title: Dynamics of reversals and condensates in two-dimensional Kolmogorov flows.
abstract: We present numerical simulations of the different two-dimensional flow regimes generated by a constant spatially periodic forcing balanced by viscous dissipation and large-scale drag with a dimensionless damping rate 1/Rh. The linear response to the forcing is a 6×6 square array of counterrotating vortices, which is stable when the Reynolds number Re or Rh are small. After identifying the sequence of bifurcations that lead to a spatially and temporally chaotic regime of the flow when Re and Rh are increased, we study the transitions between the different turbulent regimes observed for large Re by varying Rh. A large-scale circulation at the box size (the condensate state) is the dominant mode in the limit of vanishing large-scale drag (Rh large). When Rh is decreased, the condensate becomes unstable and a regime with random reversals between two large-scale circulations of opposite signs is generated. It involves a bimodal probability density function of the large-scale velocity that continuously bifurcates to a Gaussian distribution when Rh is decreased further.
authors.affiliation: Laboratoire de Physique Statistique, Ecole Normale Superieure, 24 Rue Lhomond, Paris, France.
authors.forename: Pankaj Kumar
authors.initials: PK
authors.lastname: Mishra
authors.affiliation: Laboratoire de Physique Statistique, Ecole Normale Superieure, 24 Rue Lhomond, Paris, France.
authors.forename: Johann
authors.initials: J
authors.lastname: Herault
authors.affiliation: Laboratoire de Physique Statistique, Ecole Normale Superieure, 24 Rue Lhomond, Paris, France.
authors.forename: Stephan
authors.initials: S
authors.lastname: Fauve
authors.affiliation: Department of Physics, Indian Institute of Technology, Kanpur 208 016, India.
authors.forename: Mahendra K
authors.initials: MK
authors.lastname: Verma