Magnetic field generation by coherent turbulence structures

It is thought that the small-scale magnetic fields observed in accretion discs, galaxies and galactic clusters are generated by a dynamo process in which the turbulent plasma amplifies small initial magnetic fluctuations. Numerical simulations of turbulence have revealed that turbulence consists of filamentlike vortex structures superimposed on an incoherent background, which carry a considerable amount of the energy. The natural questions to ask are whether these coherent structures can generate a magnetic field and, if so, if the generated magnetic field is also filament-like. After setting up a turbulence model which consists only of vortex filaments, we show in an unambiguous way that the coherent structure can sustain kinematic dynamo action and that the magnetic field thus generated consists of relatively thick ribbons (flattened tubes) located in between vortices

Elementary Vortex Processes in Thermal Superfluid Turbulence

By solving pertinent mathematical models with numerical and computational methods, we analyze the formation of superfluid vorticity structures in a turbulent normal fluid with an inertial range exhibiting Kolmogorov scaling. We demonstrate that mutual friction forcing causes quantum vortex instabilities whose signature is spiral vortical configurations. The spirals expand until they accidentally meet metastable, intense normal fluid vorticity tubes of similar curvature and vorticity orientation that trap them by driving them towards low mutual friction sites where superfluid bundles are formed. The bundle formation sites are located within the tube cores, but, due to tube curvature and many-tube interaction effects, are displaced by variable distances from the tube centerlines as they follow the contours of the latter. We analyze possible implications of these processes in fully developed thermal superfluid turbulence dynamics

Vortex spectrum in superfluid turbulence: interpretation of a recent experiment

We discuss a recent experiment in which the spectrum of the vortex line density fluctuations has been measured in superfluid turbulence. The observed frequency dependence of the spectrum, $f^{-5/3}$, disagrees with classical vorticity spectra if, following the literature, the vortex line density is interpreted as a measure of the vorticity or enstrophy. We argue that the disagrement is solved if the vortex line density field is decomposed into a polarised field (which carries most of the energy) and an isotropic field (which is responsible for the spectrum).Comment: Submitted for publication http://crtbt.grenoble.cnrs.fr/helio/GROUP/infa.html http://www.mas.ncl.ac.uk/~ncfb

A Theory for steady and self-sustained premixed combustion waves

Based on the compressible Navier – Stokes equations for reactive flow problems, an eigenvalue problem for the steady and self-sustained premixed combustion wave propagation is developed. The eigenvalue problem is analytically solved and a set of analytic formulae for description of the wave propagation is found out. The analytic formulae are actually the exact solution of the eigenvalue problem in the form of integration, based on which author develops an iterative and numerical algorithm for calculation of the steady and self-sustained premixed combustion wave propagation and its speed. In order to explore the mathematical model and test the computational method developed in this paper, three groups of combustion wave propagation modes are calculated. The computational results show that the non-trivial modes of the combustion wave propagation exist and their distribution is not continuous but discrete

ソフトテニスのグラウンドストローク技術における筋活動と動作画像分析 : フットワークに着目して

The average properties of Lagrangian motion of test solid particles in helium II counterflow are analyzed. We consider the case where the flow of normal fluid is laminar and uniform, and the turbulence in the superfluid component manifests itself as a tangle of quantized vortices. The model employed in this paper has certain limitations: It assumes that particles do not disturb superfluid vortices and neglects the possibility of trapping particles by vortices. We estimate the time and length scales of the particle motion, and calculate the statistical properties of the particle motion as well as the statistical properties of superfluid turbulence along particle trajectories. We analyze the alignment between particle velocities and the superfluid velocity induced by the vortex tangle, and calculate the statistical properties of proximity between particles and quantized vortices. We expect these statistical properties to be important in the context of experimental PIV measurements in the thermal counterflow

Differential approximation for Kelvin-wave turbulence

I present a nonlinear differential equation model (DAM) for the spectrum of Kelvin waves on a thin vortex filament. This model preserves the original scaling of the six-wave kinetic equation, its direct and inverse cascade solutions, as well as the thermodynamic equilibrium spectra. Further, I extend DAM to include the effect of sound radiation by Kelvin waves. I show that, because of the phonon radiation, the turbulence spectrum ends at a maximum frequency $\omega^* \sim (\epsilon^3 c_s^{20} / \kappa^{16})^{1/13}$ where $\epsilon$ is the total energy injection rate, $c_s$ is the speed of sound and $\kappa$ is the quantum of circulation.Comment: Prepared of publication in JETP Letter

Vortex density spectrum of quantum turbulence

The fluctuations of the vortex density in a turbulent quantum fluid are deduced from local second-sound attenuation measurements. These measurements are performed with a micromachined open-cavity resonator inserted across a flow of turbulent He-II near 1.6 K. The power spectrum of the measured vortex line density is compatible with a (-5/3) power law. The physical interpretation, still open, is discussed.Comment: Submitted to Europhys. Let