8 research outputs found
Dynamic Procedure for Filtered Gyrokinetic Simulations
Large Eddy Simulations (LES) of gyrokinetic plasma turbulence are
investigated as interesting candidates to decrease the computational cost. A
dynamic procedure is implemented in the GENE code, allowing for dynamic
optimization of the free parameters of the LES models (setting the amplitudes
of dissipative terms). Employing such LES methods, one recovers the free energy
and heat flux spectra obtained from highly resolved Direct Numerical
Simulations (DNS). Systematic comparisons are performed for different values of
the temperature gradient and magnetic shear, parameters which are of prime
importance in Ion Temperature Gradient (ITG) driven turbulence. Moreover, the
degree of anisotropy of the problem, that can vary with parameters, can be
adapted dynamically by the method that shows Gyrokinetic Large Eddy Simulation
(GyroLES) to be a serious candidate to reduce numerical cost of gyrokinetic
solvers.Comment: 10 pages, 10 figures, submitted to Physics of Plasma
Sub-grid-scale effects in magnetised plasma turbulence
In the present paper, we use a coarse-graining approach to investigate the
nonlinear redistribution of free energy in both position and scale space for
weakly collisional magnetised plasma turbulence. For this purpose, we use
high-resolution numerical simulations of gyrokinetic (GK) turbulence that span
the proton-electron range of scales, in a straight magnetic guide field
geometry. Accounting for the averaged effect of the particles' fast gyro-motion
on the slow plasma fluctuations, the GK approximation captures the dominant
energy redistribution mechanisms in strongly magnetised plasma turbulence.
Here, the GK system is coarse-grained with respect to a cut-off scale,
separating in real space the contributions to the nonlinear interactions from
the coarse-grid-scales and the sub-grid-scales (SGS). We concentrate on the
analysis of nonlinear SGS effects. Not only that this allows us to investigate
the flux of free energy across the scales, but also to now analyse its spatial
density. We find that the net value of scale flux is an order of magnitude
smaller than both the positive and negative flux density contributions. The
dependence of the results on the filter type is also analysed. Moreover, we
investigate the advection of energy in position space. This rather novel
approach for GK turbulence can help in the development of SGS models that
account for advective unstable structures for space and fusion plasmas, and
with the analysis of the turbulent transport saturation.Comment: 15 figures Accepted for publication by Journal of Plasma Physic
Structure of Plasma Heating in Gyrokinetic Alfvénic Turbulence
We analyze plasma heating in weakly collisional kinetic Alfv\'en wave (KAW)
turbulence using high resolution gyrokinetic simulations spanning the range of
scales between the ion and the electron gyroradii. Real space structures that
have a higher than average heating rate are shown not to be confined to current
sheets. This novel result is at odds with previous studies, which use the
electromagnetic work in the local electron fluid frame, i.e. , as a proxy for
turbulent dissipation to argue that heating follows the intermittent spatial
structure of the electric current. Furthermore, we show that electrons are
dominated by parallel heating while the ions prefer the perpendicular heating
route. We comment on the implications of the results presented here.Comment: 5 pages, 3 figure
Locality and universality in gyrokinetic turbulence
The nature of nonlinear interactions in gyrokinetic turbulence, driven by the ion-temperature gradient instability, is investigated using direct numerical simulations in toroidal flux tube geometry. To account for the level of separation existing between scales involved in an energetic interaction, the degree of locality of the free energy scale flux is analyzed employing Kraichnan's infrared (IR) and ultraviolet locality functions. Because of the nontrivial dissipative nature of gyrokinetic turbulence, an asymptotic level for the locality exponents, indicative of a universal dynamical regime for gyrokinetics, is not recovered and an accentuated nonlocal behavior of the IR interactions is found instead, in spite of the local energy cascade observed. © 2012 American Physical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe