Littérature scientifique sur le sujet « Band-superposition approach »

A vibration transfer analysis method based on polynomial chaos expansion (PCE) is proposed in this study and is used to analyze the stochastic dynamic compliance of uncertain systems with the Gaussian distribution. The random dynamic compliance is established by utilizing mode superposition on the system as the parameters of system uncertainties are regarded as input variables. Considering the asymptotic probability density function of mode shape, the dynamic compliance is decomposed into the mean of mode shape and the subsystem represented as an orthogonal polynomial expansion. Following this, the vibration transmission analysis approach is proposed for the random vibration. Results of a numerical simulation carried out employing the PCE approach show that broad-band spectrum analysis is more effective than narrow-band spectrum analysis because the former jump of the dynamic compliance amplitude is weakened. This proposed approach is valid and feasible, but since broad-band spectrum analysis loses some important information about the random vibration, both the aforementioned processes need to simultaneously be applied to analyze the random vibration transmission of low-medium frequency systems.

Spatio‐temporal analysis of seismic records is of particular relevance in many geophysical applications, e.g., vertical seismic profiles, plane‐wave slowness estimation in seismographic array processing and in sonar array processing. The goal is to estimate from a limited number of receivers the 2-D spectral signature of a group of events that are recorded on a linear array of receivers. When the spatial coverage of the array is small, conventional f-k analysis based on Fourier transform leads to f-k panels that are dominated by sidelobes. An algorithm that uses a Bayesian approach to design an artifacts‐reduced Fourier transform has been developed to overcome this shortcoming. A by‐product of the method is a high‐resolution periodogram. This extrapolation gives the periodogram that would have been recorded with a longer array of receivers if the data were a limited superposition of monochromatic planes waves. The technique is useful in array processing for two reasons. First, it provides spatial extrapolation of the array (subject to the above data assumption) and second, missing receivers within and outside the aperture are treated as unknowns rather than as zeros. The performance of the technique is illustrated with synthetic examples for both broad‐band and narrow‐band data. Finally, the applicability of the procedure is assessed analyzing the f-k spectral signature of a vertical seismic profile (VSP).

Experimental and numerical investigations of the modal behavior of a prototype Kaplan turbine runner in air have been conducted in this paper. The widely used roving accelerometer method was used in the experimental modal analysis. A systematic approach from a single blade model to the whole runner has been used in the simulation to get a thorough understanding. The experimental results show that all the detected modes concentrate their displacements on the impacted blade. The numerical results show that the modes of the single blade form different mode families of the runner, and each mode family corresponds to a narrow frequency band. Harmonic response analysis shows that, at the response peak point, the single blade excitation can only get mode shapes with concentrations on the exciting blade due to the superposition of the close modes in each mode family, which explains the experimental results well, while the mode superposition can be avoided by the order excitation method. With the reduction of the connection stiffness between the blades and hub/control system, the frequencies of most modes change from insensitive to more and more sensitive to the connection stiffness change, which results in a sensitive area and an insensitive area. Through comparison with the experimental results, it is indicated that the natural frequencies of the runner can probably be predicted by merging the runner into a whole body.

The Operational Land Imager (OLI) onboard Landsat 8 has found successful application in inland and coastal water remote sensing. Its radiometric specification and high spatial resolution allows quantification of water-leaving radiance while resolving small water bodies. However, its limited multispectral band set restricts the range of water quality parameters that can be retrieved. Identification of cyanobacteria biomass has been demonstrated for sensors with a band centered near 620 nm, the absorption peak of the diagnostic pigment phycocyanin. While OLI lacks such a band in the orange region, superposition of the available multispectral and panchromatic bands suggests that it can be calculated by a scaled difference. A set of 428 in situ spectra acquired in diverse lakes in Belgium and The Netherlands was used to develop and test an orange contra-band retrieval algorithm, achieving a mean absolute percentage error of 5.39% and a bias of −0.88% in the presence of sensor noise. Atmospheric compensation error propagated to the orange contra-band was observed to maintain about the same magnitude (13% higher) observed for the red band and thus results in minimal additional effects for possible base line subtraction or band ratio algorithms for phycocyanin estimation. Generality of the algorithm for different reflectance shapes was tested against a set of published average coastal and inland Optical Water Types, showing robust retrieval for all but relatively clear water types (Secchi disk depth > 6 m and chlorophyll a < 1.6 mg m − 3 ). The algorithm was further validated with 79 matchups against the Ocean and Land Colour Imager (OLCI) orange band for 10 globally distributed lakes. The retrieved band is shown to convey information independent from the adjacent bands under variable phycocyanin concentrations. An example application using Landsat 8 imagery is provided for a known cyanobacterial bloom in Lake Erie, US. The method is distributed in the ACOLITE atmospheric correction code. The contra-band approach is generic and can be applied to other sensors with overlapping bands. Recommendations are also provided for development of future sensors with broad spectral bands with the objective to maximize the accuracy of possible spectral enhancements.

At present, double expansion chamber structures are widely used in the field of acoustic attenuation, and two kinds of double-chamber compound structures for hydraulic attenuators are proposed in this paper. A one-dimensional analytical approach was developed to predict the pressure pulsation attenuation performance of these two structures, and comparisons of insertion loss predictions with experimental results illustrated that the one-dimensional approach is suitable for accurate prediction among the research frequency band. This approach was then used to investigate the effects of porosity and geometrical parameters on the pressure pulsation performance of these two double-chamber compound hydraulic attenuators. To optimize the pressure pulsation attenuation performance at the backwash frequency, parameter optimization was performed for these double-chamber compound structures, and a genetic algorithm based on double-precision floating-point encoding was proposed. The results showed that the range of attenuation frequency bands was widened; however, the effect on low frequency filtering characteristics was limited. The insertion loss of the second structure, which had a partially perforated tube, exhibits a superposition of dome attenuation and axial resonance in the plane wave region. By choosing the length and location of the perforated section to match resonances with the troughs of the pulsation attenuator, a desirable broadband pressure pulsation attenuation can be obtained.

Vibration energy harvesters in industrial applications usually take the form of cantilever oscillators covered by a layer of piezoelectric material and exploit the resonance phenomenon to improve the generated power. In many aeronautical applications, the installation of cantilever harvesters is not possible owing to the lack of room and/or safety and durability requirements. In these cases, strain piezoelectric harvesters can be adopted, which directly exploit the strain of a vibrating aeronautic component. In this research, a mathematical model of a vibrating slat is developed with the modal superposition approach and is coupled with the model of a piezo-electric patch directly bonded to the slat. The coupled model makes it possible to calculate the power generated by the strain harvester in the presence of the broad-band excitation typical of the aeronautic environment. The optimal position of the piezoelectric patch along the slat length is discussed in relation with the modes of vibration of the slat. Finally, the performance of the strain piezoelectric harvester is compared with the one of a cantilever harvester tuned to the frequency of the most excited slat mode.

SUMMARY We consider the problem of inhomogeneous subsurface imaging using beam waves. The formulation is based on the ultra-wide-band phase-space beam summation (UWB-PS-BS) method, which is structured upon windowed Fourier transform (WFT) expansions of surface fields and sources. In this approach, the radiated field is given as a superposition of beam propagators. Here, we use the beams first for expanding the surface sources and the scattered data, and then for imaging where we use the backpropagation and cross-correlation of beams. This formulation enables a target oriented imaging approach, where we take into account only pairs of source and receiver beams that pass near a region of interest, and thus extract only the relevant data arriving from this region. It also leads to a priori sparse representation of both the beam domain data and the beam propagators. A physical cogent for the beam domain data is obtained under the Born approximation. The beam domain data can be approximated as the local interaction between the beam propagators and the medium reflectivity. Thus, one may interpret the beam domain data as a local Snell’s law reflection in the direction defined by the vector summation of the incident beam and backpropagated beam ray parameters. We demonstrate a physical model for the beam domain data and the salient features of the proposed imaging algorithm using numerical examples.

Compared with the plane-wave reflection coefficient, the spherical-wave reflection coefficient (SRC) can more accurately describe the reflected wavefield excited by a point source, especially in the case of low seismic frequency and short travel distance. However, unlike the widely used plane-wave amplitude-variation-with-offset/frequency (AVO/AVF) inversion, the practical application of spherical-wave AVO/AVF inversion in multilayer elastic media is still in the exploratory stage. One of the difficulties is how to fully use the amplitude and phase information of the complex-valued SRC and the spherical-wave response property of each frequency component to obtain the spherical-wave synthetic seismogram in multilayer elastic media. In view of this, we have developed a complex convolution model considering the amplitude and phase information of an SRC to obtain the complex synthetic seismogram of a certain frequency component. A simple harmonic superposition method is further developed. By superposing the complex synthetic seismograms of different frequency components, the synthetic seismogram of the full-frequency band can be obtained. In addition, a novel three-parameter SRC in terms of P- and S-wave moduli and density is derived. Based on the SRC and complex seismic traces with different offsets (or incidence angles) and frequency components, an inversion approach of complex spherical-wave amplitude and phase variation with offset and frequency is proposed. A noisy synthetic data example verifies the robustness of our complex spherical-wave inversion approach. Field data examples indicate that the P- and S-wave moduli estimated by the complex spherical-wave inversion approach can reasonably match the filtered well-logging data. Considering spherical waves rather than plane waves can improve the accuracy of seismic inversion results.

Purpose This paper aims to investigate whether electroencephalography (EEG) technology is effective in qualifying the tactile sensation evoked by non-steady cutaneous electrical stimulation. EEG is a novel method for electrotactile analysis and has demonstrated the discrimination ability for electrotactile sensation under steady contact conditions in recent years. However, in non-steady contact conditions, it is necessary to test its effectiveness. This study aims to explore an objective analysis method in comparison to psychophysical approach and to provide a methodology for non-steady electrotactile research. Design/methodology/approach With EEG experimentation on 13 volunteers, the authors collected evoked potentials by the predesigned “1” and “0” stimulation events. In addition, with a series of data preprocessing including artifact elimination, band-pass filtering, baseline normalization, data superposition and fast Fourier transform transformation, the authors got the power spectrum of alpha, beta and gamma rhythms. Furthermore, statistics analysis and ANOVA test were adopted for exploring the discrepancy of the spectrum characterizations for different non-steady electrostimulation events. Findings The EEG power spectrum of the central cortical brain is valuable in discriminating the two types of stimulation events. The power of alpha rhythm especially in the central cortical brain evoked by event “1,” whose current level is equal to the threshold, was significantly lower than that evoked by event “0,” whose level is less than the threshold (p < 0.05). Then, the power of the beta rhythm presented counter-change (p < 0.05). This study suggests that EEG may have the potential to qualify non-steady electrotactile sensation for engineering applications. Research limitations/implications Limiting factors of non-steady electrotactile stimulation were considered in this study. Different tapping frequency and contact time should be investigated in future studies. Originality/value This paper fulfills a challenge in qualifying the tactile sensations evoked by non-steady electrical stimulation with EEG characteristics.

In the present review investigation results are presented on the elastic wave propagation in laminated and fibrous unidirectional composite materials modeled by the piecewise-homogeneous medium (the structural model). In contrast to continual theories, the model of such a type does not essentially impose additional restrictions (other than those postulated by solid deformable body mechanics) on frequencies and gradients of the spatial alteration of the wave processes investigated. However, it should be stressed that, within the framework of this approach, it is necessary to solve complicated boundary-value or initial-boundary-value problems, which considerably complicate investigations in this area. Currently, regularly laminated materials are most thoroughly investigated. A detailed, comprehensive analysis is given of bulk, surface, and normal waves. The rule of selection, choice of modes, is formulated for piecewise-inhomogeneous spectra, the structure of the pass bands zones for bulk, shear and longitudinal–transverse waves is described with its dependence of the relative thickness and mechanical properties of layers, and vibration modes on transmission zone boundaries are determined. The theory of Love- and Rayleigh-type surface waves is presented. These waves may propagate in regularly laminated composites at frequencies corresponding to the stop band zones of bulk waves. A highly significant influence of the correspondence of the set of frequency and other composition properties to the pass band or stop band zones for bulk waves in the reflecting materials is noted on the reflection character of shear and longitudinal–transverse plane waves. The existence of frequency intervals and several incidence angles in the cases of the complete internal wave reflection is shown. Penetration of surface disturbances deep into the regularly laminated materials was investigated on plane cylindrical and spherical interface surfaces of properties. The character of the displacements and the stress attenuation is essentially different for pass band and stop band zones of bulk longitudinal and transverse waves in the medium with plane boundaries. Results are also presented for thermoelastic, magnetoelastic and electroelastic (acoustoelectric) waves. The application of the superposition principle and the summation theorems for cylindrical functions for unidirectional fibrous materials with regular packing allows us to construct formal solutions both for doubly periodic media and for the separately situated row of elastic inclusions with periodic location. Solutions of such a type may be extended to smooth inclusions of noncircular cylindrical form. In all cases boundary value problems are reduced to infinite systems of algebraic equations with complex coefficients containing cylindrical functions. For the row of periodically located fibers, the informal character of solutions is shown, and infinite systems of equations are investigated. Specific quantitative results are also obtained. The diffraction of shear and longitudinal-transverse waves on solid and hollow fibers was analyzed. In the discrete frequency spectrum, the existence of resonance effects of the Wood-anomaly type is shown. For shear waves on separately located inclusion, the influence of the noncircular fiber form on the stress distribution was investigated. The prospects for development of wave theory are pointed out within the framework of the structural composite model.

In the last thirty years, the aerodynamic nonlinearities related to the slow variation of the angle of attack produced by large-scale atmospheric turbulence and their impact on the buffeting response of long-span suspension bridges have been a hot topic in wind engineering research. Self-excited forces accounting for such an effect of turbulence have been crucial in predicting the dynamic response of bridge sectional models and long-span suspension bridges subjected to multi-harmonic gusts and the turbulent wind, respectively. Despite several nonlinear aerodynamic models produced by the scientific community throughout the last years, only few studies on full suspension bridges nonlinear buffeting response in realistic turbulent flows are available. This doctoral work addresses this topic, aiming to enlarge the understanding of the effects of turbulence on the suspension bridge buffeting response. The first contribution of this work concerns nonlinear aerodynamic load modelling. Indeed, large-scale atmospheric turbulence produces large-amplitude low-frequency fluctuations of the angle of attack that can significantly change the self-excited and the external buffeting forces acting on a bridge deck. Assuming that the angle of attack varies slowly compared to the bridge motion, a time-variant linear model relying on Roger’s rational function approximation (RFA) of the force transfer function is proposed for modelling self-excited forces. In particular, an existing model is improved by a flexible fitting of the RFA directly in the multivariate space of reduced velocity and angle of attack. Another contribution of the present work is setting up an experimental procedure based on bi- or multi-harmonic forced-vibration tests to underscore the variation of magnitude and phase of self-excited forces under a time-variant angle of attack. These wind tunnel tests also allowed a sound experimental validation of the proposed model, considering two quite different bridge deck cross-sections as case studies. Aerodynamic derivatives for various angles of attack were measured to determine the model parameters. Despite its simplicity, the model yields accurate results up to relatively fast variations of the angle of attack, and it can reproduce the complicated behaviour of the self-excited forces revealed by the experiments. The model performance strongly depends on the goodness of the RFA-based fitting of aerodynamic derivatives, and the excellent results obtained were possible thanks to the high flexibility of the proposed method. Then, the nonlinear external buffeting forces are formulated to achieve a reasonable compromise between the conflicting needs of modelling both nonlinear and unsteady effects of wind velocity fluctuations. Then, the proposed 2D RFA model for self-excited forces and the nonlinear buffeting forces are incorporated into a stochastic time-variant state-space framework to assess the nonlinear buffeting response of a suspension bridge. The most important feature of this model is the modulation of the self-excited forces due to the spatio-temporal fluctuation of the angle of attack produced by low-frequency turbulence. Such an angle of attack accounts for the spatial wind correlation along the bridge girder. The model is applied to the Hardanger Bridge in Norway, considering different wind conditions. Indeed, the aerodynamic derivatives of this bridge deck cross-section present a strong dependence on the mean angle of attack. Moreover, a novel approach is suggested, diversifying the cut-off frequencies for the considered input motion components in the self-excited forces. In the wake of this, the thesis also investigates the sensitivity of the response statistics to the model cut-offs used to separate the low-frequency and the high-frequency turbulence band. The results emphasise the significant impact on the buffeting response and flutter stability of considering time-variant self-excited forces, though in specific cases the classical linear time-invariant approach is found to provide accurate predictions of the bridge vibrations.

Periodic structures provide filtering behavior for vibrations, as a result of the repetition in space of unit blocks, or unit cells. In general, they are characterized by an internal mechanical impedance mismatch, so that waves are reflected and transmitted every time a discontinuity is present. The global behavior given by waves superposition is their cancellation, only for specific frequency ranges, generally called stop-bands or band-gaps. The variation of non-dimensional parameters shows how these attenuation regions move in the frequency domain: the correspondent diagrams are the main tools for the design problem and are known as band-maps. The selection of the geometrical, physical and elastic properties of the unit cell is therefore dependent on the designer experience and nothing can be said about the optimality of the proposed solution. Numerical methods are used for the selection of the best cell geometry, in order to get optimal attenuation. Generally, this is a time consuming approach. In this paper, an new method is presented, based on how the waves are reflected and transmitted at cells interface. Both beam and rod case studies are investigated. The algorithm allows matching between band-gap central frequency and the desired value, while the designed attenuation is optimal there, under certain physical and geometrical constraints. Moreover, the design of the bandgap location has been decoupled from the design of the magnitude of attenuation. This approach is purely analytic, therefore the computational efforts required are minimum. In order to validate the analytical model, a passive periodic beam has been manufactured. Its real frequency response is therefore compared to the expected one.

Photostrictive materials are lanthanum-modified lead zirconatetitanate (Pb, La)(Zr, Ti) O3 ceramics doped with WO3, called PLZT, exhibit large photostriction under uniform illumination of high-energy light. Photostrictive materials are ferrodielectric ceramics that have a photostrictive effect. Photostriction arises from a superposition of the photovoltaic effect, i.e. the generation of large voltage from the irradiation of light, and the converse-piezoelectric effect, i.e. expansion or contraction under the voltage applied. Photostrictive materials offer the potential for actuators with many advantages over traditional transducing electromechanical actuators made of shape memory alloys and electroceramics (piezoelectric and electrostrictive). Drawback of traditional actuators is that they require hard-wired connections to transmit the control signals which introduce electrical noise into the control signals; on the other hand PLZT actuators offer non-contact actuation, remote control, and are immune from electric/magnetic disturbances. The main goal of the research work is to investigate the feasibility of utilizing photostrictive materials as an optical actuator for Micro-Electro-Mechanical-Systems (MEMS) applications. In this investigation process both experimental and computational approaches have been implemented. In the experimental part of this research, a test set-up has been designed and developed to measure the photostriction effect of a PLZT thin film on a silicon wafer as smart beams. The experimental set-up includes high pressure short arc xenon lamp with lamp housing, power supply, lamp igniter, hot mirror, band pass filters, optical chopper, and laser sensor with sensor head and controller.1 μm PLZT thin film on the silicon wafer sample has been tested as a cantilever beam with different light intensities, and focusing the light at the different locations on the PLZT cantilever beam. The experiment has been performed for continuous and pulses of lights focusing on the PLZT optical actuator. An optical chopper was used to make pulses of light on the PLZT cantilever beam. Also, a computational finite element method useful for design of systems incorporating thin film photostrictive actuators has already been developed by the authors. The element has been implemented in an in-house finite element code. This derived finite element for continuous illumination of light on the photostrictive thin film has been used to investigate the application of photostrictive actuators for the different structures and various boundary conditions of microbeams with various actuator locations and length intensities. A successful conclusion of these tasks will affirm the potential of the PLZT optical actuator to use in the MEMS devices.