Готові списки джерел за темами / Normal Lamb's waves

Добірка наукової літератури з теми "Normal Lamb's waves"

Автор: Grafiati

Опубліковано: 30 травня 2022

Оновлено: 31 травня 2022

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Статті в журналах з теми "Normal Lamb's waves"

Schiel, Klaus, and João S. Protázio. "Transient-wave solution for Lamb's problem at the free surface." Bulletin of the Seismological Society of America 79, no. 6 (December 1, 1989): 1956–71. http://dx.doi.org/10.1785/bssa0790061956.

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Abstract An exact analytic solution of Lamb's problem is presented with no approximation or restriction for material parameters or source observer geometries. The transient-wave field for a directional point force normal to the surface of an elastic half-space is given in terms of its constituent components: the body waves, the surface wave, and the near field. In contrast to other known solutions, ours enables us to characterize these wave phenomena separately. Some outstanding features should be mentioned: the singularities appearing in the Green's function solution of the problem are integrable. The transient-wave solution presented is well-suited for numerical computation.

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Xu, Heming, Steven M. Day, and Jean-Bernard H. Minster. "Two-dimensional linear and nonlinear wave propagation in a half-space." Bulletin of the Seismological Society of America 89, no. 4 (August 1, 1999): 903–17. http://dx.doi.org/10.1785/bssa0890040903.

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Abstract We examine a staggered pseudospectral method to solve a two-dimensional wave propagation problem with arbitrary nonlinear constitutive equations, and evaluate a general image method to simulate the traction-free boundary condition at the surface. This implementation employs a stress-velocity formulation and satisfies the free surface condition by explicitly setting surface shear stress to zero and making the normal stress antisymmetric about the free surface. Satisfactory agreement with analytical solutions to Lamb's problem is achieved for both vertical point force and explosion sources, and with perturbation solutions for nonlinearly elastic wave propagation within the domain of validity of such solutions. The Rayleigh wave, however, suffers much more severe numerical dispersion than do body waves. At four grids per wavelength, the relative error in the Rayleigh-wave phase velocity is 25 times greater than the corresponding error in the body-wave phase velocity. Thus for the Rayleigh wave, the pseudospectral method performs no better than a low-order finite difference method. A substantial merit of the image approach is that it does not assume any particular rheology, the method is readily applicable even when stresses are not analytically related to kinematic variables, as is the case for most nonlinear models. We use this scheme to investigate the response of a nonlinear half-space with endochronic rheology, which has been fit to quasi-static and dynamic observations. We find that harmonics of a monochromatic source are generated and evolve with epicentral range, and energy is transferred from low to higher frequencies for a broadband source. This energy redistribution characteristic of the propagation is strain-amplitude dependent, consistent with laboratory experiments. Compared with the linear response, the nonlinear response of an endochronic layer near the surface shows a deamplification effect in the intermediate-frequency band and an amplification effect in the higher-frequency band. The computational method, with modifications to accommodate realistic nonlinear soil characteristics, could be applied to estimate earthquake strong ground motions and path effects.

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Burton, D. M. F., and H. Babinsky. "Corner separation effects for normal shock wave/turbulent boundary layer interactions in rectangular channels." Journal of Fluid Mechanics 707 (August 2, 2012): 287–306. http://dx.doi.org/10.1017/jfm.2012.279.

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AbstractExperiments are conducted to examine the mechanisms behind the coupling between corner separation and separation away from the corner when holding a high-Mach-number ${M}_{\infty } = 1. 5$ normal shock in a rectangular channel. The ensuing shock wave interaction with the boundary layer on the wind tunnel floor and in the corners was studied using laser Doppler anemometry, Pitot probe traverses, pressure sensitive paint and flow visualization. The primary mechanism explaining the link between the corner separation size and the other areas of separation appears to be the generation of compression waves at the corner, which act to smear the adverse pressure gradient imposed upon other parts of the flow. Experimental results indicate that the alteration of the $\lambda $-region, which occurs in the supersonic portion of the shock wave/boundary layer interaction (SBLI), is more important than the generation of any blockage in the subsonic region downstream of the shock wave.

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Amin, Ushtar, Lucy Sullivan, Pat Trudeau, and Selim R. Benbadis. "Association Between Positive Occipital Sharp Transients of Sleep and Lambda Waves." Clinical EEG and Neuroscience 50, no. 3 (November 14, 2018): 219–21. http://dx.doi.org/10.1177/1550059418813020.

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Positive occipital sharp transients of sleep (POSTS) and lambda waves have similar morphology and location. We studied a possible association between these 2 normal EEG patterns. We reviewed a series of consecutive unselected ambulatory EEGs during a 3-month period (October 16, 2017 to January 19, 2018) and identified records with POSTS and records with lambda waves. Statistical analysis was performed using a chi-square test. A total of 140 ambulatory EEGs were reviewed. Duration of EEGs ranged from 24 to 168 hours (mean 76 hours). The population was 34% males, with ages ranging from 3 to 93 years (mean 48 years). Of the 140 records, 30 were abnormal, including 20 with epileptiform abnormalities. A chi-square test for independence (with Yates continuity correction) indicated a significant association between POSTS and lambda waves, χ2(1, n = 140) = 69.208, p < .001, φ = 0.72. In addition, 100% of records with lambda waves had POSTS, and 72% of records with POSTS had lambda waves. There is a high association between lambda waves and POSTS. This suggests a strong similarity between the 2 waveforms, and possibly a common occipital generator.

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Beamish, J. R., and K. Warner. "Ultrasonic studies of liquid helium in porous media." Canadian Journal of Physics 65, no. 11 (November 1, 1987): 1557–59. http://dx.doi.org/10.1139/p87-254.

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We have studied the propagation of 12 MHz transverse ultrasonic waves in a porous ceramic containing liquid 4He. Both the sound velocity and the attenuation clearly show the superfluid nature of helium. The helium in the pores increases the system's effective density by an amount proportional to the normal-fluid density and so decreases the sound speed. The decoupling of the superfluid fraction below the lambda transition allows us to use the shear wave essentially as a "high-frequency torsional oscillator" to determine the superfluid density and pore tortuosity. The sound attenuation in this system is due to the same mechanism as for fourth sound, namely, viscous losses due to motion of the normal-fluid component. We observed an attenuation proportional to the normal-fluid density and compare this result to predictions of the Biot theory of sound propagation in fluid-filled porous media.

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Sedaghat, A., J. A. D. Ackroyd, and N. J. Wood. "Turbulence modelling for supercritical flows including examples with passive shock control." Aeronautical Journal 103, no. 1020 (February 1999): 113–25. http://dx.doi.org/10.1017/s0001924000027810.

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Abstract The aerodynamic performance of supercritical aerofoils at transonic speeds is strongly influenced by the shock wave-boundary-layer interaction. Passive shock control is one of the techniques used for controlling the undesirable effects of strong shock wave-boundary-layer interaction leading to extensive separation. Using passive shock control, the stall margin is increased and the onset of buffeting is delayed. Passive shock control is modelled by introducing a closed plenum chamber underneath a perforated surface at the foot of the shock wave where a combination of blowing and suction is generated. As a result, a strong normal shock wave is changed into a series of weak shock waves with lambda shape. Since the Baldwin-Lomax turbulence model has been used extensively for passive shock control modelling, which exhibits poor predictability for separated flows, an attempt has been made to modify the Reynolds normal stresses for this model so as to improve the accuracy of numerical results for flows with separation. Further modification to the Baldwin-Lomax model has been employed so that the mass transpiration effects are taken into account for the passive shock control computations. In this paper, a brief description on an implicit finite-volume TVD scheme in general coordinates is given and the details of the Balwin-Lomax turbulence model and its modifications are presented. The validated numerical results for several RAE 2822 aerofoil problems plus corresponding results for the modelled methods are presented and compared with some experimental data and other numerical results.

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Nishawala, V. V., M. Ostoja-Starzewski, M. J. Leamy, and E. Porcu. "Lamb's problem on random mass density fields with fractal and Hurst effects." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 472, no. 2196 (December 2016): 20160638. http://dx.doi.org/10.1098/rspa.2016.0638.

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This paper reports on a generalization of Lamb's problem to a linear elastic, infinite half-space with random fields (RFs) of mass density, subject to a normal line load. Both, uncorrelated and correlated (with fractal and Hurst characteristics) RFs without any weak noise restrictions, are proposed. Cellular automata (CA) is used to simulate the wave propagation. CA is a local computational method which, for rectangular discretization of spatial domain, is equivalent to applying the finite difference method to the governing equations of classical elasticity. We first evaluate the response of CA to an uncorrelated mass density field, more commonly known as white-noise, of varying coarseness as compared to CA's node density. We then evaluate the response of CA to multiscale mass density RFs of Cauchy and Dagum type; these fields are unique in that they are able to model and decouple the field's fractal dimension and Hurst parameter. We focus on stochastic imperfection sensitivity; we determine to what extent the fractal or the Hurst parameter is a significant factor in altering the solution to the planar stochastic Lamb's problem by evaluating the coefficient of variation of the response when compared with the coefficient of variation of the RF.

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Couldrick, J. S., S. L. Gai, J. F. Milthorpe, and K. Shankar. "Normal shock wave/turbulent boundary-layer interaction control using ‘smart’ piezoelectric actuators." Aeronautical Journal 109, no. 1101 (November 2005): 577–83. http://dx.doi.org/10.1017/s0001924000000919.

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Abstract This paper looks at active control of the normal shock wave/turbulent boundary layer interaction (SBLI) using smart flap actuators. The actuators are manufactured by bonding piezoelectric material to an inert substrate to control the bleed/suction rate through a plenum chamber. The cavity allows rapid thickening of the boundary-layer approaching the shock, which splits into a series of weaker shocks forming a lambda shock foot, thus reducing wave drag. Active control allows optimisation of the interaction, as it would be capable of either positioning the control region around the original shock position using a series of unimorph flaps or fixing the shock position by controlling the rate of mass transfer. The level of control achieved by unimorph piezoelectric actuators is not large because of small amounts of deflection possible. It is believed that to provide optimal control a piezoelectric material, which can provide greater strain and hence higher amounts of deflection is needed. However, currently such a piezoelectric material is not commercially available.

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Ha, B., C. L. Lucas, G. W. Henry, E. G. Frantz, J. I. Ferreiro, and B. R. Wilcox. "Effects of chronically elevated pulmonary arterial pressure and flow on right ventricular afterload." American Journal of Physiology-Heart and Circulatory Physiology 267, no. 1 (July 1, 1994): H155—H165. http://dx.doi.org/10.1152/ajpheart.1994.267.1.h155.

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The effects of pulsatile hemodynamics on right ventricle-pulmonary circulation interactions were studied in control lambs and in two lamb models of altered pulmonary hemodynamics induced at infancy: elevated pulmonary arterial pressure (PAP) was created by the infusion of monocrotaline pyrrole (MCTP), and elevated pulmonary arterial blood flow was obtained by the creation of an arteriovenous fistula (Shunt). High-fidelity PAP, midvessel Doppler blood velocity (PAV), and cardiac output (CO) were measured in open-chest, anesthetized lambs. PAV waveforms were normalized to match the measured CO. Measured pressure and flow signals were separated in the time domain into forward and backward components. Pulmonary input impedance and indexes quantifying the timing of the reflected wave pulse (beginning of reflected pulse, duration of reflected pulse in systole, and duration of reflected wave in diastole) were calculated for each group. Results indicate that in control animals the reflected wave returned late in systole and extended through much of diastole, thereby increasing diastolic pressure like a counterpulsation balloon. No significant differences in the timing indexes were found between Shunt and control animals. In the MCTP group, the reflected wave returned significantly earlier than normal with the peak reflected pulse occurring before valve closure. The resulting augmentation of systolic pressure and, therefore, large pulse pressure is consistent with pressure waveforms observed in clinical pulmonary hypertension. We conclude that early wave reflection exerts a detrimental effect in pulmonary hypertension by unfavorably loading the still-ejecting right ventricle.

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Carcione, José M. "The wave equation in generalized coordinates." GEOPHYSICS 59, no. 12 (December 1994): 1911–19. http://dx.doi.org/10.1190/1.1443578.

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This work introduces a spectral collocation scheme for the viscoelastic wave equation transformed from Cartesian to generalized coordinates. Both the spatial derivatives of field variables and the metrics of the transformation are calculated by the Chebychev pseudospectral method. The technique requires a special treatment of the boundary conditions, which is based on 1-D characteristics normal to the boundaries. The numerical solution of Lamb’s problem requires two 1-D stretching transformations for each Cartesian direction. The results show excellent agreement between the elastic numerical and analytical solutions, demonstrating the effectiveness of the differential operator and boundary treatment. Another example, requiring 1-D transformations, tests the propagation of a Rayleigh wave around a corner of the numerical mesh. Two‐dimensional transformations adapt the grid to topographic features: a syncline, and an anticlinal structure formed with fine layers.

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Більше джерел

Дисертації з теми "Normal Lamb's waves"

Романюк, Маргарита Игоревна. "Теоретические основы расчета ультразвуковых трактов устройств контроля поверхности металлопроката". Doctoral thesis, Киев, 2015. https://ela.kpi.ua/handle/123456789/13840.

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Paget, Christophe. "Active Health Monitoring of Aerospace Composite Structures by Embedded Piezoceramic Transducers." Doctoral thesis, KTH, Aeronautical Engineering, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3277.

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The objectives of the thesis work were to study theinteraction between embedded piezoceramic transducers andcomposite structures as well as determine techniques tosimplify the Lamb waves analysis. Firstly, this studyconsidered the design of the embedded piezoceramic transducers.Secondly, the effect of the embedded transducer on thecomposite strength as well as the influence of the mechanicallyloaded composite on the characteristics of the embeddedtransducer were investigated. Finally, to simplify the analysisof such complex Lamb wave responses, two techniques weredeveloped. They were based on the wavelet technique and amodelling technique, respectively.

The design of the embedded piezoceramic transducers wasimproved by reducing the stress concentrations in the compositeas well as in all components constituting the piezoceramictransducer, that is, the piezoceramic element, interconnectorand conductive adhesive. The numerical analysis showed that thethickness of the interconnector had no significant influence onthe stress state of the piezoceramic transducer. It was alsofound that a compliant conductive adhesive reduced the stressconcentration located at the edge of the piezoceramic element.The structural integrity of composites embedded with theimproved piezoceramic transducer was investigated. Theexperiments, performed in tensile and compressive staticloading, indicated that the strength of the composite was notsignificantly reduced by the embedded piezoceramic transducer.Further investigations were conducted to evaluate theperformance of the improved piezoceramic transducer used as aLamb wave generator embedded in composites subjected tomechanical loading. The tests were conducted in tensile andcompressive static loading as well as fatigue loading. Thestudy showed a large working range of the embedded piezoceramictransducer. A post processing technique based on the waveletswas further assessed in the detection of damage and in thedamage size evaluation. A new wavelet basis was developedspecially for processing the Lamb wave response. This method,focused on the wavelet coefficients from the decomposition Lambwave response, showed promising results in evaluating thedamage size. The wavelets offered a sensitive tool to detectsmall damage, compared to other detection methods, improvingthe damage detection capabilities. The other technique wasdevoted to the simplification of the generated Lamb waves bythe use of multi-element transducers. The transducers weredesigned using both a normal-mode expansion and a FE-method.This technique allowed reducing the effect of a Lamb wave modetowards another. This technique was successfully implemented ina damage detection system in composites.

Keywords:Embedded piezoceramic, transducer, composite,structural integrity, health monitoring, damage detection, Lambwaves, wavelets, normal-mode expansion, FE-method

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Тези доповідей конференцій з теми "Normal Lamb's waves"

Santoni, Giola B., and Victor Giurgiutiu. "Lamb Wave Tuning Between Piezoelectric Wafer Active Sensors and Host Structure: Experiments and Modeling." In ASME 2006 International Mechanical Engineering Congress and Exposition. ASMEDC, 2006. http://dx.doi.org/10.1115/imece2006-13533.

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In structural health monitoring (SHM), a network of embedded sensors permanently bonded to the structure is used to monitor the presence and extent of damage. The sensors can actively interrogate the structure through ultrasonic waves. Among the ultrasonic waves, Lamb waves are quite convenient because they can propagate at large distances in plates and then interrogate a large area. Lamb waves in a plate can be produced with piezoelectric wafer active sensors (PWAS) that are small, inexpensive, unobtrusive transducers. PWAS can be surface-mounted on an existing structured or placed inside composite materials. PWAS sensors use the piezoelectric principle. An alternating voltage applied to the PWAS terminals produces an oscillatory expansion and contraction of the PWAS. An oscillatory expansion and contraction of the PWAS produces an alternating voltage at the PWAS terminals. PWAS are bonded to the structure through an adhesive layer; the coupling with the investigated structure is higher then conventional transducers. If the PWAS bonded to the structure is excited, it couples its in-plane motion with the Lamb wave particle motion on the material surfaces. In previous studies, the Lamb wave mode tuning between PWAS and isotropic plates has been observed experimentally and theoretically. Recently experiments have been performed to verify the presence of tuning between bonded PWAS and composite plates. In the present paper, it will be discussed a method, normal mode expansion (NME), for predicting the tuning frequencies of the PWAS-plate structure. This method can be used for both isotropic and non-isotropic material. Experimental values for the tuning frequencies in isotropic plates are compared with the theoretically data obtain with integral transform solution and NME.

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Lin, Bin, and Victor Giurgiutiu. "PVDF and PZT Piezoelectric Wafer Active Sensors for Structural Health Monitoring." In ASME 2005 International Mechanical Engineering Congress and Exposition. ASMEDC, 2005. http://dx.doi.org/10.1115/imece2005-80400.

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Piezoelectric wafer active sensors (PWAS) used in structural health monitoring (SHM) applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive and inexpensive. PWAS achieve direct transduction between electric and elastic wave energies. PWAS are essential elements in the Lamb-wave SHM with pitch-catch, pulse-echo, phased array system and electromechanical impedance methods. PWAS are charge mode sensors and they can be used as both transmitters and receivers. A model of PWAS is shown in this paper. In vibration, impact detections applications, the PWAS response is strong due to the large dynamic change of strain. In pitch-catch, pulse-echo and phased array applications, PWAS are used to generate and receive Lamb waves and the PWAS response is small. A charge amplifier for PWAS applications is introduced in this paper. PWAS are normally made of piezoceramic Lead Zirconate Titanate (PZT). The structural integrity tests require attachment of PWAS to the material surface and there are critical applications where the rigid piezoceramic wafers cannot conform to curved surfaces. As alternative one can use flexible piezopolymer such as polyvinylidene fluoride (PVDF); such PVDF-PWAS have been studied in this paper. PVDF-PWAS were mounted on a cantilever beam for the free vibration test and on a long rod for the longitudinal impact test. The experimental results of the PZT-PWAS and PVDF-PWAS have been compared with the conventional strain gauge. The theoretical and experimental results in this study gave the basic demonstration of the piezoelectricity of PZT-PWAS and PVDF-PWAS.

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Bonello, Bernard, Rémi Marchal, Rayisa Moiseyenko, Yan Pennec, Bahram Djafari-Rouhani, Jinfeng Zhao, and Olga Boyko. "Silicon Pillars as Resonators in an Acoustical Metamaterial." In ASME 2014 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/imece2014-37914.

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We have investigated the propagation of Lamb waves in structures made of either an isolated resonant pillar or a set of pillars arranged in a line on a thin plate. The resonators as well as the plate are made of silicon. FEM computations show that two bending modes and one compressional mode are unambiguously identified in the frequency range of interest (0–10 MHz). We used a laser ultrasonic technique to map both the amplitude and the phase of the normal displacements on top of the pillars and at the surface of the sample. When the frequency is tuned to a resonant mode, either compressional or bending, the pillars vibrate 180° out-of-phase with respect to the Lamb waves, resulting in a negative modulus or negative mass density respectively.

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Baillard, André, Jean-Marc Conoir, Dominique Décultot, Gérard Maze, Aleksander Klauson, and Jaan Metsaveer. "A0 Wave in a Submerged Stiffened Cylindrical Shell." In ASME 1999 Design Engineering Technical Conferences. American Society of Mechanical Engineers, 1999. http://dx.doi.org/10.1115/detc99/vib-8092.

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Abstract The backscattering of a normal incident plane acoustic wave by a stiffened cylindrical shell is considered. The studied cylindrical shell is made of stainless steel and its internal to external radius ratio is b/a = 0.98. The shell submerged in water is strengthened by an internal plate of the same material and thickness, which is diametrically soldered. As for the unstiffened shell, A wave (Scholte-Stoneley wave) and S0 wave (first symmetrical Lamb wave) are detected in the frequency range investigated (50–200 ka where k is the wave number in water). On the contrary, the presence of the plate into the tube allows the detection of A0 wave (flexural Lamb wave) whereas it is not observed for the unstiffened shell, owing to the attenuation of the wave during the propagation around the tube. In particularl, A0 wave could be reflected on the junction with enough energy to re-emit in water. Furthermore, wave type conversions are allowed by way of the junctions: S0–A0 wave conversions are observed.

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Schreiber, H. A., and H. Starken. "An Investigation of a Strong Shock-Wave Turbulent Boundary Layer Interaction in a Supersonic Compressor Cascade." In ASME 1991 International Gas Turbine and Aeroengine Congress and Exposition. American Society of Mechanical Engineers, 1991. http://dx.doi.org/10.1115/91-gt-092.

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Experiments have been performed in a Supersonic cascade facility to elucidate the fluid dynamic phenomena and loss mechanism of a strong shock-wave turbulent boundary layer interaction in a compressor cascade. The cascade geometry is typical for a transonic fan tip section that operates with a relative inlet Mach number of 1.5, a flow turning of about 3 degrees, and a static pressure ratio of 2.15. The strong oblique and partly normal blade passage shock-wave with a pre-shock Mach number level of 1.42 to 1.52 induces a turbulent boundary layer separation on the blade suction surface. Freestream Reynolds number based on chord length was about 2.7×106. Cascade overall performance, blade surface pressure distributions, Schlieren photographs, and surface visualisations are presented. Detailed Mach number and flow direction profiles of the interaction region (lambda shock) and the corresponding boundary layer have been determined using a Laser-2-Focus anemometer. The obtained results indicated that the axial blade passage stream sheet contraction (axial velocity density ratio) has a significant influence on the mechanism of strong interaction and the resulting total pressure losses.

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Yoo, Byungseok, Darryll J. Pines, and Ashish S. Purekar. "Multi-Location Actuators and Piezoceramic Based 2-D Spiral Array for Structural Health Monitoring: Thin Isotropic Panels." In ASME 2010 Conference on Smart Materials, Adaptive Structures and Intelligent Systems. ASMEDC, 2010. http://dx.doi.org/10.1115/smasis2010-3723.

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In this paper, a new and robust 2-D phased array technique with multiple distributed actuators is studied for damage detection application based on Guided Lamb Wave (GLW) interrogation in a thin isotropic panel. A 2-D phased array technique using a single actuator located near the center of the 2-D phased array is unable to detect a linear crack oriented normal to the wavefront of the GLW excited from the actuator. To overcome this limitation, the 2-D phased array is coupled with multiple actuators in this study where the actuators are positioned at various locations on a test panel while the 2-D phased array is mounted at the center of the panel. A piezoceramic based 2-D phased array with a spiral configuration is used as a sensor array and the corresponding 2-D phased array signal processing is used to produce array responses and detect various damages. An innovative GLW propagation and reflection analysis technique is implemented to evaluate the damage locations in the panel. Experimental results demonstrate that the 2-D phased array damage detection technique using multiple distributed actuators can provide more robust damage detection scheme in thin isotropic panels than a technique with a single actuator element.

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