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1
Chang, Lei. "Alfvénic gap eigenmode in a linear plasma with ending magnetic throats." Physics of Plasmas 25, no. 12 (December 2018): 122503. http://dx.doi.org/10.1063/1.5066047.
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2
Zhang, Chang-Qing, Cun-Jun Ruan, Ding Zhao, Shu-Zhong Wang, and Xiu-Dong Yang. "Complete eigenmode analysis of a ladder-type multiple-gap resonant cavity." Chinese Physics B 23, no. 8 (July 31, 2014): 088401. http://dx.doi.org/10.1088/1674-1056/23/8/088401.
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3
Chang, L., B. N. Breizman, and M. J. Hole. "Gap eigenmode of radially localized helicon waves in a periodic structure." Plasma Physics and Controlled Fusion 55, no. 2 (December 21, 2012): 025003. http://dx.doi.org/10.1088/0741-3335/55/2/025003.
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4
Tiainen, Teemu, and Markku Heinisuo. "Buckling length of a frame member." Rakenteiden Mekaniikka 51, no. 2 (December 8, 2018): 49–61. http://dx.doi.org/10.23998/rm.66836.
Повний текст джерелаАнотація:
In steel frame design, the definition of buckling lengths of members is a basic task. Computers can be used to calculate the eigenmodes and corresponding eigenvalues for the frames and using these the buckling lengths of the members can be defined using Euler's equation. However, it is not always easy to say, which eigenmode should be used for the definition of the buckling length of a specific member. Conservatively, the lowest positive eigenvalue can be used for all members. In this paper, methods to define the buckling length of a specific member is presented. For this assessment, two ideas are considered. The first one uses geometric stiffness matrix locally and the other one uses strain energy measures to identify members taking part in a buckling mode. The behaviour of the methods is shown in several numerical examples. Both methods can be implemented into automated frame design, removing one big gap in the integrated design. This is essential when optimization of frames is considered.
5
Chang, Lei, Ning Hu, and Jianyao Yao. "Influence of number and depth of magnetic mirror on Alfvénic gap eigenmode." Chinese Physics B 25, no. 10 (September 23, 2016): 105204. http://dx.doi.org/10.1088/1674-1056/25/10/105204.
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6
Chang, Lei. "Preliminary computation of the gap eigenmode of shear Alfvén waves on the LAPD." Chinese Physics B 27, no. 12 (December 2018): 125201. http://dx.doi.org/10.1088/1674-1056/27/12/125201.
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J Kramer, G., C. Z Cheng, M. Podestà, and R. Nazikian. "Simulation of the eigenmode spectrum below the Toroidicity-induced Alfvén eigenmode gap generated by the coupling of Alfvén and slow-magnetosonic waves in tokamaks." Plasma Physics and Controlled Fusion 62, no. 7 (June 17, 2020): 075012. http://dx.doi.org/10.1088/1361-6587/ab9153.
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8
Al Ba'ba'a, H., M. Nouh, and T. Singh. "Dispersion and topological characteristics of permutative polyatomic phononic crystals." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 475, no. 2226 (June 2019): 20190022. http://dx.doi.org/10.1098/rspa.2019.0022.
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This work presents a comprehensive mathematical treatment of phononic crystals (PCs) which comprise a finite lattice of repeated polyatomic unit cells. Wave dispersion in polyatomic lattices is susceptible to changes in the local arrangement of the monatoms (subcells) constituting the individual unit cell. We derive and interpret conditions leading to identical and contrasting band structures as well as the possibility of distinct eigenmodes as a result of cyclic and non-cyclic cellular permutations. Different modes associated with cyclic permutations yield topological invariance, which is assessed via the winding number of the complex eigenmode. Wave topology variations in the polyatomic PCs are quantified and conditions required to support edge modes in such lattices are established. Next, a transfer function analysis of finite polyatomic PCs is used to explain the formation of multiple Bragg band gaps as well as the emergence of truncation resonances within them. Anomalies arising from the truncation of the infinite lattice are further exploited to design mirror symmetrical edge modes in an extended lattice. We conclude with a generalized explanation of the band gap evolution mechanism based on the Bode plot analysis.
9
Terao, Takamichi. "Numerical methods for design of metamaterial photonic crystals and random metamaterials." EPJ Applied Metamaterials 9 (2022): 1. http://dx.doi.org/10.1051/epjam/2021012.
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Two-dimensional metamaterial photonic crystals (2DMPCs) composed of dispersive metamaterials in a positive-refractive-index medium were investigated by incorporating finite-difference time-domain calculations into the auxiliary differential equation method. A distinct band gap was formed and the effects of positional and size disorder when the dispersive metamaterials are aligned in air were elucidated. In addition, using the self-consistent finite-difference frequency-domain method, an eigenmode analysis of 2DMPCs with positional disorder was performed. Finally, a numerical method for the inverse design of binary random metamaterial multilayers was proposed.
10
Varela, J., J. Huang, D. A. Spong, J. Chen, V. Chan, L. Garcia, A. Wingen, Y. Ghai, and Y. Zou. "Theoretical study of the Alfven eigenmode stability in CFETR steady state discharges." Nuclear Fusion 62, no. 3 (January 20, 2022): 036005. http://dx.doi.org/10.1088/1741-4326/ac4778.
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Abstract The aim of this study is to analyze the stability of Alfven eigenmodes (AE) in the China Fusion Engineering Test Reactor (CFETR) plasma for steady state operations. The analysis is done using the gyro-fluid code FAR3d including the effect of the acoustic modes, EP finite Larmor radius damping effects and multiple energetic particle populations. Two high poloidal β scenarios are studied with respect to the location of the internal transport barrier (ITB) at r/a ≈ 0.45 (case A) and r/a ≈ 0.6 (case B). Both operation scenarios show a narrow TAE gap between the inner-middle plasma region and a wide EAE gap all along the plasma radius. The AE stability of CFETR plasmas improves if the ITB is located inwards, case A, showing AEs with lower growth rates with respect to the case B. The AEs growth rate is smaller in the case A because the modes are located in the inner-middle plasma region where the stabilizing effect of the magnetic shear is stronger with respect to the case B. Multiple EP populations effects (NBI driven EP + alpha articles) are negligible for the case A, although the simulations for the case B show a stabilizing effect of the NBI EP on the n = 1 BAE caused by α particles during the thermalization process. If the FLR damping effects are included in the simulations, the growth rate of the EAE/NAE decreases up to 70%, particularly for n > 3 toroidal families. Low n AEs (n < 6) show the largest growth rates. On the other hand, high n modes (n = 6 to 15) are triggered in the frequency range of the NAE, strongly damped by the FLR effects.
11
Kumar, M. Santosh, Santigopal Maity, Soumojit Shee, Ayan Kumar Bandyopadhyay, Debashish Pal, and Chaitali Koley. "Design Studies of Re-Entrant Square Cavities for V-Band Klystrons." Electronics 12, no. 1 (December 20, 2022): 6. http://dx.doi.org/10.3390/electronics12010006.
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V-band Klystrons find applications in satellite communications and 5G technology. Here, we present a V-band Klystron using radial re-entrant square cavities (RRSCs). The RRSCs are easy to fabricate, assemble, align, and tune to the operating frequency, which are significant concerns in V-band. We have optimized the number of cavities and the gap for best possible gain and bandwidth. The eigenmode and particle-in-cell (PIC) simulation results of the CST microwave studio are presented. The optimum design uses eleven equidistantly placed RRSCs, producing the maximum gain of 27.17 dB at 60.1 GHz with a bandwidth of ∼150 MHz.
12
Meliga, P. "Harmonics generation and the mechanics of saturation in flow over an open cavity: a second-order self-consistent description." Journal of Fluid Mechanics 826 (August 4, 2017): 503–21. http://dx.doi.org/10.1017/jfm.2017.439.
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The flow over an open cavity is an example of supercritical Hopf bifurcation leading to periodic limit-cycle oscillations. One of its distinctive features is the existence of strong higher harmonics, which results in the time-averaged mean flow being strongly linearly unstable. For this class of flows, a simplified formalism capable of unravelling how exactly the instability grows and saturates is lacking. This study builds on previous work by Mantič-Lugo et al. (Phys. Rev. Lett., vol. 113, 2014, 084501) to fill in the gap using a parametrized approximation of an instantaneous, phase-averaged mean flow, coupled in a quasi-static manner to multiple linear harmonic disturbances interacting nonlinearly with one another and feeding back on the mean flow via their Reynolds stresses. This provides a self-consistent modelling of the mean flow–fluctuation interaction, in the sense that all perturbation structures are those whose Reynolds stresses force the mean flow in such a way that the mean flow generates exactly the aforementioned perturbations. The first harmonic is sought as the superposition of two components, a linear component generated by the instability and aligned along the leading eigenmode of the mean flow, and a nonlinear orthogonal component generated by the higher harmonics, which progressively distorts the linear growth rate and eigenfrequency of the eigenmode. Saturation occurs when the growth rate of the first harmonic is zero, at which point the stabilizing effect of the second harmonic balances exactly the linear instability of the eigenmode. The model does not require any input from numerical or experimental data, and accurately predicts the transient development and the saturation of the instability, as established from comparison to time and phase averages of direct numerical simulation data.
13
Liu, Haixia, Weitao He, Lixia Li, and Qi Jia. "Theoretical and Experimental Investigations on the Ultra-Low-Frequency Broadband of Quasi-Static Metamaterials." Applied Sciences 12, no. 18 (September 7, 2022): 8981. http://dx.doi.org/10.3390/app12188981.
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This paper proposes an I-shaped radial elastic metamaterial with ultra-low-frequency broadband characteristics and studies the propagation characteristics of elastic waves in their quasi-static state. Through the calculation of the dispersion relationship, the frequency response function, and the eigenmode displacement field, it is found that the ultra-low-frequency wide band gap can be generated in the quasi-static metamaterial. The wide band gap is mainly caused by modal transitions. The equivalent mass–spring model reveals the modal changes of the I-shaped radial elastic metamaterial under the surface constraints. Furthermore, by studying the directional vibration displacement field of the finite period structure, it is demonstrated that the mechanism of the ultra-low-frequency broadband (0<Reduced frequency(Ω)<0.20) is the local resonance mechanism. Subsequently, the influence of the geometric and the material parameters on the location and width of the band gap is explored numerically. Finally, based on the model, through the hammer modal experiment, it is proven that the quasi-static structure yields an ultra-low-frequency stop band of 0.1–1012 Hz. The research conclusions can be applied to mechanical engineering fields such as ultra-low-frequency vibration reduction.
14
Xiong, Chengwang, Liang Cheng, Feifei Tong, and Hongwei An. "Oscillatory flow regimes for a circular cylinder near a plane boundary." Journal of Fluid Mechanics 844 (April 4, 2018): 127–61. http://dx.doi.org/10.1017/jfm.2018.164.
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Oscillatory flow around a circular cylinder close to a plane boundary is numerically investigated at low-to-intermediate Keulegan–Carpenter ($KC$) and Stokes numbers ($\unicode[STIX]{x1D6FD}$) for different gap-to-diameter ratios ($e/D$). A set of unique flow regimes is observed and classified based on the established nomenclature in the ($KC,\unicode[STIX]{x1D6FD}$)-space. It is found that the flow is not only influenced by $e/D$ but also by the ratio of the thickness of the Stokes boundary layer ($\unicode[STIX]{x1D6FF}$) to the gap size (e). At relatively large $\unicode[STIX]{x1D6FF}/e$ values, vortex shedding through the gap is suppressed and vortices are only shed from the top of the cylinder. At intermediate values of $\unicode[STIX]{x1D6FF}/e$, flow through the gap is enhanced, resulting in horizontal gap vortex shedding. As $\unicode[STIX]{x1D6FF}/e$ is further reduced below a critical value, the influence of $\unicode[STIX]{x1D6FF}/e$ becomes negligible and the flow is largely dependent on $e/D$. A hysteresis phenomenon is observed for the transitions in the flow regime. The physical mechanisms responsible for the hysteresis and the variation of marginal stability curves with $e/D$ are explored at $KC=6$ through specifically designed numerical simulations. The Stokes boundary layer over the plane boundary is found to be responsible for the relatively large hysteresis range over $0.25<e/D<1.0$. Three mechanisms have been identified to the change of the marginal stability curve over $e/D$, which are the blockage effect due to the geometry setting, the favourable pressure gradient over the gap and the location of the leading eigenmode relative to the cylinder.
15
Gao, Nansha, Hong Hou, and Hang Xin. "A single and double slotting radial acoustic metamaterial plate." Modern Physics Letters B 31, no. 12 (April 24, 2017): 1750128. http://dx.doi.org/10.1142/s0217984917501287.
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This paper presents the low-frequency acoustic properties of a new single and double slotting radial acoustic metamaterial plate which is arranged in the axial coordinate. The band structures, transmission spectra, and eigenmode displacement fields of this kind of acoustic metamaterial are different from previous studies. Numerical calculation results show that the first-order band gap (BG) of the radial flexible elastic metamaterial plate is below 200 Hz. A multiple-vibration coupling mechanism is proposed to explain the low-frequency band gaps. By changing the geometric parameters a, t, and g, the location and width of the low-frequency band gaps can be varied neatly. In summary, rational design of geometries and materials is the crucial pathway to open and lower BGs, and could restrain low-frequency vibration similarly. This can be used to protect infrasound, generate filters, and design acoustic devices.
16
Li, Shasha, Feng Zhang, Cunjun Ruan, Yiyang Su та Pengpeng Wang. "A G-Band High Output Power and Wide Bandwidth Sheet Beam Extended Interaction Klystron Design Operating at TM31 with 2π Mode". Electronics 10, № 16 (12 серпня 2021): 1948. http://dx.doi.org/10.3390/electronics10161948.
Повний текст джерелаАнотація:
In this paper, we propose a high-order mode sheet beam extended interaction klystron (EIK) operating at G-band. Through the study of electric field distribution, we choose TM31 2π mode as the operating mode. The eigenmode simulation shows that the resonant frequency of the modes adjacent to the operating mode is far away from the central frequency, so there is almost no mode competition in our high mode EIK. In addition, by studying the sensitivity of the related geometry parameters, we conclude that the height of the coupling cavity has a great influence on the effective characteristic impedance, and the width of the gap mainly affects the working frequency. Therefore, it is necessary to strictly control the fabrication tolerance within 2 μm. Finally, the RF circuit using six barbell multi-gap cavities is determined, with five gaps for the input cavity and idler cavities and seven gaps for the output cavity. To expand the bandwidth, the stagger tuning method is adopted. Under the conditions of a voltage of 16.5 kV, current of 0.5 A and input power of 0.2 W, the peak output power of 650 W and a 3-dB bandwidth of 700 MHz are achieved without any self-oscillation.
17
Sharma, Kohli, Brière, Ménard, and Nabki. "Translational MEMS Platform for Planar Optical Switching Fabrics." Micromachines 10, no. 7 (June 30, 2019): 435. http://dx.doi.org/10.3390/mi10070435.
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While 3-D microelectromechanical systems (MEMS) allow switching between a large number of ports in optical telecommunication networks, the development of such systems often suffers from design, fabrication and packaging constraints due to the complex structures, the wafer bonding processes involved, and the tight alignment tolerances between different components. In this work, we present a 2-D translational MEMS platform capable of highly efficient planar optical switching through integration with silicon nitride (SiN) based optical waveguides. The discrete lateral displacement provided by simple parallel plate actuators on opposite sides of the central platform enables switching between different input and output waveguides. The proposed structure can displace the central platform by 3.37 µm in two directions at an actuation voltage of 65 V. Additionally, the parallel plate actuator designed for closing completely the 4.26 µm air gap between the fixed and moving waveguides operates at just 50 V. Eigenmode expansion analysis shows over 99% butt-coupling efficiency the between the SiN waveguides when the gap is closed. Also, 2.5 finite-difference time-domain analysis demonstrates zero cross talk between two parallel SiN waveguides across the length of the platform for a 3.5 µm separation between adjacent waveguides enabling multiple waveguide configuration onto the platform. Different MEMS designs were simulated using static structural analysis in ANSYS. These designs were fabricated with a custom process by AEPONYX Inc. (Montreal, QC, Canada) and through the PiezoMUMPs process of MEMSCAP (Durham, NC, USA).
18
Kitayama, Daisuke, Adam Pander, and Hiroyuki Takahashi. "Analysis of Asymmetry in Active Split-Ring Resonators to Design Circulating-Current Eigenmode: Demonstration of Beamsteering and Focal-Length Control toward Reconfigurable Intelligent Surface." Sensors 22, no. 2 (January 17, 2022): 681. http://dx.doi.org/10.3390/s22020681.
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In this work, toward an intelligent radio environment for 5G/6G, design methodologies of active split-ring resonators (SRRs) for more efficient dynamic control of metasurfaces are investigated. The relationship between the excitation of circulating-current eigenmode and the asymmetric structure of SRRs is numerically analyzed, and it is clarified that the excitation of the circulating-current mode is difficult when the level of asymmetry of the current path is decreased by the addition of large capacitance such as from semiconductor-based devices. To avoid change in the asymmetry, we incorporated an additional gap (slit) in the SRRs, which enabled us to excite the circulating-current mode even when a large capacitance was implemented. Prototype devices were fabricated according to this design methodology, and by the control of the intensity/phase distribution, the variable focal-length and beamsteering capabilities of the transmitted waves were demonstrated, indicating the high effectiveness of the design. The presented design methodology can be applied not only to the demonstrated case of discrete varactors, but also to various other active metamaterials, such as semiconductor-integrated types for operating in the millimeter and submillimeter frequency bands as potential candidates for future 6G systems.
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Martinez, Y., G. Brunet, and M. K. Yau. "On the Dynamics of Two-Dimensional Hurricane-Like Vortex Symmetrization." Journal of the Atmospheric Sciences 67, no. 11 (November 1, 2010): 3559–80. http://dx.doi.org/10.1175/2010jas3499.1.
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Abstract Despite the fact that asymmetries in hurricanes, such as spiral rainbands, polygonal eyewalls, and mesovortices, have long been observed in radar and satellite imagery, many aspects of their origin, space–time structure, and dynamics still remain unsolved, particularly their role on the vortex intensification. The underlying inner-core dynamics need to be better understood to improve the science of hurricane intensity forecasting. To fill this gap, a simple 2D barotropic “dry” model is used to perform two experiments starting respectively with a monopole and a ring of enhanced vorticity to mimic hurricane-like vortices during incipient and mature stages of development. The empirical normal mode (ENM) technique, together with the Eliassen–Palm (EP) flux calculations, are used to isolate wave modes from the model datasets to investigate their space–time structure, kinematics, and the impact on the changes in the structure and intensity of the simulated hurricane-like vortices. From the ENM diagnostics, it is shown in the first experiment how an incipient storm described by a vortex monopole intensifies by “inviscid damping” of a “discrete-like” vortex Rossby wave (VRW) or quasi mode. The critical radius, the structure, and the propagating properties of the quasi mode are found to be consistent with predictions of the linear eigenmode analysis of small perturbations. In the second experiment, the fastest growing wavenumber-4 unstable VRW modes of a vortex ring reminiscent of a mature hurricane are extracted, and their relation with the polygonal eyewalls, mesovortices, and the asymmetric eyewall contraction are established in consistency with results previously obtained from other authors.
20
CATON, F., B. JANIAUD, and E. J. HOPFINGER. "Stability and bifurcations in stratified Taylor–Couette flow." Journal of Fluid Mechanics 419 (September 25, 2000): 93–124. http://dx.doi.org/10.1017/s0022112000001348.
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In this article we present new experimental and theoretical results which were obtained for the flow between two concentric cylinders, with the inner one rotating and in the presence of an axial, stable density stratification. This system is characterized by two control parameters: one destabilizing, the rotation rate of the inner cylinder; and the other stabilizing, the stratification.Two oscillatory linear stability analyses assuming axisymmetric flow conditions are presented. First an eigenmode linear stability analysis is performed, using the small-gap approximation. The solutions obtained give insight into the instability mechanisms and indicate the existence of a confined internal gravity wave mode at the onset of instability. In the second stability analysis, only diffusion is neglected, predicting accurately the instability threshold as well as the critical pulsation for all the stratifications used in the experiments.Experiments show that the basic, purely azimuthal flow (circular Couette flow) is destabilized through a supercritical Hopf bifurcation to an oscillatory flow of confined internal gravity waves, in excellent agreement with the linear stability analysis. The secondary bifurcation, which takes the system to a pattern of drifting non-axisymmetric vortices, is a saddle-node bifurcation. The proposed bifurcation diagram shows a global bifurcation, and explains the discrepancies between previous experimental and numerical results. For slightly larger values of the rotation rate, weakly turbulent spectra are obtained, indicating an early appearance of weak turbulence: stationary structures and defects coexist. Moreover, in this regime, there is a large distribution of structure sizes. Visualizations of the next regime exhibit constant-wavelength structures and fluid exchange between neighbouring cells, similar to wavy vortices. Their existence is explained by a simple energy argument.The generalization of the bifurcation diagram to hydrodynamic systems with one destabilizing and one stabilizing control parameter is discussed. A qualitative argument is derived to discriminate between oscillatory and stationary onset of instability in the general case.
21
Rühle, S., L. K. van Vugt, H. Y. Li, N. A. Keizer, L. Kuipers, and D. Vanmaekelbergh. "Nature of Sub-Band Gap Luminescent Eigenmodes in a ZnO Nanowire." Nano Letters 8, no. 1 (January 2008): 119–23. http://dx.doi.org/10.1021/nl0721867.
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KOUZAEV, GUENNADI A. "CONTROLLED LOCALIZED EIGENMODES IN PSEUDO-RANDOM MULTILAYER ELECTRON WAVEGUIDES." International Journal of Modern Physics B 28, no. 01 (December 11, 2013): 1350192. http://dx.doi.org/10.1142/s0217979213501920.
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In this paper, the localization of eigenmodes in quasi-1-D pseudo-random potential lattice electron waveguides is studied using the equivalent circuit method applied to the Schrödinger equation. It is found that the mixed deterministic and random discrete potential and limited waveguide width allow only intermediate localization of modes, and their two main types are discovered: the gap-localized and random-lattice localized modes which are stable toward variation of the waveguide's geometry. These modes can be transformed to each other by proper choosing the waveguide parameters. The discovered and studied effects and created models are interesting in the developments of spatial nano-switches of electron waves.
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Liu, Mao, Pei Li, Yongteng Zhong, and Jiawei Xiang. "Research on the Band Gap Characteristics of Two-Dimensional Phononic Crystals Microcavity with Local Resonant Structure." Shock and Vibration 2015 (2015): 1–8. http://dx.doi.org/10.1155/2015/239832.
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A new two-dimensional locally resonant phononic crystal with microcavity structure is proposed. The acoustic wave band gap characteristics of this new structure are studied using finite element method. At the same time, the corresponding displacement eigenmodes of the band edges of the lowest band gap and the transmission spectrum are calculated. The results proved that phononic crystals with microcavity structure exhibited complete band gaps in low-frequency range. The eigenfrequency of the lower edge of the first gap is lower than no microcavity structure. However, for no microcavity structure type of quadrilateral phononic crystal plate, the second band gap disappeared and the frequency range of the first band gap is relatively narrow. The main reason for appearing low-frequency band gaps is that the proposed phononic crystal introduced the local resonant microcavity structure. This study provides a good support for engineering application such as low-frequency vibration attenuation and noise control.
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Barik, A., S. A. Triana, M. Hoff, and J. Wicht. "Triadic resonances in the wide-gap spherical Couette system." Journal of Fluid Mechanics 843 (March 27, 2018): 211–43. http://dx.doi.org/10.1017/jfm.2018.138.
Повний текст джерелаАнотація:
The spherical Couette system, consisting of a viscous fluid between two differentially rotating concentric spheres, is studied using numerical simulations and compared with experiments performed at BTU Cottbus-Senftenberg, Germany. We concentrate on the case where the outer boundary rotates fast enough for the Coriolis force to play an important role in the force balance, and the inner boundary rotates slower or in the opposite direction as compared to the outer boundary. As the magnitude of differential rotation is increased, the system is found to transition through three distinct hydrodynamic regimes. The first regime consists of the emergence of the first non-axisymmetric instability. Thereafter one finds the onset of ‘fast’ equatorially antisymmetric inertial modes, with pairs of inertial modes forming triadic resonances with the first instability. A further increase in the magnitude of differential rotation leads to the flow transitioning to turbulence. Using an artificial excitation, we study how the background flow modifies the inertial mode frequency and structure, thereby causing departures from the eigenmodes of a full sphere and a spherical shell. We investigate triadic resonances of pairs of inertial modes with the fundamental instability. We explore possible onset mechanisms through numerical experiments.
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Pucella, G., E. Alessi, F. Auriemma, P. Buratti, M. V. Falessi, E. Giovannozzi, F. Zonca, et al. "Beta-induced Alfvén eigenmodes and geodesic acoustic modes in the presence of strong tearing activity during the current ramp-down on JET." Plasma Physics and Controlled Fusion 64, no. 4 (March 10, 2022): 045023. http://dx.doi.org/10.1088/1361-6587/ac4ade.
Повний текст джерелаАнотація:
Abstract The analysis of the current ramp-down phase of JET plasmas has revealed the occurrence of additional magnetic oscillations in pulses characterized by large magnetic islands. The frequencies of these oscillations range from 5 to 20 kHz , being well below the toroidal gap in the Alfvén continuum and of the same order as the low-frequency gap opened by plasma compressibility. The additional oscillations only appear when the magnetic island width exceeds a critical threshold, suggesting that the oscillations could tap their energy from the tearing mode (TM) by a non-linear coupling mechanism. A possible role of fast ions in the excitation process can be excluded, being the pulse phase considered in the observations characterized by very low additional heating. The calculation of the coupled Alfvén–acoustic continuum in toroidal geometry suggests the possibility of beta-induced Alfvén eigenmodes (BAEs) rather than beta-induced Alfvén–acoustic eigenmodes. As a main novelty compared to previous work, the analysis of the electron temperature profiles from electron cyclotron emission has shown the simultaneous presence of magnetic islands on different rational surfaces in pulses with multiple magnetic oscillations in the low-frequency gap of the Alfvén continuum. This observation supports the hypothesis of different BAE with toroidal mode number n = 1 associated with different magnetic islands. As another novelty, the observation of magnetic oscillations with n = 2 in the BAE range is reported for the first time in this work. Some pulses, characterized by slowly rotating magnetic islands, exhibit additional oscillations with n = 0, likely associated with geodesic acoustic modes (GAMs), and a cross-spectral bicoherence analysis has confirmed a non-linear interaction between TM, BAE and GAM, with the novelty of the observation of multiple triplets (twin BAEs plus GAM), due to the simultaneous presence of several magnetic islands in the plasma.
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Wang, Wenjia, Deng Zhou, Youjun Hu, Yue Ming, and Baolong Hao. "Numerical simulation of the multiple reversed shear Alfvén eigenmodes associated with the trianuglarity Alfvén gap." Chinese Physics B 27, no. 12 (December 2018): 125202. http://dx.doi.org/10.1088/1674-1056/27/12/125202.
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Kramer, G. J., G. Y. Fu, R. Nazikian, R. V. Budny, C. Z. Cheng, N. N. Gorelenkov, S. D. Pinches, S. E. Sharapov, and K.-D. Zastrow. "Reversed shear Alfvén eigenmodes in the frequency range of the triangularity induced gap on JET." Plasma Physics and Controlled Fusion 50, no. 8 (June 11, 2008): 082001. http://dx.doi.org/10.1088/0741-3335/50/8/082001.
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Oliver, H. J. C., S. E. Sharapov, B. N. Breizman, and L. J. Zheng. "Axisymmetric global Alfvén eigenmodes within the ellipticity-induced frequency gap in the Joint European Torus." Physics of Plasmas 24, no. 12 (December 2017): 122505. http://dx.doi.org/10.1063/1.5005939.
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Froes, D., M. Arana, J. P. Sinnecker, and L. C. Sampaio. "Magnetoelastic modes in Néel domain walls." Journal of Applied Physics 132, no. 22 (December 14, 2022): 223908. http://dx.doi.org/10.1063/5.0128775.
Повний текст джерелаАнотація:
Spin wave propagation over long distances in confined ferromagnetic strip lines exhibiting magnetoelasticity opens up promising perspectives for device applications. Domain walls as natural spin wave channels increase the spin wave propagation distance. We calculate the magnetic and elastic modes through micromagnetic simulations and the dispersion relation of strip lines containing a Néel wall. We show that at the crossing points in the dispersion relation, two behaviors are observed: an anticrossing gap when a strong coupling is present or a gapless point when the magnetoelastic feedback cycle is not fulfilled. For the Néel wall-confined magnetic mode, the magnetic and elastic waves oscillate independently forming a gapless crossing point. For the domain modes, both behaviors are found. We discuss the gap existence based on the symmetry of the eigenmodes.
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Suneera, T. P., and P. A. Subha. "Higher eigenmodes of nonlocal gap solitons in parity-time symmetric complex potential with a defocusing nonlinearity." Chaos, Solitons & Fractals 98 (May 2017): 183–88. http://dx.doi.org/10.1016/j.chaos.2017.03.019.
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Li, Suobin, Yihua Dou, Tianning Chen, Zhiguo Wan, Jingjing Huang, Bing Li, and Fan Zhang. "Evidence for complete low-frequency vibration band gaps in a thick elastic steel metamaterial plate." Modern Physics Letters B 33, no. 04 (February 10, 2019): 1950038. http://dx.doi.org/10.1142/s0217984919500386.
Повний текст джерелаАнотація:
Elastic steel metamaterial plates can be used for noise- and vibration-reduction due to unique physical properties related to their vibration band gap. However, obtaining a complete low-frequency vibration band gap in a thick elastic steel metamaterial plate is difficult. In this paper, we simulate a complete low-frequency vibration band gap in a thick elastic steel metamaterial plate. The structure consists of periodic, double-sided, composite stepped resonators, which were deposited on a 2D locally resonant phononic crystal plate. The phononic crystal plate consists of an array of rubber fillers embedded in a thick steel plate. The dispersion relations, power-transmission spectra, and the displacement fields of the eigenmodes are calculated using the finite-element method. The results show that, for the proposed structure, the opening of the first complete vibration band gap is reduced by a factor of 9.5 compared to a conventional thick elastic steel metamaterial plate. This causes attenuation of low-frequency elastic waves. The formation mechanisms for the vibration band gap are also explored numerically. The results indicate that the formation mechanism for the new low-frequency vibration band gap can be attributed to coupling between a local resonance mode of the composite stepped resonators and the Lamb wave mode of the thick steel-plate. The location of the vibration band gap is determined by the resonator mode of the composite stepped resonators. The vibration band gap effects of the composite stepped resonators are also investigated in this paper. We find that the location of the complete vibration band gaps can be modulated with a relatively low frequency using different composite stepped resonators. Such an elastic steel metamaterial plate with a complete low-frequency vibration band gap can be used to reduce both vibration and noise in various commercial and research applications.
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Schönke, Johannes, and Eliot Fried. "Stability of vertical magnetic chains." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 473, no. 2198 (February 2017): 20160703. http://dx.doi.org/10.1098/rspa.2016.0703.
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A linear stability analysis is performed for a pair of coaxial vertical chains made from permanently magnetized balls under the influence of gravity. While one chain rises from the ground, the other hangs from above, with the remaining ends separated by a gap of prescribed length. Various boundary conditions are considered, as are situations in which the magnetic dipole moments in the two chains are parallel or antiparallel. The case of a single chain attached to the ground is also discussed. The stability of the system is examined with respect to three quantities: the number of balls in each chain, the length of the gap between the chains, and a single dimensionless parameter which embodies the competition between magnetic and gravitational forces. Asymptotic scaling laws involving these parameters are provided. The Hessian matrix is computed in exact form, allowing the critical parameter values at which the system loses stability and the respective eigenmodes to be determined up to machine precision. A comparison with simple experiments for a single chain attached to the ground shows good agreement.
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Zhou, Longwen. "Generating Many Majorana Corner Modes and Multiple Phase Transitions in Floquet Second-Order Topological Superconductors." Symmetry 14, no. 12 (December 2, 2022): 2546. http://dx.doi.org/10.3390/sym14122546.
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A d-dimensional, nth-order topological insulator or superconductor has localized eigenmodes at its (d−n)-dimensional boundaries (n≤d). In this work, we apply periodic driving fields to two-dimensional superconductors, and obtain a wide variety of Floquet second-order topological superconducting (SOTSC) phases with many Majorana corner modes at both zero and π quasienergies. Two distinct Floquet SOTSC phases are found to be separated by three possible kinds of transformations, i.e., a topological phase transition due to the closing/reopening of a bulk spectral gap, a topological phase transition due to the closing/reopening of an edge spectral gap, or an entirely different phase in which the bulk spectrum is gapless. Thanks to the strong interplay between driving and intrinsic energy scales of the system, all the found phases and transitions are highly controllable via tuning a single hopping parameter of the system. Our discovery not only enriches the possible forms of Floquet SOTSC phases, but also offers an efficient scheme to generate many coexisting Majorana zero and π corner modes, which may find applications in Floquet quantum computation.
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Tumin, Anatoli. "Receptivity of pipe Poiseuille flow." Journal of Fluid Mechanics 315 (May 25, 1996): 119–37. http://dx.doi.org/10.1017/s0022112096002364.
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The receptivity problem is considered for pipe flow with periodic blow–suction through a narrow gap in the pipe wall. Axisymmetric and non-axisymmetric modes (1, 2, and 3) are analysed. The method of solution is based on global eigenvalue analysis for spatially growing disturbances in circular pipe Poiseuille flow. The numerical procedure is formulated in terms of the collocation method with the Chebyshev polynomials application. The receptivity problem is solved with an expansion of the solution in a biorthogonal eigenfunction system, and it was found that there is an excitation of many eigenmodes, which should be taken into account. The result explains the non-similar character of the amplitude distribution in the downstream direction that was observed in experiments.
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Freed-Brown, Julian, Jason M. Amundson, Douglas R. MacAyeal, and Wendy W. Zhang. "Blocking a wave: frequency band gaps in ice shelves with periodic crevasses." Annals of Glaciology 53, no. 60 (2012): 85–89. http://dx.doi.org/10.3189/2012aog60a120.
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AbstractWe assess how the propagation of high-frequency elastic-flexural waves through an ice shelf is modified by the presence of spatially periodic crevasses. Analysis of the normal modes supported by the ice shelf with and without crevasses reveals that a periodic crevasse distribution qualitatively changes the mechanical response. The normal modes of an ice shelf free of crevasses are evenly distributed as a function of frequency. In contrast, the normal modes of a crevasse-ridden ice shelf are distributed unevenly. There are ‘band gaps’, frequency ranges over which no eigenmodes exist. A model ice shelf that is 50 km in lateral extent and 300 m thick with crevasses spaced 500 m apart has a band gap from 0.2 to 0.38 Hz. This is a frequency range relevant for ocean-wave/ice-shelf interactions. When the outermost edge of the crevassed ice shelf is oscillated at a frequency within the band gap, the ice shelf responds very differently from a crevasse-free ice shelf. The flexural motion of the crevassed ice shelf is confined to a small region near the outermost edge of the ice shelf and effectively ‘blocked’ from reaching the interior.
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Chen, Xu, and Wenhui Fan. "Tunable Bound States in the Continuum in All-Dielectric Terahertz Metasurfaces." Nanomaterials 10, no. 4 (March 27, 2020): 623. http://dx.doi.org/10.3390/nano10040623.
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In this paper, a tunable terahertz dielectric metasurfaces consisting of split gap bars in the unit cell is proposed and theoretically demonstrated, where the sharp high-quality Fano resonance can be achieved through excitation of quasi-bound states in the continuum (quasi-BIC) by breaking in-plane symmetry of the unit cell structure. With the structural asymmetry parameter decreasing and vanishing, the calculated eigenmodes spectra demonstrate the resonance changes from Fano to symmetry-protected BIC mode, and the radiative quality factors obey the inverse square law. Moreover, combining with graphene monolayer and strontium titanate materials, the quasi-BIC Fano resonance can be tuned independently, where the resonance amplitude can be tuned by adjusting the Fermi level of graphene and the resonance frequency can be tuned by controlling the temperature of strontium titanate materials. The proposed structure has numerous potential applications on tunable devices including modulators, switches, and sensors.
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Гусев, В. К., А. А. Мартынов, С. Ю. Медведев, Ю. В. Петров, М. И. Патров та А. Ю. Тельнова. "Тороидальные альфвеновские моды в плазме токамака Глобус-М". Письма в журнал технической физики 44, № 2 (2018): 65. http://dx.doi.org/10.21883/pjtf.2018.02.45466.17034.
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AbstractConditions of the excitation of toroidal Alfvén eigenmodes (TAEs) and their inf luence on the confinement of fast particles into the Globus-M spherical tokamak have been studied by KINX code calculations of the magnetohydrodynamic spectra of reconstructed divertor equilibrium configurations with stability margin q _0 > 1 on the magnetic axis. The sensitivity of the frequencies of TAE modes with toroidal wavenumber n = 1 to the type of boundary conditions and choice of boundary surface has been studied. It has been established that the frequencies of modes with dominating poloidal harmonics m = 1 and 2 in the gap of continuum are significantly higher than those observed in the spectra of signals measured in the Mirnov coil probes, especially under the assumption of free plasma boundary with allowance for its compressibility. The TAE modes with lower frequencies and higher poloidal wavenumbers localized near the plasma boundary may be responsible for the oscillations observed in the experiment. However, these modes are characterized by the interaction with continuum and, probably, exhibit related damping.
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Krasikova, Mariia, Anton Melnikov, Sergei Krasikov, Yuri Baloshin, David Powell, and Andrey Bogdanov. "Broadband noise mitigation using coupled Helmholtz resonators: a numerical study." INTER-NOISE and NOISE-CON Congress and Conference Proceedings 263, no. 2 (August 1, 2021): 3999–4007. http://dx.doi.org/10.3397/in-2021-2573.
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In this work we investigate a periodic structure in the frequency range from 20 Hz to 5500 Hz designed for broadband noise insulation. The considered unit cell consists of a simple structure: a pair of polymer pipes with slits carved along the axes, representing two coupled Helmholtz resonators. In order to develop a design with a broad band gap, we analyze the eigenmodes of the infinite two-dimensional structure considering their symmetry and interaction. This analysis is supported by parametric optimization of the resonator geometry. The obtained band diagram is compared with numerically determined transmission coefficient of a finite structure based on the same unit cell. The number of unit cells of the finite structure is chosen to be sufficient for demonstration of insulating properties and stop band formation. Furthermore, we analyze how the transmission coefficient is linked to the pressure field distribution inside the resonators. Owing to the simplicity of the geometry, the obtained results may become a basis for development of budget-friendly passive systems for broadband noise insulation within the audible range of frequencies.
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Chen, R., and D. L. Lin. "Electron–Interface–Phonon Interaction and Magnetopolaronic Impurity Transitions in Quantum Wells." International Journal of Modern Physics B 11, no. 08 (March 30, 1997): 991–1008. http://dx.doi.org/10.1142/s0217979297000502.
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The polaronic effect on the hydrogenic 1s–2p+ transition energy of a donor impurity located at the quantum well center in a double heterostructure is studied theoretically in detail. The electron–optical–phonon interaction Hamiltonian is derived on the basis of eigenmodes of lattice vibrations supported by the double heterostructure. Both the confined and interface phonon modes are included in the electron–phonon coupling. The transition energy is calculated as a function of the applied magnetic field for GaAs/Al 1-x Ga x As samples of well -widths d=125 Å, 210 Å and 450 Å by the second-order perturbation. Wide transition gaps are predicted around the two-level and three-level resonances for all three cases. It is found that the transition gap narrows with the increasing well-width but remains larger than the LO and TO phonon frequency difference for d=450 Å as is observed. We also perform the same calculation by assuming that the confined electron interacts with three-dimensional and two-dimensional phonon modes. The transition energy spectra from these calculations appear to be similar to those for a bulk sample, the spectrum splits at the resonance with the longitudinal optical phonon frequency only. From comparisons of our results with these calculations as well as with experiments, it is conclusively established that the wide gap of transition energy is solely due to the interface modes.
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MacAyeal, Douglas R., Julian Freed-Brown, Wendy W. Zhang, and Jason M. Amundson. "The influence of ice melange on fjord seiches." Annals of Glaciology 53, no. 60 (2012): 45–49. http://dx.doi.org/10.3189/2012/aog60a027.
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AbstractWe compute the eigenmodes (seiches) of the barotropic and baroclinic hydrodynamic equations for an idealized fjord having length and depth scales similar to those of Ilulissat Icefjord, Greenland, into which Jakobshavn Isbræ (also known as Sermeq Kujalleq) discharges. The purpose of the computation is to determine the fjord’s seiche behavior when forced by iceberg calving, capsize and melange movement. Poorly constrained bathymetry and stratification details are an acknowledged obstacle. We are, nevertheless, able to make general statements about the spectra of external and internal seiches using numerical simulations of ideal one-dimensional channel geometry. Of particular significance in our computation is the role of weakly coupled ice melange, which we idealize as a simple array of 20 icebergs of uniform dimensions equally spaced within the fjord. We find that the presence of these icebergs acts to (1) slow down the propagation of both external and internal seiches and (2) introduce band gaps where energy propagation (group velocity) vanishes. If energy is introduced into the fjord within the period range covered by a band gap, it will remain trapped as an evanescent oscillatory mode near its source, thus contributing to localized energy dissipation and ice/melange fragmentation.
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Aynaou, Hassan, Abdelkader Mouadili, Noama Ouchani, El Houssaine El Boudouti, Abdellatif Akjouj, and Bahram Djafari-Rouhani. "Scaling Law, Confined and Surface Modes in Photonic Fibonacci Stub Structures: Theory and Experiment." Applied Sciences 10, no. 21 (November 3, 2020): 7767. http://dx.doi.org/10.3390/app10217767.
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We investigate both theoretically and experimentally the properties of electromagnetic waves propagation and localization in periodic and quasi-periodic stub structures of Fibonacci type. Each block constituting the Fibonacci sequence (FS) is composed of an horizontal segment and a vertical stub. The origin of the primary and secondary gaps shown in such systems is discussed. The behaviors and scattering properties of the electromagnetic modes are studied in two geometries, when the FS is inserted horizontally between two semi-infinite waveguides or grafted vertically along a guide. Typical properties of the Fibonacci systems such as the fragmentation of the frequency spectrum, the self-similarity following a scaling law are analyzed and discussed. It is found that certain modes inside these two geometries decrease according to a power law rather than an exponential law and the localization of these modes displays the property of self-similarity around the central gap frequency of the periodic structure where the quasi-periodicity is most effective. Also, the eigenmodes of the FS of different generation order are studied depending on the boundary conditions imposed on its extremities. It is shown that both geometries provide complementary information on the localization of the different modes inside the FS. In particular, in addition to bulk modes, some localized modes induced by both extremities of the system exhibit different behaviors depending on which surface they are localized. The theory is carried out using the Green’s function approach through an analysis of the dispersion relation, transmission coefficient and electric field distribution through such finite structures. The theoretical findings are in good agreement with the experimental results performed by measuring in the radio-frequency range the transmission along a waveguide in which the FS is inserted horizontally or grafted vertically.
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Liao, Yan-Juan, Jing-Nan Huang, Jia-Yang Guo, and Shao-Ji Jiang. "Realistic FDTD Simulations of Plasmonic Properties on Ag Columnar Thin Films as SERS Biosensor." Nano 11, no. 10 (September 29, 2016): 1650113. http://dx.doi.org/10.1142/s1793292016501137.
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Noble metal sculptured thin films are of great interest during last decade as excellent surface-enhanced Raman scattering (SERS) substrates due to remarkable plasmonic properties in the visible and near-infrared range. In this work, Ag columnar thin films (Ag-CTFs) have been prepared by the glancing angle deposition technique. Finite-difference time-domain simulations has been utilized to study plasmonic properties of Ag-CTFs with a more accurate model based on binary scanning electron microscope (SEM) images by taking account of the shape irregularities, size distributions and random arrangement. The calculated absorption spectra based on the model of binarized SEM images show the best agreement with the measured spectra compared with models of periodic array with a regular shape. The near-field plasmonic properties are simulated based on the verified model. The distributions of electric field enhancement and hot spots are confirmed to be spectral and polarization dependent. There are multiple resonance peaks from visible to near-infrared and multiple eigenmodes coexist at the same wavelength and electric field enhancement are mainly excited by the polarized light vertical to the gap orientation. The electric field enhancement is found to distribute unevenly in the films with surface-localized feature. The equations to calculate the simulation SERS enhancement factor (EF) and total number of hot spots (tHN) are modified according to the above discussions. The experimental SERS EFs are on the order of 107–108, which indicates the high sensitivity of the films and the simulation SERS EFs and tHNs show good agreement with the experimental EFs. It is found that the SERS performance of Ag-CTFs is decided by both the cross-section structural characteristics and film thickness, which affect the electric filed enhancement and number of adsorbed molecules, respectively. Our work could be helpful in understanding the SERS mechanism and useful to the optimization of metal sculptured thin films for designing SERS biosensor.
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Mahmoud, Kamel G., Oliver Knaus, Tigran Parikyan, Guenter Offner, and Stjepan Sklepic. "An integrated model for the performance of piston ring pack in internal combustion engines." Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics 232, no. 3 (October 25, 2017): 371–84. http://dx.doi.org/10.1177/1464419317736676.
Повний текст джерелаАнотація:
Piston rings are important components in internal combustion engines. Their primary function is to seal dynamically the gap between moving piston and cylinder liner surface in order to prevent the combustion gases from penetrating into the crankcase. The rings also control the oil leakage from the crankcase to the combustion chamber. The performance of the piston ring pack impacts the engine efficiency, durability and emissions. The recognition of the impact of the ring-pack performance on the engine design resulted in a sustained effort of research and development aimed at understanding the operation of the piston ring pack. Most of the published models developed in this field are two-dimensional assuming that the ring and liner are perfect circles for the purpose of modelling the axial and radial dynamics. Although this approach has proved to be useful, there exist a number of asymmetrical characteristics of the power cylinder system that can be crucial to the ring-pack performance and therefore it is considered to be appropriate. In this work, an integrated methodology that handles the complex ring-pack mechanism is presented. The physics of the ring-pack mechanism covers the three-dimensional piston ring dynamics of asymmetric engine cylinder due to bore distortion, the mixed lubrication at ring running face as well as the ring flanks and the interring gas dynamics. The modelling method is verified in two steps. In the first step, the dynamic behaviour of the three-dimensional ring model is verified against a commercial finite element software by comparing the eigenmodes up to a frequency of about 1 kHz. In the second step, the ring-pack modelling approach using three-dimensional ring models is also verified against a commercial ring dynamics program, which is based on the two-dimensional modelling. It is shown that the three-dimensional ring dynamics modelling method has advantages over the two-dimensional modelling approach as it facilitates studying the influence of the non-uniform twist along its circumference (ring winding), the effect of bore distortion on blow-by, ring friction, friction power losses and wear.
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Schlosser, Jessica, Michael Keller, Kamran Fouladi, and Babak Eslami. "Strengthening Polylactic Acid by Salification: Surface Characterization Study." Polymers 15, no. 3 (January 17, 2023): 492. http://dx.doi.org/10.3390/polym15030492.
Повний текст джерелаАнотація:
Polylactic acid (PLA) is one of the market’s most commonly used biodegradable polymers, with diverse applications in additive manufacturing, specifically fused deposition modeling (FDM) 3D printing. The use of PLA in complex and sophisticated FDM applications is continually growing. However, the increased range of applications requires a better understanding of the material properties of this polymer. For example, recent studies have shown that PLA has the potential to be used in artificial heart valves. Still, the durability and longevity of this material in such a harsh environment are unknown, as heart valve failures have been attributed to salification. Additionally, there is a gap in the field for in situ material characterization of PLA surfaces during stiffening. The present study aims to benchmark different dynamic atomic force microscopy (AFM) techniques available to study the salification phenomenon of PLA at micro-scales using different PLA thin films with various salt concentrations (i.e., 10%, 15%, and 20% of sodium chloride (NaCl)). The measurements are conducted by tapping mode AFM, bimodal AFM, the force spectroscopy technique, and energy quantity analysis. These measurements showed a stiffening phenomenon occurring as the salt solution is increased, but the change was not equally sensitive to material property differences. Tapping mode AFM provided accurate topographical information, while the associated phase images were not considered reliable. On the other hand, bimodal AFM was shown to be capable of providing the topographical information and material compositional mapping through the higher eigenmode’s phase channel. The dissipated power energy quantities indicated that how the polymers become less dissipative as salt concentration increases can be measured. Lastly, it was shown that force spectroscopy is the most sensitive technique in detecting the differences in properties. The comparison of these techniques can provide a helpful guideline for studying the material properties of PLA polymers at micro- and nano-scales that can prove beneficial in various fields.
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Gorelenkov, N. N. "Double-Gap Alfvén Eigenmodes: Revisiting Eigenmode Interaction with the Alfvén Continuum." Physical Review Letters 95, no. 26 (December 27, 2005). http://dx.doi.org/10.1103/physrevlett.95.265003.
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Qu, Zhisong, and Matthew J. Hole. "The shear Alfvén continuum with a magnetic island chain in tokamak plasmas." Plasma Physics and Controlled Fusion, December 8, 2022. http://dx.doi.org/10.1088/1361-6587/aca9f8.
Повний текст джерелаАнотація:
Abstract The shear Alfv én continuum spectrum is studied for a tokamak with a single island chain using the ideal Magnetohydrodynamics (MHD) theory. We have taken into account the toroidal geometry and toroidal mode coupling with the island considered as a highly-shaped stellarator. Various new frequency gaps open up inside the island due to its asymmetry both poloidally and toroidally, such as the Mirror-induced Alfv én Eigenmode (MAE) gap and the Helicity-induced Alfv én Eigenmode (HAE) gap. We have shown that the MAE gap acts as the continuation of the outside Toroidal Alfv én Eigenmode (TAE) gap into the island. However, the combined TAE/MAE gap is getting narrower as the island grows, leaving only half of its original width with a moderate island size as much as 3.2% of the minor radius. In addition, the two-dimensional eigenfunction of the continuum mode on the lower tip of the MAE gap now has highly localised structures around the island's long axis, contrary to the usual oscillatory global solutions found with no or a low level of toroidal asymmetry - an indication of the continuous spectrum becoming discrete and dense. These results have implications for the frequency, mode structure and continuum damping of global TAEs residing in the gap.
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Bueno Lamas, Patrick, and Rodrigo Nicoletti. "Experimental Evidence of the Band Gap Formation in Rotors with Longitudinal Periodicity." Journal of Engineering for Gas Turbines and Power, September 26, 2022. http://dx.doi.org/10.1115/1.4055776.
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Abstract The longitudinal periodicity of the rotating elements in a rotating machine can impose band gaps (modal spacing) on the frequency spectrum of the system. These band gaps are characterized by a large distance between two adjacent modes with a low vibrating response of the system. Here, the rotating elements of the machine (e.g. the stages or the impellers) are considered to be the periodic elements of the rotor. In this disk-like configuration of the rotor, the system can present band gaps due to two different reasons: the matching between the number of disks and the eigenmode wavenumber (usually in slender rotors); the presence of local-mode shapes (usually in large rotors). This work presents experimental evidence of the band gap formation in a slender periodic rotor. The obtained results validate the theoretical predictions of previous works.
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Lamas, Patrick B., and Rodrigo Nicoletti. "The Band Gap Formation in Rotors With Longitudinal Periodicity and Quasi-Periodicity." Journal of Engineering for Gas Turbines and Power 144, no. 5 (February 21, 2022). http://dx.doi.org/10.1115/1.4053193.
Повний текст джерелаАнотація:
Abstract Modal spacing (band gaps) in the frequency spectrum of rotating machines can be imposed by geometric periodicity. By designing the rotor with a geometry that repeats periodically, we can impose to the vibration response of the rotor a modal “gap” considerably large, where no resonances appear. In this work, we consider that the rotating elements of the machine (e.g., the stages or the impellers) are the periodic elements of the rotor. In this disk-like configuration of the rotor, the system can present band gaps due to two different reasons: due to matching between the number of disks and the eigenmode wavenumber (usually in slender rotors); due to the presence of local-mode shapes (usually in large rotors). Analytical modeling of the system is presented, whose approximated solution can be used to predict the start and stop frequencies of the band gaps. The limitations in band gap formation are also shown when the rotor is not perfectly periodic (quasi-periodic geometry). In this case, disk positioning plays an important role in the band gap formation.
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Möller, Daniel, Maximilian Jüngst, Felix Holzinger, Christoph Brandstetter, Heinz-Peter Schiffer, and Sebastian Leichtfuß. "Mechanism of Nonsynchronous Blade Vibration in a Transonic Compressor Rig." Journal of Turbomachinery 139, no. 1 (August 2, 2016). http://dx.doi.org/10.1115/1.4034029.
Повний текст джерелаАнотація:
This paper presents a numerical study on blade vibration for the transonic compressor rig at the Technische Universität Darmstadt (TUD), Darmstadt, Germany. The vibration was experimentally observed for the second eigenmode of the rotor blades at nonsynchronous frequencies and is simulated for two rotational speeds using a time-linearized approach. The numerical simulation results are in close agreement with the experiment in both cases. The vibration phenomenon shows similarities to flutter. Numerical simulations and comparison with the experimental observations showed that vibrations occur near the compressor stability limit due to interaction of the blade movement with a pressure fluctuation pattern originating from the tip clearance flow. The tip clearance flow pattern travels in the backward direction, seen from the rotating frame of reference, and causes a forward traveling structural vibration pattern with the same phase difference between blades. When decreasing the rotor tip gap size, the mechanism causing the vibration is alleviated.
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Shi, P. W., Y. R. Yang, W. Chen, Z. B. Shi, Z. C. Yang, L. M. Yu, T. B. Wang, et al. "Observation and simulation of n = 1 reversed shear Alfvén eigenmode on the HL-2A tokamak." Chinese Physics Letters, September 1, 2022. http://dx.doi.org/10.1088/0256-307x/39/10/105201.
Повний текст джерелаАнотація:
Abstract A branch of high frequency Alfvénic mode is observed on the HL-2A tokamak. The electromagnetic mode can be driven unstable in the plasma with an off-axis neutral beam heating. Its mode frequency keeps almost unchanged or presents a slow-sweeping behavior, depending on the detail current evolution. The poloidal and toroidal mode numbers are $m/n=1/1$. The mode has a quite short duration ($\leq20 $ ms) and usually appears 5-10 ms after the neutral beam being injected into the plasma. Hybrid simulations based on M3D-K have also been carried out. The result suggests that co-passing energetic particles are responsible for the mode excitation. The simulated mode structures are localized nearby location of minimum safety factor ($q_{min}$) and agree with the structures obtained through tomography of soft x-ray arrays. Further, the modes are localized in the continuum gap and their frequencies increase with variation of $q_{min}$ in a wide range. Last but not least, the characteristic of unchanged frequency on experiment is also reproduced by the nonlinear simulation with a fixed safety factor. All those evidences indicate that the $n=1$ high frequency mode may belong to a reversed shear Alfvén eigenmode (RSAE).