Showing papers on "Longitudinal wave published in 1993"

Importance of anelasticity in the interpretation of seismic tomography

Abstract: Temperature dependence of seismic wave velocities comes both from anharmonicity and anelasticity. The contribution from anelasticity is shown to be important in the Earth's mantle particularly for shear waves. In the low Q (Qµ∼100) regions in the upper mantle, the correction due to anelasticity will roughly double the temperature derivatives due to anharmonicity alone. The correction for anelasticity will also be important in the deep mantle where Q is larger, if temperature derivatives due to anharmonicity will decrease significantly with pressure. These results imply that the temperature anomalies associated with low velocity anomalies in the mantle will be significantly smaller than previously considered on the basis of anharmonic effect alone and that the amplitude of velocity anomalies will be significantly larger for shear waves than for compressional waves.

Measurements of the primary instabilities of film flows

Abstract: We present novel measurements of the primary instabilities of thin liquid films flowing down an incline. A fluorescence imaging method allows accurate measurements of film thickness h(x, y, t) in real time with a sensitivity of several microns, and laser beam deflection yields local measurements with a sensitivity of less than one micron. We locate the instability with good accuracy despite the fact that it occurs (asymptotically) at zero wavenumber, and determine the critical Reynolds number R, for the onset of waves as a function of angle ,8. The measurements of R,(/3) are found to be in good agreement with calculations, as are the growth rates and wave velocities. We show experimentally that the initial instability is convective and that the waves are noisesustained. This means that the waveform and its amplitude are strongly affected by external noise at the source. We investigate the role of noise by varying the level of periodic external forcing. The nonlinear evolution of the waves depends strongly on the initial wavenumber (or the frequency f). A new phase boundary e(R) is measured, which separates the regimes of saturated finite amplitude waves (at high f) from multipeaked solitary waves (at low f). This boundary probably corresponds approximately to the sign reversal of the third Landau coefficient in weakly nonlinear theory. Finally, we show that periodic waves are unstable over a wide frequency band with respect to a convective subharmonic instability. This instability leads to disordered two-dimensional waves.

Low‐frequency dispersion and attenuation in partially saturated rocks

Abstract: A theory is developed for the attenuation and dispersion of compressional waves in inhomogeneous fluid‐saturated materials. These effects are caused by material inhomogeneity on length scales of the order of centimeters and may be most significant at seismic wave frequencies, i.e., on the order of 100 Hz. The micromechanism involves diffusion of pore fluid between different regions, and is most effective in a partially saturated medium in which liquid can diffuse into regions occupied by gas. The local fluid flow effects can be replaced on the macroscopic scale by an effective viscoelastic medium, and the form of the viscoelastic creep function is illustrated for a compressional wave propagating normal to a layered medium. The wave speeds in the low‐ and high‐frequency limits are associated with conditions of uniform pressure and of uniform ‘‘no‐flow,’’ respectively. These correspond to the isothermal and isentropic wave speeds in a disordered thermoelastic medium.

Theory of magnetostatic waves

Abstract: Magnetic materials can support propagating waves of magnetization. Since these are oscillations in the magnetostatic properties of the material, they are called magnetostatic waves (sometimes "magnons" or "magnetic polarons"). Under the proper circumstances, these waves can exhibit, for example, either dispersive or nondispersive isotropic or anisotropic propagation, nonreciprocity, frequency-selective nonlinearities, soliton propagation and chaotic behaviour. This variety of behaviour has led to a number of proposed applications in microwave and optical signal processing. The book begins by discussing the basic physics of magnetism in magnetic insulators and the propagation of electromagnetic waves in anisotropic dispersive media. It then treats magnetostatic modes, describing how the modes are excited, how they propagate, and how they interact with light. There are problems at the end of each chapter; many of these serve to expand or explain the material in the text.

Nonlinear evolution of waves on a vertically falling film

Abstract: Wave formation on a falling film is an intriguing hydrodynamic phenomenon involving transitions among a rich variety of spatial and temporal structures. Immediately beyond an inception region, short, near-sinusoidal capillary waves are observed. Further downstream, long, near-solitary waves with large tear-drop humps preceded by short, front-running capillary waves appear. Both kinds of waves evolve slowly downstream such that over about ten wavelengths, they resemble stationary waves which propagate at constant speeds and shapes. We exploit this quasi-steady property here to study wave evolution and selection on a vertically falling film. All finite-amplitude stationary waves with the same average thickness as the Nusselt flat film are constructed numerically from a boundary-layer approximation of the equations of motion. As is consistent with earlier near-critical analyses, two travelling wave families are found, each parameterized by the wavelength or the speed. One family γ1 travels slower than infinitesimally small waves of the same wavelength while the other family γ2 and its hybrids travel faster. Stability analyses of these waves involving three-dimensional disturbances of arbitrary wavelength indicate that there exists a unique nearly sinusoidal wave on the slow family γ1 with wavenumber αs (or α2) that has the lowest growth rate. This wave is slightly shorter than the fastest growing linear mode with wavenumber αm and approaches the wave on γ1 with the highest flow rate at low Reynolds numbers. On the fast γ2 family, however, multiple bands of near-solitary waves bounded below by αf are found to be stable to two-dimensional disturbances. This multiplicity of stable bands can be interpreted as a result of favourable interaction among solitary-wave-like coherent structures to form a periodic train. (All waves are unstable to three-dimensional disturbances with small growth rates.) The suggested selection mechanism is consistent with literature data and our numerical experiments that indicate waves slow down immediately beyond inception as they approach the short capillary wave with wavenumber α2 of the slow γ1 family. They then approach the long stable waves on the γ2 family further downstream and hence accelerate and develop into the unique solitary wave shapes, before they succumb to the slowly evolving transverse disturbances.

Mechanics of continua and wave dynamics

Abstract: I Theory of Elasticity.- 1. The Main Types of Strain in Elastic Solids.- 1.1 Equations of Linear Elasticity Theory.- 1.1.1 Hooke's Law.- 1.1.2 Differential Form of Hooke's Law. Principle of Superposition.- 1.2 Homogeneous Strains.- 1.2.1 An Elastic Body Under the Action of Hydrostatic Pressure.- 1.2.2 Longitudinal Strain with Lateral Displacements Forbidden.- 1.2.3 Pure Shear.- 1.3 Heterogeneous Strains.- 1.3.1 Torsion of a Rod.- 1.3.2 Bending of a Beam.- 1.3.3 Shape of a Beam Under Load.- 1.4 Exercises.- 2. Waves in Rods, Vibrations of Rods.- 2.1 Longitudinal Waves.- 2.1.1 Wave Equation.- 2.1.2 Harmonic Waves.- 2.2 Reflection of Longitudinal Waves.- 2.2.1 Boundary Conditions.- 2.2.2 Wave Reflection.- 2.3 Longitudinal Oscillations of Rods.- 2.4 Torsional Waves in a Rod. Torsional Vibrations.- 2.5 Bending Waves in Rods.- 2.5.1 The Equation for Bending Waves.- 2.5.2 Boundary Conditions. Harmonic Waves.- 2.5.3 Reflection of Waves. Bending Vibrations.- 2.6 Wave Dispersion and Group Velocity.- 2.6.1 Propagation of Nonharmonic Waves.- 2.6.2 Propagation of Narrow-Band Disturbances.- 2.7 Exercises.- 3. General Theory of Stress and Strain.- 3.1 Description of the State of a Deformed Solid.- 3.1.1 Stress Tensor.- 3.1.2 The Strain Tensor.- 3.1.3 The Physical Meaning of the Strain Tensor's Components.- 3.2 Equations of Motion for a Continuous Medium.- 3.2.1 Derivation of the Equation of Motion.- 3.2.2 Strain-Stress Relation. Elasticity Tensor.- 3.3 The Energy of a Deformed Body.- 3.3.1 The Energy Density.- 3.3.2 The Number of Independent Components of the Elasticity Tensor.- 3.4 The Elastic Behaviour of Isotropic Bodies.- 3.4.1 The Generalized Hooke's Law for an Isotropic Body.- 3.4.2 The Relationship Between Lame's Constants and E and v.- 3.4.3 The Equations of Motion for an Isotropic Medium.- 3.5 Exercises.- 4. Elastic Waves in Solids.- 4.1 Free Waves in a Homogeneous Isotropic Medium.- 4.1.1 Longitudinal and Transverse Waves.- 4.1.2 Boundary Conditions for Elastic Waves.- 4.2 Wave Reflection at a Stress-Free Boundary.- 4.2.1 Boundary Conditions.- 4.2.2 Reflection of a Horizontally Polarized Wave.- 4.2.3 The Reflection of Vertically Polarized Waves.- 4.2.4 Particular Cases of Reflection.- 4.2.5 Inhomogeneous Waves.- 4.3 Surface Waves.- 4.3.1 The Rayleigh Wave.- 4.3.2 The Surface Love Wave.- 4.3.3 Some Features of Love's Waves.- 4.4 Exercises.- 5. Waves in Plates.- 5.1 Classification of Waves.- 5.1.1 Dispersion Relations.- 5.1.2 Symmetric and Asymmetric Modes.- 5.1.3 Cut-Off Frequencies of the Modes.- 5.1.4 Some Special Cases.- 5.2 Normal Modes of the Lowest Order.- 5.2.1 Quasi-Rayleigh Waves at the Plate's Boundaries.- 5.2.2 The Young and Bending Waves.- 5.3 Equations Describing the Bending of a Thin Plate.- 5.3.1 Thin Plate Approximation.- 5.3.2 Sophie Germain Equation.- 5.3.3 Bending Waves in a Thin Plate.- 5.4 Exercises.- II Fluid Mechanics.- 6. Basic Laws of Ideal Fluid Dynamics.- 6.1 Kinematics of Fluids.- 6.1.1 Eulerian and Lagrangian Representations of Fluid Motion.- 6.1.2 Transition from One Representation to Another.- 6.1.3 Convected and Local Time Derivatives.- 6.2 System of Equations of Hydrodynamics.- 6.2.1 Equation of Continuity.- 6.2.2 The Euler Equation.- 6.2.3 Completeness of the System of Equations.- 6.3 The Statics of Fluids.- 6.3.1 Basic Equations.- 6.3.2 Hydrostatic Equilibrium. Vaisala Frequency.- 6.4 Bernoulli's Theorem and the Energy Conservation Law.- 6.4.1 Bernoulli's Theorem.- 6.4.2 Some Applications of Bernoulli's Theorem.- 6.4.3 The Bernoulli Theorem as a Consequence of the Energy-Conservation Law.- 6.4.4 Energy Conservation Law in the General Case of Unsteady Flow.- 6.5 Conservation of Momentum.- 6.5.1 The Specific Momentum Flux Tensor.- 6.5.2 Euler's Theorem.- 6.5.3 Some Applications of Euler's Theorem.- 6.6 Vortex Flows of Ideal Fluids.- 6.6.1 The Circulation of Velocity.- 6.6.2 Kelvin's Circulation Theorem.- 6.6.3 Helmholtz Theorems.- 6.7 Exercises.- 7. Potential Flow.- 7.1 Equations for a Potential Flow.- 7.1.1 Velocity Potential.- 7.1.2 Two-Dimensional Flow. Stream Function.- 7.2 Applications of Analytical Functions to Problems of Hydrodynamics.- 7.2.1 The Complex Flow Potential.- 7.2.2 Some Examples of Two-Dimensional Flows.- 7.2.3 Conformal Mapping.- 7.3 Steady Flow Around a Cylinder.- 7.3.1 Application of Conformal Mapping.- 7.3.2 The Pressure Coefficient.- 7.3.3 The Paradox of d'Alembert and Euler.- 7.3.4 The Flow Around a Cylinder with Circulation.- 7.4 Irrotational Flow Around a Sphere.- 7.4.1 The Flow Potential and the Particle Velocity.- 7.4.2 The Induced Mass.- 7.5 Exercises.- 8. Flows of Viscous Fluids.- 8.1 Equations of Flow of Viscous Fluid.- 8.1.1 Newtonian Viscosity and Viscous Stresses.- 8.1.2 The Navier-Stokes Equation.- 8.1.3 The Viscous Force.- 8.2 Some Examples of Viscous Fluid Flow.- 8.2.1 Couette Flow.- 8.2.2 Plane Poiseuille Flow.- 8.2.3 Poiseuille Flow in a Cylindrical Pipe.- 8.2.4 Viscous Fluid Flow Around a Sphere.- 8.2.5 Stokes' Formula for Drag.- 8.3 Boundary Layer.- 8.3.1 Viscous Waves.- 8.3.2 The Boundary Layer. Qualitative Considerations.- 8.3.3 Prandl's Equation for a Boundary Layer.- 8.3.4 Approximate Theory of a Boundary Layer in a Simple Case.- 8.4 Exercises.- 9. Elements of the Theory of Turbulence.- 9.1 Qualitative Considerations. Hydrodynamic Similarity.- 9.1.1 Transition from a Laminar to Turbulent Flow.- 9.1.2 Similar Flows.- 9.1.3 Dimensional Analysis and Similarity Principle.- 9.1.4 Flow Around a Cylinder at Different Re.- 9.2 Statistical Description of Turbulent Flows.- 9.2.1 Reynolds' Equation for Mean Flow.- 9.2.2 Turbulent Viscosity.- 9.2.3 Turbulent Boundary Layer.- 9.3 Locally Isotropic Turbulence.- 9.3.1 Properties of Developed Turbulence.- 9.3.2 Statistical Properties of Locally Isotropic Turbulence.- 9.3.3 Kolmogorov's Similarity Hypothesis.- 9.4 Exercises.- 10. Surface and Internal Waves in Fluids.- 10.1 Linear Equations for Waves in Stratified Fluids.- 10.1.1 Linearization of the Hydrodynamic Equations.- 10.1.2 Linear Boundary Conditions.- 10.1.3 Equations for an Incompressible Fluid.- 10.2 Surface Gravity Waves.- 10.2.1 Basic Equations.- 10.2.2 Harmonic Waves.- 10.2.3 Shallow- and Deep-Water Approximations.- 10.2.4 Wave Energy.- 10.3 Capillary Waves.- 10.3.1 "Pure" Capillary Waves.- 10.3.2 Gravity-Capillary Surface Waves.- 10.4 Internal Gravity Waves.- 10.4.1 Introductory Remarks.- 10.4.2 Basic Equation for Internal Waves. Boussinesq Approximation.- 10.4.3 Waves in an Unlimited Medium.- 10.5 Guided Propagation of Internal Waves.- 10.5.1 Qualitative Analysis of Guided Propagation.- 10.5.2 Simple Model of an Oceanic Waveguide.- 10.5.3 Surface Mode. "Rigid Cover" Condition.- 10.5.4 Internal Modes.- 10.6 Exercises.- 11. Waves in Rotating Fluids.- 11.1 Inertial (Gyroscopic) Waves.- 11.1.1 The Equation for Waves in a Homogeneous Rotating Fluid.- 11.1.2 Plane Harmonic Inertial Waves.- 11.1.3 Waves in a Fluid Layer. Application to Geophysics.- 11.2 Gyroscopic-Gravity Waves.- 11.2.1 General Equations. The Simplest Model of a Medium.- 11.2.2 Classification of Wave Modes.- 11.2.3 Gyroscopic-Gravity Waves in the Ocean.- 11.3 The Rossby Waves.- 11.3.1 The Tangent of ?-Plane Approximation.- 11.3.2 The Barotropic Rossby Waves.- 11.3.3 Joint Discussion of Stratification and the ?-Effect.- 11.3.4 The Rossby Waves in the Ocean.- 11.4 Exercises.- 12. Sound Waves.- 12.1 Plane Waves in Static Fluids.- 12.1.1 The System of Linear Acoustic Equations.- 12.1.2 Plane Waves.- 12.1.3 Generation of Plane Waves. Inhomogeneous Waves.- 12.1.4 Sound Energy.- 12.2 Sound Propagation in Inhomogeneous Media.- 12.2.1 Plane Wave Reflection at the Interface of Two Homogeneous Media.- 12.2.2 Some Special Cases. Complete Transparency and Total Reflection.- 12.2.3 Energy and Symmetry Considerations.- 12.2.4 A Slowly-Varying Medium. Geometrical-Acoustics Approximation.- 12.2.5 Acoustics Equations for Moving Media.- 12.2.6 Guided Propagation of Sound.- 12.3 Spherical Waves.- 12.3.1 Spherically-Symmetric Solution of the Wave Equation.- 12.3.2 Volume Velocity or the Strength of the Source. Reaction of the Medium.- 12.3.3 Acoustic Dipole.- 12.4 Exercises.- 13. Magnetohydrodynamics.- 13.1 Basic Concepts of Magnetohydrodynamics.- 13.1.1 Fundamental Equations.- 13.1.2 The Magnetic Pressure. Freezing of the Magnetic Field in a Fluid.- 13.1.3 The Poiseuille (Hartmann) Flow.- 13.2 Magnetohydrodynamic Waves.- 13.2.1 Alfven Waves.- 13.2.2 Magnetoacoustic Waves.- 13.2.3 Fast and Slow Magnetoacoustical Waves.- 13.3 Exercises.- 14. Nonlinear Effects in Wave Propagation.- 14.1 One-Dimensional Nonlinear Waves.- 14.1.1 The Nonlinearity Parameter.- 14.1.2 Model Equation. Generation of Second Harmonics.- 14.1.3 The Riemann Solution. Shock Waves.- 14.1.4 Dispersive Media. Solitons.- 14.2 Resonance Wave Interaction.- 14.2.1 Conditions of Synchronism.- 14.2.2 The Method of Slowly-Varying Amplitudes.- 14.2.3 Multiwave Interaction.- 14.2.4 Nonlinear Dispersion.- 14.3 Exercises.- Appendix: Tensors.- Bibliographical Sketch.

A new interpretation of the redshift observed in optically thin transition region lines

Abstract: We propose that the pervasive redshift observed in transition region spectral lines is caused by downward propagating acoustic waves. The waves are assumed to be generated in the corona as a result of nanoflares or some other form of episodic heating. The dynamic response of a coronal loop to energy released as heat near the loop apex is studied by solving the hydrodynamic equations numerically, consistently including the effects of nonequilibrium ionization on the radiative losses and on the internal energy.

Multiple-scattering theory for electromagnetic waves.

Abstract: In this paper, a multiple-scattering formalism for electromagnetic waves is presented. Its application to the three-dimensional periodic dielectric structures is given in a form similar to the usual Korringa-Kohn-Rostoker form of scalar waves. Using this approach, the band-structure results of touching spheres of diamond structure in a dielectric medium with dielectric constant 12.96 are calculated. The application to disordered systems under the coherent-potential approximation is discussed.

Basic wave mechanics : for coastal and ocean engineers

Abstract: Sea Surface Gravity Waves. Small Amplitude Wave Theory and Characteristics. Two-Dimensional Wave Transformation. Finite Amplitude Wave Theory. Three-Dimensional Wave Transformations. Wind-Generated Waves. Design Wave Determination. Wave-Structure Interaction. Long Waves. Laboratory Investigation of Surface Waves. Index.

Propagation and Structural Interpretation of Non-Plane Waves

Abstract: SUMMARY As a model for the 2-D horizontal propagation of seismic surface waves, we study the propagation of non-plane acoustic waves in homogeneous and inhomogeneous media. We find that their phase velocity depends not only on the medium but also on the local geometry of the wavefield, especially on the distribution of amplitudes around the point of observation. the phase velocity of a wave is therefore conceptually and in most cases numerically different from the phase velocity parameter in the wave equation, which is determined by the elastic properties of the medium. the same distinction must be made for seismic surface waves. Although it is a common observation that waves of the same period can propagate with different phase velocities over the same path, the fundamental character of this observation has apparently not been recognized, and the two phase velocities are frequently confused in the seismological literature. We derive a local relationship between the two phase velocities that permits a correct structural interpretation of acoustic waves in inhomogeneous media, and also of non-plane seismic surface waves in laterally homogeneous parts of the medium.

Surface‐breaking fatigue crack detection using laser ultrasound

Abstract: Surface‐breaking tight fatigue cracks in mild steel have been examined with laser‐generated ultrasonic pulses. Before the arrival of transmitted Rayleigh waves arriving at the detector, evidence is presented of a fast skimming longitudinal pulse which is also transmitted through the crack. Additionally, another ultrasonic feature is consistent with a longitudinal wave which is mode converted to a diffracted shear pulse by the tip of the fatigue crack. Such an interaction mechanism can form the basis of laser‐based fatigue crack depth measurements.

Internal waves produced by the turbulent wake of a sphere moving horizontally in a stratified fluid

Abstract: The internal gravity wave field generated by a sphere towed in a stratified fluid was studied in the Froude number range 1.5 < F < 12.7, where Fis defined with the radius of the sphere. The Reynolds number was sufficiently large for the wake to be turbulent (Re~[380,30000]). A fluorescent dye technique was used to differentiate waves generated by the sphere, called lee waves, from the internal waves, called random waves, emitted by the turbulent wake. We demonstrate that the lee waves are well predicted by linear theory and that the random waves due to the turbulence are related to the coherent structures of the wake. The Strouhal number of these structures depends on F when F 5 4.5. Locally, these waves behave like transient internal waves emitted by impulsively moving bodies.

Turbulence amplification by a shock wave and rapid distortion theory

Abstract: Amplification of turbulent kinetic energy in an axial compression is examined in the frame of homogeneous rapid distortion theory (RDT) by using the Craya–Herring formalism. By separating the turbulent field into solenoidal and dilatational modes (Helmholtz decomposition), one can show the dilatational mode is mediated by the parameter Δm0=D0/a0k0, which corresponds to the initial ratio between the acoustic time scale (a0k0)−1 and the compression time scale D0−1, with D0 the compression rate. It is shown here that amplification of total kinetic energy is then limited by two analytical solutions obtained for Δm0=0 (purely solenoidal‐acoustical regime) and for Δm0≫1 (‘‘pressure released’’ regime), respectively. The results of the theory are first compared to results of direct numerical simulations (DNS) on homogeneous axial compression. The applicability of this homogeneous approach to the shock wave turbulence interaction, is then discussed. Considering a shock‐induced compression at given Mach number, it ...

Bivectors and waves in mechanics and optics

Abstract: The ellipse bivectors complex symmetric matrices complex orthogonal matrices ellipsoids homogenous and inhomogeneous plane waves description of elliptical polarization energy flux electromagnetic plane waves plane waves in linearized elasticity theory plane waves in viscous fields. Appendix: spherical trigonometry.

On the propagation of ideal, linear Alfvén waves in radially stratified stellar atmospheres and winds

Abstract: The propagation of Alfven waves through isothermal, radially stratified, spherically symmetrical models of stellar atmospheres and winds is discussed. The transmission coefficient for the waves is calculated as a function of frequency, magnetic field base intensity, surface gravity and atmospheric temperature. When a wind is present, the wave energy flux is no longer conserved, but the conservation of the wave-action flux (Heinemann & Olbert 1980) allows the definition of an analogous transmission coefficient, giving the relative amount of waves reaching the super-Alfvenic regions of the wind. It is shown that for high-frequency waves the transmission coefficient for static and wind models is identical, while for low-frequency waves the presence of a wind enhances the transmission considerably

On slow‐mode waves in an anisotropic plasma

Abstract: The properties of small-amplitude waves propagating in a homogeneous anisotropic plasma are investigated using an MHD double-polytropic model that incorporates the CGL double-adiabatic model in one extreme and the isothermal model in the other. It is found that the properties of fast and intermediate mode waves remain qualitatively the same as in ordinary MHD but that, in certain parameter regimes, three inversions occur for slow-mode waves: (1) their phase speed exceeds that of intermediate waves; (2) they behave like fast-mode waves in that, across them, the plasma density and magnetic field increase or decrease together; (3) rarefaction waves rather than compression waves steepen.

Envelope solitons with stationary crests

Abstract: Recent analytical and numerical work has shown that gravity–capillary surface waves as well as other dispersive wave systems support symmetric solitary waves with decaying oscillatory tails, which bifurcate from linear periodic waves at an extremum value of the phase speed. It is pointed out here that, for small amplitudes, these solitary waves can be interpreted as particular envelope‐soliton solutions of the nonlinear Schrodinger equation, such that the wave crests are stationary in the reference frame of the wave envelope. Accordingly, these waves (and their three‐dimensional extensions) are expected to be unstable to oblique perturbations.

A survey of low frequency waves at Jupiter: The Ulysses encounter

Abstract: We report the results of a survey of low-frequency (LF) plasma waves detected during the Ulysses Jupiter flyby. In the Jovian foreshock, two predominant wave periods are detected: 10(exp 2)-s and 5-s, as measured in the spacecraft frame. The 10(exp 2)-s waves are highly nonlinear propagate at large angles to vector-B(sub 0) (typically 50 deg), are steepened, and sometimes have attached whistler packets. For the interval analyzed the 10(exp 2)-s waves had mixed right-and left-hand polarizations. We argue that these are all consistent with being right-hand magnetosonic waves in the solar wind frame. The 10(exp 2)-s waves with attached whistler are similar to cometary waves. The trailing portions are linearly polaraized and the whistler portions circularly polarized with amplitudes decreasing linearly with time. The emissions are generated by approximately 2-keV protons flowing from the Jovian bow shock/magnetosheath into the upstream region. The instability is the ion beam instability. Higher Z ions were considered as a source of the waves but have been ruled out because of the low sunward velocities needed for their resonance. The 5-s waves have delta vector-B/B(sub 0 approximately = 0.5, are compressive and are left-hand polarized in the spacecraft frame. Local generation by three different resonant interactions were considered and have been ruled out. One possibility is that these waves are whistler mode by-products of the steepened lower-frequency magnetosonic waves. Mirror mode structures were detected throughout the outbound magnetosheath passes. For these structures, the theta(sub kB) values were consistently in the range of 80 deg to 90 deg, exceptionally high values.

The laser‐generated ultrasonic phased array: Analysis and experiments

Abstract: Focused ultrasonicwaves have been generated in a solid by irradiating its surface with a multiple beam‐pulsed YAG laser. A set of 16 rectilinear sources is used, equivalent to a phased array of ultrasonic transducers. Longitudinal waves are focused in the sample by introducing an appropriate time delay between each laser pulse. The elastic waves are detected either by a broadband optical heterodyne probe to analyze the wide ultrasonic signal spectrum (0–20 MHz), or by a narrow‐band piezoelectric transducer to achieve the sectorial acoustic beam scanning of the sample. Neglecting heat diffusion in the solid and considering the source as a surface center of expansion, the impulse directivity patterns of laser‐generated longitudinal acoustic waves have been computed. Experiments performed on duraluminum samples in the thermoelastic regime and steel samples in the ablation regime are presented and compared with this analysis. It is shown that a high focusing and a significant improvement of the signal sensitivity for longitudinal waves can be achieved with this technique.

Scattering of plane compressional waves by spherical inclusions in a poroelastic medium

Abstract: This study treats the scatter of a plane compression wave by a spherical inclusion embedded in an infinite poroelastic medium. The pressure‐solid displacement form of the harmonic equations of motion for a poroelastic solid is developed from the form of the equations originally presented by Biot. Then these equations are solved for the case of a plane wave impinging on a spherical inclusion. Solutions are obtained for the cases when the inclusion is composed of elastic solid or fluid, and when the sphere is fixed and rigid. The incident wave may be composed of any linear combination of Biot ‘‘fast’’ and ‘‘slow’’ waves.

A two parameter coupling-of-modes model for shear horizontal type SAW propagation in periodic gratings

Abstract: A closed form dispersion relation for shear surface acoustic waves (BGW, STW, leaky waves) propagating in periodic structures in the frequency range corresponding to the Bragg stopband is found. The consideration includes the changes in spatial structure of the waves mutually reflecting on the grating as well as bulk wave scattering. A comparison with numerically obtained dispersion curves for leaky waves on 36-LiTaO3 and with experimental data on STW in quartz shows good agreement

Repeated reflection of waves in the systemic arterial system

Abstract: Traditional analysis of pulse-wave propagation and reflection in the arterial system treats measured pressure and flow waves as the sum of a single forward wave (traveling away from the heart) and ...

Scattering of elastic waves by a spherical inclusion—I. Theory and numerical results

Abstract: SUMMARY A complete and exact solution for the problem of an incident P wave scattered by an elastic spherical inclusion is presented and described. The solution can be obtained from either analytical formulae or stable numerical procedures. A method of estimating the number of terms that must be retained in the harmonic series in order to achieve a specified accuracy is given. The results are investigated by calculating synthetic seismograms, scattering diagrams, and scattering cross-sections for a broad frequency band and for both low-velocity and high-velocity inclusions. The fields within the shadow zone are formed primarily from three different types of waves, P waves transmitted through the sphere, P waves diffracted around the sphere, and S waves converted at the boundary of the sphere. The relative contribution from these different waves depends upon the distance of the observation point from the sphere.

A comparison between wave propagation in water-saturated and air-saturated porous materials

Abstract: The classical Biot theory [J. Acoust. Soc. Am. 28, 168 (1956)] predicts the existence of three waves that can propagate in a fluid‐saturated porous material: A fast compressional wave, a slow compressional wave, and a shear wave. Through use of this theory, propagation characteristics within water‐filled and air‐filled materials were compared in the 10 Hz–100 kHz band. Numerical calculations show that the ratio of fluid to solid motion for the slow compressional wave is around 2 in water‐filled sand, but greater than 300 in air‐filled sand. In addition, calculations of plane wave transmission from a fluid into a fluid‐saturated porous solid were investigated. The calculations show that when the fluid is water, nearly all of the incident energy is transferred to the reflected wave and to the transmitted fast compressional wave that is traveling mainly in the solid frame. Only a slight frequency dependence occurs in the energy transfer. When the fluid is air, however, the interaction of the waves with the boundary becomes strongly dependent upon frequency, and most of the incident energy is transferred to the reflected wave and to the transmitted slow compressional wave traveling mainly in the pores. These theoretical results justify the different approaches used to treat reflections from porous materials in underwater and aeroacoustics. For reflections, air‐filled soil or snow can be approximately modeled as a modified fluid (ignoring motion in the frame) rather than as a viscoelastic solid (ignoring motion in the pores), the approximation commonly used to model saturated undersea sediments.

Head-on Collision of Normal Shock Waves with Rigid Porous Materials

Abstract: The head-011 collision of a planar shock wave with a rigid porous material has been investigated experimentally. The study indicated that unlike the reflection from a flexible porous material, where the transmitted compression waves do not converge to a sharp shock wave, in the case of a rigid porous material the transmitted compression waves do converge to a sharp shock wave, which decays as it propagates along the porous material.

A liquid sensor based on a shear horizontal saw device

Abstract: The surface acoustic wave (SAW) changes its propagation characteristic if the region above the propagation surface is altered. This phenomenon can be used for realization of an SAW sensor. The Rayleigh mode known as a general SAW can be used as a gas phase sensor. If, however, the propagation surface is loaded with liquid, a longitudinal wave is radiated into the liquid and the Rayleigh wave decays. Hence, this wave cannot be used for a liquid sensor. To realize a liquid sensor, the SH mode must be used in which the particle displacement is normal to the propagation direction and parallel to the propagation surface. It has been reported that the SAWs propagating on piezoelectric crystals such as 36° rotated Y-cut X propagation LiTaO3 and X-cut 150° propagation LiTaO3 are SH-mode SAWs (SH-SAW). In this paper, it is found theoretically and experimentally that the shear horizontal- (SH-) SAW devices using these piezoelectric crystal cannot only detect the mechanical effects such as the viscosity of the liquid and the mass loading in the liquid but also the electrical effects such as the permittivity and conductivity of the liquid. By way of comparison with other liquid sensors using high-frequency ultrasonic devices, the characteristics of the SH-SAW sensors are clarified.