270 research outputs found
Fluctuations and control in the Vlasov-Poisson equation
In this paper we study the fluctuation spectrum of a linearized
Vlasov-Poisson equation in the presence of a small external electric field.
Conditions for the control of the linear fluctuations by an external electric
field are established.Comment: 8 pages late
Diffusion and dispersion of passive tracers: Navier-Stokes versus MHD turbulence
A comparison of turbulent diffusion and pair-dispersion in homogeneous,
macroscopically isotropic Navier-Stokes (NS) and nonhelical magnetohydrodynamic
(MHD) turbulence based on high-resolution direct numerical simulations is
presented. Significant differences between MHD and NS systems are observed in
the pair-dispersion properties, in particular a strong reduction of the
separation velocity in MHD turbulence as compared to the NS case. It is shown
that in MHD turbulence the average pair-dispersion is slowed down for
, being
the Kolmogorov time, due to the alignment of the relative Lagrangian tracer
velocity with the local magnetic field. Significant differences in turbulent
single-particle diffusion in NS and MHD turbulence are not detected. The fluid
particle trajectories in the vicinity of the smallest dissipative structures
are found to be characterisically different although these comparably rare
events have a negligible influence on the statistics investigated in this work.Comment: Europhysics Letters, in prin
Statistical properties of an ensemble of vortices interacting with a turbulent field
We develop an analytical formalism to determine the statistical properties of
a system consisting of an ensemble of vortices with random position in plane
interacting with a turbulent field. We calculate the generating functional by
path-integral methods. The function space is the statistical ensemble composed
of two parts, the first one representing the vortices influenced by the
turbulence and the second one the turbulent field scattered by the randomly
placed vortices.Comment: Third version; Important corrections in the normalization for the gas
of vortices, et
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