Contribution Of Modes Research Articles

Modal contribution of bundle oscillation to induced flow

Bundle motion of a pressurized water reactor core under seismic excitation induces a pulsatile oscillatory flow which can strongly affect the behaviour of the whole reactor. Understanding these phenomena is thus a primary concern for the nuclear industry. An index matched experimental facility was recently built allowing the study of a surrogate fuel bundle under axial flow undergoing a seismic excitation. The use of a fluid which is refractive index matched with the transparent surrogate bundle allows to perform non-intrusive particle image velocimetry measurements over the whole fluid domain. This paper focuses on experiments performed with a shaking table frequency close to the first resonance mode of the bundle under otherwise stagnant water. The local flow induced by the oscillation of the fuel bundle is then observed. Three significant dataset are analysed to understand how very small changes in the structural behaviour of the fuel bundle can lead to important changes in the flow field. A proper orthogonal decomposition of the bundle excitation shows that the higher modes (even if they have small amplitudes) can contribute significantly to the flow induced in the channel. A numerical simulation based on the potential flow theory simulates the experimental conditions and provides a physical interpretation of the observed phenomena.

Quasinormal mode theory for nanoscale electromagnetism informed by quantum surface response

We report a self-consistent quasinormal mode theory for nanometer scale electromagnetism where the possible nonlocal and quantum effects are treated through quantum surface responses. With Feibelman's frequency-dependent \textit{d} parameters to describe the quantum surface responses, we formulate the source-free Maxwell's equations into a generalized linear eigenvalue problem to define the quasinormal modes. We then construct an orthonormal relation for the modes and consequently unlock the powerful toolbox of modal analysis. The orthonormal relation is validated by the reconstruction of the full numerical results through modal contributions. Significant changes in the landscape of the modes are observed due to the incorporation of the quantum surface responses for a number of nanostructures. Our semi-analytical modal analysis enables transparent physical interpretation of the spontaneous emission enhancement of a dipolar emitter as well as the near-field and far-field responses of planewave excitations in the nanostructures.

Optical resonances in graded index spheres: A resonant-state-expansion study and analytic approximations

Recent improvements in the resonant-state expansion (RSE), focusing on the static mode contribution, have made it possible to treat transverse-magnetic (TM) modes of a spherically symmetric system with the same efficiency as their transverse-electric (TE) counterparts. We demonstrate here that the efficient inclusion of static modes in the RSE results in its quick convergence to the exact solution regardless of the static mode set used. We then apply the RSE to spherically symmetric systems with continuous radial variations of the permittivity. We show that in TM polarization, the spectral transition from whispering gallery to Fabry-Perot modes is characterized by a peak in the mode losses and an additional mode as compared to TE polarization. Both features are explained quantitatively by the Brewster angle of the surface reflection which occurs in this frequency range. Eliminating the discontinuity at the sphere surface by using linear or quadratic profiles of the permittivity modifies this peak and increases the Fabry-Perot mode losses, in qualitative agreement with a reduced surface reflectivity. These profiles also provide a nearly parabolic confinement for the whispering gallery modes, for which an analytical approximation using the Morse potential is presented. Both profiles result in a reduced TE-TM splitting, which is shown to be further suppressed by choosing a profile radially extending the mode fields. Based on the concepts of ray optics, phase analysis of the secular equation, and effective quantum-mechanical potential for a wave equation, we have further developed a number of useful approximations which shed light on the physical phenomena observed in the spectra of graded-index systems.

Nanoscale Mapping and Defect‐Assisted Manipulation of Surface Plasmon Resonances in 2D Bi2Te3/Sb2Te3 In‐Plane Heterostructures

AbstractThe Bi2Te3/Sb2Te3 in‐plane heterostructure is reported as a low‐dimensional tunable chalcogenide well suited as plasmonic building block for the visible−UV spectral range. Electron‐driven plasmon excitations of low‐dimensional Bi2Te3/Sb2Te3 are investigated by monochromated electron energy loss spectroscopy spectrum imaging. To resolve the nanoscale spatial distribution of various local plasmonic resonances, singular value decomposition is used to disentangle the spectral data and identify the individual spectral contributions of various corner, edge, and face modes. Furthermore, defect‐plasmon interactions are investigated both for nanoscale intrinsic and thermally induced extrinsic polygonal defects (in situ sublimation). Signature of defect‐induced red shift ranging from a several hundreds of millielectronvolts to a few electronvolts, broadening of various plasmon response, together with selective enhancement and significant variations in their intensity are detected. This study highlights the presence of a heterointerface and identifies defects as physical tuning pathways to modulate the plasmonic response over a broad spectral range. Finally, the experimental observations are compared qualitatively and validated with numerical simulations using the electron‐driven discrete dipole approximation. Low‐dimensional Bi2Te3/Sb2Te3 as a less explored plasmonic system holds great promises as emerging platform for integrated plasmonics. Furthermore, introducing controlled structural defects can open the door for nanoengineering of plasmonic properties in such systems.

Development of fragility curves in adjacent steel moment-resisting frames considering pounding effects through improved wavelet-based refined damage-sensitive feature

Fragility curves present useful information related to earthquake-induced probability assessment of steel moment-resisting frames (MRFs) and determine the probability of the damage exceedance at different floor levels of MRFs. The review of the literature shows that most of the previous studies dealing with the fragility curves were based on conventional measures, such as spectral acceleration at the first mode period, peak ground acceleration, and/or engineering demand parameters (e.g. maximum story drift ratio (max SDR)) to estimate the structure's damage state. In this article, a new approach is developed to map the fragility curves in adjacent MRFs including effects of pounding through improved wavelet-based refined damage-sensitive feature (WB-rDSF) which considers contributions of the upper modes. Morlet and complex Morlet (cmorfb-fc) wavelets, which is known as extremely precise rDSF, were extended to map the fragility curves. The correlation coefficient between rDSF and max SDR is evaluated as a criterion to determine the efficiency of wavelets-based damage index (WB-DI). The steel MRFs with six and nine stories are selected to implement the proposed approach concerning adjacent MRFs prone to structural pounding during earthquakes. Acceleration responses recorded at the roof of both colliding and non-colliding MRFs were determined using incremental dynamic analysis (IDA) including different seismic ground motion records to formulate the damage index. Moreover, the first mode structural period, used in assembling the WB-rDSF, is estimated via auto-regressive moving-average with exogenous input method along with a stabilization diagram. The results show that the fragility curves, derived from cmorfb-fc WB-rDSF for both colliding and non-colliding MRFs due to higher correlation coefficient, have lower damage probabilities and are more efficient than the estimated fragility curves based on Morlet WB-rDSF which consider only the structural period of the first mode. Furthermore, due to the pounding phenomenon, the lower MRF experiences more damages with a larger probability.

Vibrational fingerprints of ferroelectric HfO2

Hafnia (HfO2) is a promising material for emerging chip applications due to its high-κ dielectric behavior, suitability for negative capacitance heterostructures, scalable ferroelectricity, and silicon compatibility. The lattice dynamics along with phononic properties such as thermal conductivity, contraction, and heat capacity are under-explored, primarily due to the absence of high quality single crystals. Herein, we report the vibrational properties of a series of HfO2 crystals stabilized with yttrium (chemical formula HfO2: xY, where x = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. We untangle the effects of Y by testing our calculations against the measured Raman and infrared spectra of the cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal the signature modes of polar orthorhombic hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO2 and paves the way for an analysis of mode contributions to high-κ dielectric and ferroelectric properties for chip technologies.

Effects of chemical energy accommodation on nonequilibrium flow and heat transfer to a catalytic wall

The accurate prediction of the aeroheating performance of hypersonic vehicles requires more detailed modeling of the catalysis process, rather than merely employing a catalytic coefficient. In this paper, the theoretical modeling, as well as the direct simulation Monte Carlo method, is used to preliminarily study the incomplete chemical energy accommodation effects, that is, only a part of the potential energy released in the heterogenous recombination reaction is transferred to the surface, while the remaining is retained as the vibrational energy of the desorbed molecule. An integrated model is proposed to describe the contribution of each energy mode in the rarefied nonequilibrium heat and mass transfer process. Based on the model and several Damköhler numbers, an analytical formula is derived, and is also shown to compare well with the numerical results. On account of the incomplete accommodations of the chemical and vibrational energy on the wall, a variation up to 20% is observed in predicting the stagnation point heat flux under typical nonequilibrium flow conditions. This study could enrich our understanding of the nonequilibrium heat transfer phenomenon and also shows a potential practical value.

Fatigue Crack Growth Behavior of CP-Ti Cruciform Specimens with Mixed Mode I-II Crack under Biaxial Loading.

Investigations on the fatigue crack growth of commercial pure titanium are carried out with cruciform specimens under different biaxial load ratios (λ = 0, 0.5, and 1) and crack inclination angles (β = 90°, 60°, and 45°) in this paper. Based on the finite element results, the modified solution of stress intensity factors KI and KII for cruciform specimens containing mixed mode I-II crack is obtained by considering crack size, biaxial load ratio, and crack inclination angles. The experimental results show that the maximum tangential stress criterion is fit for the prediction of crack initiation angles for mixed model I-II crack under uniaxial or biaxial loading condition. When the biaxial load ratio increases, the crack propagation angle becomes smaller, and so does the fatigue crack growth rate of mode I crack or mixed mode I-II crack. Based on an equivalent stress intensity factor, a new valid stress intensity factor is proposed to better describe the biaxial fatigue crack growth behavior, which can demonstrate the contribution of mode I and mode II of stress intensity factor.

Towards understanding, estimating and mitigating higher-mode effects for more resilient tall buildings

Rapid urbanization has resulted in increased demand for tall buildings in many large and medium-sized cities around the world. Current code-based standards for seismic design are primarily aimed at minimizing life-safety risks under major earthquakes. While reinforced concrete (RC) high-rise buildings designed following current code requirements are expected to achieve collapse-prevention, the contribution of higher modes of vibrations to the dynamic response of these structures can produce seismic demands significantly larger than those obtained from prescriptive code-based procedures, causing unexpectedly higher structural and non-structural damage to these buildings. These imply considerable costs associated with the loss of residences and business operations as well as the post-earthquake recovery of cities. This paper presents a concise review of the current state-of-the-art and state of research pertaining to the understanding, estimation and mitigation of higher-mode effects on the seismic response of tall and slender RC structures. The paper is organized into four main foci: (1) analytical studies on understanding and quantifying higher-mode effects, (2) available experimental work on this topic, (3) advances in code practices in accounting for higher-mode effects in seismic design of RC tall buildings, and (4) recent developments in innovative systems intended to mitigate higher-mode effects in RC tall buildings. The paper concludes by briefly summarizing future challenges facing the construction of earthquake-resilient RC tall buildings that are essential in building resilient cities of the future.

Spatial-Temporal Mode Analysis and Prediction of Outgoing Longwave Radiation over China in 2002–2021 Based on Atmospheric Infrared Sounder Data

Outgoing longwave radiation (OLR) is a key factor to study the radiation balance of the earth–atmosphere system. It is of great significance to explore the temporal and spatial variation characteristics over the OLR value in China region and to predict its future variation trend. We investigate the characteristic distribution of OLR value over China and predict its results in time series using the seasonal autoregressive integrated moving average (SARIMA) and long short-term memory (LSTM) methods based on the OLR data by the Atmospheric Infrared Sounder (AIRS). The Mann–Kendall (MK) mutation test was used to analyze the annual average of OLR values in China and the mutation points in the four seasons. The empirical orthogonal function (EOF) is used to decompose the spatial characteristics and temporal variation of OLR values in China. The MK mutation test is used to obtain the mutation points in the three seasons of spring, summer and autumn. The cumulative variance contribution of the four modes obtained by EOF decomposition exceeds 70%, and the variance contribution of the first mode exceeds 50%. The prediction accuracy with SARIMA model is 99% and LSTM algorithm is 97%. The results of spatiotemporal analysis show that the OLR value near the equator is significantly higher than that of the north and south poles and decreases with the increase of latitude; the OLR value in spring, summer and autumn is higher than that in winter. The results of the MK test show that there are many mutation points in autumn, and the location of the mutation points cannot be determined. The mutation points in spring and summer meet the confidence interval; the first mode of EOF decomposition has a meridional structure, and the OLR value is dropped within 18 years as a whole. The spatial characteristics of modes 1 and 3 have obvious changes in the Qinghai-Tibet Plateau and Northeast China. The prediction results show that the prediction accuracy of SARIMA is higher than that of LSTM. Therefore, the results predicted by SARIMA may provide a reference for the study of the radiation balance of the earth–atmosphere system in China.

Mod-two APS index and domain-wall fermion

We show a mathematical relation between the mod-two Atiyah–Patodi–Singer (APS) index of a massless Dirac operator and massive domain-wall fermion determinant. The domain-wall fermion is given on a closed manifold, which is extended from the original manifold with boundary, where we instead give a fermion mass term changing its sign at the location of the original boundary. This new setup does not need the APS boundary condition, which is non-local. A mathematical proof of equivalence between the two different formulations is given by two different evaluations of the same index of a Dirac operator on a higher-dimensional manifold. The domain-wall fermion allows us to separate the edge and bulk mode contributions in a natural but not in a gauge invariant way, which offers a straightforward description of the global anomaly inflow.

Influence of a Central Jet on Isothermal and Reacting Swirling Flow in a Model Combustion Chamber

We studied flow dynamics in a model combustion chamber using Large-eddy simulations (LES) and Particle image velocimetry (PIV) at the Reynolds number Re of 15,000. The swirl is produced using a Turbomeca swirler and air flow, while combustion is supported by a central methane/air jet. We compared four flow regimes, assessing the effect of the central jet for isothermal and lean reacting conditions. A detailed comparison for isothermal and reactive cases without the central jet is described, validating the LES results against PIV. We observe that unsteady dynamics are governed by global instability in the form of a well-known precessing vortex core (PVC). The central jet slightly changes the dynamics of PVC in the isothermal case where a strong recirculation zone is still formed. However, for the reacting case, the bubble is completely destroyed with no signs of strong vortical structures in the inner shear layer. These observations are confirmed using spectral analysis and proper orthogonal decomposition, describing the contribution of different flow modes in terms of azimuthal harmonics.

A Computational Approach to Nontraditional Intrinsic Luminescence: Vibrationally Resolved Absorption and Fluorescence Spectra of DABCO.

Recently, so-called "nontraditional intrinsic luminescence" has been reported in several macromolecular systems. Although DABCO (1,4-diazabicyclo[2.2.2]octane) is the first system in which the effect was observed, a thorough analysis of the optical properties of the molecule, which would reveal the origin of this mysterious effect, is still pending. We perform an advanced post-Hartree-Fock treatment of the low-lying electronic states of this molecule, which need to be described with care because of their pronounced Rydberg character. We take a deeper look into the low-lying electronic transitions of DABCO targeting the explanation of the complex vibronic structures of its absorption and fluorescence spectra. Two electronic states, the 1E'(n+3pxy) and 1A2″(n+3pz) states, contribute to the absorption spectrum in the 39000-46000 cm-1 spectral range. We also reveal the spectroscopic signature of the 1A2″(n+3pz) state. The analyses of the contributions of individual vibrational normal modes allowed the identification of those giving rise to the complex vibronic structures of the spectra. Fluorescence emission arises from the vibronic coupling of the one-photon forbidden transition between the 1A1'(n+3s) state and the electronic ground state. The spectrum, which can be interpreted in terms of populating a few vibrational normal modes, is shifted toward visible wavelengths mostly due to the forced interaction of the lone pair electrons of the two nitrogen atoms. Our work on DABCO may help to rationalize the luminescence of more complex systems containing tertiary amine groups.

Aging effect on the mixed-mode (I/III) fracture toughness of cement emulsified asphalt composite: Experimental and statistical investigation

Aging can affect the fracture behavior of bitumen-containing composites due to an increase in the total stiffness of the material. In this study, the effect of aging on the behavior of Cement Emulsified Asphalt Mortar (CEAM) under the mixed-mode I/III fractures was investigated at an intermediate temperature, and its fracture behavior was modeled with the aid of Response Surface Methodology (RSM). To collect the experiential data, several mix designs with different cement contents (C: [460, 500] kg/m3), bitumen-to-cement ratios (A/C: [0.3, 0.5]), and water-to-cement ratios (W/C: [0.4, 0.6]) were generated by Central Composite Circumscribed (CCC) method. Then, a series of fracture tests were performed on aged and unaged Edge Notched Disc Bend (ENDB) specimens. The results showed that for both aged and unaged (control) specimens, the highest fracture toughness is obtained under pure tensile mode and by increasing the contribution of mode III the corresponding value of fracture toughness decreases. The developed statistical models revealed that for unaged specimens, the increase in the A/C ratio has a significant effect on the stress intensity factors of mode I, mode III and mixed-mode I/III. However, for aged specimens, the effect of A/C is significantly observed in its interaction with W/C. The effect of aging on the fracture toughness of mortar was also measured with an index called the ratio of Aged to Unaged Fracture Toughness (FTRiAged/Unaged). The results showed that aging decreases the critical stress intensity factor of the mortar in the mode mixities, with the highest decrease of 15% under pure mode I.

Entropy-stable schemes in the low-Mach-number regime: Flux-preconditioning, entropy breakdowns, and entropy transfers

Entropy-Stable (ES) schemes, specifically those built from Tadmor (1987) [54], have been gaining interest over the past decade, especially in the context of under-resolved simulations of compressible turbulent flows using high-order methods. These schemes are attractive because they can provide stability in a global and nonlinear sense (consistency with thermodynamics). However, fully realizing the potential of ES schemes requires a better grasp of their local behavior. Entropy-stability itself does not imply good local behavior (Gouasmi et al., 2019 [17], Gouasmi et al., 2020 [18]). In this spirit, we studied ES schemes in problems where global stability is not the core issue. In the present work, we consider the accuracy degradation issues typically encountered by upwind-type schemes in the low-Mach-number regime (Turkel, 1999 [59]) and their treatment using Flux-Preconditioning (Turkel, 1987 [58], Miczek et al., 2015 [41]). ES schemes suffer from the same issues and Flux-Preconditioning can improve their behavior without interfering with entropy-stability. This is first demonstrated analytically: using similarity and congruence transforms we were able to establish conditions for a preconditioned flux to be ES, and introduce the ES variants of the Miczek's and Turkel's preconditioned fluxes. This is then demonstrated numerically through first-order simulations of two simple test problems representative of the incompressible and acoustic limits, the Gresho Vortex and a right-moving acoustic wave. The results are overall consistent with previous studies. For instance, we observe that Turkel's preconditioner improves accuracy in the incompressible limit with the downside of overly damping acoustic waves (Bruel et al., 2019 [4]). For Miczek's matrix however, we came across unexpected spurious transients in both problems (a small left-moving acoustic wave in the latter), motivating further analysis. We revisited the pressure fluctuation argument of Guillard and Viozat (1999) [26] in terms of entropy, showing how the standard ES dissipation operator (Ismail and Roe, 2009 [30]) can introduce inconsistent discrete entropy fluctuations in space. These analytical results are achieved by introducing mode-by-mode decompositions of the dissipation operator, similar to Roe and Pike (1984) [51], Tadmor (2003) [55], leading to what we call discrete Entropy Production Breakdowns (EPBs) in space. These EPBs outline the contributions of convective and acoustic modes to the discrete entropy production, both locally and globally. Ultimately, these EPBs enable us to single-out in the ES Miczek flux a skew-symmetric matrix component which we believe causes entropy transfers between acoustic waves. Removing this contribution eliminates the spurious transients without interfering with entropy-stability. This conjecture is explored numerically and analytically.

Sparsity of Radiating Characteristic Modes on Infinite Periodic Structures

Characteristic modes on infinite periodic structures are studied using spectral dyadic Green's functions. This formulation demonstrates that, in contrast to the modal analysis of finite structures, the number of radiating characteristic modes is limited by unit cell size and incident wavevector (i.e., scan angle or phase shift per unit cell). The reflection tensor is decomposed into modal contributions from radiating modes, indicating that characteristic modes are a predictably sparse basis in which to study reflection phenomena.

Dark matter from entropy perturbations in curved field space

The accumulated energy density of the excited entropy modes in multiple field inflationary scenarios can play the role of dark matter. In the usual case of a flat field space without any turning trajectory, only light superhorizon entropy modes can be excited through the gravitational instability. In the case of a negatively curved field space, we show that subhorizon entropy modes can be excited as well through the tachyonic instability induced by the negative curvature of the field space. The latter allows for production of entropy modes with masses larger than or at the order of the Hubble expansion rate during inflation, leading to a new dark matter scenario. Due to the contribution of subhorizon modes, the corresponding spectral density has a peak at a scale smaller than its counterpart in the models based on a flat field space. This difference makes our model observationally distinguishable.

Decoupling catalyst aggregation, ripening, and coalescence processes inside operating fuel cells

Simultaneous in situ X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) of hydrogen fuel cell electrocatalyst layers allows nanoparticle degradation mechanisms of aggregation, coalescence and ripening to be independently decoupled during cell operation. The ratio of particle size to crystallite size is proposed as a direct measurement of catalyst particle–particle interactions. This metric is applied to track the aggregation of a practical industrial fuel cell catalyst in situ, during accelerated degradation. The dominant catalyst degradation mode changes over the course of the device’s life cycle, passing through distinct phases of aggregation, coalescence and ripening. Understanding the relative contribution of each degradation mode is necessary for engineering next-generation catalysts with commercially acceptable durability.

Multifrequency superscattering driven by symmetry-reduced resonators

We unveil novel mechanisms to achieve superscattering by investigating the resonances’ profile of non-Hermitian Hamiltonian structures lacking spherical symmetry. We show that superscattering can be obtained within a single scattering channel due to a contribution of a single strongly-coupled mode. Such phenomenon can’t be observed in Mie-based resonators. We then spatially and spectrally engineer modes of multi-resonators in a cluster to realize broadband superscattering with ultra-strong resonances at several frequency points.

A Study of Cybersecurity Education Using a Present-Test-Practice-Assess Model

<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Contribution:</i> Based on a previous exploratory study, this research further investigated learning mode contributions from simulations and live competitive activities compared to a traditional classroom, laboratory, and testing approach to instruction in order to assess the applied behavioral contributions of each. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Background:</i> Cybersecurity capabilities in organizations lag behind the threats; consequently, there is a renewed emphasis on cybersecurity education. However, education appears to lack its full potential in most settings. Few empirical studies have systematically tested the efficacy of various training methods and modes, and those that have been conducted have yielded inconsistent findings. Recent literature on the use of gamified simulations have suggested that they may improve cybersecurity behaviors. Similarly, live activities, such as hackathons and capture the flag events, have been surmised to augment learning and capabilities. This study sought to systematically investigate this question. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Research Questions:</i> 1) Do cybersecurity simulations improve applied learning performance; 2) do live competitive activities improve applied learning performance; and 3) do any or each of these modes amplify applied learning performance when used in combination? <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Methodology:</i> This study randomly assigned computer science students to one of four sections using different modalities. It used pretest scores on an applied exam as the co-variate, with post-test scores as the dependent variable. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Results:</i> Simulations improved learning performance over traditional classroom/lab instruction alone whereas live competitive activities did not. However, the greatest learning outcomes achieved was structured cybersecurity simulated environments combined with live competitive activities.