Search

The main goal of this paper is to establish a better understanding of the relationship between a Strouhal number and surface roughness. Hot-wire anemometry was used to evaluations the characteristics of turbulent flow behind circular cylinders with different relative roughness 0% (smooth surface) 0.83%, 1.67%, 3.33%, and 6.67%. At the experimental investigation, the Reynolds number based on the cylinder diameter was 5 × 103 < Red < 2 × 104. The obtained data showed that the Strouhal number decreased with increasing roughness. While, the dissipation rate decreases, and the value of the Kolmogorov and Taylor microscales increases. Also, spectral analysis of streamwise velocity fluctuations allowed us to estimate the location of the vortex-shedding frequency which at growing roughness tends to reduce.

The precise energy level alignment between a metal electrode and an organic semiconductor is required to reduce contact resistance and enhance the efficiency of organic-semiconductor-based devices. One of the ways is to include interlayers that mediate the energy level alignment, i.e., charge injection layers (CILs). Here we introduce the monolayer thick CILs based on the aromatic carboxylic acids that can induce the energy level shift in the subsequent layers by up to 0.8 eV. By gradual chemical transformation of the as-deposited molecules, we achieve a highly tunable energy level shift in the range of 0.5 eV. We reveal that the position of both the work function and energy-level position in the CIL increases linearly with the density of induced dipoles. The energy level position of subsequent layers changes in the same way as the CIL. Our results thus connect the energy alignment quantities, i.e., energy level positions of both CIL and subsequent layers and sample work function. The high tunability would allow precise tuning of the active layers deposited on the CIL, which marks the path towards efficient charge injection layers on metal electrodes.

International audience; Graphene is a 2D material that displays excellent electronic transport properties with prospective applications in many fields. Inducing and controlling magnetism in the graphene layer, for instance by proximity of magnetic materials, may enable its utilization in spintronic devices. This paper presents fabrication and detailed characterization of single-layer graphene formed on the surface of epitaxial FeRh thin films. The magnetic state of the FeRh surface can be controlled by temperature, magnetic field or strain due to interconnected order parameters. Characterization of graphene layers by X-ray Photoemission and X-ray Absorption Spectroscopy, Low-Energy Ion Scattering, Scanning Tunneling Microscopy, and Low-Energy Electron Microscopy shows that graphene is single-layer, polycrystalline and covers more than 97% of the substrate. Graphene displays several preferential orientations on the FeRh(001) surface with unit vectors of graphene rotated by 30°, 15°, 11°, and 19° with respect to FeRh substrate unit vectors. In addition, the graphene layer is capable to protect the films from oxidation when exposed to air for several months. Therefore, it can be also used as a protective layer during fabrication of magnetic elements or as an atomically thin spacer, which enables incorporation of switchable magnetic layers within stacks of 2D materials in advanced devices.; Le graphène est un matériau 2D qui présente d’excellentes propriétés de transport électronique avec des applications potentielles dans de nombreux domaines. L’induction et le contrôle du magnétisme dans la couche de graphène, par exemple par la proximité de matériaux magnétiques, peuvent permettre son utilisation dans des dispositifs spintroniques. Une collaboration entre le LPMC, CEITEC-BUT à Brno (République tchèque), CFEL à Hambourg (Allemagne) et la ligne TEMPO à Soleil, présente la fabrication et la caractérisation détaillée du graphène monocouche formé à la surface de couches minces épitaxiales de FeRh. L’état magnétique de la surface FeRh peut être contrôlé par la température, le champ magnétique ou la déformation en raison de paramètres d’ordre interconnectés. La caractérisation des couches de graphène par photoémission de rayons X et spectroscopie d’absorption de rayons X, diffusion d’ions à faible énergie, microscopie à effet tunnel et microscopie électronique à faible énergie montre que le graphène est monocouche, polycristallin et couvre plus de 97% du substrat. Le graphène affiche plusieurs orientations préférentielles sur la surface de FeRh (001) par rapport aux vecteurs unitaires de substrat FeRh. Finalement, la couche de graphène est capable de protéger les films de l’oxydation lorsqu’ils sont exposés à l’air pendant plusieurs mois. Par conséquent, il peut également être utilisé comme couche protectrice lors de la fabrication d’éléments magnétiques ou comme séparateur atomique, ce qui permet l’incorporation de couches magnétiques actives entre matériaux bidimensionnels pour les dispositifs du futur.

Circular cylinder in cross-flow was studied experimentally using time-resolved PIV technique. Reynolds numbers based on the cylinder diameter were in range from 3 850 up to 19 260. Effect of the Reynolds number on wake time-mean topology as well as the dynamical behavior is studied. The wake dynamics is analyzed using the OPD method. The Oscillation Pattern Decomposition method provides unique information on dynamical patterns topology and oscillations in the flow-field. The pseudo-periodical wavy patterns have been detected within the cylinder wake. The parameters of this behavior are Reynolds number dependent, especially their sizes and shapes. Typical flow-field topologies are presented in the paper. The study is of qualitative character as flow boundary conditions are not well defined.

A novel method of contactless measurement of turbine blade vibration during increasing (decreasing) operational speed is presented. The method is based on evaluation of time differences of blade passages along contactless sensors placed on the stator, and substitutional data correction using a numerical model. The method has been verified both numerically and experimentally. For the experimental research on the bladed model wheel of the Institute of Thermomechanics and subsequent applications under operating conditions, two new types of sensors functioning on magnetoresistive and induction principle have been developed.

The new test approach for evaluation of thermo-visco-elastic parameters of elastomers is designed, realized and discussed herein. The main attention is devoted to a kinematically excited rubber beam specimen under transient resonant sweep excitation. Mechanical material characteristics, i.e. Young modulus and loss angle, are identified from analytical formulas of frequency response function based on measured dynamic loading and response signals. Heat material constants were estimated indirectly by numerical thermo-mechanical FE model and matching numerical and experimental results. The 'sweep' regime tests serve for estimation of thermal and mechanical dependences. Long-term fatigue tests with cyclic resonant loading enable analysis of material degradation, such as hardening and permanent deformations.

The vibrodiagnostic system VDS-UT2 was developed for the investigation of a tram wheel operational vibration and deformation. The system consists of measuring, acquisition and processing tools for monitoring of tram wheels and their operational parameters, e.g. static and dynamic deformation, displacement, velocity, acceleration and temperature. The measuring on a rotating wheel is realized both by telemetric and by top-timing system. Measured data can be remote-observed via Internet connection with the control PC. The acquisition system by Starmans Electronics s.r.o. provides continuous analog and digital data logging. The control PC processes digital logical signals and evaluates time intervals representing circumferential turning angles between disk and rim. In the contribution, the diagnostic system is described and the measured data including damage analysis are presented.