Chaotic Motion of the N-Vortex Problem on a Sphere: I. Saddle-Centers in Two-Degree-of-Freedom Hamiltonians |
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Authors: | Takashi Sakajo Kazuyuki Yagasaki |
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Institution: | (1) Department of Mathematics, Hokkaido University, Sapporo Hokkaido 060-8610, Japan;(2) Department of Mechanical and Systems Engineering, Gifu University, Gifu 501-1193, Japan |
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Abstract: | We study the motion of N point vortices with N∈ℕ on a sphere in the presence of fixed pole vortices, which are governed by a Hamiltonian dynamical system with N degrees of freedom. Special attention is paid to the evolution of their polygonal ring configuration called the N
-ring, in which they are equally spaced along a line of latitude of the sphere. When the number of the point vortices is N=5n or 6n with n∈ℕ, the system is reduced to a two-degree-of-freedom Hamiltonian with some saddle-center equilibria, one of which corresponds
to the unstable N-ring. Using a Melnikov-type method applicable to two-degree-of-freedom Hamiltonian systems with saddle-center equilibria
and a numerical method to compute stable and unstable manifolds, we show numerically that there exist transverse homoclinic
orbits to unstable periodic orbits in the neighborhood of the saddle-centers and hence chaotic motions occur. Especially,
the evolution of the unstable N-ring is shown to be chaotic.
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Keywords: | Hamiltonian system Point vortex Flow on sphere Chaos Melnikov method |
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