Abstract
The quasi-coherent effects in two-dimensional incompressible turbulence are analyzed starting from the test particle trajectories. They can acquire coherent aspects when the stochastic potential has slow time variation and the motion is not strongly perturbed. The trajectories are, in these conditions, random sequences of large jumps and trapping or eddying events. Trapping determines quasi-coherent trajectory structures, which have a micro-confinement effect that is reflected in the transport coefficients. They determine non-Gaussian statistics and flows associated to an average velocity. Trajectory structures also influence the test modes on turbulent plasmas. Nonlinear damping and generation of zonal flow modes is found in drift turbulence in uniform magnetic field. The coupling of test particle and test mode studies permitted to evaluate the self-consistent evolution of the drift turbulence in an iterated approach. The results show an important nonlinear effect of ion diffusion, which can prevent the transition to the nonlinear regime at small drive of the instability. At larger drive, quasi-coherent trajectory structures appear and they have complex effects on turbulence.
Highlights
The direct numerical simulations, which have obtained important results in the last decades, largely dominate the actual research in turbulence
The cause is the basic problem of particle trajectories in stochastic velocity fields, which can be rather complex in important special cases
The decorrelation trajectory method (DTM, [12]) and the nested subensemble approach (NSA, [13]) are the first semi-analytical methods that are able to go beyond the quasilinear regime that corresponds to quasi-Gaussian transport
Summary
The direct numerical simulations, which have obtained important results in the last decades, largely dominate the actual research in turbulence. The decorrelation trajectory method (DTM, [12]) and the nested subensemble approach (NSA, [13]) are the first semi-analytical methods that are able to go beyond the quasilinear regime that corresponds to quasi-Gaussian transport. They were developed for the case of two-dimensional incompressible turbulence that is characterized by trajectory trapping or eddying. We present a theoretical approach to the study of turbulence evolution, the iterated self-consistent method (ISC). It is based on the analysis of the test particles and test modes in turbulent plasmas.
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