Spectral Nature Research Articles

Influences in Experimental Studies on the Characteristics of Transonic Shock Buffet

This article examines the evolution of transonic shock buffet on the OAT15A supercritial airfoil, aspiring to elucidate some of the causes of disparate buffet-relevant settings reported in the literature. Experimental campaigns on three distinct chord lengths are conducted thus resulting in chord Reynolds numbers of 1.3×106, 2.0×106, and 2.7×106. Continuous variations in Mach and AoA allow mapping the shock wave behavior across a large parameter space and capturing the transition between prebuffet, onset, developed buffet, and offset. High-speed focusing schlieren imaging is employed to extract the temporal and spectral nature of buffet and to deduce key quantities that characterize the unsteadiness. Three-dimensional effects along the span due to interference with the side walls are assessed thoroughly, revealing that phases of upstream shock motion and large separation are accompanied by 3D distortion. Even though this effect gains relevance at reduced aspect ratio, a substantially 2D character of the shock surface around the centerplane is confirmed. Fully developed buffet governed by a 2D buffet mode is proven to exist across all test cases, although the buffet peak condition is gradually shifted toward reduced angle of attack, yet greater Mach number with increased Reynolds number. Conversely, accompanied by severely increased angles of attack, the buffet mechanism appears more volatile for small chord length and is consolidated with enlarged chord, which strongly suggests a dependence on the state of the turbulent boundary layer.

Unraveling Plasmonic Tilted Fiber Bragg Gratings (TFBG): A Journey From “Anomalous Resonances” to Refined Refractometry

AbstractPlasmonic tilted fiber Bragg gratings (TFBGs) have emerged as versatile tools for refractometric analyses and biochemical sensing. Their applications have significantly blossomed these last years, from proteins and cellular bioassays to operando monitoring in batteries, to cite just a few. They are widely recognized for their cutting‐edge performance and low limits of detection, arising from their dense multimodal spectral nature featuring tens of narrowband cladding mode resonances. Their comb‐like spectrum is so rich that numerous demodulation techniques have been reported, without benchmark of their relative performance while they possess important distinctions. This review highlights developments in detangling techniques from the pioneering works based on single‐peak analysis up to the most recent approaches involving Fourier analysis, the implementation of machine learning, and cascaded spectral decomposition processes. To fairly compare the different techniques of the literature, we implemented each analysis on original experimental refractometric calibrations, revealing the assets of the most updated methods. This paper therefore reviews these demodulation techniques based on the same datasets, obtained under the same conditions. We show and discuss the results obtained from bioassays and pinpoint the importance of advanced analytical methodologies to maximize the reproducibility, reliability and performance of plasmonic‐based TFBGs biosensors.

On the Flux Density Spectral Property of High Linearly Polarized Signal from Pulsar J0332+5434

The polarization position angles (PPA) of time samples with high linear polarization often show two parallel tracks across the pulsar profile that follow the rotating vector model (RVM). This feature supports coherent curvature radiation (CCR) as the underlying mechanism of radio emission from pulsars, where the parallel tracks of the PPA represent the orthogonal extraordinary (X) and ordinary (O) eigenmodes of strongly magnetized pair plasma. However, the frequency evolution of these high linearly polarized signals remains unexplored. In this work, we explore the flux density spectral nature of high linearly polarized signals by studying the emission from PSR J0332+5434 over a frequency range between 300 and 750 MHz, using the Giant Metrewave Radio Telescope. The pulsar average profile comprises a central core and a pair of conal components. We find the high linearly polarized time samples to be broadband in nature, and in many cases, they resemble a narrow spiky feature in the conal regions. These spiky features are localized within a narrow pulse longitude over the entire frequency range, and their spectral shapes sometimes resemble an inverted parabolic shape. In all such cases, the PPA is exclusively along one of the orthogonal RVM tracks, likely corresponding to the X-mode. The inverted spectral shape can, in principle, be explained if the high linearly polarized emission in these time samples is formed due to the incoherent addition of CCR from a large number of charged solitons (charge bunches) exciting the X-mode.

Unveiling the short and faint X-ray transient nature of IGR J17419-2802

We report new X-ray results from the INTErnational Gamma-Ray Astrophysics Laboratory ( and observations of the hitherto poorly studied unidentified X-ray transient We studied in detail the temporal, spectral, and energetic properties of three hard X-ray outbursts detected above 20 keV by They are all characterized by an average X-ray luminosity of 3times 1035 erg $ and a constrained duration of a few days. This marks a peculiarly short and faint X-ray transient nature for From archival unpublished soft X-ray observations, we found that the source spends most of the time undetected at very low X-ray fluxes (down to $<4.7 $ erg cm $^ $ s$^ $) for a dynamic range $>$2,000 when in outburst. We provided an accurate arcsecond-sized source error circle. Inside it, we pinpointed the best candidate near-infrared counterpart whose photometric properties are compatible with a late-type spectral nature. Based on our new findings, we suggest that is a new member of the very faint X-ray transients (VFXTs) class. Detailed investigations of VFXT outbursts above 20 keV are particularly rare. In this respect, our reported outbursts are among the best studied to date; in particular, their constrained duration of a few days is among the shortest ever measured for a VFXT. X-rays: binaries: individual: src

Inquiring into a spectral concept in the physics classroom

Abstract We designed an inquiry activity to investigate the question ‘How transparent are transparent films and papers?’ Using an easily-replicable set up, we observed the effect of increasing the number of transparent films, thin papers and general papers between a light source and a light sensor. For each material, one sheet was added each time. The amount of light received was collected and graphed by a data logger. Our findings show that, as the number of sheets increases, the amount of light received at the receiver decreases. The general paper and thin paper stacks took 4 sheets and 10 sheets respectively to achieve negligible light transmittance. The transparent film stack did not achieve negligible light transmittance, but successive addition of sheets did lower transmittance. Evidently, transparent films are not perfectly transparent. Transparency (and opacity) is not a binary condition, but rather a continuum based on boundary conditions. The inquiry activity developed through this study, which investigates a spectrum of transparency in films and papers, may be useful for students to appreciate the spectral nature of the transparency concept across different materials.

Grain boundary segregation for the Fe-P system: Insights from atomistic modeling and Bayesian inference

In this work we re-assess experimental data for grain boundary (GB) segregation of P in bcc Fe with thermodynamic and statistical methods. The data are based on Auger-Electron-Spectroscopy (AES) measurements which have provided P GB concentrations for various bulk contents and temperatures for ferrite. While in the previous investigations of this system a single-site McLean equation was used to extract segregation enthalpy and entropy, we employ multi-site segregation models as suggested by atomistic simulations. We use a recently introduced methodology based on inter-atomic potentials to calculate the segregation energy spectrum, as well as vibrational entropy and solute-solute interactions of P in polycrystalline Fe, thereby obtaining a three-dimensional distribution of energy, entropy and interactions. Using this trivariate distribution we predict P GB concentrations and compare them to the experimental measurements. Furthermore, we calibrate the parameters of physical models of increasing complexity to the experimental data with an inverse modeling approach based on Bayesian inference. While it is not possible to seamlessly link the results from AES to atomistic modeling, our investigation provides significant new insights for thermodynamic modeling of GB segregation. The vibrational entropy deduced from experiment differs from physics based atomistic simulations even when taking the spectral nature of energy, entropy and interactions into account. However, the correlations of the quantities are in good agreement when considering the trivariate model. Our investigation highlights the need for new and more accurate experimental datasets of GB segregation.

Back to the Future: Everything You Wish You’d Asked Derrida About ChatGPT When You Had the Chance!

This article considers and then reconsiders what ChatGPT produces and how it produces it, using the work of a range of critical theorists and authors. In particular, it imagines what different philosophers, thinkers, and writers would say about this most recent technological leap, if they were somehow brought back from the past, into this, our new future. To ventriloquise for them, this article plays fast and loose with the work and styles of Douglas Adams, Virginia Woolf, and Alan Turing, among others, to try to demonstrate what ChatGPT can do, having been potentially “trained” on their work, as well as highlighting the nuances of allusion, subtext, paradox, and contradiction as possibly more human aspects of both writing and reading. Such “play” is followed by a more serious analysis of writing and the suggestion of a “Double Signature Signification” at work in the text produced by ChatGPT, meaning one system of signification for writing the text (schematic) and one for reading it (referent), which overlap perfectly. The article concludes by arguing that it is not the consciousness of ChatGPT that beguiles us, it is the possibility of that consciousness, and what gives rise to that sense of possibility is partly the spectral and haunting nature of dialoguing with it. A dialogue with ChatGPT has all the excitement of a séance: it is uncertain, unknown, yet with its traces of the familiar it is also like talking to the dead.

Stability of the gapless pure point spectrum of self-adjoint operators

We consider a self-adjoint operator T on a separable Hilbert space, with pure-point and simple spectrum with accumulations at finite points. Explicit conditions are stated on the eigenvalues of T and on the bounded perturbation V ensuring the global stability of the spectral nature of T + ɛV, ε∈R.

General spectral characteristics of human activity and its inherent scale-free fluctuations

The scale-free nature of daily human activity has been observed in different aspects; however, the description of its spectral characteristics is incomplete. General findings are complicated by the fact that—although actigraphy is commonly used in many research areas—the activity calculation methods are not standardized; therefore, activity signals can be different. The presence of 1/f noise in activity or acceleration signals was mostly analysed for short time windows, and the complete spectral characteristic has only been examined in the case of certain types of them. To explore the general spectral nature of human activity in greater detail, we have performed Power Spectral Density (PSD) based examination and Detrended Fluctuation Analysis (DFA) on several-day-long, triaxial actigraphic acceleration signals of 42 healthy, free-living individuals. We generated different types of activity signals from these, using different acceleration preprocessing techniques and activity metrics. We revealed that the spectra of different types of activity signals generally follow a universal characteristic including 1/f noise over frequencies above the circadian rhythmicity. Moreover, we discovered that the PSD of the raw acceleration signal has the same characteristic. Our findings prove that the spectral scale-free nature is generally inherent to the motor activity of healthy, free-living humans, and is not limited to any particular activity calculation method.

Implication of site-specific segregation on grain boundary structural transition and deformation response in nanocrystalline Ni-Nb alloy

In this study, atomistic simulation technique is utilized to examine the impact of grain boundary (GB) character (disorientation angle), structure (boundary-free volume) and energy (boundary excess energy) on site-specific Nb solute segregation behaviour and structural transition in nanocrystalline (NC) Ni. Further, the role of site-specific solute segregation on deformation response in NC Ni is also studied. Towards this, hybrid Monte Carlo/molecular dynamics simulations is performed to obtain the equilibrium structure in a Ni-Nb binary alloy at 500 K and 1200 K. The spectral nature of GB atoms is explored in NC Ni, and found that the GBs have a skew-normal distribution for excess atomic volume and excess atomic energy. Furthermore, comprehensive segregation energy calculations are performed for each GB site, revealing the preferential affinity of Nb towards GB sites in NC Ni. The study also examined interfacial solute excess for each boundary as a function of GB energy and Voronoi volume. The findings indicate that the GB energy decreased as the solute excess increased, whereas the Voronoi volume demonstrated an increment in response to the solute excess. Moreover, simulations conducted at 1200 K reveal that not all GBs within the microstructure are transformed to amorphous complexions; rather some GBs remain ordered. The simulated tensile tests conducted at 500 K and 1200 K reveal that the strengthening in Ni-Nb enhances with the increase in dopant concentration. Additionally, the simulated shear test conducted at 1200 K shows that the deformation in Ni-5Nb occurs more uniformly than pure NC Ni due to formation of thicker amorphous GBs in the former one.

Correlation between absorbed acetylene spectral characteristic and nature of ionic liquids studied by in situ ATR-FTIR spectroscopy

Ionic liquids (ILs) are believed to be a prospective replacement for molecular solvents and are suggested to be used for storing and/or purifying acetylene. Systematic studies of the nature of the interaction between acetylene and different ILs may be necessary to predict new types of ionic liquids. Therefore, in order to investigate the absorption of acetylene by ionic liquids, in situ ATR-FTIR spectroscopy has been applied for the first time. Two series of ILs, such as [C2‍–‍10MIM][BF4] and ILs based on imidazolium with various anions, have been considered. A detailed analysis of the spectra of absorbed acetylene is given, with special attention being paid to the basicity of IL anions as well as the influence of the cation alkyl chain length. It has been found that the magnitude of the band shift associated with the CH stretching vibration and the antisymmetric CH bending vibration of absorbed acetylene correlates well with the change in the Kamlet-Taft β parameter of the ILs. Moreover, neither the free volume nor the IL polarity plays a key role in acetylene solubility. In addition, the enthalpy and entropy of absorption have been calculated for several ILs.

Experimental investigation on the turbulent wake flow in fully established transonic buffet conditions

The transonic flight regime is often dominated by transonic buffet, a highly unsteady and complex shock-wave/boundary-layer interaction involving major parts of the flow field. The phenomenon is associated with a large-amplitude periodic motion of the compression shock coupled with large-scale flow separation and intermittent re-attachment. Due to the resulting large-scale variation of the global flow topology, also the turbulent wake of the airfoil or wing is severely affected, and so are any aerodynamic devices downstream on which the wake impinges. To analyze and understand the turbulent structures and dynamics of the wake, we performed a comprehensive experimental study of the near wake of the supercritical OAT15A airfoil in transonic buffet conditions at a chord Reynolds number of 2×106\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2\ imes 10^{6}$$\\end{document}. Velocity field measurements reveal severe global influences of the buffet mode on both the surface-bound flow field on the suction side of the airfoil and the wake. The flow is intermittently strongly separated, with a significant momentum deficit that extends far into the wake. The buffet motion induces severe disturbances and variations of the turbulent flow, as shown on the basis of phase-averaged turbulent quantities in terms of Reynolds shear stress and RMS-values. The spectral nature of downstream-convecting fluctuations and turbulent structures are analyzed using high-speed focusing schlieren sequences. Analyses of the power spectral density pertaining to the vortex shedding in the direct vicinity of the trailing edge indicate dominant frequencies one order of magnitude higher than those associated with shock buffet (Stc=O(1)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$St_c=\\mathcal {O}({1})$$\\end{document}) vs. Stc=O(0.1)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$St_c=\\mathcal {O}({0.1})$$\\end{document}). It is shown that the flapping motion of the shear layer is accompanied by the formation of a von Kármán-type vortex street of fluctuating strength. These wake structures and dynamics will impact any downstream aerodynamic devices affected by the wake. Our study, therefore, allows conclusions regarding the incoming flow of devices such as the tail plane.

Characterizing the Spectral Structure of Weak Impulsive Narrowband Quiet Sun Emissions

Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs) are a newly discovered class of radio emission from the solar corona. These emissions are characterized by their extremely impulsive, narrowband, and ubiquitous nature. We have systematically been working on their detailed characterization, including their strengths, morphologies, temporal characteristics, energies, etc. This work is the next step in this series and focuses on the spectral nature of WINQSEs. Given that their strength is only a few percent of the background solar emission, we have adopted an extremely conservative approach to reliably identify WINQSES. Only a handful of WINQSEs meet all of our stringent criteria. Their flux densities lie in the 20–50 Jy range and they have compact morphologies. For the first time, we estimate their bandwidths and find them to be less than 700 kHz, consistent with expectations based on earlier observations. Interestingly, we also find similarities between the spectral nature of WINQSEs and the solar radio spikes. This is consistent with our hypothesis that the WINQSEs are the weaker cousins of the type III radio bursts and are likely to be the low-frequency radio counterparts of the nanoflares, originally hypothesized as a possible explanation for coronal heating.

DSSFN: A Dual-Stream Self-Attention Fusion Network for Effective Hyperspectral Image Classification

Hyperspectral images possess a continuous and analogous spectral nature, enabling the classification of distinctive information by analyzing the subtle variations between adjacent spectra. Meanwhile, a hyperspectral dataset includes redundant and noisy information in addition to larger dimensions, which is the primary barrier preventing its use for land cover categorization. Despite the excellent feature extraction capability exhibited by convolutional neural networks, its efficacy is restricted by the constrained receptive field and the inability to acquire long-range features due to the limited size of the convolutional kernels. We construct a dual-stream self-attention fusion network (DSSFN) that combines spectral and spatial information in order to achieve the deep mining of global information via a self-attention mechanism. In addition, dimensionality reduction is required to reduce redundant data and eliminate noisy bands, hence enhancing the performance of hyperspectral classification. A unique band selection algorithm is proposed in this study. This algorithm, which is based on a sliding window grouped normalized matching filter for nearby bands (SWGMF), can minimize the dimensionality of the data while preserving the corresponding spectral information. Comprehensive experiments are carried out on four well-known hyperspectral datasets, where the proposed DSSFN achieves higher classification results in terms of overall accuracy (OA), average accuracy (AA), and kappa than previous approaches. A variety of trials verify the superiority and huge potential of DSSFN.

Decoding transport selectivity of ions in polymer membranes by In-situ impedance spectroscopy

ABSTRACT Depending upon charge and chemical affinity, interplay between resistances of membrane (Rmem) and membrane–solution interface (RHT) may lead to preferential transport of an ion. Here, intermittent in-situ electrochemical impedance spectroscopy (EIS) was done during electrodriven transport (radiotracer based) to analyze the transport selectivity of Cs+ over Na+ in different membranes. Preference of the membranes for Cs+ was reflected in the time-dependent Nyquist plots itself. Bode plot analysis also indicated dominant Cs+ transport in terms of phase and frequency shift in crown ether (DB21C7) based membrane. In Cs+ selective polymer incluion membranes, irrespective of carrier, RHT contributed majorly to overall resistance. However, time dependence of RHT/Rmem was carrier as well as ion dependent. Interestingly, for nonselective ionic carrier, RHT/Rmem was majorly close to 1 and a reverese transport order than previous membranes were obtained. A higher Na+ transport (than Cs+) was also obtained for DB21C7 loaded Nafion, where, due to ion templating effect, Rmem was the governing factor. EIS spectral nature of a mixed feed solution follows that of the most preferred ion, thus suggesting that EIS can be used to study prospective real-life systems and can be used as a significant tool in designing ion-selective membranes.

Real-World Urban Light Emission Functions and Quantitative Comparison with Spacecraft Measurements

We provide quantitative results from GIS-based modelling of urban emission functions for a range of representative low- and mid-rise locations, ranging from individual streets to residential communities within cities, as well as entire towns and city regions. Our general aim is to determine whether lantern photometry or built environment has the dominant effect on light pollution and whether it is possible to derive a common emission function applicable to regions of similar type. We demonstrate the scalability of our work by providing results for the largest urban area modelled to date, comprising the central 117 km2 area of Dublin City and containing nearly 42,000 public lights. Our results show a general similarity in the shape of the azimuthally averaged emission function for all areas examined, with differences in the angular distribution of total light output depending primarily on the nature of the lighting and, to a smaller extent, on the obscuring environment, including seasonal foliage effects. Our results are also consistent with the emission function derived from the inversion of worldwide skyglow data, supporting our general results by an independent method. Additionally, a comparison with global satellite observations shows that our results are consistent with the deduced angular emission function for other low-rise areas worldwide. Finally, we validate our approach by demonstrating very good agreement between our results and calibrated imagery taken from the International Space Station of a range of residential locations. To our knowledge, this is the first such detailed quantitative verification of light loss calculations and supports the underlying assumptions of the emission function model. Based on our findings, we conclude that it should be possible to apply our approach more generally to produce estimates of the energy and environmental impact of urban areas, which can be applied in a statistical sense. However, more accurate values will depend on the details of the particular locations and require treatment of atmospheric scattering, as well as differences in the spectral nature of the sources.

Spectral Luminescent Properties and Nature of Electronically Excited States of Sulfaguanidine in Water

Spectral Luminescent Properties and Nature of Electronically Excited States of Sulfaguanidine in Water

Understanding Pseudocapacitance Mechanisms by Synchrotron X‐ray Analytical Techniques

Pseudocapacitive materials that store charges via reversible surface or near‐surface faradaic reactions are capable of overcoming the capacity limitations of electrical double‐layer capacitors. Revealing the structure–activity relationship between the microstructural features of pseudocapacitive materials and their electrochemical performance on the atomic scale is the key to build high‐performance capacitor‐type devices containing ideal pseudocapacitance effect. Currently, the high brightness (flux), and spectral and coherent nature of synchrotron X‐ray analytical techniques make it a powerful tool for probing the structure–property relationship of pseudocapacitive materials. Herein, we report a comprehensive and systematic review of four typical characterization techniques (synchrotron X‐ray diffraction, pair distribution function [PDF] analysis, soft X‐ray absorption spectroscopy, and hard X‐ray absorption spectroscopy) for the study of pseudocapacitance mechanisms. In addition, we offered significant insights for understanding and identifying pseudocapacitance mechanisms (surface redox pseudocapacitance, intercalation pseudocapacitance, and the extrinsic pseudocapacitance phenomenon in battery materials) by combining in situ hard XAS and electrochemical analyses. Finally, a perspective for further depth of understanding into the pseudocapacitance mechanism using synchrotron X‐ray analytical techniques is proposed.

Synthesis, characterization and structural study of new nickel(II) and mercury (II) complexes with imidazole oxime ligand

In this work, we reported the experimental study of two new complexes named bis[chloro(1-methyl-1H-imidazole-2-carbaldehyde oxime) Nickel(II) {Ni(L)2Cl2} (I) and Di-μ-chloro-bis[chloro(1-methyl-1H-imidazole-2-carbaldehyde oxime) mercury(II) {Hg2(L)2Cl4}, DMF (II) based on (1-methylimidazole-2-aldoxime, L). The synthesis, crystallization, and characterization were detailed. Complex I display a hexa-coordinated compound with a distorted octahedral geometry. The NiII ion is coordinated to two ligands in bidentate mode through nitrogen atoms. Complex II has one unique HgII atom and forms a dimeric complex that contains two HgII atoms related by a crystallographic inversion center. In this complex, the oxime ligand is ligated to the metal ion in a monodentate fashion through the nitrogen atoms. On the basis of bond distances, this complex exhibits a distorted tetrahedral geometry around each mercury center metal. The complexes are analyzed using single-crystal diffraction, FT-IR, NMR, and UV–visible. Cyclic voltammetry was also used to understand the electrochemical behavior of the ligand and both complexes. Furthermore, we have performed DFT calculations to get more about the spectral electronic transitions' nature, reactivity, and electronic structure of the ligand and complexes.

Temperature response in skin tissue during hyperthermia based on three-phase-lag bioheat model using RBF meshfree method

Various non-Fourier heat conduction models have been proposed to overcome the paradox of the infinite propagation speed of heat signals in Fourier’s law. The three-phase-lag (TPL) heat model is a non-Fourier model based on the linearized theory of coupled thermoelasticity that combines heat flux, temperature gradient, and thermal displacement gradient. The present paper proposes a novel method with meshfree and spectral nature to solve the two-dimensional three-phase lag (TPL) bioheat model for tissue heating during hyperthermia. In the space domain, radial basis functions (RBFs) and, in the time direction, shifted Chebyshev polynomials are employed to solve the model. The use of Chebyshev polynomials in the time direction enables one to get the solution in the whole time domain simultaneously with fewer nodes. Further, there is no need for background meshes in space due to the meshless nature of RBFs, and the proposed method is equally applicable to irregular domains. The proposed method is validated with the existing semi-analytical solution of the TPL model in one spatial dimension and found to be in good agreement. The temperature profile and the impact of various parameters, such as phase lag times, blood perfusion rate, and heat source parameters on heat transfer in tissue, have also been discussed.