Non-linear dynamics of Kelvin-Helmholtz unstable magnetized jets: three-dimensional effects

A numerical study of the Kelvin-Helmholtz instability in compressible magnetohydrodynamics is presented. The three-dimensional simulations consider shear flow in a cylindrical jet configuration, embedded in a uniform magnetic field directed along the jet axis. The growth of linear perturbations at specified poloidal and axial mode numbers demonstrate intricate non-linear coupling effects. The physical mechanims leading to induced secondary Kelvin-Helmholtz instabilities at higher mode numbers are identified. The initially weak magnetic field becomes locally dominant in the non-linear dynamics before and during saturation. Thereby, it controls the jet deformation and eventual breakup. The results are obtained using the Versatile Advection Code [G. Toth, Astrophys. Lett. Comm. 34, 245 (1996)], a software package designed to solve general systems of conservation laws. An independent calculation of the same Kelvin-Helmholtz unstable jet configuration using a three-dimensional pseudo-spectral code gives important insights into the coupling and excitation events of the various linear mode numbers.Comment: 10 (+7) pages, 6 figures, accepted for Phys. Plasmas 6, to appear 199

A Comparison of LPV Gain Scheduling and Control Contraction Metrics for Nonlinear Control

Gain-scheduled control based on linear parameter-varying (LPV) models derived from local linearizations is a widespread nonlinear technique for tracking time-varying setpoints. Recently, a nonlinear control scheme based on Control Contraction Metrics (CCMs) has been developed to track arbitrary admissible trajectories. This paper presents a comparison study of these two approaches. We show that the CCM based approach is an extended gain-scheduled control scheme which achieves global reference-independent stability and performance through an exact control realization which integrates a series of local LPV controllers on a particular path between the current and reference states.Comment: IFAC LPVS 201

Sums of products of Ramanujan sums

The Ramanujan sum $c_n(k)$ is defined as the sum of $k$-th powers of the primitive $n$-th roots of unity. We investigate arithmetic functions of $r$ variables defined as certain sums of the products $c_{m_1}(g_1(k))...c_{m_r}(g_r(k))$, where $g_1,..., g_r$ are polynomials with integer coefficients. A modified orthogonality relation of the Ramanujan sums is also derived.Comment: 13 pages, revise

Growth of ZnO nanostructures on Si by means of plasma immersion ion implantation and deposition

Crystalline zinc oxide (ZnO) nanostructures have been grown on Si substrates by means of Plasma Based Ion Implantation and Deposition (PIII&D) at temperature of about 300 0C and in the presence of an argon glow discharge. In the process a crucible filled with small pieces of metallic zinc plays the role of the anode of the discharge itself, being polarized by positive DC voltage of about 400V. Electrons produced by thermionic emission by an oxide cathode (Ba, Sr, Ca)O impact this crucible, causing its heating and vaporization of Zn. Partial ionization of Zn atoms takes place due to collisions with plasma particles. High negative voltage pulses (7 kv/40μs/250Hz) applied to the sample holder cause the implantation of metallic zinc into Si surface, while Zn deposition happens between pulses. After annealing at 700 0C, strong UV and various visible photoluminescence bands are observed at room temperature, as well as the presence of ZnO nanoparticles. The coated surface was characterized in detail using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. XRD indicated the presence of only ZnO peaks after annealing. The composition analysis by EDS revealed distinct Zn/O stoichiometry relation depending on the conditions of the process. AFM images showed the formation of columns in the nanoscale range. Topography viewed by SEM showed the formation of structures similar to cactus with nanothorns. Depth analysis performed by XPS indicated an increase of concentration of metallic Zn with increasing depth and the exclusive presence of ZnO for outer regions. PIII&D allowed to growing nanostructures of ZnO on Si without the need of a buffer layer

The effect of periphyton on the light environment and production of Potamogeton perfoliatus L. in the mesotrophic basin of Lake Balaton

Light within the littoral zone affects the productivity and interaction between periphyton and its macrophyte substrate. The effect of periphyton on macrophyte photosynthesis, seasonal variation and vertical distribution of periphyton on artificial substrates (plastic strips), and the effect of periphyton on the light environment was studied in Lake Balaton. Data showed that an average of 4.1±0.4 mg (dry weight) cm-2 of periphyton had accumulated on the plastic strips after 8.8±0.4 days. This biomass corresponded to 294±30 μg m-2 chl-a of epiphytic algae and blocked 92.3±0.8 % of the depth specific radiation. Seasonal variation and specific vertical distribution of periphyton were observed. The most active time of periphyton accumulation corresponded to spring up until mid-June. Later in the year, the amount of periphyton significantly decreased. The optimal conditions for periphyton accumulation were at 30-40 cm depth. Most of the light reaching the adaxial leaf surface was attenuated by periphyton, decreasing the production of Potamogeton perfoliatus by 60-80%. This increased the importance of backscattered light that corresponded to 10-15% of the macrophyte production. A smaller part of the periphyton consisted of precipitated inorganic material, while epiphytic algae, making up the majority of the periphyton, were connected to both benthic (dominantly benthic penales) and pelagic (very close seasonal dynamics of pelagic and epiphytic biomass) algae. Periphyton affects macrophyte production especially in spring and in the upper water layers even in a mesotrophic water body. This increases the importance of the light absorbed through the abaxial side of the leaf and confirm the role of periphyton in transition from clear to turbid water states

Papír-ceruza és számítógépes tesztek eredményeinek összehasonlító vizsgálata

The transition from paper-and-pencil (PP) to computer-based (CB) assessment has gained a growing interest in the last decades. This shift may lead to achievement differences. Several analyses did not find significant differences between students' PP and CB performance, however, a number of studies report substantial differences across media. This paper provides an overview about the nature and importance of CB testing. Then, the most important results of the first Hungarian pilot study exploring the medium effect are presented. The PP inductive reasoning test created by Csapó (1994) was administered to a sample of $`h graders (N=843). Few weeks later, the same items along with questionnaires for background variables in an online form were administered by means of the TAO platform. Analysis of the data showed that students performed significantly better at the PP test compared to the CB one. On the other hand, the subtest-level analysis highlighted various achievement differences e.g. the multiple choice items indicated better results in a CB environment and the subtests with open-ended items seemed easier in PP format. This paper presents the achievement differences based on background variables between genders and the relationship between computer use and standardized test scores

Periodic forcing in viscous fingering of a nematic liquid crystal

We study viscous fingering of an air-nematic interface in a radial Hele-Shaw cell when periodically switching on and off an electric field, which reorients the nematic and thus changes its viscosity, as well as the surface tension and its anisotropy (mainly enforced by a single groove in the cell). We observe undulations at the sides of the fingers which correlate with the switching frequency and with tip oscillations which give maximal velocity to smallest curvatures. These lateral undulations appear to be decoupled from spontaneous (noise-induced) side branching. We conclude that the lateral undulations are generated by successive relaxations between two limiting finger widths. The change between these two selected pattern scales is mainly due to the change in the anisotropy. This scenario is confirmed by numerical simulations in the channel geometry, using a phase-field model for anisotropic viscous fingering.Comment: completely rewritten version, more clear exposition of results (14 pages in Revtex + 7 eps figures