Broadband Wave Research Articles

Broadband wave attenuation and topological transport in novel periodic pile barriers

Topological insulator (TI) has received enormous attention in recent years due to its intriguing dynamic characterizes of topologically protected wave transport and defect immunity. Besides, the elastic waves can propagate along the pre-designed interface but body, which may dramatically promote practical applications of novel devices. This work introduces the concept of TI into the design of periodic pile barriers, which is expected to realize vibration mitigation and waveguiding simultaneously. By tuning the difference of pile radius, the inversion symmetry is broken and the attenuation zone (AZ) emerges. Then, the topological phase transitions happen when pile positions are interchanged in a unit cell and the opposite valley Chern numbers (Cv) are obtained meanwhile. Based on the complex dispersion (k(ω) method) analysis of supercell, the existence of topological edge states between two types of unit cells with distinct topological phases is confirmed at first. Second, some key parameters affecting the design of periodic pile barriers is discussed comprehensively, especially the influence of soil damping on attenuation zones and edge states. Compared to the traditional real dispersion (ω(k) method), the complex dispersion can describe the propagation property and attenuation property synchronously, providing useful guidances for this novel multifunctional periodic pile barriers. Subsequently, broadband wave attenuation and topological wave transport of novel periodic pile barriers are further validated by the analysis in both frequency domain and time domain, showing high vibration reduction and transmission efficiency. This new kind of wave barriers may have great potential for both vibration mitigation and elastic wave energy harvesting.

A Multi-Modal Energy Harvesting Device for Multi-Directional and Low-Frequency Wave Energy

As a kind of sustainable energy source, ocean wave energy has always attracted attention. Because of the characteristics of multi-directions and low-bandwidth, the harvesting of wave energy has always been difficult. To harvest broadband wave energy in multiple directions and improve power generation efficiency, we present a multi-modal energy harvesting device by using a cross beam structure. In this device, the efficient harvesting of multi-frequency vibration energy in multiple directions has been achieved by using the multi-modal characteristics of the structure and the high power generation efficiency of the electric device based on liquid metal. Contrast experiments in multiple directions show that the device not only has the capability of multi-directional energy harvesting but also can work in the sweep range of the full band. Under horizontal excitation, compared with traditional single cantilever structure, the peak power density increased to 2227% and the working frequency band increased to 6.25 times. The experimental results show that the device significantly improves the efficiency of low-bandwidth multi-directional energy harvesting, providing a new method for vibration energy harvesting.

Dependence of Nonlinear Effects on Whistler‐Mode Wave Bandwidth and Amplitude: A Perspective From Diffusion Coefficients

AbstractThe electron resonant interaction with whistler‐mode waves is characterized by transport in pitch angle–energy space. We calculate electron diffusion and advection coefficients (a simplified characterization of transport) for a large range of electron pitch angle and energy using test particle simulations. Nonlinear effects are analyzed by comparing the diffusion coefficients using test particle simulations and quasilinear theory, and by evaluating the advection rates. Dependence of nonlinear effects on the wave amplitude and bandwidth of whistler‐mode waves is evaluated by running test particle simulations with a broad range of wave amplitude and bandwidth. The maximum amplitudes where the quasilinear approach is valid are found to increase with increasing bandwidth, from 50 pT for narrowband waves to 300 pT for broadband waves at L‐shell of 6. Moreover, interactions between intense whistler‐mode waves and small pitch angle electrons lead to large positive advection, which limits the applicability of diffusion‐based models. This study demonstrates the parameter range of the applicability of quasilinear theory and diffusion model for different wave amplitudes and frequency bandwidths of whistler‐mode waves, which is critical for evaluating the effects of whistler‐mode waves on energetic electrons in the Earth’s magnetosphere.

Exceeding the classical time-bandwidth product in nonlinear time-invariant systems

The classical “time-bandwidth” limit for linear time-invariant (LTI) devices in physics and engineering asserts that it is impossible to store broadband propagating waves (large $${\Delta }\omega$$ ’s) for long times (large Δt’s). For standing (non-propagating) waves, i.e., vibrations, in particular, this limit takes on a simple form, $${\Delta }t \,{\Delta }\omega = 1$$ , where $${\Delta }\omega$$ is the bandwidth over which localization (energy storage) occurs, and $${\Delta }t$$ is the storage time. This is related to a well-known result in dynamics, namely that one can achieve a high Q-factor (narrowband resonance) for low damping, or small Q-factor (broadband resonance) for high damping, but not simultaneously both. It thus remains a fundamental challenge in classical wave physics and vibration engineering to try to find ways to overcome this limit, not least because that would allow for storing broadband waves for long times, or achieving broadband resonance for low damping. Recent theoretical studies have suggested that such a feat might be possible in LTI terminated unidirectional waveguides or LTI topological “rainbow trapping” devices, although an experimental confirmation of either concept is still lacking. In this work, we consider a nonlinear but time-invariant mechanical system and demonstrate experimentally that its time-bandwidth product can exceed the classical time-bandwidth limit, thus achieving values both above and below unity, in an energy-tunable way. Our proposed structure consists of a single-degree-of-freedom nonlinear oscillator, rigidly coupled to a nondispersive waveguide. Upon developing the full theoretical framework for this class of nonlinear systems, we show how one may control the nonlinear flow of energy in the frequency domain, thereby managing to disproportionately decrease (increase) $${\Delta }t$$ , the storage time in the resonator, as compared with an increase (decrease) of the system’s bandwidth $${\Delta }\omega$$ . Our results pave the way toward conceiving and harnessing hitherto unattainable broadband and simultaneously low-loss wave-storage devices, both linear and nonlinear, for a host of key applications in wave physics and engineering.

Metal-organic framework-derived hierarchical porous carbon fiber bundles/Y2Co8Fe9 composite as a thin and broadband electromagnetic wave absorber

The development of broadband and thin electromagnetic wave absorbing materials is urgently required for use in practical applications. In this study, microwave absorption composites comprising Y2Co8Fe9 and various weight ratios of metal-organic framework (MOF)-derived carbon fiber bundles (0%, 0.1%, 0.3%, 0.5%) were prepared. The composite filled with 0.1 wt% MOF-derived carbon fiber bundles had a minimum reflection loss (RL) of -64.79 dB at a thickness of 1.44 mm and an effective absorption bandwidth (with an RL of less than -10 dB) of 5.02 GHz. Furthermore, a wide effective absorption bandwidth of 6.52 GHz was also acquired at 1.6 mm. The microwave absorption performance was significantly enhanced owing to the improved impedance matching, as well as the synergetic effect between the dielectric loss and magnetic loss. This study provides a new paradigm for fabricating composites consisting of MOF-derived materials and rare-earth materials for thin and broadband microwave absorption performance.

Cross-scale energy cascade powered by magnetospheric convection

Plasma convection in the Earth’s magnetosphere from the distant magnetotail to the inner magnetosphere occurs largely in the form of mesoscale flows, i.e., discrete enhancements in the plasma flow with sharp dipolarizations of magnetic field. Recent spacecraft observations suggest that the dipolarization flows are associated with a wide range of kinetic processes such as kinetic Alfvén waves, whistler-mode waves, and nonlinear time-domain structures. In this paper we explore how mesoscale dipolarization flows produce suprathermal electron instabilities, thus providing free energy for the generation of the observed kinetic waves and structures. We employ three-dimensional test-particle simulations of electron dynamics one-way coupled to a global magnetospheric model. The simulations show rapid growth of interchanging regions of parallel and perpendicular electron temperature anisotropies distributed along the magnetic terrain formed around the dipolarization flows. Unencumbered in test-particle simulations, a rapid growth of velocity-space anisotropies in the collisionless magnetotail plasma is expected to be curbed by the generation of plasma waves. The results are compared with in situ observations of an isolated dipolarization flow at one of the Magnetospheric Multiscale Mission spacecraft. The observations show strong wave activity alternating between broad-band wave activity and whistler waves. With estimated spatial extent being similar to the characteristic size of the temperature anisotropy patches in our test-particle simulations, the observed bursts of the wave activity are likely to be produced by the parallel and perpendicular electron energy anisotropies driven by the dipolarization flow, as suggested by our modeling results.

Tailoring Self‐Polarization of Bimetallic Organic Frameworks with Multiple Polar Units Toward High‐Performance Consecutive Multi‐Band Electromagnetic Wave Absorption at Gigahertz

AbstractMultiple relaxation behaviors are promising for broad frequency band and strong electromagnetic wave (EMW) absorption based on polarization‐controlled electromagnetic (EM) attenuation. However, rational selection of materials and structure manipulation through tunable substitution or phase control are challenging toward optimization of EMW absorption. Herein, bi‐metallic organic frameworks (B‐MOFs) with various morphologies are employed as EMW absorbers. Remarkably, the polar units can be enhanced by introducing Ni‐metal nodes into the Cu‐coordinated MOFs, rendering the B‐MOFs with self‐polarized properties and consecutive multifrequency EMW absorption behaviors. The maximum reflection loss of acetylene black (ACET) filled NiCu‐MOFs can reach –40.54 dB together with a wide bandwidth (<‐10 dB) of 5.87 GHz at a thickness of 2.5 mm. As a counterpart of the Ni/Cu/C derivatives, significantly increased broad band absorption (6.93 GHz) and multifrequency absorbing and polarization characteristics are also maintained in bimetal coexisting carbonized architectures as prepared by calcination of CuNi‐MOFs. This work demonstrates that the performance of effective absorbing frequency band can be enhanced in multi‐metallic organic frameworks‐based architectures, and paves a novel avenue for developing broadband and strong EMW absorbers.

Ultra-compact metafence to block and channel mechanical waves

Wave steering by artificial materials (for example, phononic crystals and acoustic metamaterials) is a fascinating frontier in modern physics and engineering, but suffers from bulky sizes and intractable challenges in fabrication. A sparse layer of identical tiny scatters, which we call metafence, is presented here in a non-destructive way to omnidirectionally block and arbitrarily channel flexural mechanical waves in plates. The underlying mechanism is that the restraining force and moment of the scatter are tuned simultaneously to counter-balance the incident wave. Both our experimental results and numerical analysis have demonstrated that broadband wave sources ranging from 3 to 7 kHz can be segregated from the protected area by the metafence. The metafence is also assembled into a waveguide routing with an arbitrary configuration. Compared with previous isolators and waveguides, our metafences exhibit absolute advantages in compact size, flexible configuration, and high structural strength. The current scenario sheds light on the design of lightweight-and-strong architectures for vibration control and energy harvesting with a high efficiency, and can be extended to microfluidics, acoustics, seismology and other fields.

Hanbury Brown and Twiss effect demonstrated for sound waves from a waterfall: An experimental, numerical, and analytical study

Hanbury Brown and Twiss determined the angular size of a visible light source (the star Sirius) by studying how the cross-correlation in intensity fluctuation recorded by two detectors changes with the distance between the detectors. We find that this principle works equally well for sound waves from a waterfall. This is remarkable, since sound is a completely different kind of wave from the HBT case. The frequency of the waves differs by a factor ∼1012 and the wavelength as well as the angular extension of the source seen from the observer's position differ by a factor ∼107. Our analysis is based on the general properties of broadband waves. We start with broadband waves at the amplitude level (not at intensity level) and demonstrate a HBT-like effect. We follow up with an explanation and demonstrations showing how the effect also manifests itself at the intensity level, providing a bridge to the original HBT work. We use the same reasoning in our numeric and analytical treatments, as well as in the experimental work, with identical results. The presentation is simple enough to be introduced even for second year bachelor students. Computer programs (in Matlab), including software for time-resolved frequency analysis, as well as original sound files are available from the authors.

In situ growth of CoNi bimetallic alloys inside polyetheretherketone-derived hierarchical porous carbon as a broadband and efficient electromagnetic wave absorber.

Both morphological structure and chemical composition are the important factors that determine the electromagnetic wave (EMW) absorption properties of EMW absorbers. Herein, hierarchical porous carbon (HPPC) was synthesized by using polyetheretherketone (PEEK) as the starting material via the salt template method combined with KOH activation. Then, taking advantage of the hierarchical porous characteristics of the carbon, a novel EMW absorber (HPPC/CoNi) was synthesized by in situ growth of CoNi bimetallic alloys inside the above porous carbon. The as-synthesized HPPC/CoNi exhibits excellent EMW absorption performance with a minimum reflection loss (RLmin) value of -65.56 dB at 2.00 mm and a maximum effective absorption bandwidth (EAB) of 5.92 GHz at 1.80 mm. The superior EMW absorption property is ascribable to the multiple reflection/scattering induced by the hierarchical porous structure of HPPC, dielectric loss, magnetic loss, and good impedance matching. The results show that HPPC/CoNi is a potential EMW absorber with characteristics of ultra-light weight, ultrathin thickness, broad bandwidth, and strong absorption.

Graded elastic meta-waveguides for rainbow reflection, trapping and mode conversion

Precise control of elastic waves is a challenge for many applications in the field of mechanical vibrations, ultrasonic inspection, and energy harvesting. Graded arrays of resonators on elastic substrates recently revealed superior performances for broadband wave trapping and mode conversion. In this study we present elastic waveguides able to govern waves at different scales exploiting rainbow reflection, trapping and mode conversion. We investigate whether these mechanisms, and the associated control, can be used for energy harvesting or signal conversion devices.

Broadband-excitation-based mechanical spectroscopy of highly viscous tissue-mimicking phantoms.

Standard rheometers assess mechanical properties of viscoelastic samples up to 100 Hz, which often hinders the assessment of the local-scale dynamics. We demonstrate that high-frequency analysis can be achieved by inducing broadband waves and monitoring their media-dependent propagation using optical coherence tomography. Here, we present a new broadband wave analysis based on two-dimensional Fourier transformation. We validated this method by comparing the mechanical parameters to monochromatic excitation and a standard oscillatory test data. Our method allows for high-frequency mechanical spectroscopy, which could be used to investigate the local-scale dynamics of different biological tissues and the influence of diseases on their microstructure.

Flexural wave mitigation in metamaterial cylindrical curved shells with periodic graded arrays of multi-resonator

This paper aims to present a theoretical model of flexural wave bandgaps in locally resonant cylindrical curved shell and investigates a technique for broadband wave mitigation using a metamaterial cylindrical curved shell. Firstly, cartesian coordinates are converted to cylindrical coordinates in the cylindrical curved shell structure to construct periodic conditions in the circumferential direction. Then, governing equations are derived by using the extended Hamilton principle, and plane wave expansion (PWE) method is implemented to calculate the band structures of acoustic metamaterial cylindrical curved shells with different multi-resonator parameters. In addition, a periodic graded arrays technique is also utilized to broaden the frequency range of bandgap. Lastly, finite element method (FEM) is used to calculate the band structures and vibration transmission characteristics to verify the accuracy of PWE results. The results disclose that the proposed structure has a good attenuation effect on flexural wave and the periodic graded arrays technique can effectively splice bandgaps of each resonator to enlarge frequency range of the flexural wave mitigation. Hence, the proposed structure can be used to achieve a good attenuation effect on flexural wave in the future.

First-year ion-acoustic wave observations in the solar wind by the RPW/TDS instrument on board Solar Orbiter

Context.Electric field measurements of the Time Domain Sampler (TDS) receiver, part of the Radio and Plasma Waves (RPW) instrument on board Solar Orbiter, often exhibit very intense broadband wave emissions at frequencies below 20 kHz in the spacecraft frame. During the first year of the mission, the RPW/TDS instrument was operating from the first perihelion in mid-June 2020 and through the first flyby of Venus in late December 2020.Aims.In this paper, we present a year-long study of electrostatic fluctuations observed in the solar wind at an interval of heliocentric distances from 0.5 to 1 AU. The RPW/TDS observations provide a nearly continuous data set for a statistical study of intense waves below the local plasma frequency.Methods.The on-board and continuously collected and processed properties of waveform snapshots allow for the mapping plasma waves at frequencies between 200 Hz and 20 kHz. We used the triggered waveform snapshots and a Doppler-shifted solution of the dispersion relation for wave mode identification in order to carry out a detailed spectral and polarization analysis.Results.Electrostatic ion-acoustic waves are the most common wave emissions observed between the local electron and proton plasma frequency by the TDS receiver during the first year of the mission. The occurrence rate of ion-acoustic waves peaks around perihelion at distances of 0.5 AU and decreases with increasing distances, with only a few waves detected per day at 0.9 AU. Waves are more likely to be observed when the local proton moments and magnetic field are highly variable. A more detailed analysis of more than 10 000 triggered waveform snapshots shows the mean wave frequency at about 3 kHz and wave amplitude about 2.5 mV m−1. The wave amplitude varies asR−1.38with the heliocentric distance. The relative phase distribution between two components of the E-field projected in theY − ZSpacecraft Reference Frame (SRF) plane shows a mostly linear wave polarization. Electric field fluctuations are closely aligned with the directions of the ambient field lines. Only a small number (3%) of ion-acoustic waves are observed at larger magnetic discontinuities.

Tunable band structures design for elastic wave transmission in tension metamaterial chain

This paper reports a novel band structure manipulation mechanism stemmed from taut strings to shift the Bragg and local resonance band gaps simultaneously. We first developed a metamaterial chain encompassing periodic tighten strings, which are essentially elastic foundations with tunable effective stiffnesses controlled by the tensions of the grounded strings. Subsequently, band edge functions were analyzed against the tension applied, thereby revealing an ultra-wide band gap with a tunable pass band. Nonlinear dispersion characteristics of the proposed system were further corrected by multiple-scale perturbation, and the band gap boundaries are reduced slightly with larger incoming amplitudes. Transmission spectra of the finite specimen indicated reasonable agreement with theoretical analysis. With respect to the band-pass property, a very narrow band filter was achieved with a precise frequency selection. It was shown that, by actively tuning string tensions, broader attenuation bands appear; tightening strings in different locations could be used to control different frequency branches. The space-time distribution of the transmitted wave also confirmed that the pass band would narrow to the specified band by the tension design. This work enables broadband wave blocking and meticulous wave filtering through proper string tensions.

Influence of Ca/Al ratio on physical and dielectric properties of Al2O3/CaO-B2O3-SiO2 glass-ceramics composite coatings prepared by high enthalpy atmospheric plasma spraying

The dielectric layer plays a key role in regulating electromagnetic wave broadband scattering based on meta-surface technology. Herein, the physical properties of composite powder, prepared by spray drying of CaO-B2O3-SiO2 (CBS) glass-ceramic powder and Al2O3 in different mass ratios, are systematically investigated. Meanwhile, a high enthalpy atmospheric plasma spraying equipment is utilized to prepare CBS/Al2O3 composite coatings, and the morphology, physical properties and dielectric properties of the composite coating are analyzed. The XRD and DSC data of the composite coating reveal that the crystallization behavior of β-CaSiO3 and CaB2O4 gradually disappear with the increase of Al2O3 content. Hence, only CaAl2Si2O8 phase is observed during heat treatment. The experimental results confirm that the dielectric properties of CBS/Al2O3 composite coating conform to the rule of mixture for composite materials. Also, the dielectric properties are affected by porosity and crystallization rate.

Fixed-points-theory-based critical condition for [formula omitted] sprung mass isolation of a two-body point absorber in a sea wave environment

The present research concerns a system that is composed of a heaving point absorber coupled with a sprung (overlying) mass. For this system we impose and address the requirement of sprung mass isolation from the buoy’s oscillation movement induced by the incident waves. To this end, the proposed passive control methodology builds on the related isolation approaches that concern the implementation of the H∞ optimization criterion in the Dynamic Vibration Absorbers (DVAs). Then, a novel H∞ optimization process is developed for the proposed wave-excited system to accommodate for the distinct challenges that arise due to the frequency dependence of the two-body system. To accomplish this, a “virtual” system is realized to temporarily bypass the frequency dependence of the actual system by capitalizing on the existence of the fixed points in the “virtual” system. The realized “virtual” system is embedded in an iterative framework to derive the pair of critical parameters (the tuning ratio δcrit and the damping ratio ζscrit) resulting in the formation of equal double peaks at the frequency dependent actual system. It is proved that any tuning ratio δ<δcrit combined with its corresponding damping ratio ensures the fulfillment of the H∞ optimization criterion. Moreover, any value of this pair is capable of controlling the global sprung mass acceleration maximum. Any conditions leading the “virtual” system to undesired solutions are investigated and eliminated from the optimization process of the actual system. The potential of the critical parameters to yield simultaneous broadband wave energy extraction is also demonstrated.

On an eddy viscosity model for energetic deep-water surface gravity wave breaking

We present an investigation of the fundamental physical processes involved in deep-water gravity wave breaking. Our motivation is to identify the underlying reason causing the deficiency of the eddy viscosity breaking model (EVBM) in predicting surface elevation for strongly nonlinear waves. Owing to the limitation of experimental methods in the provision of high-resolution flow information, we propose a numerical methodology by developing an EVBM enclosed standalone fully nonlinear quasi-potential (FNP) flow model and a coupled FNP plus Navier–Stokes flow model. The numerical models were firstly verified with a wave train subject to modulational instability, then used to simulate a series of broad-banded focusing wave trains under non-, moderate- and strong-breaking conditions. A systematic analysis was carried out to investigate the discrepancies of numerical solutions produced by the two models in surface elevation and other important physical properties. It is found that EVBM predicts accurately the energy dissipated by breaking and the amplitude spectrum of free waves in terms of magnitude, but fails to capture accurately breaking induced phase shifting. The shift of phase grows with breaking intensity and is especially strong for high-wavenumber components. This is identified as a cause of the upshift of the wave dispersion relation, which increases the frequencies of large-wavenumber components. Such a variation drives large-wavenumber components to propagate at nearly the same speed, which is significantly higher than the linear dispersion levels. This suppresses the instant dispersive spreading of harmonics after the focal point, prolonging the lifespan of focused waves and expanding their propagation space.

Parametric optimization of an aperiodic metastructure based on genetic algorithm

The paper presents an integrated analytical, numerical and experimental study on a kind of one-dimensional finite aperiodic metastructure, with mass-in-mass unit cells optimized for the broadband wave attenuation via genetic algorithm. The study begins with the analytic wave solution of the aperiodic structure and numerically validates the correctness of the wave solution in both frequency domain and time domain. Then, the paper outlines the optimization scheme in order to connect multiple separated narrow bandgaps into a wider continuous one, including the objective function of the wave attenuation, the design variables of inner masses and their constraints. The numerical studies show the successful broadband wave attenuation with a low vibration transmissibility in one direction and two opposite directions, respectively, based on the genetic algorithm method. The maximal attenuation bandwidth of the optimized aperiodic structure increases about 90% compared with the conventional repetitive local resonance, without any mass increase. Finally, the paper gives experimental studies of a 3D-printed lattice structure with an adjustable inner mass in each cell. The measured vibrations of the fabricated structure experimentally validate the optimized aperiodic structure with the broadband wave attenuation, as well as the effectiveness of proposed optimization method.

Magnetic Field and Energetic Particle Flux Oscillations and High‐Frequency Waves Deep in the Inner Magnetosphere During Substorm Dipolarization: ERG Observations

AbstractUsing Exploration of energization and Radiation in Geospace (ERG or Arase) spacecraft data, we studied low‐frequency magnetic field and energetic particle flux oscillations and high‐frequency waves deep in the inner magnetosphere at a radial distance of ~4–5 during substorm dipolarization. The magnetic field oscillated alternately between dipole‐like and taillike configuration at a period of 1 min during dipolarization. When the magnetic field was dipole‐like, the parallel magnetic component of the Pi2 waves was at trough. Both energetic ion and electron fluxes with a few to tens of kiloelectronvolts enhanced out of phase, indicating that magnetosonic waves were in slow mode. Field‐aligned currents also oscillated. These observations are consistent with signatures of ballooning instability. In addition, we found that broadband waves from the Pi1 range to above the electron cyclotron frequency tended to appear intermittently in the central plasma sheet near dipole‐like configuration.