18 results found
Colombi A, Craster R, Colquitt D, et al., 2017, Elastic wave control beyond band-gaps: shaping the flow of waves in plates and half-spaces with subwavelength resonant rods., Frontiers in Mechanical Engineering, Vol: 3, ISSN: 2297-3079
In metamaterial science, local resonance and hybridization are key phenomena strongly influencing the dispersion properties; the metasurface discussed in this article created by a cluster of resonators, subwavelength rods, atop an elastic surface being an exemplar with these features. On this metasurface, band-gaps, slow or fast waves, negative refraction, and dynamic anisotropy can all be observed by exploring frequencies and wavenumbers from the Floquet–Bloch problem and by using the Brillouin zone. These extreme characteristics, when appropriately engineered, can be used to design and control the propagation of elastic waves along the metasurface. For the exemplar we consider, two parameters are easily tuned: rod height and cluster periodicity. The height is directly related to the band-gap frequency and, hence, to the slow and fast waves, while the periodicity is related to the appearance of dynamic anisotropy. Playing with these two parameters generates a gallery of metasurface designs to control the propagation of both flexural waves in plates and surface Rayleigh waves for half-spaces. Scalability with respect to the frequency and wavelength of the governing physical laws allows the application of these concepts in very different fields and over a wide range of lengthscales.
Colombi A, Ageeva V, Smith RJ, et al., 2017, Enhanced sensing and conversion of ultrasonic Rayleigh waves by elastic metasurfaces, Scientific Reports, Vol: 7, ISSN: 2045-2322
Recent years have heralded the introduction of metasurfaces that advantageously combine the vision of sub- wavelength wave manipulation, with the design, fabrication and size advantages associated with surface excitation. An important topic within metasurfaces is the tailored rainbow trapping and selective spatial frequency separation of electromagnetic and acoustic waves using graded metasurfaces. This frequency dependent trapping and spatial frequency segregation has implications for energy concentrators and associated energy harvesting, sensing and wave filtering techniques. Different demonstrations of acoustic and electromagnetic rainbow devices have been performed, however not for deep elastic substrates that support both shear and compressional waves, together with surface Rayleigh waves; these allow not only for Rayleigh wave rainbow effects to exist but also for mode conversion from surface into shear waves. Here we demonstrate experimentally not only elastic Rayleigh wave rainbow trapping, by taking advantage of a stop-band for surface waves, but also selective mode conversion of surface Rayleigh waves to shear waves. These experiments performed at ultrasonic frequencies, in the range of 400-600 kHz, are complemented by time domain numerical simulations. The metasurfaces we design are not limited to guided ultrasonic waves and are a general phenomenon in elastic waves that can be translated across scales.
Skelton EA, Craster RV, Colombi A, et al., 2017, Fluid-loaded metasurfaces
We consider wave propagation along fluid-loaded structures which take theform of an elastic plate augmented by an array of resonators forming ametasurface, that is, a surface structured with sub-wavelength resonators. Suchsurfaces have had considerable recent success for the control of wavepropagation in electromagnetism and acoustics, by combining the vision ofsub-wavelength wave manipulation, with the design, fabrication and sizeadvantages associated with surface excitation. We explore one aspect of recentinterest in this field: graded metasurfaces, but within the context offluid-loaded structures. Graded metasurfaces allow for selective spatial frequency separation and areoften referred to as exhibiting rainbow trapping. Experiments, and theory, havebeen developed for acoustic, electromagnetic, and even elastic, rainbow devicesbut this has not been approached for fluid-loaded structures that supportsurface waves coupled with the acoustic field in a bulk fluid. This surfacewave, coupled with the fluid, can be used to create an additional effect bydesigning a metasurface to mode convert from surface to bulk waves. Wedemonstrate that sub-wavelength control is possible and that one can createboth rainbow trapping and mode conversion phenomena for a fluid-loaded elasticplate model.
Colquitt DJ, Colombi A, Craster RV, et al., 2017, Seismic metasurfaces: Sub-wavelength resonators and Rayleigh wave interaction, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, Vol: 99, Pages: 379-393, ISSN: 0022-5096
Colombi A, 2016, Resonant metalenses for flexural waves in plates., Journal of the Acoustical Society of America, Vol: 140, Pages: EL423-EL428, ISSN: 0001-4966
The dispersion curves of a cluster of closely spaced rods supported by a thin plate are characterised by subwavelength bandgaps and slow group velocities induced by local resonance effects. A recent analytical study [Williams, Roux, Rupin, and Kuperman (2015). Phys. Rev. B 91, 104307], has shown how the slow velocity branch depends, amongst other parameters, on the height of the rods that make up the cluster. Such metamaterial, offering easy-to-tune spatial velocity gradients, is a perfect candidate for building gradient index lenses such as Luneburg, Maxwell, and 90° rotating. Here theoretical results are combined with numerical simulations to design and test metalenses for flexural waves. The lenses are obtained by tuning the height of the cluster of rods such that they provide the required refractive index profile. Snapshots and videos from three-dimensional numerical simulations in a narrow band centered at ∼4 kHz are used to analyse the performances of three types of gradient index metalens (Luneburg, Maxwell, and 90° rotating).
Colombi A, Gueguen P, 2016, Experimental and Numerical Evidence of the Clustering Effect of Structures on Their Response during an Earthquake: A Case Study of Three Identical Towers in the City of Grenoble, France, Bulletin of the Seismological Society of America, Vol: 106, Pages: 2855-2864, ISSN: 1943-3573
In this article, interpretation of an equivalent to a macroseismic intensitysurvey, performed in three identical stand-alone buildings located in Grenoble, France,after anML4.1 earthquake, reveals a clustering effect, resulting in different levels ofperception of seismic loading by inhabitants. The clustering effect is confirmed usingnumerical simulation; the variation of the seismic response of the building in the middleof the cluster depends on the azimuth of the seismic source relative to the buildingcluster. The major effect is the splitting of its resonance frequency, accompanied by adecrease in vibration amplitude. We conclude that clustering has an impact on urbaneffects, calling into question the validity of seismic design, which considers buildings inurban areas as stand-alone constructions, and the interpretation of macroseismic inten-sity surveys conducted in dense urban areas.
Critical concepts from three different fields, elasticity, plasmonics and metamaterials, are brought together to design a metasurface at the geophysical scale, the resonant metawedge, to control seismic Rayleigh waves. Made of spatially graded vertical subwavelength resonators on an elastic substrate, the metawedge can either mode convert incident surface Rayleigh waves into bulk elastic shear waves or reflect the Rayleigh waves creating a "seismic rainbow" effect analogous to the optical rainbow for electromagnetic metasurfaces. Time-domain spectral element simulations demonstrate the broadband efficacy of the metawedge in mode conversion while an analytical model is developed to accurately describe and predict the seismic rainbow effect; allowing the metawedge to be designed without the need for extensive parametric studies and simulations. The efficiency of the resonant metawedge shows that large-scale mechanical metamaterials are feasible, will have application, and that the time is ripe for considering many optical devices in the seismic and geophysical context.
Colombi A, Guenneau S, Roux P, et al., 2016, Transformation seismology: composite soil lenses for steering surface elastic Rayleigh waves., Scientific Reports, Vol: 6, ISSN: 2045-2322
Metamaterials are artificially structured media that exibit properties beyond those usually encountered in nature. Typically they are developed for electromagnetic waves at millimetric down to nanometric scales, or for acoustics, at centimeter scales. By applying ideas from transformation optics we can steer Rayleigh-surface waves that are solutions of the vector Navier equations of elastodynamics. As a paradigm of the conformal geophysics that we are creating, we design a square arrangement of Luneburg lenses to reroute Rayleigh waves around a building with the dual aim of protection and minimizing the effect on the wavefront (cloaking). To show that this is practically realisable we deliberately choose to use material parameters readily available and this metalens consists of a composite soil structured with buried pillars made of softer material. The regular lattice of inclusions is homogenized to give an effective material with a radially varying velocity profile and hence varying the refractive index of the lens. We develop the theory and then use full 3D numerical simulations to conclusively demonstrate, at frequencies of seismological relevance 3-10 Hz, and for low-speed sedimentary soil (vs: 300-500 m/s), that the vibration of a structure is reduced by up to 6 dB at its resonance frequency.
Colombi A, Roux P, Guenneau S, et al., 2016, Forests as a natural seismic metamaterial: Rayleigh wave bandgaps induced by local resonances, Scientific Reports, Vol: 6, ISSN: 2045-2322
We explore the thesis that resonances in trees result in forests acting as locally resonant metamaterials for Rayleigh surface waves in the geophysics context. A geophysical experiment demonstrates that a Rayleigh wave, propagating in soft sedimentary soil at frequencies lower than 150 Hz, experiences strong attenuation, when interacting with a forest, over two separate large frequency bands. This experiment is interpreted using finite element simulations that demonstrate the observed attenuation is due to bandgaps when the trees are arranged at the sub-wavelength scale with respect to the incident Rayleigh wave. The repetitive bandgaps are generated by the coupling of the successive longitudinal resonances of trees with the vertical component of the Rayleigh wave. For wavelengths down to 5 meters, the resulting bandgaps are remarkably large and strongly attenuating when the acoustic impedance of the trees matches the impedance of the soil. Since longitudinal resonances of a vertical resonator are inversely proportional to its length, a man-made engineered array of resonators that attenuates Rayleigh waves at frequency ≤10 Hz could be designed starting from vertical pillars coupled to the ground with longitudinal resonance ≤10 Hz.
Colombi A, Roux P, Colquitt D, et al., 2016, Conversion and reflection of Rayleigh waves with the seismic metawedge, 10th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics (METAMATERIALS), Publisher: IEEE, Pages: 313-315
Pedersen HA, Boue P, Poli P, et al., 2015, Arrival angle anomalies of Rayleigh waves observed at a broadband array: a systematic study based on earthquake data, full waveform simulations and noise correlations, Geophysical Journal International, Vol: 203, Pages: 1626-1641, ISSN: 1365-246X
Deviation of seismic surface waves from the great-circle between source and receiver is illustrated by the anomalies in the arrival angle, that is the difference between the observed backazimuth of the incident waves and the great-circle. Such arrival angle anomalies have been known for decades, but observations remain scattered. We present a systematic study of arrival angle anomalies of fundamental mode Rayleigh waves (20–100 s period interval) from 289 earthquakes and recorded by a broadband network LAPNET, located in northern Finland. These observations are compared with those of full waveform synthetic seismograms for the same events, calculated in a 3-D Earth and also compared with those of seismograms obtained by ambient noise correlation. The arrival angle anomalies for individual events are complex, and have significant variations with period. On average, the mean absolute deviation decreases from ∼9° at 20 s period to ∼3° at 100 s period. The synthetic seismograms show the same evolution, albeit with somewhat smaller deviations. While the arrival angle anomalies are fairly well simulated at long periods, the deviations at short periods are very poorly modelled, demonstrating the importance of the continuous improvement of global crustal models. At 20–30 s period, both event data and numerical simulations have strong multipathing, and relative amplitude changes between different waves will induced differences in deviations between very closely located events. The source mechanism has only limited influence on the deviations, demonstrating that they are directly linked to propagation effects, including near-field effects in the source area. This observation is confirmed by the comparison with seismic noise correlation records, that is where the surface waves correspond to those emitted by a point source at the surface, as the two types of observations are remarkably similar in the cases where earthquakes are located close to seism
Colombi A, Roux P, Guenneau S, et al., 2015, Directional cloaking of flexural waves in a plate with a locally resonant metamaterial, JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol: 137, Pages: 1783-1789, ISSN: 0001-4966
Colombi A, Chaput J, Brenguier F, et al., 2014, On the temporal stability of the coda of ambient noise correlations, Comptes Rendus Geoscience, Vol: 346, Pages: 307-316, ISSN: 1631-0713
We analyze the sensitivity of cross correlations to the anisotropy of the incident field in the context of seismic ambient noise monitoring of small velocity changes. Numerical simulations of elastic waves are performed in a 2D scattering plate with a focus on the comparative character of the direct and coda waves in the cross-correlation. We show that coda waves reconstructed from cross-correlations are far more robust than direct waves in the presence of azimuthal anisotropy of the incident field. We observe similar behavior with real data recorded on Erebus volcano, where a database of impulsive icequakes is used to simulate an anisotropic source field. We propose a simplified approach to evaluate the sensitivity of scattered waves to the anisotropy of the incoming noise field. We rely on previous results obtained for direct waves and on intrinsic properties of scattered waves to predict the errors produced by strong source anisotropy with numerical experiments. These results also yield realistic values for monitoring the accuracy to be expected with real data at crustal scales. Our analysis shows that high-precision noise-based monitoring could be performed with coda waves in the correlation functions, even in the presence of variations in the azimuthal distribution of the ambient noise field.
Colombi A, Roux P, Rupin M, 2014, Sub-wavelength energy trapping of elastic waves in a metamaterial, JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol: 136, Pages: EL192-EL198, ISSN: 0001-4966
Colombi A, Nissen-Meyer T, Boschi L, et al., 2014, Seismic waveform inversion for core-mantle boundary topography, GEOPHYSICAL JOURNAL INTERNATIONAL, Vol: 198, Pages: 55-71, ISSN: 0956-540X
Nissen-Meyer T, van Driel M, Stähler SC, et al., 2014, AxiSEM: broadband 3-D seismic wavefields in axisymmetric media, Solid Earth, Vol: 5, Pages: 425-445, ISSN: 1869-9529
We present a methodology to compute 3-D global seismic wavefields for realistic earthquake sources in visco-elastic anisotropic media, covering applications across the observable seismic frequency band with moderate computational resources. This is accommodated by mandating axisymmetric background models that allow for a multipole expansion such that only a 2-D computational domain is needed, whereas the azimuthal third dimension is computed analytically on the fly. This dimensional collapse opens doors for storing space–time wavefields on disk that can be used to compute Fréchet sensitivity kernels for waveform tomography. We use the corresponding publicly available AxiSEM (www.axisem.info) open-source spectral-element code, demonstrate its excellent scalability on supercomputers, a diverse range of applications ranging from normal modes to small-scale lowermost mantle structures, tomographic models, and comparison with observed data, and discuss further avenues to pursue with this methodology.
Colombi A, Boschi L, Roux P, et al., 2014, Green's function retrieval through cross-correlations in a two-dimensional complex reverberating medium, JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol: 135, Pages: 1034-1043, ISSN: 0001-4966
Colombi A, Nissen-Meyer T, Boschi L, et al., 2012, Seismic waveform sensitivity to global boundary topography, GEOPHYSICAL JOURNAL INTERNATIONAL, Vol: 191, Pages: 832-848, ISSN: 0956-540X
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