Showing papers on "Lamb waves published in 1982"

Propagation Speeds and Acoustic Damping of Waves in Magnetic Flux Tubes

Abstract: Propagation speeds are derived for the wave modes of a thin magnetic tube in an otherwise homogeneous magnetized or unmagnetized fluid. These results generalize results obtained by previous authors. There are three types of wave, a (torsional) Alfven wave and two waves which are specific for the thin tube. These are named the longitudinal and transversal tube waves, according to their polarization properties. They can be camped by radiating an MHD or acoustic wave into the surroundings of the tube. Conditions for occurrence of this acoustic damping, and the damping rates, are derived. The behavior of the waves in the solar convection zone and corona is discussed.

Experimental investigations of the propagation of surface waves along a plasma column

Abstract: Plasma surface waves were discovered in 1958. The experiments done since then on these waves are critically reviewed and analysed in terms of appropriate models and corresponding dispersion relations. Discrepancies and unresolved issues are identified and discussed. Wave damping experiments and analysis, and results of more recent works on nonlinear aspects of these waves, are also reported.

Effect of surface topography on the diffraction of P, SV, and Rayleigh waves

Abstract: A generalized inverse method for approximate boundary conditions is adapted for boundary value problems in elastic wave propagation. The diffraction of P, SV , and Rayleigh waves at the vicinity of a semi-elliptical canyon is considered. The effects of mode conversion from compressional to shear waves, or vice versa, are examined in detail. The maximum amplification for each plane wave is also studied.

Characteristics of a PVDF Membrane Hydrophone for Use in the Range 1-100 MHz

Abstract: Abstrocr-The experimental determination of the frequency response, directionality, and electrical characteristics of membrane hydrophones are reported. These characteristics are interpreted in terms of theoretical models, based mainly on published values for the properties of polyvinylidene fluoride (pvdf). The frequency response was determined using both linear and nonlinear acoustic fields. The directionality is affected by the angular variation of piezoelectric sensitivity and also, for large angles of incidence, by Lamb wave propagation in the membrane. The hydrophones are shown to be useful experimental devices both for studying acoustic waves in the frequency range 1-100 MHz and for investigating the properties of pvdf. They are well-suited to the study of nonlinear ultrasonic fields, and those used in medicine and nondestructive testing.

Characteristics of the ULF waves associated with upstream ion beams

Abstract: A new class of upstream wave is reported with relatively high frequencies of about 1 Hz and small amplitudes compared to the more common larger amplitude, low-frequency (0.03 Hz) upstream wave. The waves were first noted in association with beams of ions reflected back upstream at the bowshock, and although beam presence appears to be a necessary condition for the observation of the waves, it is not a sufficient condition for the existence of the waves. Magnetometer measurements are used to determine intrinsic properties of the waves, and simultaneous two point measurements are used to calculate and eliminate Doppler shifting effects. Results indicate that the waves are right-hand elliptically polarized whistler mode waves with plasma rest frame frequencies of about 20-100 times the proton gyrofrequency and wavelengths of about 100 km.

Measurement of velocities in solitary waves

Abstract: The water surface profiles and corresponding water particle velocities of several solitary waves have been obtained. The measurements of the horizontal and vertical velocity components are conducted using a two-dimensional laser-Doppler velocimeter (LDV). Results are presented for three different wave height-to-depth ratios: ϵ = 0.11, 0.19 and 0.29. The experimental results are compared with existing theories which follow different orders of approximation, and the theories are found to agree well with the experiments. It has been convincingly demonstrated that the LDV measurement technique offers an unmatched advantage to determine the water particle velocities under water waves, especially in the region above the still water level.

Direct imaging of travelling Rayleigh waves by stroboscopic X-ray topography

Abstract: We report here the first successful experiments which exploit the time structure and single bunch mode of operation of the Daresbury Synchrotron Radiation Source (SRS). Rayleigh waves travelling on the surface of a piezoelectric crystal have been imaged by stroboscopic X-ray topography, in which the generation of the waves is synchronized with X rays emitted by the orbiting electrons in the storage ring. Interactions between the Rayleigh waves and microscopic crystalline defects have been observed and provide new insights into the factors affecting surface acoustic wave (SAW) device operation. It has been demonstrated that this novel technique has considerable potential for the study of periodic phenomena in crystals.

Strong interference effects in surface Brillouin scattering from a supported transparent film

Abstract: A measurement of surface Brillouin scattering from a 2250-\AA{} film of silica grown on (001) silicon is presented. The spectrum shows many structures related to Rayleigh, Sezawa, and Lamb waves. We calculate the Brillouin cross section taking the ripple and the elasto-optic coupling mechanisms into account in the two media. Both the mechanisms are found to be important and there is evidence of strong interference effects between the ripple and the elasto-optic contributions to the scattering amplitude.

Coupled amplitude theory of nonlinear surface acoustic waves

Abstract: The nonlinear propagation characteristics of surface acoustic waves on an isotropic elastic solid have been studied in this paper. The solution of the harmonic boundary value problem for Rayleigh waves is obtained as a generalized Fourier series whose coefficients are proportional to the slowly varying amplitudes of the various harmonics. The infinite set of coupled equations for the amplitudes when solved exhibit an oscillatory slow variation signifying a continuous transfer of energy back and forth among the various harmonics. A conservation relation is derived among all the harmonic amplitudes.

Acoustic dipole shear wave logging device

Abstract: The preferred embodiment of this invention includes a logging sonde, an elongated pair of oppositely polarized piezoelectric plates connected to each other by their flat surfaces, and an electrical pulse applying means for applying electrical pulses across the pair of plates so as to bend and vibrate the plates in a direction perpendicular to the length of the plates. Vibration of the plates in a fluid contained in a well creates in the fluid a positive compressional wave in one direction and simultaneously a negative compressional wave in the opposite direction. The two compressional waves will interfere to produce a dipole shear wave in the earth surrounding the well. The dipole shear wave arrival is detected at two locations in the fluid spaced longitudinally along the well from each other and from the plate. The ratio of the time interval between the detections of the dipole shear wave arrival at the two locations to the distance between the two locations yields the shear wave velocity of the earth around the well. The length of the plates may be selected to generate dipole shear waves with frequencies in a preferred range so as to improve shear wave signal to compressional wave noise ratio.

Propagation of elastic surface waves along a cylindrical cavity and their excitation by a point force

Abstract: The existence of surface wave modes, propagating along an infinite cylindrical cavity in an elastic medium, is established for every integer m, where m is the azimuthal mode number. These waves are analogous to the Rayleigh wave on a half‐space, being confined to the immediate vicinity of the cavity. The modes exhibit dispersion and have a cutoff frequency which increases with m, except for the flexural (m = 1) mode which exists at all frequencies. At cutoff the phase velocity is equal to that of the shear waves and decreases, with increasing frequency, to that of the Rayleigh wave. We present results for the group velocities and displacement and stress fields of the modes and also exhibit the effect of various point forces acting near the cavity. In the vicinity of the cavity, not too near the point force, the surface wave contribution dominates the total displacement field.

Non-destructive, non-contact ultrasonic material

Abstract: The invention relates to a method for non-contact, non-destructive testing of a test body of ferromagnetic and/or electrically-conductive material with ultrasound waves, comprising the steps of: producing in a near-surface region of the test body a low-frequency alternating magnetic bias field having flux lines generally parallel to a surface of the test body; producing high frequency alternating magnetic excitation fields in said near-surface region generally parallel to said surface during a time interval when the bias field is at a quasi-static maximum, adjacent excitation fields having opposing polarity and having flux lines lying in mutually parallel directions, whereby ultrasound waves are generated in the test body; and detecting high frequency alternating magnetic fields in said near-surface region during the same time interval when the bias field is at a quasi-static maximum and producing a signal therefrom representative of said ultrasound waves. In this method, the flux lines of said excitation fields lie generally parallel to the flux lines of said bias field, such that dynamic forces are electrodynamically generated in the test body in a direction normal to the surface of the test body, which launch longitudinal waves, Rayleigh waves and Lamb waves, and dynamic forces are magnetostrictively generated in the test body in a direction parallel to said surface, which launch transversal waves, Rayleigh waves and Lamb waves.

Ultrasonic input-output for transmitting and receiving longitudinal transducers coupled to same face of isotropic elastic plate

Abstract: The quantitative understanding of ultrasonic nondestructive evaluation parameters such as the stress wave factor were studied. Ultrasonic input/output characteristics for an isotropic elastic plate with transmitting and receiving longitudinal transducers coupled to the same face were analyzed. The asymptotic normal stress is calculated for an isotropic elastic half space subjected to a uniform harmonic normal stress applied to a circular region at the surface. The radiated stress waves are traced within the plate by considering wave reflections at the top and bottom faces. The output voltage amplitude of the receiving transducer is estimated by considering only longitudinal waves. Agreement is found between the output voltage wave packet amplitudes and times of arrival due to multiple reflections of the longitudinal waves.

Energy flux of elastic waves

Abstract: The energy fluxes of complex fields in inhomogeneous media are considered.

The full acoustic wave train in a laboratory model of a borehole

Abstract: We studied the characteristics of acoustic wave propagation in a fluid-filled borehole using as a laboratory model a concrete cylinder 2 ft high and 2 ft in diameter with a 1/4-inch diameter borehole along its axis. The model represents sonic logging in the field reduced by a factor of 40. We recorded the full wave train consisting of a refracted compressional P wave, a refracted shear S wave, and guided waves including a number of normal modes and a Stoneley wave. Exploiting the dispersive properties of a modal wave and the source-receiver frequency characteristics, we were able to isolate the S–wave, which contains much valuable information about the formation rock, but which has not been widely used since it is difficult to extract from the full wave train. The observed Stoneley wave had a very high amplitude at low frequency and showed little dispersion. Stoneley-wave velocity is closely related to S–wave velocity and formation density, and can be measured very accurately because the Stoneley wave gen...

Acoustic wave scattering from a fluid/solid periodic rough surface

Abstract: A rigorous theory based on the extended boundary condition method is proposed to solve the problem of elastic wave scattering from a periodic fluid/solid interface. The diffraction efficiencies of the reflected compressional wave in the fluid and the transmitted shear and compressional waves are calculated. The energy conservation criterion is used to check the accuracy of the numerical results. The effect of loss (viscoelasticity) in the solid is also included. The wave diffraction from a water and acrylic rough surface is measured. Good agreement between the theory and the experiment is obtained.

Nonlinear waves in superposed fluids

Abstract: A weakly nonlinear theory of wave propagation in superposed fluids in the presence of magnetic fields is presented in this paper. We derive the equations governing the evolution of the amplitude of the progressive as well as the standing waves. It is demonstrated that the waves can be unstable against modulation in the presence of the magnetic fields. We also obtain the nonlinear cut off wave number which separates the region of stability from instability.

Resolution transducers, systems and methods for the transmission and/or reception of waves propagated by vibration

Abstract: Problems of resolution and focussing arise in the near field in systems employing radiation, including ultrasonic systems. These problems can be alleviated by suppressing the plane waves in the combination of plane waves and edge waves usually employed. Several ultrasonic transducers with this property are described and in one a piezoelectric disc with surface electrodes is non-linearly polarized to transmit edge waves without plane waves.

Physical Characteristics of Ultrasound

Abstract: The information provided in this chapter is intended to develop an understanding of the fundamental principles of ultrasound—its basic properties and interactions—so that medical and biological applications may be understood more fully.