accepted: 17 October 2020 / Published: 23 October 2020
(This article belongs to the Special Issue Lagrangian Transport in Geophysical Fluid Flows)
There are two main strategies for improving the projection-based reduced order model (ROM) accuracy—(i) improving the ROM, that is, adding new terms to the standard ROM; and (ii) improving the ROM basis, that is, constructing ROM bases that yield more accurate ROMs. In this paper, we use the latter. We propose two new Lagrangian inner products that we use together with Eulerian and Lagrangian data to construct two new Lagrangian ROMs, which we denote α-ROM and λ-ROM. We show that both Lagrangian ROMs are more accurate than the standard Eulerian ROMs, that is, ROMs that use standard Eulerian inner product and data to construct the ROM basis. Specifically, for the quasi-geostrophic equations, we show that the new Lagrangian ROMs are more accurate than the standard Eulerian ROMs in approximating not only Lagrangian fields (e.g., the finite time Lyapunov exponent (FTLE)), but also Eulerian fields (e.g., the streamfunction). In particular, the α-ROM can be orders of magnitude more accurate than the standard Eulerian ROMs. We emphasize that the new Lagrangian ROMs do not employ any closure modeling to model the effect of discarded modes (which is standard procedure for low-dimensional ROMs of complex nonlinear systems). Thus, the dramatic increase in the new Lagrangian ROMs’ accuracy is entirely due to the novel Lagrangian inner products used to build the Lagrangian ROM basis. View Full-Text
Keywords: Lagrangian reduced order model; Lagrangian inner product; quasi-geostrophic equations; finite time Lyapunov exponent Lagrangian reduced order model; Lagrangian inner product; quasi-geostrophic equations; finite time Lyapunov exponent
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Xie, X.; Nolan, P.J.; Ross , S.D.; Mou , C.; Iliescu, T. Lagrangian Reduced Order Modeling Using Finite Time Lyapunov Exponents. Fluids 2020, 5, 189.

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