A Casimir preserving scheme for long-Time simulation of spherical ideal hydrodynamics
Artikel i vetenskaplig tidskrift, 2019

The incompressible two-dimensional Euler equations on a sphere constitute a fundamental model in hydrodynamics. The long-Time behaviour of solutions is largely unknown; statistical mechanics predicts a steady vorticity configuration, but detailed numerical results in the literature contradict this theory, yielding instead persistent unsteadiness. Such numerical results were obtained using artificial hyperviscosity to account for the cascade of enstrophy into smaller scales. Hyperviscosity, however, destroys the underlying geometry of the phase flow (such as conservation of Casimir functions), and therefore might affect the qualitative long-Time behaviour. Here, we develop an efficient numerical method for long-Time simulations that preserve the geometric features of the exact flow, in particular conservation of Casimirs. Long-Time simulations on a non-rotating sphere then reveal three possible outcomes for generic initial conditions: The formation of either 2, 3 or 4 coherent vortex structures. These numerical results contradict the statistical mechanics theory and show that previous numerical results, suggesting 4 coherent vortex structures as the generic behaviour, display only a special case. Through integrability theory for point vortex dynamics on the sphere we present a theoretical model which describes the mechanism by which the three observed regimes appear. We show that there is a correlation between a first integral (the ratio of total angular momentum and the square root of enstrophy) and the long-Time behaviour: small, intermediate and large yields most likely 4, 3 or 2 coherent vortex formations. Our findings thus suggest that the likely long-Time behaviour can be predicted from the first integral. © 2019 Cambridge University Press. All rights reserved.

Hamiltonian theory

rotating flows

computational methods

Författare

Klas Modin

Chalmers, Matematiska vetenskaper, Tillämpad matematik och statistik

Milo Viviani

Chalmers, Matematiska vetenskaper, Tillämpad matematik och statistik

Journal of Fluid Mechanics

0022-1120 (ISSN) 1469-7645 (eISSN)

A22

Ämneskategorier

Teknisk mekanik

Annan fysik

Strömningsmekanik och akustik

DOI

10.1017/jfm.2019.944

Mer information

Senast uppdaterat

2020-01-30