Hydrodynamic Lyapunov modes in coupled map lattices.

In this paper, numerical and analytical results are presented which indicate that hydrodynamic Lyapunov modes (HLMs) also exist for coupled map lattices (CMLs). The dispersion relations for the HLMs of CMLs are found to fall into two different universality classes. It is characterized by lambda approximately k for coupled standard maps and lambda approximately k2 for coupled circle maps. The conditions under which HLMs can be observed are discussed. The role of the Hamiltonian structure, conservation laws, translational invariance, and damping is elaborated. Our results are as follows: (1) The Hamiltonian structure is not a necessary condition for the existence of HLMs. (2) Conservation laws or the translational invariance alone cannot guarantee the existence of HLMs. (3) Including a damping term in the system of coupled Hamiltonian maps does not destroy the HLMs. The lambda-k dispersion relation of HLMs, however, changes to the universality class with lambda-k2 under damping. In contrast, no HLMs survives in the system of coupled circle maps under damping. (4) An on-site potential destroys the HLMs. (5) The study of zero-value Lyapunov exponents (LEs) and associated Lyapunov vectors (LVs) shows that translational invariance and conservation laws play different roles in the tangent space dynamics. (6) The dynamics of the coordinate and momentum parts of LVs in Hamiltonian systems are related but different. Furthermore, numerical results for a two-dimensional system show that the appearance of HLMs in CMLs is not restricted to the one-dimensional case.