Nonuniversal velocity probability densities in two-dimensional turbulence: The effect of large-scale dissipation

We show that some statistical properties of forced two-dimensional turbulence have an important sensitivity to the form of large-scale dissipation, which is required to damp the inverse cascade We consider three models of large-scale dissipation linear Ekman drag, nonlinear quadratic drag, and scale-selective hypo-drag that damps only low-wavenumber modes In all cases, the statistically steady vorticity field is dominated by almost axisymmetric vortices, and the probability density function of vorticity is non-Gaussian However, in the case of linear and quadratic drag, we find that the velocity statistics is close to Gaussian, with non-negligible contribution coming from the background turbulent flow On the other hand, with hypo-drag, the probability density function of velocity is non-Gaussian and is predominantly determined by the properties of the vortices With hypo-drag, the relative positions of the vortices and the exponential distribution of the vortex extremum are important factors responsible for the non-Gaussian velocity statistics (C) 2010 American Institute of Physics [doi 10 1063/1 3504377]