Showing papers on "Wave propagation published in 1998"

Laser beam propagation through random media

Abstract: Random Processes and Random Fields Optical Turbulence in the Atmosphere Free-Space Propagation of Gaussian-Beam Waves Classical Theory of Optical Wave Propagation Line-of-Sight Propagation - Weak Fluctuation Theory, Part 1 Line-of-Sight Propagation - Weak Fluctuation Theory, Part 2 Propagation Through Random Phase Screens Laser Satellite Communication Systems Propagation Through Complex Paraxial ABCD Optical Systems Doublepassage Problems - Laser Radar Systems Line-of-Sight Propagation - Strong Fluctuation Theory Appendices - Special Functions Integral Table Tables of Beam Statistics Mathematica Programmes.

Noninterferometric Phase Imaging with Partially Coherent Light

Abstract: We demonstrate that interferometric imaging may be replaced by noninterferometric propagation-based techniques in many experiments. We explore propagation through the Poynting vector and find two classes of phase, one of which is topological in origin. Only this latter class may require interferometry to be determined, and even then only in specific well-defined circumstances. Our alternative definitions of phase are readily generalized to partially coherent radiation. Our analysis leads to an approach that is able to determine the absolute phase and the amplitude of a wave.

Numerical Modeling of Tidal Wave Runup

Abstract: A numerical solution for the 2 + 1 (long-shore and onshore propagation directions and time) nonlinear shallow-water wave equations, without friction factors or artificial viscosity is presented. The models use a splitting method to generate two 1 + 1 propagation problems, one in the onshore and the other in long-shore direction. Both are solved in characteristic form using the method of characteristics. A shoreline algorithm is implemented, which is the generalization of the earlier 1 + 1 algorithm used in the code VTCS-2. The model is validated using large-scale laboratory data from solitary wave experiments attacking a conical island. The method is applied then to model the 1993 Okushiri, Japan, the 1994 Kuril Island, Russia, and the 1996 Chimbote, Peru tsunamis. It is found that the model can reproduce correctly overland flow and even extreme events such as the 30-m runup and the 20-m/s inundation velocities inferred during field surveys. The results suggest that bathymetric and topographic resolution ...

Interpretations of observed climatological patterns in stratospheric gravity wave variance

Abstract: Observational analyses of gravity waves in the stratosphere have revealed various climatological patterns in gravity wave activity. Seasonal, geographical, and vertical variations have all been observed. In this work, a linear model of gravity wave propagation is applied to investigate the underlying causes of some of the observed patterns. A collection of monochromatic gravity waves that represent a broad spectrum of wavenumbers and frequencies is input at 6-km altitude in the model. Propagation of the waves through realistic background atmospheric wind and stability fields is treated with linear ray theory and a simple saturation condition to limit amplitudes to stable values. The wave spectrum at the 6-km source height is specified to be constant at all latitudes, longitudes, and times, so the variability that appears at higher altitudes is due entirely to background atmosphere variations. Before the model results are compared to the observations, the spectrum of waves is filtered in a way that mimics the limitations of each of the observation techniques. The filtering is described in terms of vertical wavelength and is referred to as the “observational filter.” In a vertically varying background wind, gravity waves are Doppler-shifted in intrinsic frequency and refracted to different vertical wavelengths as they propagate vertically through the atmosphere. The observational filter and the wave refraction effects can thus couple in interesting ways that have not been explicitly considered in previous work. The model shows that this coupling can give rise to geographical, seasonal, and vertical variations in gravity wave observations without any variations in the spectrum or amplitude of gravity wave sources in the troposphere. Thus careful consideration of both the background wind profile and observational filter can greatly affect the interpretation of the observed climatological patterns in gravity wave activity.

Noise-supported travelling waves in sub-excitable media

Abstract: The detection of weak signals of nonlinear dynamical systems in noisy environments may improve with increasing noise, reaching an optimal level before the signal is overwhelmed by the noise This phenomenon, known as stochastic resonance1,2, has been characterized in electronic3, laser4, magnetoelastic5, physical6 and chemical7 systems Studies of stochastic resonance and noise effects in biological8,9 and excitable dynamical systems10,11,12,13 have attracted particular interest, because of the possibility of noise-supported signal transmission in neuronal tissue and other excitable biological media Here we report the positive influence of noise on wave propagation in a photosensitive Belousov–Zhabotinsky14,15,16,17 reaction The chemical medium, which is sub-excitable and unable to support sustained wave propagation, is illuminated with light that is spatially partitioned into an array of cells in which the intensity is randomly varied Wave propagation is enhanced with increasing noise amplitude, and sustained propagation is achieved at an optimal level Above this level, only fragmented waves are observed

A ray-tracing method for modeling indoor wave propagation and penetration

Abstract: A ray-tracing method for waves inside buildings is presented. Ray tubes are used to model the wave propagation and penetration and all the significantly reflected and transmitted ray tubes from interfaces are included. Also, the cross sections of the ray tubes at the field points are evaluated to find the spreading factors of the waves and then the geometrical optics (GO) contributions at the locations of the receiving antenna. A program has been developed according to this ray-tracing technique that can be applied to simulate waves transmitted through and reflected from electrically large complex 2D and 3D bodies. To verify this ray-tracing program, 2D moment method (MM) solutions for wave propagating in a two-room structure and also through a stair-shaped wall above a lossy ground are used to compare with those obtained from the ray tracing. Besides, comparisons of field measurements and ray-tracing simulations at 900 and 1800 MHz performed in a corridor on different floors and inside a staircase are shown. The effective complex dielectric constants of the buildings determined from a free-space method are employed in the simulations and a vector network analyzer is used for the field measurements. Good agreements are obtained. In addition, measured results for waves penetrating an exterior wall with metal-framed windows at 1290 MHz are employed to test the ray-tracing solutions, which indicate that scattering from the metal frames may be significant for field points near the windows. This ray-tracing program can be applied to evaluate the channel characteristics for the indoor wireless communications.

Electromechanical wave propagation in large electric power systems

Abstract: An electrical power network consisting of generators and transmission lines is treated as a continuum system. The application of the limit of zero generator spacing, with finite rotor inertia and transmission line impedance per unit length, yields a nonlinear partial differential equation in time and two spatial dimensions for the rotor phase angle. The equation is a nonlinear version of the standard second-order wave equation which exhibits an explicit expression for the finite wave phase velocity. The electromechanical wave propagation characteristics, equilibrium solutions, and linear stability are investigated and some potentially important results are presented. Numerical simulations of the usual discrete generator model, based upon the swing equation, are presented and demonstrate the electromechanical wave propagation as having interesting properties. Numerical solutions of the analogous continuum model are compared to the discrete model and are found to be in excellent agreement. A numerical estimate of the wave phase velocity for the U.S. power grid is consistent with observations of the transient wave phenomena during staged fault events. The continuum model enables an array of alternative analytic and simulation methods to be applied to the study of global power system characteristics, such as stability and transient dynamics.

Phase velocities of Rayleigh waves in the MELT experiment on the East Pacific Rise

Abstract: The phase velocities of Rayleigh waves increase more rapidly with distance from the East Pacific Rise (EPR) axis than is predicted by models of conductive cooling of the lithosphere. Low velocities near the axis are probably caused by partial melt at depths of 20 to 70 kilometers in a zone several hundred kilometers wide. The lowest velocities are offset to the west of the EPR. Wave propagation is anisotropic; the fast direction is approximately perpendicular to the ridge, parallel to the spreading direction. Anisotropy increases from a minimum near the axis to 3 percent or more on the flanks.

Electromechanical wave propagation in large electric power systems

Abstract: An electrical power network consisting of generators and transmission lines is treated as a continuum system. The application of the limit of zero generator spacing, with finite rotor inertia and transmission line impedance per unit length, yields a nonlinear partial differential equation in time and two spatial dimensions for the rotor phase angle. The equation is a nonlinear version of the standard second-order wave equation which exhibits an explicit expression for the finite wave phase velocity. The electromechanical wave propagation characteristics, equilibrium solutions, and linear stability are investigated and some potentially important results are presented. Numerical simulations of the usual discrete generator model, based upon the swing equation, are presented and demonstrate the electromechanical wave propagation as having interesting properties. Numerical solutions of the analogous continuum model are compared to the discrete model and are found to be in excellent agreement. A numerical estimate of the wave phase velocity for the U.S. power grid is consistent with observations of the transient wave phenomena during staged fault events. The continuum model enables an array of alternative analytic and simulation methods to be applied to the study of global power system characteristics, such as stability and transient dynamics.

Defect states of acoustic waves in a two-dimensional lattice of solid cylinders

Abstract: Using the plane-wave expansion method, we study the propagation of acoustic waves through two-dimensional (2D) periodic composites consisting of solid cylinders in air. Defect in those structures create localized states inside the band gaps. We study both single and line defects. Line defects can act as a waveguide for acoustic waves while single defects can be used as acoustical filters.

MSX satellite observations of thunderstorm-generated gravity waves in mid-wave infrared images of the upper stratosphere

Abstract: Data from the Midcourse Space Experiment (MSX) has provided the first observations of thunderstorm-generated gravity waves imaged from space. Gravity wave theory predicts that isolated, sufficiently convective thunderstorms can launch waves and create a unique intensity pattern of concentric circles on a radiating surface of constant altitude above such a storm. Among the MSX constant-nadir-angle mid-wave infrared (MWIR) observations, two instances of such patterns have been identified. It was confirmed from meteorological satellite images that highly convective isolated thunderstorms occurred at the locations and times expected.

FDTD analysis of wave propagation in nonlinear absorbing and gain media

Abstract: An explicit finite-difference time-domain (FDTD) scheme for wave propagation in certain kinds of nonlinear media such as saturable absorbers and gain layers in lasers is proposed here. This scheme is an extension of the auxiliary differential equation FDTD approach and incorporates rate equations that govern the time-domain dynamics of the atomic populations in the medium. For small signal intensities and slowly varying pulses, this method gives the same results as frequency-domain methods using the linear susceptibility function. Population dynamics for large signal intensities and the transient response for rapidly varying pulses in two-level (absorber) and four-level (gain) atomic media are calculated to demonstrate the advantages of this approach.

Traveltimes for infrasonic waves propagating in a stratified atmosphere

Abstract: SUMMARY The tau‐p method of Buland & Chapman (1983) is reformulated for sound waves propagating in a stratified atmosphere under the influence of a height-dependent wind velocity profile. For a given launch angle along a specified azimuth, the ray parameter is redefined to include the influence of the horizontal wind component along the direction of wave propagation. Under the assumption of negligible horizontal wind shear, the horizontal wind component transverse to the ray propagation does not aVect the direction of the wave normal, but displaces the reference frame of the moving wavefront, thus altering the observed incidence azimuth. Expressions are derived for the time, horizontal range, and transverse range of the arriving waves as a function of ray parameter. Algorithms for the location of infrasonic wave sources using the modified tau‐p formulation in conjunction with regional atmospheric wind and temperature data are discussed.

Properties of seismic fault zone waves and their utility for imaging low-velocity structures

Abstract: A two-dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study properties of seismic waves in a laterally heterogeneous low-velocity structure. The waves, referred to as seismic fault zone waves, include head waves, internal fault zone reflections, and trapped waves. The analysis aims to clarify the dependency of the waves on media velocities, media attenuation coefficients, layer widths, and source-receiver geometry. Additional calculations with extreme low-velocity layers provide examples that may be relevant for volcanic and geothermal domains. The interference patterns controlling seismic fault zone waves change with the number of internal reflections in the low-velocity structure. This number increases with propagation distance along the structure, decreases with fault zone width, and increases (for given length scales) with the velocity contrast. The relative lateral position of the source within the low-velocity layer modifies die length scales associated with internal reflections and influences the resulting interference pattern. Low values of Q affect considerably the dominant period and overall duration of the waves. Thus there are significant tradeoffs between propagation distance along the structure, fault zone width, velocity contrast, source location within the fault zone, and Q. The lateral and depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong function of both coordinates. The zone connecting sources generating fault zone waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the fault zone width. Calculations for cases with layer P wave velocity of ∼200 m s−1, modeling a vertical dike or crack with fluid and gas, show conspicuous persisting oscillations. The results resemble aspects of seismic data in volcanic domains. If these waves exist in observed records, their explicit recognition and modeling will help to separate source and structural effects and aid in the interpretation of volcano-seismology signals. Although the tradeoffs in parameters governing seismic fault zone waves are significant, each variable has its own signature, and the parameters may be constrained by additional geophysical data. Simultaneous modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high-resolution structural image.

Modulational instability of dust-acoustic and dust-ion-acoustic waves

Abstract: Using the standard reductive perturbation technique, a nonlinear Schr\"odinger equation is derived to study the modulational instability of finite amplitude dust-acoustic (DA) and dust-ion-acoustic (DIA) waves against oblique perturbations (with respect to the propagation direction of the carrier waves) in an unmagnetized dusty plasma. It is shown that both the DA and DIA waves are modulationally unstable. Possible stationary states of the wave packets can appear as envelope solitons.

Electrodynamics: A Modern Geometric Approach

Abstract: Special relativity Maxwell's equation(s) Lorentz-Force equations electromagnetic waves in 1-D electromagnetic waves in vacuum polarization waves in media waves at boundaries the field of a moving charge the Lorrentz-Dirac equation rotations and spherical harmonics radiation multipoles.

The inertial chimney: The near‐inertial energy drainage from the ocean surface to the deep layer

Abstract: A three-dimensional primitive equation numerical model is used to study the behavior of near-inertial waves generated by surface wind stress on an ƒ plane. The model is fixed depth with a rigid lid on the surface and is horizontally periodic. This study shows the behaviors of the wind-generated near-inertial waves for four different eddies: (1) a subtropical cold-core eddy, (2) a subtropical warm-core eddy, (3) a California subsurface warm-core eddy, and (4) the Gulf Stream warm-core ring. The mean secondary circulation generated by the wind-eddy interaction has magnitude to comparable to that of the Ekman current, and its characteristics are determined by the relative angle between wind and current. The propagation characteristics of near-inertial waves are quite different depending upon the sign of the relative vorticity. In a cyclonic eddy, near-inertial wave propagation is outward from the core of the eddy. The propagation direction in an anticyclonic eddy is downward and toward the core. Vertically propagating waves inside the eddy are trapped above a critical layer and slowly dissipated by parameterized viscous effects. The model shows that anticyclones efficiently drain near-inertial energy from the surface to the deep layer below the thermocline.

Introduction to Wave Propagation in Nonlinear Fluids and Solids

Abstract: Preface Nomenclature Introduction 1. Fundamentals 2. Mechanical waves 3. Thermomechanics 4. Constitutive models Appendix A: numerical methods Appendix B: material properties References Index.

Mathematical and numerical aspects of wave propagation

Abstract: This volume contains the 178 papers presented at the Fourth International Conference on Mathematical and Numerical Aspects of Wave Propagation in Colorado in June 1998. The papers include theoretical and applied wave propagation in the areas of acoustics, electromagnetism and elasticity.

The non-linear fluid dynamics of a warped accretion disc

Abstract: The dynamics of a viscous accretion disc subject to a slowly varying warp of large amplitude is considered. Attention is restricted to discs in which self-gravitation is negligible, and to the generic case in which the resonant wave propagation found in inviscid Keplerian discs does not occur. The equations of fluid dynamics are derived in a coordinate system that follows the principal warping motion of the disc. They are reduced using asymptotic methods for thin discs, and solved to extract the equation governing the warp. In general, this is a wave equation of parabolic type with non-linear dispersion and diffusion, which describes fully non-linear bending waves. This method generalizes the linear theory of Papaloizou & Pringle (1983) to allow for an arbitrary rotation law, and extends it into the non-linear domain, where it connects with a generalized version of the theory of Pringle (1992). The astrophysical implications of this analysis are discussed briefly.

Gravity waves on ice‐covered water

Abstract: Gravity waves propagating on the surface of ice-covered water of finite depth are considered. The ice layer is viewed as a suspension, with an effective viscosity much greater than that of water and a density slightly less than that of water. It is treated as a viscous liquid, and the water beneath it is treated as an inviscid liquid. The linearized motion of gravity waves is analyzed for this two-layer model, and the dispersion equation is obtained. It is solved numerically for waves of any length. It is also simplified for waves short compared to the layer thickness and for waves long compared to the layer thickness. This equation yields dispersion and strong attenuation, both of which depend upon the effective viscosity of the suspension.

Magnetic resonance imaging of shear wave propagation in excised tissue.

Abstract: The propagation of shear waves in ex vivo tissue samples, agar/gel phantoms, and human volunteers was investigated. A moving coil apparatus was constructed to generate low acoustic frequency shear perturbations of 50 to 400 Hz. Oscillating gradients phase-locked with the shear stimulus were used to generate a series of phase contrast images of the shear waves at different time-points throughout the wave cycle. Quantitative measurements of wave velocity and attenuation were obtained to evaluate the effects of temperature, frequency, and tissue anisotropy. Results of these experiments demonstrate significant variation in shear wave behavior with tissue type, whereas frequency and anisotropic behavior was mixed. Temperature-dependent behavior related mainly to the presence of fat. Propagation velocities ranged from 1 to 5 m/sec, and attenuation coefficients of from 1 to 3 nepers/unit wavelength, depending on tissue type. These results confirm the potential of elastic imaging attributable to the intrinsic variability of elastic properties observed in normal tissue, although some difficulty may be experienced in clinical implementation because of viscous attenuation in fat.