Energy Contour Research Articles

Electron scattering in a superlattice of line defects on the surface of topological insulators

The electron scattering from periodic line defects on the surface of topological insulators with hexagonal warping effect is investigated theoretically by means of a transfer matrix method. The influence of surface line defects, acting as structural ripples on propagation of electrons are studied in two perpendicular directions due to the asymmetry of warped energy contour under momentum exchange. The transmission profiles and the details of resonant peaks which vary with the number of defects and the strength of their potentials are strongly dependent on the direction in which the line defects extend. At low energies, the quantum interference between the incident and reflected propagating electrons has the dominant contribution in transmission resonances, while at high energies the multiple scattering processes on the constant-energy contour also appear because of the strong warping effect. By increasing the spatial separation between the line defects, the minimum value of electrical conductance remains significantly high at low incident energies, while the minimum value may approach zero at high energies as the number of defects is increased. Our findings suggest that the potential ripples on the surface of topological insulators can be utilized to control the local electronic properties of these materials.

Experimental and numerical investigations for turbulent air flow characteristics of circular orifice plate

The present work describes the airflow characteristics upstream and downstream of the circular orifice numerically using the standard k-ε model and RSM and experimentally employing hot-wire and Pitot tube devices to measure the flow velocities through a tube. An expectation has to bear in mind; the RSM performed well and predicted results in a good level of agreement against the experimental measurements found in preceding studies and the present experimental data as well. In terms of flow physics point of view, velocity and turbulent kinetic energy distribution are presented. Moreover, turbulent viscosity, Reynolds stresses (u2, v2, w2) are also considered. Furthermore, the contours of velocity, static pressure and turbulent kinetic energy are presented beside the wall shear stress and y+ profiles are illustrated to show the flow physics as well as to compare the results of two different turbulence models considered in this study employing Reynolds of 30000 and the ratio of orifice diameter to the pipe diameter of 0.5.

Probing the Electronic Structure of Hybrid Perovskites in the Orientationally Disordered Cubic Phase.

Hybrid organic-inorganic lead halide perovskites are projected as new generation photovoltaic and optoelectronic materials with improved efficiencies. However, their electronic structure so far remains poorly understood, particularly in the orientationally disordered cubic phase. We performed electronic structure investigations using angle-resolved photoemission spectroscopy on two prototypical samples (MAPbBr3 and MAPbCl3) in their cubic phase, and the results are compared with the calculations within two theoretical models where MA+ is orientationally (1) disordered (MA+ ion is replaced by spherically symmetric Cs+ ion) and (2) ordered (MA oriented along (100) direction) but keeping the symmetry of the unit cell cubic. Degeneracy of the valence bands and behavior of constant energy contours are consistent with model 1, which supports strongly the disordered nature of the orientation of the MA+ ions in the cubic phase. Band structure calculations also reveal that spin-orbit coupling induced Rashba splitting is suppressed by the orientational disorder.

Analysis of thermal-hydraulic performance and flow structures of nanofluids across various corrugated channels: An experimental and numerical study

The combination of nanofluid and corrugated surface is regarded as a very cost-efficient strategy for providing high heat transfer performance between the target surface and working fluid in different heating and cooling system applications. Herein a comparison study is reported on the thermal performance of different corrugated channels using nanofluids under turbulent flow and constant heat flux conditions. Three shapes, namely, semicircle corrugated channel (SCC), trapezoidal corrugated channel (TCC), and straight channel (SC) are fabricated and tested with 1% and 2% volume fraction of SiO2-water and Al2O3-water nanofluids. Nanoparticles with a size of 20 nm dispersed in water with specific volume fractions are used for comparison and evaluation. The numerical simulations present the flow structures of nanofluids in terms of velocity, isotherms, turbulent viscosity, vorticity and turbulent kinetic energy contours. The findings show that the heat transfer and pressure drop increase as volume fractions of nanofluids and Reynolds number increase. The experiments also revealed that the use of modified channels significantly increases the heat transfer ratio and the greatest enhancement ratio was achieved through the use of a trapezoidal corrugated channel with ϕ=2% of silica nanofluid. Over ranges of considered flow, silica nanofluid was better than alumina nanofluid and the best thermal performance of 1.94 was recorded using silica nanofluid at ϕ=2% and TCC at Reynolds number of 10000. Additionally, new empirical correlations were proposed and reported based on experimental data.

Cubic Rashba Effect in the Surface Spin Structure of Rare-Earth Ternary Materials.

Spin-orbit interaction and structure inversion asymmetry in combination with magnetic ordering is a promising route to novel materials with highly mobile spin-polarized carriers at the surface. Spin-resolved measurements of the photoemission current from the Si-terminated surface of the antiferromagnet TbRh_{2}Si_{2} and their analysis within an abinitio one-step theory unveil an unusual triple winding of the electron spin along the fourfold-symmetric constant energy contours of the surface states. A two-band k·p model is presented that yields the triple winding as a cubic Rashba effect. The curious in-plane spin-momentum locking is remarkably robust and remains intact across a paramagnetic-antiferromagnetic transition in spite of spin-orbit interaction on Rh atoms being considerably weaker than the out-of-plane exchange field due to the Tb 4f moments.

Using external ion irradiations for simulating self-irradiation damage in nuclear waste glasses: State of the art, recommendations and, prospects

Due to the complexities associated with handling and characterizing the radiation damage in radioactive materials such as nuclear waste forms, ion irradiation has been widely used to simulate the effects of the self-irradiation damage. This often involves ion irradiation of nuclear waste surrogate glasses using ion accelerators. Such facilities allow using a wide range of ion types ranging from H to U and energies from tens of keV to hundreds of MeV. For various reasons, the researchers often have to use ions and energies that do not necessarily match with what is expected in actual nuclear waste forms. Due to a lack of detailed and dedicated studies, there are currently no guidelines or consensus regarding the use of the ion irradiation as a means of simulating the self-irradiation damage. The purpose of this article is to delve into this subject by presenting systematic studies of a number of glasses carried out in the last decade or so. By doing so, guidelines and easy to follow ion-mass Vs ion energy contour maps are provided to best simulate the long-term radiation damage in nuclear waste forms. We also discuss how the ions of different masses and energies can be exploited for defect healing and provide a roadmap to undertake systematic studies on this subject. For the sake of brevity, a quick summary of the important results and the guidelines is presented towards the end of this article and not here. Besides the nuclear waste community, the study is also of interest to a wide research community dealing with defect formation and defect healing in glasses for use in radiation environments experienced in nuclear reactors, decommissioning and space applications to name a few.

Rheological Behavior of Sumac (Rhus coriaria L.) Extract as Affected by Temperature and Concentration and Investigation of Flow Behavior with CFD

The purpose of this study is to investigate the rheological properties of sumac extract in different concentrations at different temperatures as well as its flow behavior in sudden expansion-contraction and at 90o elbow with CFD. The rheological behaviour of sumac extract in different concentrations (45.65%, 50.44%, 55.53%, 60.32%, and 65.13% total solids) were evaluated using a rotational viscometer at different temperatures (10, 20, 30, 40 and 50 C). Sumac extract samples showed Newtonian flow properties in these temperature ranges. Arrhenius equation was used to determine the effect of temperature. Ea value varied in the range of 11.16-34.35 kJ/mol, which diminished with a decrease in concentration. Power and Exponential models were used to characterize the effect of concentration on flow behavior. Time average velocity vector and contours, vorticity contours, kinetic energy contours, and pressure contours are given to show the flow behavior of sumac extract.

Numerical parametric study on three-dimensional rectangular counter-flow thrust vectoring control

Recently, fluidic thrust vectoring control is popular for micro space launcher propulsion systems due to its several advantages, such as fast dynamic responsiveness, better control effectiveness, and no moving mechanical equipment. Counter-flow thrust vectoring control is an especially effective technique by utilizing less suction flow to control the mainstream deflection flexibly. In the current work, theoretical and numerical analyses are performed together to elaborate on the performance of the three-dimensional rectangular counter-flow thrust vectoring control system. A new propulsion nozzle of Mach 2.5 is devised by method of characteristics. To testify the feasibility and accuracy of the present research methodology, numerical results were validated against experimental data from the open literature. The computational result using the standard k-epsilon turbulence model reveals a good match with experimentally measured static pressure values along the centerline of the upper suction collar. The influence of several key parameters on vectoring performance is investigated herein, including the mainstream temperature, collar radius, horizontal collar length, and gap height. Critical parameters have been quantitatively analyzed, such as static pressure distribution along the centerline of the upper suction collar, pitching angle, suction mass flow ratio, and thrust coefficient. Furthermore, the flow-field features are qualitatively expounded based on the static pressure contour, streamline, iso-turbulent kinetic energy contour, and iso-Mach number contour. Some important conclusions are offered for further studies. The mainstream temperature mainly affects different dynamic characteristics of the mixing shear layer, including the convective Mach number of the shear layer, density ratio, and flow velocity ratio. The collar radius influences the pressure gradient near the suction collar surface significantly. The pitching angle increases rapidly with the increasing collar radius. As the horizontal collar length increases, the systematic deflection angle initially increases then decreases. The highest pitching angle is obtained for L/ H = 3.53. With regard to the gap height, a larger gap height can achieve a higher pitching angle.

Solution Blow Spinning of High-Performance Submicron Polyvinylidene Fluoride Fibres: Computational Fluid Mechanics Modelling and Experimental Results.

Computational fluid dynamics (CFD) was used to investigate characteristics of high-speed air as it is expelled from a solution blow spinning (SBS) nozzle using a k-ε turbulence model. Air velocity, pressure, temperature, turbulent kinetic energy and density contours were generated and analysed in order to achieve an optimal attenuation force for fibre production. A bespoke convergent nozzle was used to produce polyvinylidene fluoride (PVDF) fibres at air pressures between 1 and 5 bar. The nozzle comprised of four parts: a polymer solution syringe holder, an air inlet, an air chamber, and a cap that covers the air chamber. A custom-built SBS setup was used to produce PVDF submicron fibres which were consequently analysed using scanning electron microscope (SEM) for their morphological features. Both theoretical and experimental observations showed that a higher air pressure (4 bar) is more suitable to achieve thin fibres of PVDF. However, fibre diameter increased at 5 bar and intertwined ropes of fibres were also observed.

Quasiparticle interference testing the possible pairing symmetry in Sr2RuO4**Project supported by the National Natural Science Foundataion of China (Grant Nos. 11604168, 11604166, and 11604303). WSW also acknowledges the supports by K. C. Wong Magna Fund in Ningbo University.

The quasiparticle interference (QPI) patterns of the superconducting state in Sr2RuO4 are theoretically studied by taking into account the spin–orbital coupling and two different pairing modes, chiral p-wave pairing and equal d-wave pairing, in order to propose an experimental method to test them. Both of the QPI spectra for the two pairing modes have clearly peaks evolving with energy, and their locations can be determined from the tips of the constant energy contour. But the number, location, and evolution of these peaks with energy are different between the two pairing modes. The different behaviors of the QPI patterns in these two pairing modes may help to resolve whether Sr2RuO4 is a chiral p-wave or d-wave superconductor.

Generalized quadrature for finite temperature Green’s function methods

In electronic structure and quantum transport calculations, many physical quantities are integrations over the electron energy, weighted by the Fermi–Dirac distribution function. Green’s function based approaches commonly circumvent the numerically difficult real energy integration by extending the integrand analytically into the complex energy plane, and using a Gaussian quadrature integration over a complex energy contour for zero temperature. For finite temperatures, a much slower convergent sum over the Matsubara frequencies is necessary. We present a generalized quadrature method that uses orthogonal polynomials on the manifold of Matsubara frequencies to enable rapid convergence. Both Gaussian quadrature integration and Matsubara frequency summation methods are shown to be limiting cases of the generalized method. Tests on an all-electron ab initio code and total energy calculation of interacting Anderson impurity model show convergence with a small number of energy mesh points.

Anisotropic electronic structure of antimonene

Antimonene is a recently discovered two-dimensional semiconductor with high carrier mobility, suitable bandgap, and strong spin–orbit coupling, which is of great interest for future electronic, optoelectronic, and spintronics device applications. However, as the basic properties of materials, the electronic structure of antimonene is not fully understood. In this work, the whole and anisotropic electronic structures of antimonene modulated by a horizontal crystal field have been studied in detail by angle-resolved photoemission spectroscopy. The valence band of antimonene for semiconductor properties is characterized by a three-dimensional anisotropic “M”-shape with hexagonal deformation in (E, kx, ky) space. Moreover, the surface state of antimonene for metallic properties is characterized by an upward conelike structure with a stronger warping effect. The anisotropic electronic structure of antimonene is systematically depicted along the high symmetric direction and constant energy contours.

Large-eddy simulation of counter-rotating Taylor–Couette flow: The effects of angular velocity and eccentricity

In the present work, turbulent flow in the annulus of a counter-rotating Taylor-Couette (CRTC) system is studied using large-eddy simulation. The numerical methodology employed is validated, for both the mean and second-order statistics, with the direct numerical simulation (DNS) data available in the literature, for a range of Reynolds numbers from 500 to 4000. Thereafter, turbulent flow occurring in this system at Reynolds numbers of 8000 and 16000 are studied, and the results obtained are analyzed using mean and second-order statistics, vortical structures, velocity vector plots and power energy spectra. Further, the spatio-temporal variation of azimuthal velocity, extracted near the inner cylinder, shows the existence of herringbone like patterns similar to that observed in the previous studies. The effect of eccentricity of the inner cylinder with respect to the outer cylinder is studied, on the turbulent flow in the CRTC system, for two different eccentricity ratios of 0.2 and 0.5 and for two different Reynolds numbers of 1500 and 4000. The results of the eccentric CRTC are analyzed using contours of pressure, mean and second-order statistics, velocity vectors, vortical structures, and turbulence anisotropy maps. It is observed from the eccentric CRTC simulations that the smaller-gap region seems to contain higher amplitude fluctuations and more vortical structures when compared with the larger-gap region. The mean turbulent kinetic energy contours do not change qualitatively with the Reynolds number, however, quantitatively a higher turbulent kinetic energy is observed in the higher Reynolds number case of 4000.

Ring opening dissociation of cationic twisted conformer by conformer-specific cyclopentanone photoionization.

We investigated the dissociation processes of a cationic conformer, induced by conformer-specific photoionization of cyclopentanone (CP) using a one-photon vacuum ultraviolet (VUV) laser pulse of energy in the range 9.24-9.92 eV for a few nanoseconds, generated by four-wave difference frequency mixing in a Kr cell. The adiabatic ionization energy of the CP was accurately determined to be 9.2697 ± 0.0009 eV, based on the VUV photoionization efficiency curve obtained using high-resolution VUV-photoionization time-of-flight (TOF) mass spectroscopy. The constructed potential energy contours, associated with the twisting and out-of-plane motions in the S0 and D0 states, revealed that the ionization energy value corresponded to a twisted conformer with C2 symmetry at the global minimum. Subsequently, the low photon energy above the ionization onset of the twisted conformer in the CP led to C2H4 elimination, producing a C3H4O+ fragment directly prior to CO elimination for the C4H8 + fragment. The appearance energies for the C3H4O+ and C4H8 + were determined to be 9.7068 ± 0.0017 eV and 9.7483 ± 0.0017 eV, respectively, by measuring the fragmentation yield curves for two fragments analyzed in the TOF mass spectra. The formation enthalpy for each fragment ion at 0 K, evaluated using the measured and thermochemical data, enabled the realization of plausible structures for the produced fragment ions. Consequently, based on the results of the quantum chemical calculation on the dissociation processes of the twisted CP cation (t-CP+), we suggest that the fragmentation processes to C3H4O+ and C4H8 + correspond to the methylketene and (E)-2-butene cations, respectively.

Fréedericksz-like positional transition

In this paper, a Freedericksz-like positional transition is found for a spherical micro-droplet suspended in a nematic liquid crystal cell in the presence of an external electric field. Based on the numerical calculation of elastic energy using Green function method, the equilibrium position of micro-droplet is decided through a competition between the buoyant force and the effective force built by the elastic energy gradient existing inside the nematic liquid crystal(NLC) cell. It is shown that the elastic energy dominates the kinetics of micro-droplet until the external field applied reaches a critical value large enough to flatten the elastic energy contour in the central region, which enables the asymmetric buoyant force to drive the liquid droplet abruptly from the cell midplane to a new equilibrium position. It is also found that such a threshold value of external field, which triggers positional transition, depends on thickness $L$ and Frank elastic constant $K$, in a Freedericksz-like manner, but multiplied by a factor of $3\sqrt{\pi}$. An explicit formula proposed for the critical electric field agrees extremely well with the numerical calculation.

Silindir Üzerindeki Yarı-Yuvarlak Çıkıntıların Akış Yapısı Üzerine Etkisi

In this study; two dimensional, viscous, incompressible, fully developed and turbulent flows on the surface of the cylinder placed in a channel were analyzed using numerical methods. Momentum and continuity equations were solved numerically by ANSYS FLUENT program using finite volume method. The effect of the half cylinder (d = 4.4 mm) placed on the cylinder diameter (D) 44 mm in different positions (0°, 45°, 90°, 135° and 180°) on the flow structure was examined. Streamlines and the turbulence kinetic energy contours were drawn for the Reynolds number 104 value depending on cylinder diameter. As a result, downstream flow characteristics of cylinders having semi-circular protrusions placed in different positions were examined and drag force coefficients (CD) were obtained. It was concluded that the protrusion on the cylinder placed at different angles changed the flow structure and controlled the separation.

Observations of nodal lines in the topological semimetal ZrSnTe

Performing angle-resolved photoemission spectroscopy (ARPES) and theoretical calculations for ZrSnTe, we find a nodal line in the k x - k y plane with a tiny gap and two nodal lines without gaps along the k z direction. The constant energy contours are plotted in the k x - k y plane at a photon energy of 30 eV to examine the gapping feature of the nodal line. In addition, we depict the band dispersions along the Γ-M and Γ-X directions from the ARPES images at photon energies ranging from 24 to 40 eV. The k z mapping confirms the presence of stable Dirac points with obvious k z dependences along the M-A and X-R directions. In this paper, we report the two types of nodal lines associated with the theoretical local-density approximation calculations, broadening the horizons of nodal-line semimetal research.

Shore type effect on onshore wave energy converter performance

Oscillating Water Column (OWC) is one of the WEC devices, which converts wave energy to electrical power. In this study, analysis of OWC is made in Numerical Wave Tank. Finite Volume Method based on Reynolds-Averaged Navier-Stokes equations is used. Turbulence model as k-epsilon, Volume of Fluid Method and Dynamic Mesh Technique are chosen options to have transient simulation in Fluent software. The numerical wave tanks are modelled at the length of 400m for 9s wave period and length of 211m for 6s wave period to obtain the desired wave properties because of the wave generation method used. The wave lengths are 55.8m for 6s wave period and 111.4 for 9s wave period. The initial water depth is 24.5m for all cases. The obtained results are volume fraction, velocity, vorticity and turbulence kinetic energy contours. Pressure and velocity data are used for computation and comparison of power output of cases. Power output results vary from 34.5 to 45.0 Watt for 1m wave height, 121.5 to 240.8 Watt for 2m wave height and 425.3 to 1227.6 Watt for 4m wave height. The best performances are obtained from convex arc structure at 4m wave height at 6s and 9s wave periods.

A new method for discrimination of internal fault from other transient states in power transformer using Clarke’s transform and modified hyperbolic S-transform

In this paper, a new method is proposed for discriminating internal faults from other abnormal conditions in power transformers such as inrush currents, external faults, over-excitation, and ultra-saturation. Since the differential current signals are non-stationary, a powerful tool such as S-transform is needed for their time-frequency analysis. In this study, some important characteristics are extracted using the Clark’s transform and modified hyperbolic S-transform according to which different conditions are distinguished and classified. These features include energy contour of first level, variance index, standard deviation of second harmonic component, and a special criterion. In this paper, all possible transient conditions and the effects of the saturation of current transformers are considered. Various simulations for different operating conditions of transformer are performed via PSCAD/EMTDC software. Also, the entire process of modified hyperbolic S-transform and the proposed algorithm are done in MATLAB software. Simulation results show that the proposed algorithm can be reliably used for the identification and classification of different faults in transformers.

Numerical simulation of allocator annular flow field

The allocator is the key tool for the oilfield to realize the multi-layered polymer flooding. The most common allocator is the annular pressure-reducing groove structure. The selection of reasonable structural characteristics parameters has always been a difficult problem in the design of pressure reducing structure. The structural optimization design involves many parameters, and the numerical simulation method can effectively reduce the dependence on the experiment and the number of experiments. In this paper, the parametric modelling of the annular pressure-reducing groove structure is carried out, and the numerical simulation model of the power law fluid flowing in the annular channel is established. The turbulence calculation model is used to numerically calculate the flow field. The velocity, pressure, turbulent kinetic energy and strain rate contours are obtained and analysed. The numerical calculation results of the annulus flow field with four different structural characteristic parameters are compared and the optimal structural parameters are obtained, which can provide valuable reference for the design and improvement of the annular pressure-reducing groove allocator.