Covariant Lyapunov analysis of solutions of the Navier-Stokes equations

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We studied chaotic states of Kolmogorov flows on a 2D flat torus (R/2πZ)2governed by the Navier-Stokes equations of incompressible fluids, by using the covariant Lyapunov analysis. Obtaining the bifurcation diagram of solutions, we performed the traditional Lyapunov analysis, and found that the first Lyapunov number becomes positive at Re/Rc～18, and the second one does at Re/Rc～23. Based on these data, we calculated the covariant Lyapunov vectors by the method of Ginelli et al. (2007), and found that the solution orbit is hyperbolic just after the chaotic transition, but becomes nonhyperbolic at Re/Rc～23, by observing the distribution of the angle between the stable/unstabletangent spaces of the orbit. At the hyperbolic/nonhyperbolic transition point, we found that the fuctional form of the time correlation function of the vorticity changes from oscillatory to non-oscillatory.