Stationary Regime Research Articles

Sharp Spectral Variations of the Ultrafast Transient Light Extinction by Bimetallic Nanoparticles in the Near‐UV

AbstractNoble metal nanoparticles exhibit localized plasmon resonance modes that span the visible and near‐infrared spectral ranges and have many applications. Modifying the size, shape, and composition of the nanoparticles changes the number of modes and their properties. The characteristics of these modes are transiently affected when illuminating the nano‐objects with ultrashort laser pulses. Here, core–shell gold–silver nanocuboids are synthesized and their spectral signature in the stationary and ultrafast transient regimes are measured. Their dipolar transverse mode vanishes with increasing Ag‐shell thickness, while higher‐order modes grow in the near‐ultraviolet range where no plasmon resonance can be generated with single noble metal nanoparticles. These higher‐energy modes are associated with sharp spectral variations of the ultrafast transient light extinction by the bimetallic nanocuboids. By carrying out a theoretical investigation, the different contributions to this response are broken down and a physical interpretation of its spectral profile is provided. The transient optical signal is then shown to reveal resonance modes hidden in the stationary regime spectra.

ASPECTE CALITATIVE ȘI CANTITATIVE ALE FENOMENUL DE FEROREZONANȚĂ ÎN INSTALAȚIILE ELECTRICE DE JOASĂ TENSIUNE

The phenomenon of ferroresonance is generated by the interaction between nonlinear magnetic devices and capacitive elements in an electrical installation in which losses are reduced and which are constantly supplied by at least one energy source. Feroresonance is manifested by the appearance of overvoltages and overcurrents in the installation with strongly distorted waveforms. Also, the phenomenon is accompanied by other disturbances of the quality of electricity (voltage fluctuations, asymmetries, noise, etc.), which propagate in the network affecting the proper functioning of the entire installation. In addition, unlike linear resonance, ferroresonance allows the manifestation of several stable states (modes) for the same parameters of the network, these being imposed by the initial conditions in the installation and the moment of occurrence of the phenomenon. Thus, the vulnerability to low resonance of a low voltage installation has become an indicator of electricity quality. This paper presents a procedure for calculating and investigating this phenomenon based on the analysis of numerical solutions of systems of differential equations (nonlinear and non-autonomous), which models the transient phenomena that initiate the appearance of ferroresonance (usually switching processes). Also, modern means of investigation are used (3D visualizations in the phase plan or Poincaré diagrams), imposed by the difficulty of the quantitative analysis both in dynamic regime and in stationary regime of ferroresonance. In addition, methods and procedures are proposed to mitigate the effects of the ferroresonance phenomenon on equipment or network elements in electrical distribution installations.

On the effect of reversing the direction of convection in a two-component fluid

The theoretical problem of convection in a two-component medium (for example, in salt water) after “switching on” the sources/sinks of heat and admixture on an infinite vertical boundary is considered. Generally speaking, the degree of influence of the two components on convection is different due to the difference in their diffusion velocities. We suppose that heat diffuses much faster than the admixture and then the heated/cooled region at the lateral boundary is much thicker than the region of propagation of the impurity concentration disturbance. Therefore, convection due to temperature deviations is less affected by viscosity than due to deviations in admixture concentration. As a result, even a relatively weak “thermal” source of buoyancy can determine the direction of convection for some time in spite of the action of a more intense “concentration” source of buoyancy of a different sign. But if a vertical layer of a medium of finite thickness is considered, then a stationary regime is established over time, in which the direction of convection is determined by the total source of buoyancy, so that the direction of convection can change sign.

Mathematical modeling of phytoplankton populations evolution in the Azov Sea

The paper describes the construction of a three-dimensional mathematical model of biogeochemical processes, considering the salinity and temperature influence on the phytoplankton populations development. The paper proposes a new difference scheme for solving convection-diffusion-reaction problems at large values of the Peclet grid number (2<Pe≤20), which is a linear combination of the central and the upwind leapfrog difference schemes. The three-layer difference scheme is more accurate than the traditional upwind leapfrog difference scheme for problems where convection prevails over diffusion. The construction of discrete equations for solving the problem of biogeochemical cycles on the basis of the scheme considering the filling of cells is described. The stationary regimes of phytoplankton dynamics problem were researched considering the transformation of phosphorus, nitrogen and silicon forms. Results of software complex, which allows to simulate biogeochemical processes in the Azov Sea, were described. The software package allows to forecast the dynamics of the Azov Sea ecosystem development in the conditions of modern salinization.

Wurtzite phase control for self-assisted GaAs nanowires grown by molecular beam epitaxy

The accurate control of the crystal phase in III–V semiconductor nanowires (NWs) is an important milestone for device applications. Although cubic zinc-blende (ZB) GaAs is a well-established material in microelectronics, the controlled growth of hexagonal wurtzite (WZ) GaAs has thus far not been achieved successfully. Specifically, the prospect of growing defect-free and gold catalyst-free wurtzite GaAs would pave the way towards integration on silicon substrate and new device applications. In this article, we present a method to select and maintain the WZ crystal phase in self-assisted NWs by molecular beam epitaxy. By choosing a specific regime where the NW growth process is a self-regulated system, the main experimental parameter to select the ZB or WZ phase is the V/III flux ratio. Using an analytical growth model, we show that the V/III flux ratio can be finely tuned by changing the As flux, thus driving the system toward a stationary regime where the wetting angle of the Ga droplet can be maintained in the range of values allowing the formation of pure WZ phase. The analysis of the in situ reflection high energy electron diffraction evolution, combined with high-resolution scanning transmission electron microscopy (TEM), dark field TEM, and photoluminescence all confirm the control of an extended pure WZ segment, more than a micrometer long, obtained by molecular beam epitaxy growth of self- assisted GaAs NWs with a V/III flux ratio of 4.0. This successful controlled growth of WZ GaAs suggests potential benefits for electronics and opto-electronics applications.

Effect of Temperature on Channel Compensation in Optical Camera Communication

General-purpose Complementary Metal Oxide Semiconductor (CMOS) sensors perform the image desegregation in three channels (red, green, and blue) as a result of a band-pass wavelength filtering carried out using Foveon or Bayer filters. This characteristic can be used in Optical Camera Communication (OCC) systems for increasing the links’ data rate by introducing Wavelength Division Multiplexing (WDM) or Color Shift Keying (CSK) modulation schemes. However, these techniques need a compensation stage to mitigate the cross-talk between channels introduced by the filters. This compensation is performed by a Channel State Information (CSI) estimation and a zero-forcing compensation scheme. The impact of the temperature effects of light-emitting diode (LED) emissions on the zero-forcing compensation scheme’s performance has not been analyzed in depth. This work presents a comprehensive methodology and experimental characterization of this impact for Foveon and Bayer-based image sensors, assuming that the CSI is estimated under temperature conditions different from the LED’s stationary temperature regime. Besides, Signal-to-Interference-plus-Noise Ratio (SINR) and Bit Error Rate (BER) performance metrics are presented in order to estimate the repercussion in an OCC link. The results reveal that the Foveon sensor obtains more unsatisfactory performance than the Bayer-based sensor. On the other hand, the blue band is the most penalized by the thermal effect.

Diffusion Entropy vs. Multiscale and Rényi Entropy to Detect Progression of Autonomic Neuropathy.

We review the literature to argue the importance of the occurrence of crucial events in the dynamics of physiological processes. Crucial events are interpreted as short time intervals of turbulence, and the time distance between two consecutive crucial events is a waiting time distribution density with an inverse power law (IPL) index μ, with μ < 3 generating non-stationary behavior. The non-stationary condition is characterized by two regimes of the IPL index: (a) perennial non-stationarity, with 1 < μ < 2 and (b) slow evolution toward the stationary regime, with 2 < μ < 3. Human heartbeats and brain dynamics belong to the latter regime, with healthy physiological processes tending to be closer to the border with the perennial non-stationary regime with μ = 2. The complexity of cognitive tasks is associated with the mental effort required to address a difficult task, which leads to an increase of μ with increasing task difficulty. On this basis we explore the conjecture that disease evolution leads the IPL index μ moving from the healthy condition μ = 2 toward the border with Gaussian statistics with μ = 3, as the disease progresses. Examining heart rate time series of patients affected by diabetes-induced autonomic neuropathy of varying severity, we find that the progression of cardiac autonomic neuropathy (CAN) indeed shifts μ from the border with perennial variability, μ = 2, to the border with Gaussian statistics, μ = 3 and provides a novel, sensitive index for assessing disease progression. We find that at the Gaussian border, the dynamical complexity of crucial events is replaced by Gaussian fluctuation with long-time memory.

Large-scale thermalization, prethermalization, and impact of temperature in the quench dynamics of two unequal Luttinger liquids

We study the effect of a quantum quench between two tunnel coupled Tomonaga-Luttinger liquids (TLLs) with different speed of sound and interaction parameter. The quench dynamics is induced by switching off the tunnelling and letting the two systems evolve independently. We fully diagonalize the problem within a quadratic approximation for the initial tunnelling. Both the case of zero and finite temperature in the initial state are considered. We focus on correlation functions associated with the antisymmetric and symmetric combinations of the two TLLs (relevant for interference measurements), which turn out to be coupled due to the asymmetry in the two systems' Hamiltonians. The presence of different speeds of sound leads to multiple lightcones separating different decaying regimes. In particular, in the large time limit, we are able to identify a prethermal regime where the two-point correlation functions of vertex operators of symmetric and antisymmetric sector can be characterized by two emerging effective temperatures, eventually drifting towards a final stationary regime that we dubbed $quasi-thermal$, well approximated at large scale by a thermal-like state, where these correlators become time independent and are characterized by a unique correlation length. If the initial state is at equilibrium at non-zero temperature $T_0$, all the effective temperatures acquire a linear correction in $T_0$, leading to faster decay of the correlation functions. Such effects can play a crucial role for the correct description of currently running cold atoms experiments.

Kinetic Monte Carlo simulation of heterogeneous and homogeneous radio-oxidation of a polymer

Ion irradiation is known to generate well-localized cylindrical ionization and excitation tracks in materials. The resulting radiation chemistry in organic material is thus initiated by a strongly heterogeneous source of radicals. We present a comparison of heterogeneous and homogeneous radio-oxidation kinetics of a polymer. Our analysis is based on a kinetic Monte Carlo simulation of the diffusion-reaction process of radicals in the material. We consider, as a numerical test case, the radio-oxidation of polyethylene by 5 MeV He ions. For heterogeneous kinetics, we show that the spatial distribution of radicals reached in the stationary regime is not homogeneous at the sub-micrometer scale. Both kinetics are quite comparable regarding the variation of the radical concentration with the irradiation time. However, the yield of peroxy radicals reached in stationary regime is significantly lower in the case of heterogeneous kinetics. Moreover, the propagation kinetics characterized by the ratio of POOH over POOP products exhibits very different behaviors with respect to oxygen concentration.

Thermal imaging of vibrational energy dissipated in a 2D acoustic black hole pit

An Acoustic Black Hole (ABH) is an efficient way to passively damp flexural vibrations in plates. It consists of an axisymmetric pit whose thickness gradually decreases down to a small value at its center, thus reducing progressively the local wavelength and increasing the amplitude of vibrations. A trapping effect occurs in the central area, in which a small patch of visco-elastic coating is sufficient to dissipate the localized energy in a very efficient way. The dissipated mechanical power produces a local heat release and so a local increase in temperature that is recorded by means of a high-speed infrared camera. In the stationary regime, the local temperature increase is the signature of the ABH efficiency. In the transient regime and before a characteristic time, the zones where the damping mechanisms are active and where vibrational energy is dissipated can be localized. The key role played by the constraining layer of visco-elastic material in the ABH efficiency is notably shown experimentally.

Теплосъём и термостабилизация высокотемпературных поверхностей диспергированным потоком теплоносителя

The effectiveness of stabilizing the surface temperature by a dispersed coolant flow is experimentally studied on a bench simulating energy intensive elements of thermonuclear installations A test section in which the maximum heat flux density can be obtained when being subjected to high-frequency heating was developed, manufactured, and assembled. The test section was heated using a VCh-60AV HF generator with a frequency of not lower than 30 kHz. A hydraulic nozzle with a conical insert was used as the dispersing device. Techniques for carrying out an experiment on studying a stationary heat transfer regime and for calculating thermophysical quantities were developed. The experimental data were obtained in the stationary heat transfer regime with the following range of coolant operating parameters: water pressure equal to 0.38 MPa, water mass flow rate equal to 5.35 ml/s, and induction heating power equal to 6--19 kW. Based on the data obtained, the removed heat flux density and the heat transfer coefficients were calculated for each stationary heat transfer regime. The dependences of the heat transfer coefficient on the removed heat flux density and of the removed heat flux density on the temperature difference have been obtained. High values of heat transfer coefficients and heat flux density at a relatively low coolant flow rate were achieved in the experiments.

COMBUSTION SYNTHESIS OF COMPOSITE IN THE Ti-Al-C POWDERS MIXTURE: MODEL AND NUMERICAL SIMULATION

The work uses the basis of conventional combustion theory to propose and study a model of combustion synthesis of a composite material in a three-component powder mixture Ti-Al-C. The model takes into account the dependence of the properties on the composition and structure of the reaction system. The composition of the composite material was analyzed from the reaction initiation stage and to the product cool down. It was shown that the reaction front propagation along the specimen occurs in a stationary regime (with constant rate) for any of the specimens under study. The main synthesis products in the studied concentration range are titanium carbides, titanium aluminide and ternary phases (MAX-phases) Ti2AlC and Ti3AlC2. It was shown that the preliminary heating of the initial reaction mixture prevents the formation of MAX-phases in obtained composites and facilitates the increase in the fraction of carbide phases. Within the framework of the proposed model, the change in an initial porosity of the compact has very little effect neither on the composition evolution of the product. We have found the ranges of concentrations in the initial reaction mixture that provide the prevalence of either intermetallic (TiAl) or MAX-phase (Ti2AlC, Ti3AlC2) in the matrix. The obtained theoretical results are consistent with the experimental results.

Vortex Dynamo in an Obliquely Rotating Stratified Nanofluid by Small-Scale Non-Helical Forces

In this work, a large-scale instability of the hydrodynamic -effect in an obliquely rotating stratified nanofluid taking into account the effects of Brownian diffusion and particle flux under the influence of a temperature gradient (thermophoresis) is obtained. The instability is caused by the action of an external small-scale non-spiral force, which excites small-scale velocity oscillations with zero helicity and a low Reynolds number. Nonlinear equations for large-scale motions are obtained using the method of multiscale asymptotic expansions by a small parameter (Reynolds number). A linear large-scale instability of hydrodynamic -effect is investigated depending on the parameters of rotation , temperature stratification , and concentration of nanoparticles . A new effect of the generation of large-scale vortex structures in nanofluid at is associated with an increase in the concentration of nanoparticles is obtained. The maximum instability increment is reached at inclination angles for the Prandtl numbers , and for the Prandtl numbers at inclination angles . It has been found that the frequency changing of the parametric impact will make it possible to control and track the generation of large-scale vortex structures. It is shown that circularly polarized Beltrami vortices appear in nanofluid as the result of new large-scale instability development. In this paper, the saturation regime of large-scale instability in an obliquely rotating stratified nanofluid with an external small-scale non-spiral force is investigated. In the stationary regime was obtained a dynamic system of equations for large-scale perturbations of the velocity field. Numerical solutions of this system of equations are obtained, which show the existence of localized vortex structures in the form of nonlinear Beltrami waves and kinks. The velocity profile of kink tends to be constant at large Z values.

ПОСТРОЕНИЕ ГРАНИЦ ОБЛАСТЕЙ УСТОЙЧИВОСТИ РЕЖИМА СТАЦИОНАРНОГО ВРАЩЕНИЯ РОТОРНОЙ СИСТЕМЫ С ЖИДКОСТЬЮ, ОСЬ КОТОРОЙ РАСПОЛОЖЕНА В АНИЗОТРОПНЫХ ЗАКРЕПЛЕНИЯХ

Earlier, the authors generalized the original method for studying the stability of stationary rotation of rotor systems containing a viscous incompressible fluid, the axis of which is located in isotropic anchors, in the case when the viscoelastic anchors of the axis of the rotor system are anisotropic. The generalization is based on two theorems that say that finding the stability conditions of such systems is associated with the possibility of elliptical precession-type motion, and with such motion there is a special non-inertial reference frame in which the hydrodynamic elements of the system periodically change in time. The study of such movements allows us to construct the boundaries of regions with different degrees of instability, in particular, the boundaries of the stability regions of the stationary rotation regime in the parameter space of the problem. The boundaries of the stability regions are constructed for cases when the anchoring of the rotor axis is anisotropic. In the space of the anchorage parameters, a parametrically defined D-curve is obtained as a function of the dimensionless frequency of the rotor precession. The two most interesting cases are considered – anisotropic stiffness of anchors (damping is isotropic in this case) and the opposite situation: isotropic stiffness of anchors with anisotropic damping. The obtained results are compared with the known results for the case of isotropic anchoring of the rotor axis. It is shown that the anisotropy of anchors, which is always present in real rotary systems due to the imperfection of technologies for the production of anchors, does not lead to negative effects. Moreover, using the obtained D-curves, it is possible to obtain technological tolerances for the production of fasteners, using what is known as the permissible deviation of the stiffness or damping value along the axes.

АППАРАТУРНОЕ ОФОРМЛЕНИЕ И РЕЖИМНЫЕ ПАРАМЕТРЫ ПРОЦЕССА БЛОЧНОЙ ПОЛИМЕРИЗАЦИИ ИЗОПРЕНА

The stationary bed type regimes of apparatuses for catalytic isoprene polymerization process in bulk are investigated. If the polymerization process is performed with the bed height equal to 3 mm, the thermal apparatus regime is close to isothermic one. The increase of the bed height up to 6 mm makes the apparatus regime wide of the mark. To remove the lack there is proposed the regime providing the one time stepwise increase of the wall apparatus temperature. Due to proposed regime the apparatus efficiency makes gain while polymerization is performed up to incomplete conversions

ИССЛЕДОВАНИЕ УСТОЙЧИВОСТИ СТАЦИОНАРНОГО ВРАЩЕНИЯ РОТОРНОЙ СИСТЕМЫ С ЖИДКОСТЬЮ, ОСЬ КОТОРОЙ РАСПОЛОЖЕНА В АНИЗОТРОПНЫХ ЗАКРЕПЛЕНИЯХ

Earlier, one of the authors proposed and developed (together with coworkers) an original method to study the stability of stationary rotation of rotary systems containing a viscous liquid and having a drive that maintains the angular velocity of rotation constant. It was assumed that the rotor has axial symmetry, the anchors of its axis are isotropic. The method is based on two theorems, according to which a change in the degree of instability is associated with the possibility of a perturbed motion of the circular precession type. This motion has a remarkable property: the velocity field and the shape of the liquid surface do not depend on time in a specially selected non-inertial reference frame associated with the line of centers. Finding the conditions for the feasibility of circular precession makes it possible to effectively construct the boundaries of the stability regions of the stationary rotation regime in the space of problem parameters. In addition, the study of the occurrence of circular precession allows us to find the conditions under which a subcritical (supercritical) Andronov-Hopf bifurcation takes place in the rotor system and to identify "dangerous" (“safe”) sections of the boundaries of the stability regions. In this paper, the previously proposed method of stability research applies to systems in which the rotor axis is located in anisotropic Laval type anchors. In the study of rotary systems of this type, it is possible to link the change in the degree of instability with the feasibility of perturbed movements of the elliptical precession type. It can be shown that the imaginary characteristic numbers of the equations in deviations from the stationary rotation mode are possible only in the case when there is a perturbed motion in the form of an elliptical precession. An example of a study of the stability of stationary rotation of a typical rotary system is given. Mechanical effects caused by the fact that gyroscopic stabilization becomes impossible with anisotropic fixing of the rotor axis are noted.

Impact of Stationary and Dynamic Conditions on the U-Value Measurements of Heavy-Multi Leaf Walls by Quantitative IRT

Infrared thermography (IRT) has become a commonly applied non-destructive testing method for assessing building envelopes. Like any diagnosis tool, IRT requires an appropriate experience and principle understanding, mainly when the method is used for quantitative analyses. The challenges of the IRT often deal with the dynamic properties of building partitions. Climatic conditions have a certain variability, and the accumulated energy storage in the building components can affect their temperature as well as the calculated thermal performance. This paper aims to analyze how stationary and dynamic regimes of a quantitative IRT test could impact the measured thermal transmittance of heavy multi-leaf walls. Investigation in two European countries with different climatic conditions are reported. In this way, it is discussed which boundary conditions should be guaranteed to provide reliable information about a building envelope using quantitative IRT. In order to check the quality of the measurements, the heat flux meter (HFM) method was also implemented, following the ISO 9869. The research revealed that it could be possible to use short-lasting tests in the climatic conditions of Southern Europe, while long-term tests should be implemented in Northern European countries where climatic conditions are less regular.

Computation of the Linear and Nonlinear Stability of a Two-Layer Couette Flow

Abstract—This work considers the linear and nonlinear stability of a two-layer Couette flow in a horizontal channel. At the first stage, the Navier–Stokes equations are linearized in both phases. We then solve the spectral problem and investigate the dynamics of periodic disturbances in a wide range of changes in the volumetric content of liquids and the velocity of the top plate. Both the neutral and the fastest growing disturbances of the unstable interface mode are calculated. At the second stage, nonlinear stationary traveling wave regimes were calculated for the Couette flow in a horizontal channel using the complete Navier–Stokes equations for both liquids. The results are compared to the experimental data available in the literature.

Automated data-processing technique: 2D Map for identifying the distribution of the U-value in building elements by quantitative internal thermography

Computing a 2D colour map of average U-values pixel-by-pixel could become a challenging task in terms of complexity and time, especially for entire façades under the influence of anomalies. In a quantitative IRT test, a thermal image with a resolution of 320 × 240 pixels involves 76,800 elements with different TWALL for each instant “t”. This research aims to create a thermographic 2D U-value map for the characterization of heavy walls in a stationary regime. The method was divided into three steps: (i) metrology; (ii) assessment of how mesh discretization affects the image quality by MATLAB; (iii) development of a 2D map by SURFER. The results demonstrated that all 2D maps were a great reproduction of the original image, considering as optimum a TWALL mesh comprised of 1600 elements of 8 × 6 pixels. The automated data-processing method only took 20 min and image quality losses were estimated at 6.65%.

Experimental Research and Numerical Analysis of Flow Phenomena in Discharge Object with Siphon

This article presents results of the experimental research and numerical simulations of the flow in a pumping system’s discharge object with the welded siphon. The laboratory simplified model was used in the study. Two stationary flow regimes characterized by different volume flow rates and water level heights have been chosen. The study concentrates mainly on the regions below and behind the siphon outlet. The mathematical modelling using advanced turbulence models has been performed. The free-surface flow has been carried out by means of the volume-of-fluid method. The experimental results obtained by the particle image velocimetry method have been used for the mathematical model validation. The evolution and interactions of main flow structures are analyzed using visualizations and the spectral analysis. The presented results show a good agreement of the measured and calculated complex flow topology and give a deep insight into the flow structures below and behind the siphon outlet. The presented methodology and results can increase the applicability and reliability of the numerical tools used for the design of the pump and turbine stations and their optimization with respect to the efficiency, lifetime and environmental demands.