Geometrical Cross Section Research Articles

Multipole engineering for enhanced backscattering modulation

An efficient modulation of backscattered energy is one of the key requirements for enabling efficient wireless communication channels. Typical architectures, being based on either electronically or mechanically modulated reflectors, cannot be downscaled to subwavelengths dimensions by design. Here we show that integrating high-index dielectric materials with tunable subwavelength resonators allows achieving an efficient backscattering modulation, keeping a footprint of an entire structure small. An interference between high-order Mie resonances leads to either enhancement or suppression of the backscattering, depending on a control parameter. In particular, a ceramic core-shell, driven by an electronically tunable split ring resonator was shown to provide a backscattering modulation depth as high as tens of the geometrical cross-section of the structure. The design was optimized towards maximizing reading range of radio frequency identification (RFID) tags and shown to outperform existing commercial solutions by orders of magnitude in terms of the modulation efficiency. The proposed concept of multipole engineering allows one to design a new generation of miniature beacons and modulators for wireless communication needs and other relevant applications.

Study on influence of notch triggers on absorption of energy for composite automobile crash box under impact loads

In case of a crash due to collision of vehicles, effective impact energy absorption by the vehicle safety structure is significant as it reduces the damage to the vehicle and its occupants. Components made of composite materials have a more complex mode of deformation, unlike metals, which in general deform with buckling or bending modes. The deformation mode of the component made of a composite is of great importance as the energy absorption level depends on it. Deformation mode can be modified by introducing special geometric features known as triggers in the design of the crash box. A trigger can also be useful in achieving required deformation mode and thereby helpful in attaining target energy and force values. The present study focuses on the effect of geometric shapes along with three novel trigger types on the level of energy absorbed for the GFRP composite material passenger car crash box in a low-velocity impact, named as “RCAR Test” in the automobile industry. All possible combinations of four different types of geometric cross sections, namely, square, cylindrical, hexagonal, and decagonal, and three novel patterns of notch trigger, imparted to a GFRP crash box were considered for study in the present work. The force distribution, energy absorbed, and SEA (specific energy absorption) were estimated. The relative merit of crash boxes with different combinations of cross-sections and trigger types for these parameters was assessed. The best performing combinations made from various notch triggers and different cross sections of crash boxes have been identified.

Strong, omnidirectional radar backscatter from subwavelength, 3D printed metacubes

Metallic metacubes formed of six metal plate faces connected via a metal jack are shown to backscatter microwave radiation extremely powerfully. Experimental radar scattering cross-section (RCS) data from 3D printed samples agrees very well with numerical model predictions, showing an RCS of fifteen times the geometrical cross section. The principal resonance of the metacubes demonstrates near-complete independence of the incident angle or polarisation of the radiation, making the metacube an omnidirectional scatterer. The metacubes are fabricated via additive manufacturing from metal-coated polymer, and are extremely lightweight, making them excellent candidates for improving the radar return signals from small objects such as drones and cubesats.

Addendum: Radiation force and torque of light-sheets (2017 J. Opt. 19 065403)

The purpose of this Addendum is to provide some clarifications and additional discussions related to the formalism of the generalized Lorenz–Mie theories in two-dimensions presented in the manuscript (Mitri F G 2017 J. Opt. 19 065403). The generalized formalism provides exact mathematical expressions that are used to compute the optical scattering, and time-averaged radiation force and torque of any light-sheets of arbitrary wavefront illuminating a homogeneous dielectric cylindrical particle of arbitrary geometrical cross-section, suspended in a lossless medium of wave propagation. In particular, it is emphasized here that concerning the analytical derivations and calculations based on far-field limits of the scattering, the Poynting vector formalism can only be utilized to determine the optical radiation force vector and its components, a result that is commensurate with the same outcome of an analysis based on the integration of the time-averaged Maxwell stress tensor. Nevertheless, in the far-field, the Poynting vector formalism is not valid to evaluate the optical radiation torque because it is radial, and the integration of its moment over a surface of large radius enclosing the particle leads to a zero torque, not in agreement with the derivation based on Maxwell’s stress tensor. Therefore, the analytical expression required to derive the optical radiation torque component (either in the near-field or far-field limits of the scattering, which lead to equivalent expressions without any approximations in the case of a lossless medium of wave propagation) should always involve the integration of the moment of the time-averaged Maxwell stress tensor.

Quasinormal modes, shadow, and greybody factors of 5D electrically charged Bardeen black holes

We study quasinormal modes (QNMs) in 5D electrically charged Bardeen black holes spacetime by considering the scalar and electromagnetic field perturbations. The black holes spacetime is an exact solution of Einstein gravity coupled to nonlinear electrodynamics in five dimensions, which has nonsingular behavior. To calculate QNMs, we use the WKB approximation method up to sixth order. Due to the presence of electric charge $q_e > 0$, both the scalar and electromagnetic field perturbations decay more slowly when compared to the Schwarzschild-Tangherlini black holes. We discover that the scalar field perturbations oscillate more rapidly when compared to the electromagnetic field perturbations. In terms of damping, the scalar field perturbations damp more quickly. Graphically we show that the transmission (reflection) coefficients decrease (increase) with an increase in the magnitude of the electric charge $q_e$. The emission of gravitational waves allows spacetime to undergo damped oscillations due to the nonzero value of the imaginary part, which is always negative. The imaginary part of the QNMs frequencies is continuously decreasing with an increase in the magnitude of the electric charge $q_e$ for a given mode ($l,n$). A connection between the QNMs frequencies and the black hole shadow, as well as the geometric cross-section in the eikonal limit, is also described.

Giant cross sections for L changing in Rydberg-Rydberg collisions

State changing in thermal-energy collisions between strontium atoms in very-high-$n$, $n\ensuremath{\approx}300$, $n{\phantom{\rule{0.16em}{0ex}}}^{1}{F}_{3}$ Rydberg states is studied. In collisions between Rydberg atoms, the dipole-dipole interaction generates an effective electric field at each atom, which triggers Stark precession and the evolution of their angular momenta, $L$. Such $L$-changing collisions are examined using both quantum and classical theory. The theoretical predictions are experimentally verified by monitoring the damping of quantum beats induced by the sudden application of a DC ``pump'' field. The present work shows that, due to the long-range nature of the interactions, the cross sections, $\ensuremath{\sigma}$, for $L$ changing are considerably larger than the geometric collision cross section. For thermal collisions the cross section $\ensuremath{\sigma}$ for $n{\phantom{\rule{0.28em}{0ex}}}^{1}{F}_{3}$ Rydberg states with $n\ensuremath{\simeq}300$ is $\ensuremath{\approx}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$. Scaling rules derived in this paper predict that for cold collisions $\ensuremath{\sigma}$ may increase by a further factor of 4.

Radiation force and torque of light-sheets illuminating a cylindrical particle of arbitrary geometrical cross-section exhibiting circular dichroism

Unlike standard dielectric materials, the interaction of light-sheets of arbitrary wavefronts with a particle exhibiting circular dichroism, such as chiral, topological insulator, liquid crystal, or metamaterials to name a few examples, requires additional fundamental developments from the standpoint of scattering, radiation force and torque theories.The purpose of this work is therefore directed toward developing an exact analytical formalism applicable to a 2D object exhibiting TM ⇄ TE mode conversion and possessing an arbitrary geometrical cross-section. Exact mathematical expressions for the longitudinal and transverse optical radiation force and axial torque components are derived stemming from series expansions for the incident and scattered electromagnetic fields using the mode matching method in cylindrical coordinates. The generalized radiation force and torque expressions depend on the beam-shape coefficients (BSCs) of the incident light-sheet, and the scattering coefficients of the object. Extra new terms describing the generation of the scattered cross-polarized waves induced due to TM ⇄ TE mode conversion contribute to the radiation force and torque series. Numerical illustrative examples for a circular lossy electromagnetic conductor (LEMC) cylinder are provided assuming illumination by various wavefronts, ranging from plane progressive, standing and quasi-standing waves to other non-paraxial Airy light-sheets with linear and circular polarization. In essence, the present theoretical formalism provides a complete analysis in the framework of the generalized Lorenz–Mie theory in 2D for any particle exhibiting circular dichroism in any structured light-sheet. Potential applications are in the design, optimization and the numerical predictions and computation of the optical radiation force and torque for particle transport and rotation in the realm of optical tweezers, particle manipulation and stabilization. Some perspectives are discussed and replies to misleading comments made in the literature on the publication [F.G. Mitri, J Quant Spectro Rad Transf 2020;250:106994] given by Ref. [33] are provided.

Dataset on blood flow and instantaneous wave-free ratio in normal and stenosed coronary arteries.

Instantaneous wave-free ratio (iFR) has been proposed as a hemodynamic parameter that can reliably reflect the blood flow in stenosed coronary arteries. Currently, there are few investigations on the quantitative analysis of iFR in the patients regarding the variation of microcirculatory resistance (MR). The data aim to provide geometric (cross-section area of branches) and hemodynamic (flow rate and iFR of branches) parameters of normal and stenosed coronary arteries derived from CFD simulation. The CFD simulation was performed on the three-dimensional artery models reconstructed from computed tomography (CT) images of four subjects. The hemodynamic parameters were obtained in six situations of MR to simulate coronary microvascular dysfunction (CMD). This dataset could be used as the reference to estimate the iFR and flow rate in patients with CMD and stenosis in coronary arteries. The geometric parameters could be used in the modelling of coronary arteries.

Geometrical modeling of yarn’s cross-section towards a realistic unit cell of 2D and 3D woven composites

Due to the complex microstructure of textile composites, the modeling of physical properties is challenging. The mechanical properties of textiles depend on different geometrical parameters such as the yarn's geometrical properties, undulation, geometrical cross-section, gap between two adjacent yarns, and other parameters. In this paper, a geometrical modeling of 2D and 3D woven composites is presented. The novelty of this paper is related to a more realistic modeling of yarn's cross-section. The presented modeling technique takes into account that yarns constituting 2D or 3D woven composite do not have the same cross-section shape and undulated part as usually considered with an ideal unit cell modeling. The proposed technique leads to calculate the fiber volume fraction of yarns, then the fiber volume fraction of the unit cell of the studied composite. It was applied to different kinds of woven composites where the modeled unit cell converged to a more realistic shape.

Optical cross-sections and energy efficiencies of a cylindrical material exhibiting circular dichroism in arbitrary-shaped monochromatic light-sheets

Optical scattering, extinction and absorption cross-sections are derived for a cylindrical material exhibiting rotary polarization. A structured light-sheet of arbitrary shape illuminates the cylindrical particle of arbitrary geometrical cross-section. The partial-wave series expansion method in cylindrical coordinates is used and Poynting’s theorem is applied. Exact mathematical series expansions are obtained without any approximations. They demonstrate that the optical cross-sections are not the mere sum of linearly polarized components. Additional terms contributing to the series arise from the cross-interaction of the linearly polarized modes (known also as mode conversion TM ⇆ TE) in the scattering field. Numerical examples for the scattering or extinction energy efficiencies of a left-handed circularly polarized Airy light sheet illustrate the analysis for a circular perfect electromagnetic conductor cylinder in air. The results clearly demonstrate the contribution of mode conversion for a cylinder exhibiting circular dichroism. Under some specific conditions determined by the incident light-sheet characteristics and cylinder size, mode conversion can be zero, or it can contribute in the enhancement or reduction of the scattering.

Reassesment of lateral torsional buckling in hot-holled I-beams

Lateral torsional buckling (LTB) of I-beams is characterized by simultaneous lateral deflection and twist. In this paper the influence of destabilizing and neutral effects of loadings and the possibility of the occurrence of web distortion, which characterizes the lateral distortional buckling (LDB), is numerically investigated. Recent research has shown divergences between the standard procedures of the American Institute of Steel Construction, Australian Standards and Eurocode with LTB numerical results. In addition, some procedures have recently been proposed for correcting possible deviations between the results of numerical analysis and standard procedures. In order to investigate this situation, elastic stability and physical and geometrical nonlinear analyzes are performed, using GBTul and ABAQUS softwares, respectively. The numerical model is calibrated considering experimental tests. The beams are simply supported, with fork supports at the ends, and subject to neutral and destabilizing effects of loading. The beams are subject to uniform bending, mid-span concentrated load and uniformly distributed loads. The parametric study is performed by varying the geometric cross section parameters. The results are compared with international standards and analytical procedures. It is concluded that the influence of destabilizing and neutral effects of loadings is responsible for providing non-conservative results when compared to the American Institute of Steel Construction procedure. The occurrence of LDB in hot-holled I sections influences the lateral stability strength of I-beams, but it is not predominant. With the possibility of using the elastic critical moment obtained by the elastic stability analysis, the conservative situation of EC3 is verified. The Australian Standards procedure is in agreement with all the situations analyzed.

Absorption spectra of Müller cell intermediate filaments: Experimental results and theoretical models.

Experimental spectra of Müller cell (MC) intermediate filaments (IFs) isolated from porcine retina are reported in this work. The absorption spectra recorded at different MC IF concentrations were used to estimate their absorption cross-sections at different wavelengths. The average absorption cross-section of a single MC IF was ca. (0.97 … 2.01)×10-10cm2 in the 650-445nm spectral range. To interpret these experimental absorption spectra, we made ab initio calculations of the optical spectra of α-helix polypeptides, and also used a simplified theoretical approach that modeled an IF by a conductive wire. The energy spectra of the refractive index, extinction coefficient (absorption cross-section), energy loss and reflectivity functions for different photon polarizations, with strong anisotropy with respect to the system axis, were calculated ab initio for polyglycine α-helix molecule containing 1000 glycine residues. Strong anisotropy of these parameters was explained by photons interacting with different electronic transitions. Note that similarly strong anisotropy was also obtained for the optical absorption cross-sections in the simplified model. Both modeling approaches were used for calculating the absorption cross-sections of interest. As a result, the absorption cross-section for photons propagating axially along MC IFs was much larger than their geometrical cross-section. The latter result was explained taking into account the density of electronic states, with numerous electrons contributing to the transition intensity at a given energy. We found that the simple conductive wire model describes the MC IF absorption spectrum better than the ab initio spectra. The latter conclusion was explained by the limitations of ab initio analysis, which only took into account one α-helix with 1000 aminoacids, whereas each porcine Müller cell IF is assembled of thousands of protein molecules, reaching the total length of ca. 100μm. The presently reported results contribute to the understanding of the quantum mechanism of high-contrast vision of vertebrate eyes.

Dynamic oscillatory radiation force in optical heterodyning

In contrast with the optical radiation force induced by conventional lasers of continuous mono-frequency wave-fields, a dynamic or oscillatory mode arises when a beating effect caused by intensity changes over time (i.e., amplitude modulation) occurs. The purpose of this analysis is directed toward examining theoretically the dynamic (oscillatory) radiation force in optical heterodyning, caused by interfering/mixing two electromagnetic/optical plane waves driven at slightly different frequencies. The example of a dielectric cylinder material having a circular geometric cross-section is considered. Based on the integration of Maxwell's stress tensor over the surface of the object in the near-field, the modal expansion method in cylindrical coordinates is used in conjunction with the short-term time average to obtain mathematical series for the longitudinal dynamic optical radiation force per-length (i.e. acting along the direction of wave propagation). The cases of incident TM and TE polarized plane progressive waves are considered. Numerical illustrative results for the dimensionless dynamic radiation force function are performed with particular emphasis on changing the size parameter of the cylinder as well as the normalized difference-frequency and time parameters. The results show that the total force is not the mere sum of the components originated by the primary waves. There is an additional cross-term factor related to the dynamic component which cannot be neglected, suggesting that the radiation force phenomenon using beating optical waves is slowly time-varying (oscillatory). Moreover, a resonance splitting effect in the plots of the dynamic radiation force is observed and discussed. The present analysis has the potential to open a novel method in the development of dynamic/oscillatory optical heterodyne tweezers and tractor beams for particle manipulation and characterization.

Study on Crush Tube Geometric Cross sections and Topology for Axial Crashworthiness

Crush tubes are used as crash impact energy absorbing structure (EAS) and are located in the frontal compartment of road vehicles. Ideal crashworthiness of an EAS mandates that the equivalent decelerations due to impact forces should to be ≤ 20g; and crush force and stroke efficiencies should tend to unity. It is understood from the literature that no single geometric cross-section shape exhibits a near-ideal crashworthiness; and most EAS members exhibit a high initial peak crush force which is detrimental to the occupant safety, and moderate stroke and crush force efficiencies leading to a compromise in the total energy absorbed. In this paper, finite element analysis (FEA) methodology is formulated and experimentally validated for axial crush of a crush tube of SS304 material with circular cross section. Subsequently, plastic deformation phenomenon and folding patterns in relation to crush force behaviour of crush tubes with various basic cross-sections of polygonal geometric shapes from triangle to octagon and circle are extensively studied through FEA. Further, two new geometric cross-section profiles with combination of basic shapes are proposed to combine the merits of different basic shapes. The crashworthiness of all basic cross-sections including the two proposed cross-section profiles is assessed based on standard parameters. The proposed new geometries may form a basis for the development of new EAS configurations for enhanced crashworthiness.

Performance of self-compacting concrete used in congested reinforcement structural element

Self-Compacting Concrete (SCC) is a concrete which can be placed under its own weight without any means of vibration. Furthermore, it can be handled without segregation and bleeding. This research compares the structural performance of congested reinforcement beams cast with Self Compacting Concrete (SCC) and identical beams cast with Vibrated Concrete (VC). In this research, beam specimens with same geometric cross-section, length, and stirrup configuration and different longitudinal reinforcement ratios were casted. A total of six beams (four cast with SCC and two cast with VC) are tested in the experimental investigation under monotonic mid-span concentrated loading. The test results showed that SCC performed better in congested reinforcement beam than the referenced VC. The study also indicated that the difference in mix composition of SCC from that of VC has no effects on the service load deflection response of reinforced concrete beam.

Reconsidering astrophysical constraints on macroscopic dark matter

Macroscopic dark matter---``macros''---refers to a broad class of alternative candidates to particle dark matter with still unprobed regions of parameter space. These candidates would transfer energy primarily through elastic scattering with approximately their geometric cross section. For sufficiently large cross sections, the linear energy deposition could produce observable signals if a macro were to pass through compact objects such as white dwarfs or neutron stars in the form of thermonuclear runaway, leading to a type Ia supernova or superburst, respectively. We update the constraints from white dwarfs. These are weaker than previously inferred in important respects because of more careful treatment of the passage of a macro through the white dwarf and greater conservatism regarding the size of the region that must be heated to initiate runaway. On the other hand, we place more stringent constraints on macros at a low cross section, using new data from the Montreal White Dwarf Database. New constraints are inferred from the low mass x-ray binary 4U 1820-30, in which more than a decade passed between successive superbursts. Updated microlensing constraints are also reported.

Identify the limits of geometric optics ray tracing by numerically solving the vector Kirchhoff integral.

The properties of a pencil of light as defined approximately in the geometric optics ray tracing method are investigated. The vector Kirchhoff integral is utilized to accurately compute the electromagnetic near field in and around the pencil of light with various beam base sizes, shapes, propagation directions and medium refractive indices. If a pencil of light has geometric mean cross section size of the order p times the wavelength, it can propagate independently to a distance p2 times the wavelength, where most of the beam energy diffuses out of the beam region. This is consistent with a statement that van de Hulst made in a classical text on light scattering. The electromagnetic near fields in the pencil of light are not uniform, have complicated patterns within short distances from the beam base, and the fields tend to converge to Fraunhofer diffraction fields far away from the base.

Impact of Various Parameters on Convergence Performance of CMFD Acceleration for MOC in Multigroup Heterogeneous Geometry

The impact of various parameters in the coarse mesh finite difference (CMFD) acceleration method on overall convergence behavior is investigated through numerical calculations using the method of characteristics (MOC). Four parameters appearing in the CMFD acceleration with MOC, i.e., scalar flux distribution in flat flux regions (FFRFlux), the scalar flux distribution in CMFD meshes (CMFDFlux), homogenized cross sections (HXSs) in CMFD meshes, and current correction factors (CCFs), are considered. Parts of these four parameters are fixed to the converged values throughout iterations in order to estimate their impact on convergence. Numerical calculations are carried out for Korea Advanced Institute of Science and Technology’s (KAIST’s) benchmark problem KAIST-2A, which is a heterogeneous and multigroup problem, and the number of outer iterations to reach convergence is evaluated. The impact of geometric heterogeneity and cross-section homogenization in the CMFD acceleration has not been considered in linearized Fourier analysis so far. The calculation results indicate that (1) convergence of HXS has little impact on the overall convergence, (2) convergence of FFRFlux is dominant followed by CCF when a CMFD mesh is optically thin, and (3) convergence of FFRFlux is dominant when a CMFD mesh is optically thick and contains many flat flux regions.

Variations in Aerosol Microstructure under the Influence of Smokes from Forest Fires according to Inversion of Spectral Extinction Characteristics in the Near-Surface Layer and in Vertical Atmospheric Column

We studied the specific features of aerosol microstructure in the near-surface layer and in the vertical column of the atmosphere after smoke from fires enters the atmosphere. The aerosol microstructure parameters were determined by solving the inverse problem for spectral measurements of the aerosol extinction coefficient by the path method and aerosol optical depth by the sun photometry method in a clean atmosphere and during its pollution by smoke near Tomsk. The geometric cross section, volume concentration, and mean radius of submicron and coarse aerosol particles were calculated. The coarse fraction is typically predominant in the total volume of aerosol under the background conditions. The submicron fraction of aerosol particles becomes dominant after air is polluted by smoke. As a result, the particle radius averaged over the total ensemble decreases in the smoke-polluted atmosphere. Also, the character of regression interrelations between the microstructure parameters of different aerosol fractions and the spectral characteristics of aerosol extinction significantly change after the atmosphere is polluted by smoke.

Death and serious injury from dark matter

Macroscopic dark matter (macros) refers to a class of dark matter candidates that scatter elastically off of ordinary matter with a large geometric cross-section. A wide range of macro masses MX and cross-sections σX remain unprobed. We show that over a wide region within the unexplored parameter space, collisions of a macro with a human body would result in serious injury or death. We use the absence of such unexplained impacts with a well-monitored subset of the human population to exclude a region bounded by σX>10−8−10−7 cm2 and MX<50 kg. Our results open a new window on dark matter: the human body as a dark matter detector.