Showing papers on "Phase velocity published in 1983"
TL;DR: In this article, a finite-difference solution of the two-dimensional acoustic wave equation is proposed to migrate an observed zero-offset wavefield as the solution of a boundary value problem in which the data are extrapolated backward in time.
Abstract: Migration of an observed zero-offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time. This concept is implemented through a finite-difference solution of the two-dimensional acoustic wave equation. All depths are imaged simultaneously at time 0 (the imaging condition), and all dips (right up to vertical) are correctly migrated. Numerical examples illustrate this technique in both constant and variable velocity media.
756 citations
TL;DR: In this paper, the effect of the phase velocity of the running disturbance on the aspect ratio of the column was investigated and a unique non-dimensional wavenumber ≈ 2·2 (near Mac) of the distortion was found.
Abstract: In liquid columns (Prandtl number 8·9) with free cylindrical surface heated from above, strong thermocapillary convection (TC) has been observed. Stationary thermocapillary convection exists in the form of a single axially symmetric roll bound to the free surface. For aspect ratios l/a < 1 the radial extension of the roll equals the zone length. The stream velocities and the temperature distribution were measured.The influence of buoyant forces due to horizontal temperature gradients in the experiments was also studied. Buoyant forces become obvious for a contaminated free surface and in bulk regions far from the cylinder surface.The thermocapillary convection shows a transition to time-dependent oscillatory motion when a critical Marangoni number Mac is exceeded. A unique Mac = 7 × 103 has been found for zones with lengths l < 3·5 mm. The oscillatory state of thermocapillary convection has experimentally been proved to be a distortion of the laminar state in form of a wave travelling in the azimuthal direction. A unique non-dimensional wavenumber ≈ 2·2 (near Mac) of the distortion has been found. The non-dimensional frequency of the temperature oscillations is independent of zone size if the aspect ratio is held constant. However, the non-dimensional frequency of temperature oscillations increases linearly with the aspect ratio of the zone. This result is interpreted as a dependence of the phase velocity of the running disturbance on the aspect ratio.
396 citations
TL;DR: In this article, the Kelvin-Helmholtz instability of a tangential discontinuity in a compressible plasma is reexamined in the linear magnetohydrodynamic (MHD) approximation.
Abstract: The Kelvin-Helmholtz (K-H) instability of a tangential discontinuity in a compressible plasma is reexamined in the linear magnetohydrodynamic (MHD) approximation. For fixed plasma conditions, two different kinds of surface waves (labeled F for fast and S for slow) may exist simultaneously with different tangential wave vectors k/sub t/. The surface waves can be excited only for a limited range of U, the relative flow speed of the plasmas on the two sides of the interface. Thus the instability requires U/sub c/m or approx. =U/sub i/, but the slow wave growth rate is small in comparison with both epsilon/sub F/ and epsilon/sub i/. Consequently, plasma compressibility is relatively ineffective in reducing the critical velocity for surface wave growth below that for an incompressible plasma.
223 citations
TL;DR: In this paper, the effects of both phase velocity and group velocity mismatch between fundamental and second harmonic fields are analyzed for optical pulse width measurements. And an expression for the time average second harmonic energy, which except for special cases differs from the intensity autocorrelation function, is derived.
Abstract: The use of second harmonic generation as a technique for optical pulse width measurements is analyzed to determine the effects of both phase velocity and group velocity mismatch between fundamental and second harmonic fields. An expression for the time average second harmonic energy, which except for special cases differs from the intensity autocorrelation function, is derived. For Type I phase matching, the measurement yields an apparent correlation width which can be either shorter or longer than the actual intensity correlation width, depending on the specific pulse shape. When the group velocity mismatch is nonzero, the measurement becomes sensitive to the phase matching condition. Two special pulse shapes for which the measurement is independent of group and phase velocity mismatch are the Gaussian and the single-sided exponential.
162 citations
TL;DR: In this article, the amplitude, frequency, wavenumber and phase speed of an unstable deep-water wavetrain were measured using a Hilbert transform technique, showing that the modulation variables evolve from sinusoidal perturbations that are well described as slowly varying Stokes waves, through increasingly asymmetric modulations that finally result in very rapid jumps or phase reversals.
Abstract: Time series of amplitude, frequency, wavenumber and phase speed are measured in an unstable deep-water wavetrain using a Hilbert-transform technique. The modulation variables evolve from sinusoidal perturbations that are well described as slowly varying Stokes waves, through increasingly asymmetric modulations that finally result in very rapid jumps or ‘phase reversals’. These anomalies appear to correspond to the ‘crest pairing’ described by Ramamonjiarisoa & Mollo-Christensen (1979). The measurements offer a novel local description of the instability of deep-water waves which contrasts markedly with the description afforded by conventional Fourier decomposition. The measurements display very large local modulations in the phase speed, modulations that may not be anticipated from measurements of the phase speeds of individual Fourier components travelling (to leading order) at the linear phase speed (Lake & Yuen 1978).
147 citations
TL;DR: In this article, the Swedish twin auroral radar experiment (STARE) has been used to estimate the ionospheric electron drift velocities, which are useful for a wide range of geophysical studies.
Abstract: The Scandinavian twin auroral radar experiment (STARE) has a demonstrated capability of providing estimates of the ionospheric electron drift velocities, which are useful for a wide range of geophysical studies. The accuracy of such estimates has for the first time been tested by comparison with simultaneous velocity measurements made with the European incoherent scatter facility (EISCAT). The magnitudes of the estimated drift velocities are in agreement with the EISCAT measurements for small velocities (<700 ms−1), but we find the estimates to be increasingly too low as the velocities become larger. The directions of the estimated vectors are in agreement with the EISCAT measurements for all drift magnitudes. The measured phase velocities are even lower than those predicted by the kinetic theory of the two stream instability. Agreement can only be obtained for relatively large neutral densities. The data are not inconsistent with an assumption that the possible phase velocities in the plasma are limited upward by the ion-acoustic velocity (which is an increasing function of the ionospheric electric field). Power spectral observations and an improved modelling with the kinetic theory suggest that a backscatter region, extended in altitude and allowing several values of k∥ to influence observations, leads to an improved theoretical fit to the measurements.
135 citations
TL;DR: In this article, two-dimensional solitary and periodic waves in water of finite depth were considered and it was shown that elevation solitary waves cannot be obtained as the continuous limit of periodic waves as the wavelength tends to infinity.
Abstract: Two-dimensional solitary and periodic waves in water of finite depth are considered. The waves propagate under the combined influence of gravity and surface tension. The flow, the surface profile and the phase velocity are functions of the amplitude of the wave and the parameters l = λ/H and τ = T/ρgH2. Here λ is the wavelength, H the depth, T the surface tension, ρ the density and g the acceleration due to gravity. For . In addition, it is shown that elevation solitary waves cannot be obtained as the continuous limit of periodic waves as the wavelength tends to infinity. Graphs of the results are included.
132 citations
TL;DR: In this article, compressive wave phase velocity and attenuation have been measured in several high-porosity sandstones as a continuous function of frequency from 400 to ∼2000 kHz.
Abstract: Compressional wave phase velocity and attenuation have been measured in several high-porosity (20–26%) sandstones as a continuous function of frequency from 400 to ∼2000 kHz. Samples include Massilon, Berea, and Boise sandstones and a synthetic fused glass bead sample. Both dry and brine saturated samples have been studied at effective pressures up to 40 MPa. Dry samples all show negative velocity dispersion (velocity decreasing with increasing frequency) and attenuation (dB/m) increasing as the third to fourth power of frequency. These data are interpreted as evidence of scattering within the samples. Brine saturated rocks show positive velocity dispersion and attenuation increasing with a first- to second-power frequency dependence. These data are interpreted as evidence for a local fluid-flow loss mechanism. Fused glass beads show no indication of flow related losses.
117 citations
TL;DR: In this article, the first-arrival times of two closely spaced vertical seismic profiles were used to estimate the elastic coefficients of a transversely isotropic medium for the Pierre Shale, assuming that the P-and SV-wave velocities have only a small dependence on the angle of propagation.
Abstract: The five elastic coefficients which characterize a transversely isotropic medium have been measured for the Pierre Shale. The first-arrival times deduced from two closely spaced vertical seismic profiles provide values of magnitude and direction of the phase velocity at a given depth for different locations of the sources. From these velocities, we can estimate the five elastic coefficients by assuming that the P- and SV-wave velocities have only a small dependence on the angle of propagation. On the other hand, it is shown that the accuracy in the difference between the directions of propagation and particle motion is not sufficient to determine the anisotropy. This work was industrially sponsored through the Integrated Geophysics Project.
116 citations
TL;DR: In this paper, the Korteweg-de Vries equation was extended to second order in wave amplitude and the second order correction to the wave profile and the phase speed and the first order correction of the wavelength were determined.
Abstract: Solitary waves in density stratified fluids of shallow depth are described, to first order in wave amplitude, by the Korteweg–de Vries equation; the solution for a single solitary wave has the familiar ‘‘sech2’’ profile and a phase speed which varies linearly with the wave amplitude. This theory is here extended to second order in wave amplitude. The second‐order correction to the wave profile and the phase speed and the first‐order correction to the wavelength are all determined. Four special cases are discussed in detail. In certain special circumstances the first‐order theory may fail due to the vanishing of the nonlinear coefficient in the Korteweg–de Vries equation. When this occurs a different theory is required which leads to an equation with both quadratic and cubic nonlinearities.
111 citations
TL;DR: In this paper, the linear stability of steep gravity waves is investigated for superharmonic disturbances and it is found that the instability first occurs at the steepness which corresponds to the maximum not of the phase speed but of the total energy and the impulse of the wave.
Abstract: The linear stability of steep gravity waves is investigated for superharmonic disturbances As to this problem, Longuet-Higgins presented a conjecture that the gravity wave would become unstable to superharmonic disturbances at the steepness where the phase speed of the wave attains a maximum Contrary to this conjecture, it is found that the instability first occurs at the steepness which corresponds to the maximum not of the phase speed but of the total energy and the impulse of the wave This discrepancy implies the possibility of the superharmonic bifurcation of steady gravity waves in the region of large steepness
TL;DR: The temporal distortion and spectral broadening of a pulse generated by the combined effects of group-velocity dispersion and self-phase modulation is removed by reflection of a cw-pumped, broadband, unityreflecting Kerr-like optical phase conjugator followed by retraversal of the nonlinear medium.
Abstract: We show that the temporal distortion and spectral broadening of a pulse generated by the combined effects of group-velocity dispersion and self-phase modulation is removed by reflection of a cw-pumped, broadband, unity-reflecting Kerr-like optical phase conjugator followed by retraversal of the nonlinear medium We also examine numerically the effects of finite linear loss in the material, of nonunity conjugate reflectivity, and of finite conjugator thickness
TL;DR: In this article, a spherical harmonic expansion of regionalized phase velocities has been performed to measure the lateral heterogeneity of the earth and the results show that regions of convergence have the interesting property of being slow for short-period waves and fast, faster than shields, for long-period wave data.
Abstract: Long-period (100–330 s) fundamental-mode Love and Rayleigh waves have been processed to measure the great circle phase velocities for about 200 and 250 paths, respectively. The observations are inverted for regionalized phase velocities and for an even-order harmonic expansion of the lateral velocity heterogeneity. The regionalized inversions achieve a maximum variance reduction of about 65% and 85% for the Love and the Rayleigh wave data, respectively. The l_max = 2 inversions give a maximum variance reduction of about 60% and 90% for Love and Rayleigh waves, respectively. The l_max = 8 inversion does not make a large improvement in the fit. The Love wave phase velocities have more power in l = 4 and 6, relative to l = 2, than the Rayleigh waves. For both Love and Rayleigh wave data the sectoral component dominates the l = 2 harmonics, and this component is stable if we increase l_max from 2 to 6. Heat flow also has strong sectoral components (lm = 22), which are approximately in phase with those of the phase velocities. The l = 2 harmonics of the nonhydrostatic geoid are dominated by large zonal (lm = 20) and moderate sectoral components. The sectoral components are in phase with those of the phase velocities. The sectoral pattern of heat flow and phase velocity is controlled by high heat flow-low velocity of the East Pacific Rise and western North America, which is reinforced by low velocities in the antipodal region (Red Sea-Gulf of Aden-East African Rift). By contrast the geoid l = 2 pattern is dominated by geoid highs over the western Pacific subduction zones. A spherical harmonic expansion of regionalized phase velocities shows that they have l = 2 variations similar to those of the l_max = 2 nonregionalized inversions. This means that the regionalization approach is appropriate as a first step for studying lateral heterogeneity of the earth. However, the great circle phase velocities are not sufficient by themselves to uniquely locate the lateral heterogeneity. The same is true for free oscillation data. Regions of convergence have the interesting property of being slow for short-period waves and fast, faster than shields, for long-period waves.
TL;DR: In this article, the propagation of an isolated mesoscale eddy onto a western bounding topographic slope is examined through the use of a two-layer, primitive equation, numerical model.
Abstract: The propagation of an isolated mesoscale eddy onto a western bounding topographic slope is examined through the use of a two-layer, primitive equation, numerical model. Variable parameters in the study are sense of rotation (cyclonic/anticyclonic), vertical structure (baroclinic, barotropic), and layer thickness ratio (H1/H2). Eddy size and strength (rotational velocity) and frictional parameterization are fixed. Dispersion in the isolated eddy is induced by planetary and topographic beta-effects, giving the eddy an asymmetric distribution. This asymmetric distribution allows for nonlinear self-advective propagation tendencies. These nonlinear tendencies play a key role in the direction of propagation of the eddy. Specifically, in a quiescient background, an anticyclone can have eastward propagation tendencies which can overcome planetary and topographic beta-effects. Conversely, cyclones can have westward propagation velocities which are augmented (greater than maximum Rossby wave phase speed) b...
TL;DR: In this paper, a series of severe storms which occurred in the north central United States on 9 May 1979 and whose spatial distribution and movement correlate well with observed gravity waves were identified through power spectra analysis and cross-correlation techniques applied to National Weather Service barograph traces.
Abstract: An analysis is presented of a series of severe storms which occurred in the north central United States on 9 May 1979 and whose spatial distribution and movement correlate well with observed gravity waves. Two gravity wave trains of 2.1-3 mb amplitude, 2.5-3.3 h period and 240-265 km horizontal wavelength were isolated through power spectra analysis and cross-correlation techniques applied to National Weather Service barograph traces. The wave trains propagated in the 200 deg direction, which coincided with the jet axis, with a phase velocity of 20-30 m/s and within a 300 km wide band. The storms were identified on enhanced infrared GOES satellite pictures with the help of radar summaries. These convective systems initially developed in Nebraska and propagated north-northeast at 25 m/s, revealing wave-like characteristics with a separation of 300-400 km. The convective systems were closely linked to the observed wave trains with cell intensity, height and associated rainfall maximized at the wave ridge. One of the two wave trains developed in regions of weak or no convection and appeared to initiate more intense convective clusters downstream from the point of origin. It is shown that the characteristics of the wave trains are consistent with those of gravity waves generated in a region of strong vertical shear associated with the jet. It is suggested that the wave trains continue to extract energy from the basic state all along their track through critical level interaction.
TL;DR: In this paper, a more general dispersion relation is obtained to determine the effects of rotation, relaxation time and the external magnetic field on the phase velocity of the plane harmonic waves in an infinite conducting thermo-elastic solid permeated by a primary uniform magnetic field when the entire elastic medium is rotating with a uniform angular velocity.
TL;DR: In this article, a nonlinear dispersion relation for Type I irregularities is obtained which predicts: isotropy of the Doppler shift with elevation, limiting phase velocity equal to ion acoustic speed, and saturation amplitude which is maximum for horizontally propagating waves and decreases with elevation.
Abstract: By taking into account the effect of wave electric fields on electron orbits, a nonlinear dispersion relation for Type I irregularities is obtained which predicts: (1) isotropy of the Doppler shift with elevation, (2) limiting phase velocity equal to ion acoustic speed, and (3) saturation amplitude which is maximum for horizontally propagating waves and decreases with elevation.
TL;DR: Voyager 2 images provide a basis for detailed study of the morphology and circulation of Saturn's northern midlatitudes as mentioned in this paper, where the region between 30 N and 45 N contains two oppositely directed jets in close proximity, with many bright, active features in the westward jet, and an unusual ribbonlike wave feature encircling the planet in the eastward jet.
Abstract: Voyager 2 images provide a basis for detailed study of the morphology and circulation of Saturn's northern midlatitudes. Both Saturn's large-scale cloud bands and the distribution of its local cloud features have a characteristic zonal organization. The region between 30 N and 45 N contains two oppositely directed jets in close proximity, with many bright, active features in the westward jet, and an unusual ribbonlike wave feature encircling the planet in the eastward jet. Several of the smaller features within the westward jet do not remain at fixed latitudes and interact with each other. One group of v-shaped features is found to have periods of high activity correlated with the passage of a cyclonic bright spot. The ribbon wave was Fourier analyzed to determine its spectral composition. The greatest power is near wave number 9, with significant additional peaks appearing at planetary wave numbers 19, 25-27, 35-38, and 47-51. The phase velocity increases with wave number but is not well described by a Rossby-Haurwitz dispersion relation. The curvature of the mean wind profile obtained from cloud tracking indicates that the westward jet exceeds the standard barotropic instability condition, while the eastward jet marginally exceeds the deep-circulation instability condition of Ingersoll and Pollard (1982). The rms eddy velocities on Saturn are less than half as large as those observed on Jupiter.
TL;DR: Yamasaki et al. as discussed by the authors used a fine-resolution model to deal with the tropical cyclone with realistic horizontal scale, even when a sufficiently small grid size is used, and the numerical experiments revealed many important aspects of the formation and intensification processes and the structures of tropical cyclones and their mechanisms.
Abstract: As a continuation of previous studies (Yamasaki, 1977a, b) numerical experiments of axially symmetric tropical cyclones are performed. As in the previous studies, a fine resolution model is used in which convective clouds are not parameterized but explicitly resolved. A recent advance in the computer has enabled us to deal with the tropical cyclone with realistic horizontal scale even when a sufficiently small grid size is used. Although assumption of axial symmetry restricts very realistic simulation of real tropical cyclones, it seems that the numerical experiments have revealed many important aspects of the formation and intensification processes and the structures of the tropical cyclone and their mechanisms. At the early stage before the tangential velocity attains about 10 ms-1, the area of convective activity and the vortex size expand with time because convection at the outermost part of the convective area propagates outward. Individual convective clouds are usually organized as a convective system with a time scale of about 3 hours, which is referred to as ‘mesoscale convection’ in this paper. As one mesoscale convection weakens, another forms at some distance. As a result of successive formation, convective activity and rainfall propagate outward or inward and persist for a long period of time. A large-scale (cyclone-scale) meridional circulation is intensified by an ensemble of several mesoscale convections, whereas the continuous formation of mesoscale convection is maintained by the large-scale circulation. The mechanism of such cooperative interaction between moist convection and large-scale motion at this stage, however, is different from that of the original CISK found by Ooyama (1964) and Charney and Eliassen (1964). That is, surface friction does not play any significant role, but instead, the downdraft and cooling due to evaporation of rainwater play an essential role. Such a new type of CISK was also discussed in Yamasaki (1975, 1979). When the rotational winds are intensified, surface friction becomes important. That is, the radial positions of the outermost convection and of maximum tangential velocity begin to shift inward by frictional inflow. Such an inward shift occurs when the tangential velocity attains 10-15 ms-1. Even at this stage it appears that cooling due to evaporation of rainwater and downdraft have significant effects on the large-scale dynamics. When the tangential velocity near the vortex center exceeds about 20 ms-1, an eye and eyewall are formed. Then rapid fall of the central surface pressure as well as rapid intensification of the tangential winds takes place. Surface friction plays an essential role in the formation and maintenance of the eye and eyewall. Several small-scale features, which have not been studied with coarse grid models with parameterized convection, are found in the eye and eyewall, including time variation with a period of about 10 minutes. It is suggested that the long-lasting convections obtained in this study may correspond to observed spiral rainbands, which have been interpreted by many authors as internal gravity waves modified by convective heating. The structure and the phase velocity of the long-lasting convections are different from those of internal gravity waves.
TL;DR: In this article, the authors examined the conductivity of ion waves in plasmas and found that the reduction in conductivity is independent of the wave amplitude, thus differing from the reduction due to saturation found recently by numerical solution of the Fokker-Planck equation.
Abstract: Electron energy transport in plasmas is examined in the context of ion waves which are intermediate between collisionless isothermal ion acoustic waves and collisional adiabatic sound waves. The conductivity is found to be much less than the Spitzer‐Harm result for wavelengths less than 1000 electron mean free paths. This is expected to be relevant to laser‐produced ablating plasmas in which the temperature can vary considerably over a distance of 10 to 100 mean free paths. The reduction in conductivity is independent of the wave amplitude thus differing from the reduction due to saturation found recently by numerical solution of the Fokker–Planck equation. At short wavelengths the heat flow approaches an upper limit which depends on the phase velocity of the wave. Diffusive ion wave damping is strong over a large range of wavelengths.
TL;DR: In this article, a variational method of deriving equations of motion for mixtures is used to obtain a theory for a granular sediment saturated by a liquid which contains bubbles of gas.
Abstract: A variational method of deriving equations of motion for mixtures is used to obtain a theory for a granular sediment saturated by a liquid which contains bubbles of gas. The theory is an extension of the Biot–Stoll model for wave propagation in saturated sediments. Wave velocity and attenuation are determined for a water‐saturated model sediment containing a small volume of air bubbles. It is verified that the model predicts the qualitative features that have been observed experimentally in gassy sediments. At frequencies below the bubble resonance frequency (BRF), the phase velocity is substantially decreased by the presence of the bubbles. Near the BRF, the attenuation increases markedly. Above the BRF, the phase velocity and attenuation approach the values which they would have if no bubbles were present.
TL;DR: In this article, the validity of the assumptions built into the two-phase theory has been re-examined for a range of fine powders, and the systematic deviations from the above assumptions demonstrated.
TL;DR: In this article, the authors show that the high-frequency electrostatic bursts are caused by a type of two-stream instability called the resistive medium instability, which is characterized by a reduction in the electrostatic burst frequency below the electron plasma frequency.
Abstract: Recent studies of wideband plasma wave data from the ISEE 1 and ISEE 2 spacecraft have revealed that whistler mode chorus emissions in the earth's outer magnetosphere are often accompanied by high-frequency bursts of electrostatic waves with a frequency slightly below the electron plasma frequency Investigations have shown that in some cases the electrostatic waves are modulated at the chorus frequency Further studies using the plasma analyzer (LEPEDEA) data on ISEE 1 indicate that these bursts are produced by a ‘beam’ of electrons in Landau (longitudinal) resonance with the chorus wave and thus moving at the chorus phase velocity A threshold exists in the chorus intensity below which the electrostatic bursts do not appear The high-frequency electrostatic waves are believed to be caused by a type of two-stream instability called the resistive medium instability The resistive medium instability is characterized by a reduction in the electrostatic burst frequency below the electron plasma frequency, The instability is applicable only in the regime where V0/VT is on the order of 1, where V0 is the velocity of the beam and VT is the average thermal velocity of the plasma electrons Our derivation assumes cold ions but warm electrons The instability requires Landau damping to operate Thus the beam velocity must be in the region of steep slope on the electron distribution function rather than in the high-velocity tail region In the cases examined from the LEPEDEA data the electron thermal energies are on the order of a few hundred eV The beam velocities in the observed cases were ≈ 400 eV and ≈ 630 eV, thus verifying that the electrostatic bursts are in the proper regime for the resistive medium instability
TL;DR: In this article, an experiment to measure near-bottom currents on the Madeira Abyssal Plain is described, where moorings placed near 33°N, 22°W were separated by 5-40 km with instruments at 10, 100 and 600 m above the bottom (depth ∼5300 m).
Abstract: An experiment to measure near-bottom currents on the Madeira Abyssal Plain is described. The moorings placed near 33°N, 22°W were separated by 5–40 km with instruments at 10, 100 and 600 m above the bottom (depth ∼5300 m). Rotor stalling occurred ¼ to ⅓ of the time but does not hinder the analysis which separated currents into tidal (3 cm s−1, inertial (1.3 cm s−1) and low frequency (2.5 cm s−1) components. The M2 tide is found to be principally barotropic with magnitude, ellipticity, orientation and phase adequately predicted by the tidal model of Schwiderski (1979). Oscillations of near-inertial frequency are found to be bottom intensified, have a wavelength of 100 km directed nearly due south and 3 km vertically: their phase velocity is directed downward suggesting the bottom as the source. The vertical group velocity is estimated at ∼150 m day−1 upward and corresponds to the 4–6 day lag observed between 10 and 600 m for the envelope of inertial amplitude. Low-frequency statistics are presente...
01 Jul 1983
TL;DR: In this paper, a portable coherent pulsed-Doppler radar system was developed at Stanford and used in several experiments, both on land on the California coast and on board a ship during the Joint Air-Sca Interaction (JASIN) experiment.
Abstract: Techniques have been developed for using high-frequency (HF) surface-wave radar to measure ocean currents and vertical current shears in the upper 1 or 2 m of the ocean surface. An HF radar can precisely measure the phase velocity and direction of propagation of ocean waves whose wavelength is one.half the radar wavelength. In the absence of a current, the speed of the waves is given by the still-water dispersion relation. An underlying current will modify this speed. The radar measures the actual phase velocity through a Doppler shift, and the wavelength of the ocean wave is known through the first-order Bragg scattering relation, so a difference between observed and theoretical stillwater phase velocity can be calculated. In addition, longer ocean waves are affected by currents at deeper depths than are shorter ocean waves. By measuring the phase velocity at several different wavelengths, it is possible to measure a vertical current shear in the top 1 or 2 m of the ocean surface. This is a measurement that is very difficult to make by any other means. A portable coherent pulsed-Doppler HF radar system was developed at Stanford and used in several experiments, both on land on the California coast and on board a ship during the Joint Air-Sca Interaction (JASIN) experiment. The land-based experiments demonstrated that a current could be measured by an HF radar, and that its value agreed well with that measured by in-situ drifting spar buoys. In addition, there was evidence of a vertical current shear, both from the radar measurements and from the buoy measurements. The JASIN experiment was an attempt to apply these techniques to the measurement of surface current and current shear in the open ocean. The radar system was installed on board a ship, along with a receiving antenna consisting of a steerable phased array of eight wide-band loops. The steerable antenna was quite rugged and performed as expected. It produced antenna patterns consistent with the physical aperture of the array. The wind velocity during the JASIN experiment was quite low, so wind- and wave-generated currents were quite small. Nevertheless, there is some evidence of a current shear. Its magnitude is small and near the resolution limit of the radar, but it appears to be somewhat higher than estimates based on either the wind or wave conditions alone, but less than the estimates based on the sum of the two components.
TL;DR: In this article, it was shown that the variation of azimuthal phase velocity with latitude is significantly different for different time intervals during individual events, so that a mean phase velocity for a given latitude cannot be defined.
TL;DR: In this article, a two-dimensional array measurements of near-source accelerations have been conducted 6 km from the 5.6 M L explosion Colwick at Pahute Mesa, Nevada Test Site.
Abstract: Two-dimensional array measurements of near-source accelerations have been conducted 6 km from the 5.6 M L explosion Colwick at Pahute Mesa, Nevada Test Site. The three-component, nine-station array measured 400 m across with interstation spacing of 100 m. Wavenumber spectra and beam forming (stacking) were used to measure apparent velocities and directions of propagation of acceleration waveforms. The robustness of individual station accelerograms was measured with frequency-domain coherency estimates and cross-correlation functions. The dependence of interstation coherency upon station spacing and frequency band was also examined. Wave propagation in Pahute Mesa can be characterized by P-SV and SH wave motion in a horizontally homogeneous medium for frequencies below 5 Hz. Strong incoherent scattered signals are present above 5 Hz on all components. The fraction of scattered energy can be estimated and treated as signal-generated noise. A strong SH pulse was observed to propagate across the array. Measured phase velocity, arrival time, and direction of propagation require cogeneration of SH with P-SV energy by the explosion Colwick. Weaker and more diffuse transverse acceleration signals arrive coincident with the P wave and during the time prior to S. These arrivals are interpreted as forward-scattered energy from the P wave. The small array permitted a refinement of the horizontally averaged S -wave velocity model for Pahute Mesa. The Mesa is characterized by a low velocity layer, 600 m thick, above the water table with a steep velocity gradient immediately below. The coherency measurements indicate the upper 1 km or more of Pahute Mesa is heterogeneous at scale lengths less than 1 km.
TL;DR: In this article, the evaluation of auroral radar measurements in terms of the Farley-Buneman instability is reviewed and adjusted to recent observational (Te > Ti) and theoretical (propagation not exactly perpendicular to the magnetic field, high growth rates of the unstable waves) evidence that applies for the high-latitude E region.
Abstract: The evaluation of auroral radar measurements in terms of the Farley-Buneman instability is reviewed and adjusted to recent observational (Te > Ti) and theoretical (propagation not exactly perpendicular to the magnetic field, high growth rates of the unstable waves) evidence that applies for the high-latitude E region. The kinetic dispersion relation of the instability is studied in detail for a wavelength of 1 m in order to facilitate comparisons with the STARE auroral radar. The results of this new approach are compared with the results obtained with the simple dispersion relation which has so far been used to evaluate auroral radar experiments. It is shown that the new method results in considerably higher electron drift velocities in the case of strong convection. This means that with the former simple dispersion relation the electron drift velocities and electric fields have often been underestimated.
TL;DR: In this paper, a pair of IDT with 140μm period was applied to a composite Cu(3 μm)-ZnO (0.45 μm) film that is much thinner than the acoustic wavelength in a bulk material.
Abstract: Interdigital transducers (IDT’s) have been used to generate and detect ultrasonic Lamb waves in a composite Cu(3 μm)‐ZnO (0.45 μm) film that is much thinner than the acoustic wavelength in a bulk material. rf pulses of 29.0 and 1.31 MHz are applied to one of the pair of IDT’s with 140‐μm period, giving rise to a nondispersive quasi‐symmetric mode of phase velocity 4170 m/s and a dispersive quasi‐antisymmetric mode having a phase velocity of 184 m/s and a group velocity of 362 m/s, respectively. Possible applications to electronic signal processing and material studies are described.
TL;DR: In this article, the authors obtained the streamwise derivative of temperature fluctuations on the centerline and at one streamwise station in the self-preserving region of a turbulent plane jet by two techniques.
Abstract: Statistics of the streamwise derivative of temperature fluctuations on the centerline and at one streamwise station in the self‐preserving region of a turbulent plane jet have been obtained by two techniques. In one, the derivative is approximated by the difference between the signals from two cold wires separated in the streamwise direction. In the other, the streamwise derivative is inferred from the temporal derivative and different convection velocities. Instantaneous values, root‐mean‐square values, skewness, and flatness factors are in reasonable agreement for a sufficiently large range of separations regardless of whether the mean velocity (Taylor’s hypothesis) or the instantaneous velocity are used as the convection velocity. Corrections for turbulence intensity, that are often applied to Taylor’s hypothesis, do not seem warranted. The difference between spectra of the temperature difference and temporal derivative spectra cannot be resolved by identifying the convection velocity with a frequency‐dependent phase velocity of the temperature fluctuation.