Lagrangian Particle Statistics in Turbulent Flows from a Simple Vortex Model

The statistics of Lagrangian particles in turbulent flows is considered in the framework of a simple vortex model. Here, the turbulent velocity field is represented by a temporal sequence of Burgers vortices of different circulation, strain, and orientation. Based on suitable assumptions about the vortices' statistical properties, the statistics of the velocity increments is derived. In particular, the origin and nature of small-scale intermittency in this model is investigated both numerically and analytically

Fe-Si-B čestice praha studirane EBSD analizom područja kod velikog uvećanja

We produced amorphous Fe–Si–B soft-magnetic powder using water atomisation. During annealing the powder particles developed a nanocrystalline structure, and annealing at over 700°C led to the formation of ferrite and boride phases. Here we present a high-magnification electron backscatter diffraction (EBSD) mapping analysis of the powder particles, in combination with a field-emission-gun scanning electron microscopy (FEGSEM) analysis. Some of the problems associated with the preparation of the powder particles for the EBSD analysis as well as the drift problems occurring during the EBSD mapping are reported.Amorfni Fe-Si-B magnetni prahovi su bili proizvedeni vodenom atomizacijom. Za vrijeme procesa žarenja čestice praha dobiju nanokristaliničnu strukturu. Žarenje iznad 700 °C prouzroči nastanak feritne i boridne faze. U ovom članku je prikazana EBSD analiza područja kod velikog uvećanja prašnih čestica u kombinaciji s FESEM. Istaknuti su problemi pripreme čestica praha za EBSD analizu kao i problem pomicanja uzorka tokom EBSD analize

Multiscale nature of the dissipation range in gyrokinetic simulations of Alfv\'enic turbulence

Nonlinear energy transfer and dissipation in Alfv\'en wave turbulence are analyzed in the first gyrokinetic simulation spanning all scales from the tail of the MHD range to the electron gyroradius scale. For typical solar wind parameters at 1 AU, about 30% of the nonlinear energy transfer close to the electron gyroradius scale is mediated by modes in the tail of the MHD cascade. Collisional dissipation occurs across the entire kinetic range $k_\perp\rho_i\gtrsim 1$. Both mechanisms thus act on multiple coupled scales, which have to be retained for a comprehensive picture of the dissipation range in Alfv\'enic turbulence.Comment: Made several improvements to figures and text suggested by referee

Gyrokinetic Large Eddy Simulations

The Large Eddy Simulation (LES) approach is adapted to the study of plasma microturbulence in a fully three-dimensional gyrokinetic system. Ion temperature gradient driven turbulence is studied with the {\sc GENE} code for both a standard resolution and a reduced resolution with a model for the sub-grid scale turbulence. A simple dissipative model for representing the effect of the sub-grid scales on the resolved scales is proposed and tested. Once calibrated, the model appears to be able to reproduce most of the features of the free energy spectra for various values of the ion temperature gradient

Stability of MC Carbide Particles Size in Creep Resisting Steels

Theoretical analysis of the dependence microstructure creep rate. Discussion on the effects of carbide particles size and their distribution on the base of accelerated creep tests on a steel X20CrMoV121 tempered at 800 °C. Analysis of the stability of carbide particles size in terms of free energy of formation of the compound. Explanation of the different effect of VC and NbC particles on accelerated creep rate

Free energy cascade in gyrokinetic turbulence

In gyrokinetic theory, the quadratic nonlinearity is known to play an important role in the dynamics by redistributing (in a conservative fashion) the free energy between the various active scales. In the present study, the free energy transfer is analyzed for the case of ion temperature gradient driven turbulence. It is shown that it shares many properties with the energy transfer in fluid turbulence. In particular, one finds a forward (from large to small scales), extremely local, and self-similar cascade of free energy in the plane perpendicular to the background magnetic field. These findings shed light on some fundamental properties of plasma turbulence, and encourage the development of large eddy simulation techniques for gyrokinetics.Comment: 4 pages, 2 Postscript figure