Lyapunov spectra and nonequilibrium ensembles equivalence in 2D fluid mechanics

We perform numerical experiments to study the Lyapunov spectra of dynamical systems associated with the Navier-Stokes (NS) equation in two spatial dimensions truncated over the Fourier basis. Recently new equations, called GNS equations, have been introduced and conjectured to be equivalent to the NS equations at large Reynolds numbers. The Lyapunov spectra of the NS and of the corresponding GNS systems overlap, adding evidence in favor of the conjectured equivalence already studied and partially extended in previous papers. We make use of the Lyapunov spectra to study a fluctuation relation which had been proposed to extend the "fluctuation theorem" to strongly dissipative systems. Preliminary results towards the formulation of a local version of the fluctuation formula are also presented. (C) 2003 Elsevier B.V. All rights reserved.