On the stability of discrete tripole, quadrupole, Thomson’ vortex triangle and square in a two-layer/homogeneous rotating fluid. (English) Zbl 1346.76204

Kurakin, Leonid G.; Ostrovskaya, Irina V.; Sokolovskiy, Mikhail A.

Regul. Chaotic Dyn. 21, No. 3, 291-334 (2016).

Summary: A two-layer quasigeostrophic model is considered in the \(f\)-plane approximation. The stability of a discrete axisymmetric vortex structure is analyzed for the case when the structure consists of a central vortex of arbitrary intensity \(\Gamma\) and two/three identical peripheral vortices. The identical vortices, each having a unit intensity, are uniformly distributed over a circle of radius \(R\) in a single layer. The central vortex lies either in the same or in another layer. The problem has three parameters \((R,\Gamma,\alpha)\), where \(\alpha\) is the difference between layer thicknesses. A limiting case of a homogeneous fluid is also considered.A limiting case of a homogeneous fluid is also considered.
The theory of stability of steady-state motions of dynamic systems with a continuous symmetry group \(G\) is applied. The two definitions of stability used in the study are Routh stability and \(\mathcal G\)-stability. The Routh stability is the stability of a one-parameter orbit of a steady-state rotation of a vortex multipole, and the \(\mathcal G\)-stability is the stability of a three-parameter invariant set \(O_{\mathcal G}\), formed by the orbits of a continuous family of steady-state rotations of a multipole. The problem of Routh stability is reduced to the problem of stability of a family of equilibria of a Hamiltonian system. The quadratic part of the Hamiltonian and the eigenvalues of the linearization matrix are studied analytically.
The cases of zero total intensity of a tripole and a quadrupole are studied separately. Also, the Routh stability of a Thomson vortex triangle and square was proved at all possible values of problem parameters. The results of theoretical analysis are sustained by numerical calculations of vortex trajectories.

MSC:

76U05	General theory of rotating fluids
76B47	Vortex flows for incompressible inviscid fluids
76E20	Stability and instability of geophysical and astrophysical flows

Keywords:

discrete multipole vortex structure; two-layer rotating fluid; stability

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