Unconventional Geometry Research Articles

Correlation lengths for vortex-liquid freezing in a model of layered high-temperature superconductors

We present results from extensive numerical simulations on the Lawrence-Doniach model within the lowest Landau level approximation in the unconventional spherical geometry. We study the in-layer pancake vortex density-density correlation function (intra-layer structure factor) and the layer-to-layer order-parameter correlation function along the direction perpendicular to the layers. Our results show strong evidence for the existence of a single first-order phase transition at which both inter-layer coupling and appearance of crystalline order in the in-layer vortex correlations take place as temperature is lowered.

Scanning of logs with linear cone-beam tomography

This paper reports an ongoing feasibility study in industrial X-ray tomography. Full volume scanning of logs may have a large potential in terms of increased sales value. On the other hand, there is no technology commercially available today which can meet the requirements for speed. In conventional medical 3D-tomography, the source-detector system rotates around a slowly translating patient. We do not believe such an arrangement is able to reach our speed requirements. Instead, we have been simulating an arrangement with two fixed source-detector systems rotated 90° relative to each other. Each system consists of an X-ray source and a 2D detector. The logs are to be translated through this arrangement at relatively high speed (2–3 m/s) lengthwise on a conveyor belt, while cone-beam projections are acquired by each of the source-detector systems. The 2D-detectors are composed of a number of 1D-detectors side by side. For cost reasons we will try to limit the number of rows to nine, forming a 2D-array of 256×9 detector elements. Due to the unconventional scanning geometry, we have developed a new reconstruction algorithm, which also takes special care to avoid many artifacts due to sparse and/or missing data. Experiments (still simulated), performed on voxelized CT data, indicate that the knots are reconstructed with sufficient accuracy to allow for quantitative optimization. Due to the missing data, however, heartwood can barely be distinguished from sapwood, but his fact does not seem to compromise the subsequent knot detection. Note that this paper only describes the reconstruction part of the simulated experiments. To be complete, the feasibility study should also include segmentation, which is outside the scope of this paper.

Non-coplanar reciprocal space mapping of In0.2Ga0.8As/GaAs multilayer: modelling of the composition and strain distribution

Reciprocal space mapping in non-coplanar diffraction geometry using a triple-crystal diffractometer is applied to study strained In0.2Ga0.8As/GaAs multilayer. The modelling accounts for the measured integrated intensities of the satellites in the simulation of the reciprocal space map to provide compositional profile of In distribution in the nucleated islands on the interfaces. Theoretical approach to calculate the intensity in this unconventional geometry is discussed.

Absence of stable collinear configurations in Ni(001) ultrathin films: Canted domain structure as ground state

Brillouin light scattering (BLS) measurements were performed for (17-120) Angstrom thick Cu/Ni/Cu/Si(001) films. A monotonic dependence of the frequency of the uniform mode on an in-plane magnetic field H was observed both on increasing and on decreasing H in the range (2-14) kOe, suggesting the absence of a metastable collinear perpendicular ground state. Further investigation by magneto-optical vector magnetometry (MOKE-VM) in an unconventional canted-field geometry provided evidence for a domain structure where the magnetization is canted with respect to the perpendicular to the film. Spin wave calculations confirm the absence of stable collinear configurations.

Euler and Navier-Stokes Solutions for Missiles at High Angles of Attack

Anunstructured Euler e ow solverand a structured thin-layerNavier‐ Stokessolverarevalidated againstexperimentaldata andcompared withothersolversfortwo differentmissilegeometriesata Machnumberof M =2andat angles of attack ranging between 0 and 20 deg. The e rst geometry is a conventional missile geometry with an ogive nose, a cylindrical body, and four straight tail e ns. The second geometry is an unconventional missile geometry with a lenticular body and no tail e ns. Present results are found to be in good agreement with the available results, and the differences observed between them are explained in detail. The present unstructured Euler code proved to be accurate, fast, and reliable for determining the overall aerodynamic characteristics of the missiles, whereas the thin-layer Navier‐ Stokes solutions are found to be effective in predicting the detailed viscous behavior of the e owe eld over the conventional and unconventional missile geometries studied.

Digital Photogrammetry and Microscope Photographs

The feasibility of applying commercial digital photogrammetric software to the measurement of small objects photographed through an optical microscope has been examined. The objects, about 20 mm across, were photographed using a 35 mm film camera (at the lowest magnification setting of the Olympus microscope) giving photographs at a scale of 2:1. The photographs were then scanned before processing with the VirtuoZo digital photogrammetric system. Various problems needed to be overcome, some due to the limited options available with the highly automated digital system which was not designed for such measurement tasks. The unusual image scales, the uncommon pixel sizes and the unconventional and uncertain imaging geometry, all impeded immediate photogrammetric implementation. Photographic problems with the microscope were also faced, as with all microscope photogrammetry. Creating control points and independently assessing the accuracy of results at these scales were also difficult operations, but an analytical plotter was utilized for both these purposes and to verify the imaging geometry. Once such problems were overcome, image matching proceeded well and an accurate DTM could be created successfully, provided that a suitably textured object was chosen.

Design of biplanar gradient coils for magnetic resonance imaging of the human torso and limbs

A method is described for design of gradient coils of unconventional geometry for MRI that is based on the superpositions of magnetic fields arising from individual current elements calculated by the Biot-Savart Law. Use of an optimization method based on a genetic algorithm enables a wide diversity in the shapes of coil that can be modeled. To exemplify this a two axis, biplanar gradient set is presented; this geometry offers good access for rectangular objects whilst holding the coils closer to the region of interest than is possible for cylindrical configurations. The inner dimensions of the gradient set were 40.0 × 24.4 × 40.0 cm and the gradient efficiencies were 0.3 and 0.4 mT m −1 A −1 in the z- and y- directions respectively over a 15 cm diameter region. Correction of signals arising from regions for which gradient linearity was not optimized was successful for the monotonic region within the set; the largest cuboid from which the MR signal could be processed to produce an undistorted image is of dimensions 36.3 × 17.2 × 24.4 cm.

Queenstown Bay Reservoir, New Zealand

The Queenstown Bay Reservoir project is part of a major regional infrastructure upgrade in a hilly resort residential area. The project was tendered on a design-and-construct basis and the successful tender was a joint submission from a contractor and a design engineering office. It was a requirement of the contract that the water supply to the town be maintained at all times during construction of the new storage. The new reservoir replaces two 450 m³ reservoirs on the site and will allow water supply and storage demands to be met to the year 2020. It will provide 8,600 m³ of storage when both stages are completed in December 1997. The reservoir is a water-retaining structure of unconventional geometry that has undergone specific design and concept development to accommodate challenging site constraints and two-stage construction.

Effects of geometry and polarity on the prebreakdown and flashover behavior of conical high-voltage insulators in vacuum

Concurrent optical and electrical measurements have been used to characterize the physical phenomena associated with the prebreakdown and flashover behavior of high-voltage alumina ceramic insulator samples having both conventional and unconventional conical geometries. The critical parameters are the threshold voltage for initiating flashover events, the stable interelectrode current-voltage relationship, the luminescence pattern, and the breakdown rate as a function of time. These measurements have revealed that a well-conditioned sample of unconventional conical geometry can have very low, i.e., approaching zero, breakdown rate. Physical explanations of the findings are based on the current understanding of flashover mechanisms and the prediction from simulations using a field plotting technique.

Pasisar: Performances of a High and Very High Resolution Hybrid Deep-Towed Seismic Device

The Pasisar seismic acquisition system combines a source at the sea surface and a deep-towed single channel streamer. This unconventional device geometry reduces the width of the first Fresnel zone which increases the lateral resolution. However, the device acquisition geometry generates artifacts on seismic profiles and induces large incidence angles of the seismic signal. A specific processing sequence must be applied to the data to obtain a readable seismic section. Penetration of the seismic signal depends on the energy of the signal reaching the seafloor and on its incidence angle. Because of smaller source energy, 800 Joules Sparker data cannot be acquired in water depth larger than 1500 m for example, whereas there is no depth limit for the use of this system with air gun sources. Differential acoustic absorption of seismic frequencies (below 1000 Hz) in the water column is negligible when compared with wave fronts expansion. Thus, the horizontal resolution of any seismic system strongly depends on the frequency spectrum of the seismic source and on the travel distances. Pasisar and conventional seismic profiles being usually simultaneously recorded, we illustrate the interest of using a hybrid seismic device by comparing horizontal resolutions as well as signal-to-noise ratio obtained with both the Pasisar and conventional systems. In addition, by carefully picking time arrivals of a reflection on simultaneously recorded surface and deep-towed seismic records, it is possible to estimate the average interval seismic velocity. We present the simplified example of a horizontal reflector.

Computational design study for an accelerator dipole in the range of 15-20 T

A superconducting accelerator dipole design based on the use of tape conductors in the high field region would open possibilities to reach magnetic flux densities in the range of 15-20 T. The unconventional geometry of the designed magnet avoids many problems encountered in the presently much used dipole cross-section of intersecting ellipses. The use of thin tape conductor is made possible by carefully optimizing the cross-sectional geometry of the coils. A design study of a 15 T magnet is presented and the required conductor characteristics to realize this concept are discussed.

Evidence for three-dimensional flux creep in thin-film Bi2Sr2CaCu2O8+ delta.

We have measured sweep-rate-dependent magnetization hysteresis curves of thin-film ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ using a Hall probe magnetometer in an unconventional geometry. We find a structure in the response well below the three-dimensional to two-dimensional crossover field. The observed strong enhancement of the perpendicular field component's creep rate in the presence of an in-plane applied magnetic field is in contrast to recent findings. We attribute this to the existence of a finite correlation length between vortex pancakes along the field direction larger than the interlayer spacing leading to a three-dimensional behavior of flux lines in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ at 77 K.

New airfoil design concept

The present airfoil design concept is based on utilizing unconventional geometry characteristics near the airfoil trailing edge which include a finite trailing edge thickness, strongly divergent trailing edge upper and lower surfaces, and high surface curvature on the lower surface at or near the lower surface trailing edge. This paper presents computational analyses of airfoils and a wing utilizing the concept, airfoil validation wind tunnel test results of several configurations, and wing-validation wind tunnel test results for a complete wing design. In addition to validating the concept, the airfoil and wing testing provided additional detailed data to better understand the aerodynamic advantage of such an unconventional trailing edge configuration. It is demonstrated that the concept represents a significant step in airfoil technology beyond that achieved with the Supercritical Airfoil. This concept provides the aerodynamicist an additional degree of design freedom and flexibility previously unrecognized.

An elastically bent silicon crystal as a monochromator for thermal neutrons

Some experimental results are presented for an elastically bent perfect silicon crystal in a strongly asymmetric diffraction geometry as a neutron monochromator. The use of this unconventional geometry of the monochromator appears to be suitable for a wide (several centimetres) incident polychromatic beam, when, thanks to the spatial condensation of the diffracted neutrons (Fankuchen effect), a high monochromatic beam density may be obtained. Furthermore, when using focusing in real and in momentum space by adjusting an optimum bending radius, the intensity diffracted by a sample may be comparable even with the best mosaic monochromators such as highly oriented pyrolytic graphite (PG). A comparison is demonstrated on the rocking curves of a strongly mosaic Ni–Al(020) crystal obtained with the monochromatic beam from bent Si(111), Si(400) and from PG(002), Cu(220) mosaic monochromators.

Relief mapping using nonphotographic spaceborne imagery

Three-dimensional restitution of images with unconventional imaging geometry can be performed in an operational mode using existing commercial analytical plotters without any extra investments in hardware. The paper presents two programme systems developed for the Kern DSR analytical stereo-plotter for mapping with spaceborne stereo-imagery, one for images acquired by the SPOT HRV sensor, the other one for stereo-mensuration of SAR images. Due to the movement of the satellite platforms the software requires appropriate algorithms for the consideration of time-varying orientation parameters like sensor position, sensor attitude or others. The performance of the programme systems is demonstrated with practical results derived from Space Shuttle SIR-B imagery as well as from SPOT images showing realistically achievable accuracies of a few pixels in planimetry.

First-order longitudinal Hall effect

We report the observation of first-order Hall effects determined by independent tensor elements and measured in an unconventional longitudinal geometry: magnetic field parallel to either the current or the Hall field. Monoclinic semimetallic Sr${\mathrm{As}}_{3}$ is shown to be a well-suited system for the investigation of this effect.

The interactions of polarised tritons with 32S

Cross sections and analysing powers have been measured for the elastic and inelastic scattering of tritons from 32S, along with the 32 S( t, α) 31 P reaction data, using the 17 MeV polarised triton beam available at the Los Alamos National Laboratory. An optical model analysis of the elastic scattering data has yielded a triton spin-orbit potential with an unconventional geometry. The inelastic analysing powers have shown a requirement for including the deformation of the spin-orbit potential in the coupling potential form factor. The ( t, α ) reaction data have been analysed using the zero-range DWBA. Certain features of this reaction data which could not be described by the DWBA are well predicted when the analysis is extended to include explicitly the inelastic couplings and two-step processes through a CCBA calculation.

D.C. conduction studies in evaporated films of stearic acid using an unconventional electrode geometry

Experimental studies of d.c. conduction in evaporated films of stearic acid are reported for the first time. An unconventional electrode geometry consisting of two lateral electrodes placed on the film symmetrically with a gap between of either 0.01 or 0.02 cm was used. The experimental results are found to favour the Poole-Frenkel type of electronic conduction mechanism, yielding values of the coefficient β PF that are relatively larger than the expected theoretical value. Some of the proposals suggested by other workers to explain the observed discrepancy in the β PF values are discussed, but in the present work it is mainly attributed to the different geometry of the electrodes in our test samples, and it is shown analytically that the fringe field region has an influence on the conduction characteristics that increases with increasing gap.

The identification of secondary particles by ionisation sampling (ISIS)

A large volume multi-wire proportional chamber employing drift paths of up to 2 m in an unconventional geometry is described. This device will distinguish between kaons, pions and protons for all secondaries from 5 GeV/c to over 100 GeV/c and obtain over 300 transverse position measurements on each track to an accuracy of approximately 2 mm. Results from experiments on a prototype in a test beam are given and a possible design for use with a rapid cycling bubble chamber at the CERN-SPS or NAL accelerators is discussed.

The edge-irradiated glancing-angle X-ray diffraction film technique

An analysis is given of the effects of absorption on the reflected and transmitted components of the x-ray beam diffracted from bulk specimens irradiated at glancing angles such that the edge of the specimen is in the centre of the incident beam. It is shown that this unconventional geometry leads to a situation in which the transmitted component arises mainly from the surface layers of the specimens, whereas the reflected component arises mainly from the bulk of the specimen. It is further shown that the reflected component has a linewidth not much different from that of the incident beam, whilst the transmitted component has a width at least an order of magnitude smaller than that of the incident beam. The relevance of this analysis to the identification of thin oxide layers present on bulk specimens is briefly discussed.