Helical structure of longitudinal vortices embedded in turbulent wall-bounded flow

Embedded vortices in turbulent wall-bounded flow over a flat plate, generated by a passive rectangular vane-type vortex generator with variable angle $\beta$ to the incoming flow in a low-Reynolds number flow ($Re=2600$ based on the inlet grid mesh size $L=0.039\;$m and free stream velocity $U_{\infty} = 1.0\;$m s$^{-1}$) have been studied with respect to helical symmetry. The studies were carried out in a low-speed closed-circuit wind tunnel utilizing Stereoscopic Particle Image Velocimetry (SPIV). The vortices have been shown to possess helical symmetry, allowing the flow to be described in a simple fashion. Iso-contour maps of axial vorticity revealed a dominant primary vortex and a weaker secondary one for $20^{\circ} \leq \beta \leq 40^{\circ}$. For angles outside of this range, the helical symmetry was impaired due to the emergence of additional flow effects. A model describing the flow has been utilized, showing strong concurrence with the measurements, even though the model is decoupled from external flow processes that could perturb the helical symmetry. The pitch, vortex core size, circulation and the advection velocity of the vortex all vary linearly with the device angle $\beta$. This is important for flow control, since one thereby can determine the axial velocity induced by the helical vortex as well as the swirl redistributing the axial velocity component for a given device angle $\beta$. This also simplifies theoretical studies, \eg to understand and predict the stability of the vortex and to model the flow numerically

Development of multicomponent hybrid powders based on titanium and niobium carbides as a promising material for laser cladding

Multicomponent hybrid TiC-NbC(Zr, Si) powder was developed and manufactured by mechanosynthesis in a high-energy vibratory ball mill. High-purity fragmented TiC, NbC, Zr and Si powders were selected and mixed in a ratio of 60:15:10:15 at.%, respectively, to manufacture the above powder. Several modes of mechanosynthesis were chosen for the experiment: 3, 6, 9 and 12 hours. Scanning electron microscopy (SEM) and X-Ray diffraction (XRD) analyzes were used to study the morphology, chemical and phase compositions of the obtained TiC-NbC(Zr, Si) powder. The SEM analysis confirmed the presence of all TiC, NbC, Zr and Si components in the final powder regardless of the mechanosynthesis time. However, the XRD analysis showed that after 9 and 12 hours of mechanosynthesis, the Zr and Si diffraction lines are completely absent. This occurs due to the dissolution of the Zr and Si elements in titanium and niobium carbides. In addition, it has been established that more than 6 hours are required to synthesize finely dispersed TiC-NbC(Zr, Si) powder. The study results can be useful for optimization of the mechanosynthesis process

Quantum Billiards with Surface Scattering: Ballistic Sigma-Model Approach

Statistical properties of energy levels and eigenfunctions in a ballistic system with diffusive surface scattering are investigated. The two-level correlation function, the level number variance, the correlation function of wavefunction intensities, and the inverse participation ratio are calculated.Comment: 4 pages REVTEX, two figures included as eps file

Mechanical and Structural Characterization of Laser-Cladded Medium-Entropy FeNiCr-B4C Coatings

Equiatomic medium-entropy alloy (MEA) FeNiCr-B4C (0, 1, and 3 wt.% B4C) coatings were deposited onto an AISI 1040 steel substrate using pulsed laser cladding. Based on an SEM microstructural analysis, it was found that the cross-sections of all the obtained specimens were characterized by an average coating thickness of 400 ± 20 μm, a sufficiently narrow (100 ± 20 μm) “coating–substrate” transition zone, and the presence of a small number of defects, including cracks and pores. An XRD analysis showed that the formed coatings consisted of a single face-centered cubic (FCC) γ-phase and the space group Fm-3m, regardless of the B4C content. However, additional TEM analysis of the FeNiCr coating with 3 wt.% B4C revealed a two-phase FCC structure consisting of grains (FCC-1 phase, Fm-3m) up to 1 µm in size and banded interlayers (FCC-2 phase, Fm-3m) between the grains. The grains were clean with a low density of dislocations. Raman spectroscopy confirmed the presence of B4C carbides inside the FeNiCr (1 and 3 wt.% B4C) coatings, as evidenced by detected peaks corresponding to amorphous carbon and peaks indicating the stretching of C-B-C chains. The mechanical characterization of the FeNiCr-B4C coatings specified that additions of 1 and 3 wt.% B4C resulted in a notable increase in microhardness of 16% and 38%, respectively, with a slight decrease in ductility of 4% and 10%, respectively, compared to the B4C-free FeNiCr coating. Thus, the B4C addition can be considered a promising method for strengthening laser-cladded MEA FeNiCr-B4C coatings. © 2023 by the authors.National Natural Science Foundation of China, NSFC: 52105351; Ministry of Education and Science of the Russian Federation, Minobrnauka: 121102900049-1, 122021000033-2, 122021000036-3This research was carried out within the state assignment of the Ministry of Science and Higher Education of the Russian Federation (themes “Additivity” No. 121102900049-1; “Structure” No. 122021000033-2; and “Spin” No. 122021000036-3) using the equipment of the Collaborative Access Center “Testing Center of Nanotechnology and Advanced Materials” of the IMP UB RAS, and supported by the National Natural Science Foundation of China (Grant No. 52105351)

Level and Eigenfunction Statistics in Billiards with Surface Scattering

Statistical properties of billiards with diffusive boundary scattering are investigated by means of the supersymmetric sigma-model in a formulation appropriate for chaotic ballistic systems. We study level statistics, parametric level statistics, and properties of electron wavefunctions. In the universal regime, our results reproduce conclusions of the random matrix theory, while beyond this regime we obtain a variety of system-specific results determined by the classical dynamics in the billiard. Most notably, we find that level correlations do not vanish at arbitrary separation between energy levels, or if measured at arbitrarily large difference of magnetic fields. Saturation of the level number variance indicates strong rigidity of the spectrum. To study spatial correlations of wavefunction amplitudes, we reanalyze and refine derivation of the ballistic version of the sigma-model. This allows us to obtain a proper matching of universal short-scale correlations with system-specific ones.Comment: 19 pages, 5 figures included. Minor corrections, references adde

Atmospheric precipitable water in Estonia, 1990–2001

Knowledge on atmospheric precipitable water is necessary as input to hydrological, energetic and radiation models. Short historical review of parameterization of precipitable water is given. For Tallinn (Estonia), simple formulas are proposed to calculate precipitable water from observations of surface water vapor pressure. Seasonal changes of precipitable water in Tallinn are expressed by time series for 1990–2001 as well as by tabulation of monthly averages for this period. Parameterization of precipitable water, decadal time series, and tabulation of monthly averages are also given for three neighboring stations — St. Petersburg (Russia), Jokioinen and Sodankylä (Finland)

Development of multicomponent hybrid powders based on titanium and niobium carbides as a promising material for laser cladding

Multicomponent hybrid TiC-NbC(Zr, Si) powder was developed and manufactured by mechanosynthesis in a high-energy vibratory ball mill. High-purity fragmented TiC, NbC, Zr and Si powders were selected and mixed in a ratio of 60:15:10:15 at.%, respectively, to manufacture the above powder. Several modes of mechanosynthesis were chosen for the experiment: 3, 6, 9 and 12 hours. Scanning electron microscopy (SEM) and X-Ray diffraction (XRD) analyzes were used to study the morphology, chemical and phase compositions of the obtained TiC-NbC(Zr, Si) powder. The SEM analysis confirmed the presence of all TiC, NbC, Zr and Si components in the final powder regardless of the mechanosynthesis time. However, the XRD analysis showed that after 9 and 12 hours of mechanosynthesis, the Zr and Si diffraction lines are completely absent. This occurs due to the dissolution of the Zr and Si elements in titanium and niobium carbides. In addition, it has been established that more than 6 hours are required to synthesize finely dispersed TiC-NbC(Zr, Si) powder. The study results can be useful for optimization of the mechanosynthesis process