Azimuthal Pattern Research Articles

Sound from Turbulence Convected by a Parallel Flow Within a Rectangular Duct

A previously derived high-frequency solution to Lilley's equation is used to study the far-field characteristics of the sound generated by a turbulent flow within a semi-infinite duct of nonaxisymmetric cross section. The source term is simplified by assuming the turbulence to be axisymmetric about the mean flow direction, and a relatively simple formula for the far-field azimuthal and polar sound field variation is obtained. The numerical computations involve the calculation of ray trajectories, and a parallel computer code has been developed for this purpose. It is expected that the code will be able to produce azimuthal directivity patterns in a few minutes on a large parallel computer system. The method has been applied to a rectangular duct, and numerical results for the azimuthal and polar variation of the sound field were computed on a cluster of workstations for a number of duct aspect ratios, wall impedances values, and source locations

Three-dimensional instability of axisymmetric flow in a rotating lid–cylinder enclosure

The axisymmetry-breaking three-dimensional instability of the axisymmetric flow between a rotating lid and a stationary cylinder is analysed. The flow is governed by two parameters – the Reynolds number Re and the aspect ratio γ (=height/radius). Published experimental results indicate that in different ranges of γ axisymmetric or non-axisymmetric instabilities can be observed. Previous analyses considered only axisymmetric instability. The present analysis is devoted to the linear stability of the basic axisymmetric flow with respect to the non-axisymmetric perturbations. After the linearization the stability problem separates into a family of quasi-axisymmetric subproblems for discrete values of the azimuthal wavenumber k. The computations are done using the global Galerkin method. The stability analysis is carried out at various densely distributed values of γ in the range 1 < γ < 3.5. It is shown that the axisymmetric perturbations are dominant in the range 1.63 < γ < 2.76. Outside this range, for γ < 1.63 and for γ > 2.76, the instability is three-dimensional and sets in with k = 2 and k = 3 or 4, respectively. The azimuthal periodicity, patterns, characteristic frequencies and phase velocities of the dominant perturbations are discussed.

Resonant contrawound toroidal helix antenna

AbstractThe resonant CWTHA is examined via a moment‐method simulation. The results show a very high impedance at first resonance with a narrow bandwidth and low efficiency. At second resonance, the resistance is less than an ohm, the reactance is zero, and again, the bandwidth is narrow. The azimuth patterns are not omnidirectional, but may have a dip of 10 dB or more. The simulation shows a loop‐mode cancellation of at least 140 dB. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 408–410, 2001.

Medium-term sap flux monitoring in a Scots pine stand: analysis of the operability of the heat dissipation method for hydrological purposes.

The operability of Granier-type heat dissipation sap flow meters for the medium-term monitoring of Scots pine transpiration was tested. Three sensors that had been working for 3 years were duplicated by inserting new sensors in the same trees. The new sensors operated simultaneously with the old sensors for 18 months. Analysis of the temporal patterns of thermal dissipation of the sensors showed a slight, but significant decrease in all sensors, indicating the conservation of sapwood thermal properties. Although there was a high correlation between sap flux densities registered by the old and new sensors, significant differences in sap flux densities between the duplicated sensors were detected. Such differences could not be attributed to tissue injury around the sensors or to loss of sensitivity of the old sensors, because two of the old sensors registered higher flux rates than the new sensors. No instrumental error to explain the sap flux differences was found. Because the new sensors were installed at some angular distance from the old sap flow meters to avoid thermal interferences, it was inferred that the observed sap flux differences between duplicate sensors were the result of an azimuthal sap flow pattern.

Azimuthal asymmetry in transverse energy flow in nuclear collisions at high energies

The azimuthal pattern of transverse energy flow in nuclear collisions at RHIC and LHC energies is considered. We show that the probability distribution of the event-by-event azimuthal disbalance in transverse energy flow is essentially sensitive to the presence of the semihard minijet component.

Mapping the proton unintegrated gluon distribution in dijets correlations in real and virtual photoproduction at HERA

We discuss how the dijet azimuthal correlations in DIS and real photoproduction at HERA probe the differential (unintegrated) gluon distribution in the proton. We find a strong dependence of the azimuthal correlation pattern on Bjorken- x, photon virtuality and the cut on the jet transverse momenta. A rise of the azimuthal decorrelations is observed with decreasing Bjorken- x due to the interplay of perturbative and nonperturbative effects. We predict a strong rise of the same-side jet rate with photon energy for real photoproduction. We discuss conditions for the correlation function to be dominated by hard perturbative gluons and ways of constraining the size of the nonperturbative soft component. We make some predictions for the THERA energy range. The analysis of the energy dependence of the isolated jet and two-jet cross sections in photoproduction would be a new way to study the not yet well constrained unintegrated gluon distributions and to explore the onset of the pQCD regime.

Reradiation (echo) analysis of a tapered tower section supporting a side-mounted DTV broadcast antenna and the corresponding azimuth pattern

Reradiation from a non-vertical obstruction such as a tapered section of a broadcast antenna supporting tower is analyzed. The reradiation takes on the form of a cone, which is symmetrical about the tower leg axis, modified by the reflection coefficient that varies with azimuth. The reradiation is directed either above the horizon where it is lost or below the horizon where it interferes as an echo with the direct radiation. The reradiation has an elevation pattern which is a replica of the transmitting antenna elevation pattern, but is redirected by the nonvertical tower leg. The reradiation combines with the direct radiation to form an azimuth pattern which varies with distance from the broadcast antenna. The reradiation, being an echo, is analyzed for multipath distortion of the VSB pulse. It is shown that the impact on DTV receiver threshold is minimal.

Buried target detection using synthetic aperture sonar

During the Sediment Acoustics Experiment in 1999, the Coastal Systems Station fielded a synthetic aperture sonar (SAS) operating at 20 kHz. The SAS was able to detect and image 1.5 × 0.5 m cylinders at burial depths up to 50 cm and grazing angles well below the critical angle. In order to clarify the effect of the rough surface (sand ripple field) on sound propagation into the sediment, an experiment was recently conducted using five 10-in. fluid-filled target spheres that were buried at depths ranging from 5 to 50 cm below the top of the spheres. The interior fluid was chosen to maximize the target strength of the spheres (approximately −11 dB) when buried in sediment with a sound speed of 1750 m/s. Using aspect-independent targets enables the determination of the sound level transmitted into the sediment as a function of the angle between the transmitted acoustic beam and the sand ripple direction. Complete 360 deg azimuthal patterns of target echo levels are presented, illustrating the variation due to transmit/receive azimuthal angle with respect to the ripple structure. These azimuthally dependent echo level patterns are presented for several incident grazing angles, all of which are subcritical. [Work supported by ONR Code 321TS.]

An azimuthal pattern of heat field in β-BaB2O4 crystal growth

The regime of convection in the medium of crystallization of β-BaB2O4 (BBO) was improved through crystal growth in an azimuthally distributed heat field of the 3rd-order axial symmetry. As a result BBO crystals up to 360g weight have been grown in a crucible 100mm in diameter using the Na2O solvent.

Nonlinear magneto-optical effect in Fe/Au superlattices modulated by noninteger atomic layers

Magnetic second harmonic generation (MSHG) signals from the Fe/Au superlattices grown on MgO (100) substrate have been investigated. The measurements were carried out in the longitudinal Kerr geometry with a 45° incidence. The magnetic field of ±0.2 T was applied to provide a magnetic saturation. The polar plot of azimuth dependence of the MSHG intensity showed a four-fold symmetry pattern similar to that observed in Fe/Au superlattices modulated by integer atomic layers. Azimuthal patterns show 45° rotation by reversing the magnetic field. Nonlinear Kerr rotation angle of 23.2° was observed in the x=3.75 superlattice.

Radiowave propagation from a tissue-implanted source at 418 MHz and 916.5 MHz.

Tissue-implanted ultra-high frequency (UHF) radio devices are being employed in both humans and animals for telemetry and telecommand applications. This paper describes the experimental measurement and electromagnetic modeling of propagation from 418-MHz and 916.5-MHz sources placed in the human vagina. Whole-body homogeneous and semi-segmented software models were constructed using data from the Visible Human Project. Bodyworn radiation efficiencies for a vaginally placed 418-MHz source were calculated using finite-difference time-domain and ranged between 1.6% and 3.4% (corresponding to net body losses of between 14.7 and 18.0 dB). Greater losses were encountered at 916.5 MHz, with efficiencies between 0.36% and 0.46% (net body loss ranging between 23.4 and 24.4 dB). Practical measurements were in good agreement with simulations, to within 2 dB at 418 MHz and 3 dB at 916.5 MHz. The degree of tissue-segmentation for whole-body models was found to have a minimal effect on calculated azimuthal radiation patterns and bodyworn radiation efficiency, provided the region surrounding the implanted source was sufficiently detailed.

Photoalignment and photo‐patterning of planar and homeotropic liquid‐crystal‐display configurations

Abstract— Optical alignment and micro‐patterning of the alignment of liquid‐crystal displays (LCDs) by linear photopolymerization (LPP) technology renders high‐quality multi‐domain twisted‐nematic (TN) and supertwisted‐nematic (STN) displays with broad fields of view over wide temperature ranges feasible. The prerequisites are the generation of photo‐induced high‐resolution azimuthal alignment patterns with defined bias‐tilt angles 0° ≤ θ ≤ 90°. For the first time, LPP‐aligned single‐ and dual‐domain vertically aligned nematic LCDs (VAN‐LCDs) are presented. Dual‐domain VAN‐LCDs are shown to exhibit broad fields of view which are further broadened by combining the displays with LPP‐aligned optical compensators made of liquid‐crystal polymers.

Tensor analysis of the second-order nonlinear optical susceptibility of chiral anisotropic thin films

We present a new measurement technique based on second-harmonic generation to investigate thin molecular films with in-plane anisotropy. The technique does not rely on continuous azimuthal patterns of the second-harmonic intensities. Instead, the second-harmonic intensities are recorded at a number of selected distinct azimuthal orientations of the sample. The signals are recorded as functions of the state of polarization of the fundamental laser beam. Only normalized polarization line shapes are required. As the line shapes need not be mutually calibrated, the technique is limited only by the accuracy of the individual polarization measurements and is applicable also to samples with inhomogeneous surface coverage of molecules. The technique is applied to anisotropic chiral thin films with C2 symmetry made of helicenebisquinone. Full analysis of the nonlinear susceptibility tensor of the samples is performed and the effects of anisotropy and chirality separated. The part of the nonlinearity arising from chirality is shown to dominate the nonlinear response.

非線形磁気光学効果による［Ｆｅ（ｘＭＬ）／Ａｕ（ｘＭＬ）］人工格子界面の評価

The magnetic second-harmonic generation (MSHG), one of nonlinear magneto-optical effects, was measured by using a Ti-sapphire laser (λ = 810 nm) combined with filters, a polarizer, an analyzer, a detector and a photon-counting apparatus. The polar plot of the azimuthal dependence of the MSHG from a [Fe(3.75ML)/Au(3.75ML)] superlattice showed a four fold symmetry pattern. A reversal of the magnetic field caused reflection of the azimuthal patterns with respect to a certain axis. Nonlinear magneto-optical Kerr rotation as large as 31.1° was observed in [Fe(1.75ML)/Au(1.75ML) for the S-polarization incidence. The magnetic contribution to the azimuthal pattern was explained by symmetry analysis, taking into account the surface non-magnetic (electric dipole), bulk non-magnetic (electric quadrupole) and surface magnetic (electric dipole) contributions.

Source effects on surface wave group travel times and group velocity maps

In most seismic surface wave studies observed group travel times are interpreted as time delays due entirely to the wave propagation along the wave path, and source effects are considered as negligibly small. This is in contrast with observed phase times where correction for the source phase is generally acknowledged to be mandatory. An important, yet unanswered, question is how neglecting source group time (SGT) in broadband surface wave studies will affect the accuracy of the measured group velocity curves and the tomographic maps constructed from these measurements. We consider here the effect of SGT on group velocity measurements for fundamental Rayleigh waves and report on its dependence on period (10–200 s), source mechanism, and source depth. Varying these parameters strongly affects the magnitude and azimuthal pattern of SGT shifts and we present statistics of certain salient functionals that characterize this dependence. SGT is negligible for periods less than about 75 s and for earthquake shallower than about 25 km. At longer periods and for deeper events, average SGT corrections are greater than 10 s in magnitude, which for continental scale studies translates into group velocity perturbations of 1–2%. We estimate the bias caused by uncorrected SGT in inversions for Rayleigh wave group velocity maps across the Eurasian continent. The largest perturbations to these maps (up to 1–2% for the 50-s period and up to 5% for the 100-s period) are found near the periphery of the continent where ray coverage is poor. From these results, some statistical estimates for adjacent wave paths (clusters), and the fact that SGT corrections display considerable sensitivities to earthquake depths, we conclude that the effects of SGT on group velocity tomographic images may safely be ignored at periods less than about 75 s and for shallow sources. Although such corrections are appreciable at longer periods for events deeper than about 25 km and should in principle be applied, the inherent inaccuracy of present day CMT solutions and group velocity measurements make these corrections practically non-essential for current group velocity tomographic studies.

Shear wave splitting in the Appalachians and the Urals: A case for multilayered anisotropy

Observations of shear wave splitting in the northeastern U.S. Appalachians and in the foredeep of the Urals vary significantly with the back azimuth and incidence angle of the incoming phase. These variations suggest two or more layers within the upper mantle with different anisotropic properties. Synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach. Relying on a subset of back‐azimuth and incidence angle may strongly bias the model inferred, especially if the observations are averaged. On the other hand, the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the Earth. Using a new error estimation technique for splitting, we find that individual measurements from broadband data have errors of the order of δϕ = 3°–7°for the fast direction and 0.1–0.2 s for the delay of split shear waves. The azimuthal variation of splitting parameters is broadly consistent throughout the Appalachian terranes in the northeast United States, especially for two long‐running stations in the northeast United States, HRV (Harvard, Massachusetts) and PAL (Palisades, New York). Observations can be separated into two distinct populations, with mean fast‐axis azimuths of N60°E±4°and N119°E±2°. Delay values within each population range from near zero to ∼1 s. Azimuthal splitting variation for ARU (Arti, Russia) in the foredeep of Uralian mountains is characterized by sharp transitions between different groups of observations. Using synthetic seismograms in simple structures, we develop one‐dimensional anisotropic models under stations HRV and ARU. The model for HRV includes two layers of anisotropic material under an isotropic crust, with fast‐axis azimuths N53°E and N115°E for the bottom and the top layers, respectively. The model for the upper mantle under ARU includes a layer with a fast‐axis at N50°E atop a layer with fast axis azimuth N90°E. Our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under ARU, reported by Levin and Park [1997a]. Our results suggest that both Urals and Appalachians possess a relict anisotropy in the tectosphere, associated with past continental collision and accretion, underlain by anisotropy with orientation similar to the local absolute plate motion, suggesting an asthenospheric component to the signal.

Photoelectron diffraction study of the surfaces of Si(111)√3×√3–Al and –In with Mo Mζ and Cr Lα lines

Abstract Laboratory X-ray sources of Mo M ζ and Cr L α lines were used for a photoelectron diffraction (PED) study of the well-documented surfaces of Si(111)√3×√3–Al and Si(111)√3×√3–In. Azimuthal angle PED patterns of Al 2p and In 3d core levels were measured and analyzed by a multiple scattering formalism. The determined geometric parameters showed good agreement with those already established. This indicates the good potential of a PED study with laboratory sources.

Trajectory dependence of scattered Ne+ and recoiled S− ion fractions from the Cd- and S-terminated CdS{0001} surfaces

Scattered Ne+ and recoiled S− ion fractions resulting from 4 keV Ne+ and 4 keV Kr+ impingement, respectively, on both the Cd- and S-terminated surfaces of CdS{0001} have been measured. The absolute values of these ion fractions as well as their dependence on surface structure and electron density have been determined. Using a density functional approach, a clear correlation has been demonstrated between these Ne+ and S− ion fractions and the lateral variation of the electrostatic potential along the outgoing trajectories of the scattered and recoiled atoms. The observed anisotropy in the ion fractions is a result of the variations in surface to atom electron transfer rates due to tunneling barriers introduced by the electrostatic potentials. Both the Ne+ and S− ion fractions are higher on the Cd-terminated surface than on the S-terminated surface and their azimuthal patterns are different due to the spatial modulation of the electron tunneling rates on the surface caused by the electrostatic barriers. The azimuthal anisotropies of electrons ejected during the collision indicate that they are emitted only from collisions whose impact parameters are less than a threshold value, consistent with a kinetic electron emission mechanism.

Echo analysis of side-mounted DTV broadcast antenna azimuth patterns

Azimuth pattern analysis of a side-mounted antenna is made keeping reflections separate from the main radiated signal. Each reflection is characterized by a reflection coefficient and path difference in wavelengths at a distant receiving summing point. At each azimuth, the reflections are expressed as echoes accompanying a modulated VSB data pulse. The corresponding receiver equalizer tap weights are determined from which the DTV receiver threshold impact is calculated. Measurements of a side-mounted antenna using an azimuth fly-around technique are compared to the calculated results for specific azimuth directions. It is concluded that the DTV performance degradation caused by close-in reflections from adjacent tower members may be predicted by calculation and that the degradation is small.

FDTD analysis of close-coupled 418 MHz radiating devices for human biotelemetry

This paper describes the finite-difference time-domain (FDTD) analysis of antenna-body interaction effects occurring when chest-mounted 418 MHz radio transmitters are used for medical telemetry applications. Whole-body software models (homogeneous, layered and tissue-segmented) were developed for an adult male subject. Using an electrically small (300 ) planar loop antenna, calculated radiation efficiencies ranged between 33.5% and 39.2% for a whole-body model, and between 60.7% and 66.1% for a torso; radiation patterns were found to be largely independent of model composition. The computed radiation efficiency for a 21.5 kg phantom representing a six-year-old female was within 1.1 dB of measured results (actual body mass 28 kg) and well-correlated azimuthal radiation patterns were noted.