Two-dimensional fluids via matrix hydrodynamics

Abstract: Two-dimensional (2-D) incompressible, inviscid fluids produce fascinating patterns of swirling motion. How and why the patterns emerge are long-standing questions, first addressed in the 19th century by Helmholtz, Kirchhoff, and Kelvin. Countless researchers have since contributed to innovative techniques and results, but the overarching problem of swirling 2-D motion and its long-time behavior remains largely open. Here we advocate an alternative view-point that sheds light on this problem via a link to isospectral matrix flows. The link is established through V. Zeitlin's beautiful model for the numerical discretization of Euler's equations in 2-D. When considered on the sphere, Zeitlin's model enables a deep connection between 2-D hydrodynamics and unitary representation theory of Lie algebras as pursued in quantum theory. Consequently, it provides a dictionary that maps hydrodynamical concepts to matrix Lie theory, which in turn gives connections to matrix factorizations, random matrices, and integrability theory, for example. The transferal of outcomes, from finite-dimensional matrices to infinite-dimensional fluids, is then supported by hard-fought results in quantization theory -- the field which describes the limit between quantum and classical physics. We demonstrate how the dictionary is constructed and how it unveils techniques for 2-D hydrodynamics. We also give accompanying convergence results for Zeitlin's model on the sphere.