1,647 research outputs found
Anisotropic fluxes and nonlocal interactions in MHD turbulence
We investigate the locality or nonlocality of the energy transfer and of the
spectral interactions involved in the cascade for decaying magnetohydrodynamic
(MHD) flows in the presence of a uniform magnetic field at various
intensities. The results are based on a detailed analysis of three-dimensional
numerical flows at moderate Reynold numbers. The energy transfer functions, as
well as the global and partial fluxes, are examined by means of different
geometrical wavenumber shells. On the one hand, the transfer functions of the
two conserved Els\"asser energies and are found local in both the
directions parallel (-direction) and perpendicular (-direction)
to the magnetic guide-field, whatever the -strength. On the other
hand, from the flux analysis, the interactions between the two
counterpropagating Els\"asser waves become nonlocal. Indeed, as the -intensity is increased, local interactions are strongly decreased and the
interactions with small modes dominate the cascade. Most of the energy
flux in the -direction is due to modes in the plane at , while
the weaker cascade in the -direction is due to the modes with .
The stronger magnetized flows tends thus to get closer to the weak turbulence
limit where the three-wave resonant interactions are dominating. Hence, the
transition from the strong to the weak turbulence regime occurs by reducing the
number of effective modes in the energy cascade.Comment: Submitted to PR
Scaling properties of three-dimensional magnetohydrodynamic turbulence
The scaling properties of three-dimensional magnetohydrodynamic turbulence
are obtained from direct numerical simulations of decaying turbulence using
modes. The results indicate that the turbulence does not follow the
Iroshnikov-Kraichnan phenomenology.In the case of hyperresistivity, the
structure functions exhibit a clear scaling range yielding absolute values of
the scaling exponents . The scaling exponents agree with a modified
She-Leveque model , corresponding to Kolmogorov
scaling but sheet-like geometry of the dissipative structures
Non-gaussian probability distribution functions in two dimensional Magnetohydrodynamic turbulence
Intermittency in MHD turbulence has been analyzed using high resolution 2D
numerical simulations. We show that the Probability Distribution Functions
(PDFs) of the fluctuations of the Elsasser fields, magnetic field and velocity
field depend on the scale at hand, that is they are self-affine. The departure
of the PDFs from a Gaussian function can be described through the scaling
behavior of a single parameter lambda_r^2 obtained by fitting the PDFs with a
given curve stemming from the analysis of a multiplicative model by Castaing et
al. (1990). The scaling behavior of the parameter lambda_r^2 can be used to
extract informations about the intermittency. A comparison of intermittency
properties in different MHD turbulent flows is also performed.Comment: 7 pages, with 5 figure
The role of surface chemical reactivity in the stability of electronic nanodevices based on two-dimensional materials "beyond graphene" and topological insulators
Here, we examine the influence of surface chemical reactivity toward ambient
gases on the performance of nanodevices based on two-dimensional materials
"beyond graphene" and novel topological phases of matter. While surface
oxidation in ambient conditions was observed for silicene and phosphorene with
subsequent reduction of the mobility of charge carriers, nanodevices with
active channels of indium selenide, bismuth chalcogenides and transition-metal
dichalcogenides are stable in air. However, air-exposed indium selenide suffers
of p-type doping due to water decomposition on Se vacancies, whereas the low
mobility of charge carriers in transition-metal dichalcogenides increases the
response time of nanodevices. Conversely, bismuth chalcogenides require a
control of crystalline quality, which could represent a serious hurdle for up
scaling
GPU cards as a low cost solution for efficient and fast classification of high dimensional gene expression datasets
The days when bioinformatics tools will be so reliable to become a standard aid in routine clinical diagnostics are getting very close. However, it is important to remember that the more complex and advanced bioinformatics tools become, the more performances are required by the computing platforms. Unfortunately, the cost of High Performance Computing (HPC) platforms is still prohibitive for both public and private medical practices. Therefore, to promote and facilitate the use of bioinformatics tools it is important to identify low-cost parallel computing solutions. This paper presents a successful experience in using the parallel processing capabilities of Graphical Processing Units (GPU) to speed up classification of gene expression profiles. Results show that using open source CUDA programming libraries allows to obtain a significant increase in performances and therefore to shorten the gap between advanced bioinformatics tools and real medical practic
Analysis of cancellation in two-dimensional magnetohydrodynamic turbulence
A signed measure analysis of two-dimensional intermittent magnetohydrodynamic
turbulence is presented. This kind of analysis is performed to characterize the
scaling behavior of the sign-oscillating flow structures, and their geometrical
properties. In particular, it is observed that cancellations between positive
and negative contributions of the field inside structures, are inhibited for
scales smaller than the Taylor microscale, and stop near the dissipative scale.
Moreover, from a simple geometrical argument, the relationship between the
cancellation exponent and the typical fractal dimension of the structures in
the flow is obtained.Comment: 21 pages, 5 figures (3 .jpg not included in the latex file
Numerical study of dynamo action at low magnetic Prandtl numbers
We present a three--pronged numerical approach to the dynamo problem at low
magnetic Prandtl numbers . The difficulty of resolving a large range of
scales is circumvented by combining Direct Numerical Simulations, a
Lagrangian-averaged model, and Large-Eddy Simulations (LES). The flow is
generated by the Taylor-Green forcing; it combines a well defined structure at
large scales and turbulent fluctuations at small scales. Our main findings are:
(i) dynamos are observed from down to ; (ii) the critical
magnetic Reynolds number increases sharply with as turbulence sets
in and then saturates; (iii) in the linear growth phase, the most unstable
magnetic modes move to small scales as is decreased and a Kazantsev
spectrum develops; then the dynamo grows at large scales and modifies
the turbulent velocity fluctuations.Comment: 4 pages, 4 figure
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