Resolution Of Gradients Research Articles

Boundary element analysis of adhesively bonded joints

Boundary element method has proven to have very good resolution of large stress gradients, yet its application in analysis of bonded joints is practically non-existent even though large stress gradients exist in the bonded region and bonded joints are one of the critical technology in modern design. This paper describes application of boundary element method to bonded joints, the numerical modeling considerations which have limited this application, how these limitations can be overcome, and the future research that is needed for the growth of the boundary element methodology for bonded joint application. Numerical results of single, double lap joints with several spew angles demonstrate the potential of boundary element method in analysis of bonded joints.

Heterogeneity within the subducting Pacific slab beneath the Izu–Bonin–Mariana arc: Evidence from tomography using 3D ray tracing inversion techniques

Distinct zones of seismic heterogeneity along the Izu–Bonin–Mariana arc have been investigated using complimenting regional bulk sound, shear wave speed, and P-wave tomographic images. The distribution of seismic anomalies and the inferred geometry of the subducting Pacific plate have been modelled in unprecedented detail using joint tomography and new P-wave model using a 3D inversion algorithm. The use of 3D ray tracing techniques and smaller cell parameterizations have greatly enhanced the resolution of gradients, therefore, the models show much more detail about the structure and physical properties of the subduction zone. The well-defined features from the multiple wave speed images are used to elucidate the distinct morphology change between slab beneath the Izu–Bonin (horizontal slab) and Mariana (vertical) arcs and the distribution of physical properties in the mantle and the subducting oceanic lithosphere. Changes in physical properties within the slab tear at the southern end of the Izu–Bonin arc, identified as a “gap or thinning” in the tomographic images, could be the result of the distortion of the Pacific plate as its shape transforms between near horizontal to near vertical, a decrease in the rigidity and strength of the lithosphere, the subduction of the Marcus–Necker Ridge, change in subduction velocity, or a combination of all these factors. Strong, slow anomalies exist both, in the mantle wedge, above and below the slab beneath the Izu–Bonin arc but are not present beneath the Mariana arc. At the junction between the Izu–Bonin and Mariana arcs it appears that the there are two separate pieces of the Pacific plate: the torn slab north of 26° N and a buckled near vertically dipping slab south of the tear. The transition between the two distinct slab morphologies coincides with the location of the Ogasawara Plateau and the trench.

Development and evaluation of a sampling system to determine gaseous Mercury fluxes using an aerodynamic micrometeorological gradient method

An aerodynamic gradient micrometeorological approach to the measurement of total gaseous mercury (TGM) flux has been developed. This method has been applied in many field studies for the characterization of TGM flux from various mercuriferous substrates. The resolution of the gradient method depends on the sampling systems characteristics and has been demonstrated to be on the order of 0.01 ± 0.01 ng Hg m−3 or better. The method is best suited to measuring high‐emitting sites such as studied here. The TGM flux resolution is based on the gradient resolution and depends on the site characteristics and the atmospheric condition. For a typical friction velocity u* of 0.1 m s−1 and gradient intake heights of 0.15 and 0.4 m the method can resolve a TGM flux on the order of 1.5 ng m−2 h−1. The system can be configured for two‐level or multilevel sampling, as needed. The method compares well with other micrometeorological methods as demonstrated during the Nevada storms intercomparison study. The micrometeorological method is shown to compare well with chamber techniques under comparable conditions.

SQUID technology for geophysical exploration

We report on successful tests of planar LTS SQUID gradiometers on airborne platforms such as helicopter and aircraft. The system works stable and allows profile work without any constraints. In mobile applications the gradient resolution at low frequencies is dominated by motion noise, since the parasitic areas of the SQUID gradiometer lead to strong disturbances if the gradiometer is tilted in the homogenous Earth's magnetic field. The balance can be improved further by software using data of a SQUID magnetometer triple. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of multialkali and GaAs photocathode detectors for Joint European Torus edge light detection and ranging Thomson scattering profiles

The Joint European Torus (JET) tokamak has two light detection and ranging (LIDAR) Thomson scattering systems, one for the core and one dedicated to the edge Te and ne profiles. The LIDAR scheme is unique to JET and is envisaged for use on ITER. The system’s spatial resolution is defined by the convolution product of its components: laser pulse duration, detector response time, and digitizer speed. The original multialkali photocathode microchannel plate photomultipliers dictated the response time, resulting in a 12 cm spatial resolution along the line of sight. In the edge LIDAR system, this is improved by aligning the line of sight with the flux surfaces, thus improving the effective spatial resolution to 2 cm depending on the plasma configuration. To meet demands for better edge gradient resolution, an upgrade to higher quantum efficiency detectors was proposed. Four GaAs photocathode detectors have been procured, two of which surpass expectations. These detectors are shown to have a more than two times higher effective quantum efficiency and their response time is at least twice as fast as the multialkali detectors. Combined with a fast digitizer this improves the spatial resolution by a factor of two, down to one centimeter effective, depending on plasma configuration.

The design of a new concept HTSC axial gradiometer

The design of a new-concept axial gradiometer is discussed, which employs a gradiometric pick-up loop formed by etching a transformer loop structure on flexible superconducting tape. The pick-up loop is inductively coupled to a SQUID magnetometer. Sensitivity to the magnetic field in the direction transverse to the gradiometer axis is problematic in some applications and can be reduced by matching the mutual inductance between the secondary loop of the flux transformer and the magnetometer to establish a condition of shielding. The conditions for optimum magnetic effective volume are discussed and it is shown that a gradient resolution better than 0.01 nT/(m Hz ) is achievable for a reasonable choice of device parameters.

Kinetic flux-vector splitting schemes for the hyperbolic heat conduction

A kinetic solver is developed for the initial and boundary value problems of the symmetric hyperbolic moment system. This nonlinear system of equations is related to the heat conduction in solids at low temperatures. The system consists of a conservation equation for the energy density e and a balance equation for the heat flux Q i , where e and Q i are the four basic fields of the theory. We use kinetic flux vector splitting (KFVS) scheme to solve these equations in one and two space dimensions. The flux vectors of the equations are splitted on the basis of the local equilibrium distribution of phonons. The resulting computational procedure is efficient and straightforward to implement. The second-order accuracy of the scheme is achieved by using MUSCL-type reconstruction and min-mod nonlinear limiters. The solutions exhibit second-order accuracy and satisfactory resolution of gradients with no spurious oscillations. The scheme is extended to the two-dimensional case in a usual dimensionally split manner. In order to prescribe the boundary data, we need the knowledge of the e and Q i . However, in experiments only one of the quantities can be controlled at the boundary. This problem is removed by using a continuity condition. It turned out that after some short time energy and heat flux are related to each other according to Rankine–Hugoniot jump relations. To illustrate the performance of the KFVS scheme, we perform several one- and two-dimensional test computations. For the comparison of our results, we use high order central schemes. The present study demonstrates that this kinetic method is effective in handling such problems.

A high-Tc SQUID gradiometer with integrated homogeneous field compensation

An integrated gradiometer system is presented, consisting of a single-layer high-temperaturesuperconductor dc superconducting quantum interference device (SQUID) and two coils,realized on printed circuit boards. The coils compensate the homogeneous background field,thus allowing the gradiometer to be freely moved in the Earth’s magnetic field. Thegradiometer signal is not harmed by any additional noise introduced by the homogeneousfield compensation. Therefore, a noise-limited magnetic field gradient resolution of15 pT m−1 Hz−1/2 witha 1/f corner frequency of 2 Hz was achieved. This is preserved even after motion in the Earth’smagnetic field. Because of the small dimensions of the compensation coils, the gradiometersystem can also be used for non-destructive evaluation. Preliminary measurementswere performed by moving the SQUID across a workpiece, without any shielding.

Highly balanced single-layer high-temperature superconductor SQUID gradiometer freely movable within the Earth's magnetic field

We developed a gradiometer system based on a single-layer high-temperature superconductor dc superconducting quantum interference device (SQUID), which can be freely moved within the Earth's magnetic field during measurement. The problem of circumferential shielding currents in the parallel gradiometer pick-up loop is solved by the use of an appropriately designed magnetometer SQUID integrated on the gradiometer chip. The magnetometer's feedback coil of the flux-locked loop is laid out as a small Helmholtz coil pair, thus keeping the homogeneous magnetic field constant for both the magnetometer and the gradiometer. Therefore, the balance of the directly coupled gradiometer SQUID is enhanced from 100 up to 3800. The noise limited magnetic field gradient resolution of 45 pT m−1 Hz−1/2 is preserved down to frequencies of several Hz even after strong motion in the Earth's magnetic field.

Operation of high-temperature superconductor magnetometer with submicrometer bicrystal junctions

We investigated the noise properties of dc superconducting quantum interference device flip-chip magnetometers with submicrometer-wide bicrystal junctions operating at 77.4 K. The noise of the magnetometers with electronics was about 6 fT/√Hz at frequencies above 100 Hz increasing up to about 20 fT/√Hz at 1 Hz. The operation of the magnetometers was characterized in an electronic axial first order gradiometer system, which was employed for biomagnetic measurements. The system demonstrated a gradient resolution of about 1 fT/cm√Hz at 77.4 K and stable operation in a standard magnetically shielded room under clinical conditions.

Improvement of the field gradient resolution of galvanically coupled DC-SQUID gradiometers on 10×10 mm 2 bicrystal substrates

We describe an improved design of galvanically coupled DC-SQUID gradiometers on a substrate with a restricted size of 10×10 mm 2. Considering this design the planar gradiometer antennas are placed in the diagonal of a bicrystalline substrate. The galvanically coupled DC-SQUID in the centre of the gradiometer bases on bicrystal grain boundary Josephson junctions. This new antenna geometry improves the product of the baselength and the antenna efficiency by 50% compared to usual planar gradiometer designs with the gradiometer axis parallel to the substrate edge. Field gradient resolutions of 218 fT/(cm Hz ) in the white noise region and 1.75 pT/(cm Hz ) at 1 Hz can be obtained in electrically and magnetically unshielded environment. With a considerably improved gradiometer balance higher than 100, this sensor design enables measurements in unshielded environments.

Sensitive HTS DC-SQUID gradiometers for magnetic evaluation applications

An axial HTS DC-SQUID gradiometer formed by electronic subtraction with a 10 cm baseline and a gradient resolution better than 1 fT/cm Hz at 77 K has been developed and demonstrated. The best achieved gradient resolution for the planar HTS DC-SQUID gradiometer was of about 30 fT/cm Hz for both shielded and unshielded operation. A comparison of the characteristics of the gradiometers with ones of competitive room temperature sensors like fluxgates is discussed.

The influence of source terms on a gradient-based curvilinear grid scheme applied to a reacting flow

One-dimensional simulations provide an appropriate level of detail for studying the reactor dynamics of gas–solid metal hydride absorption heat pumps, which are a class of reacting flows. Because of the high energy densities and consequent difficulty in optimizing the reactors for finite time thermal performance, steep concentration gradients develop near energy transfer boundaries. With a uniform and fixed grid capable of resolving the gradients, the computational cost of a simulation can be considerable. Adaptive grid methods have been shown to reduce the computational cost in several classes of problems by reducing the overall discretization error compared to fixed grids using the same number of nodes through redistribution of node density. However the class of problems including mass and energy source terms, which are a prominent feature in reacting flows, are not well represented in the literature. In this paper an adaptive moving grid formulation is developed for a model of a metal hydride heat pump and the influence of the source terms evaluated. The grid satisfies an elliptic mesh equation to ensure smoothness, and grid motion is determined by a gradient-based logarithmic weight function of the absorbed hydrogen concentration, which avoids grid collapse. The strategy is shown to improve the resolution of monotonic gradients near heat transfer boundaries. The scheme performs less well when features exhibiting curvature appear simultaneously.

Development and investigation of novel single-layer gradiometers using highly balanced gradiometric SQUIDs

The sensitivity of galvanically coupled single-layer gradiometers on bicrystal substrates for homogeneous magnetic fields can be minimized by an increase of the balance, which is the ratio of the effective area of the antennas and the effective area of the SQUID. In this paper, we describe possible realizations of new gradiometric SQUIDs (G-SQUIDs) on 10 × 10 mm2 STO-bicrystal substrates, which minimize the effective area of the SQUID, and therefore the parasitic area of the gradiometer, to 34 μm2. We integrate these gradiometric SQUIDs in well-defined gradiometer antennas. The G-SQUID layouts with coupling inductances between 20 pH and 100 pH do not show any dependence of the parasitic area from the SQUID inductance. With a gradient field resolution of 1.6 pT(cm√Hz)−1 (white, unshielded) at 24 pH and 1.8 pT(cm√Hz)−1 at 55 pH, these gradiometers possess outstanding noise characteristics. So the balance of comparable conventional gradiometers, which show values of about 100, can be increased by more than an order of magnitude to 1070 with the new layouts.This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.

HTS SQUID gradiometer for application without shielding

An HTS SQUID gradiometer consisting of a series gradiometer pickup loop in flip-chip configuration with a small washer SQUID is presented. The series gradiometer configuration is advantageous for applications without shielding because of the avoidance of circulating shielding currents in the pickup loop. With a 20 mm × 10 mm substrate for the pickup loop a large effective area of 1.25 mm2 and a gradiometer baseline of 9.6 mm are achieved. The balance of the gradiometer impaired by the washer read-out SQUID can significantly be enhanced by the weighted subtraction of a reference SQUID's signal. With this a balance of 5 × 104 is reached. The noise-limited magnetic field gradient resolution of the balanced SQUID gradiometer is 38 fT cm−1 Hz−1/2.This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.

Long baseline thin film SQUID gradiometers

We have developed a highly sensitive SQUID gradiometer for unshielded operation in the Earth's magnetic field. It is implemented in an all-refractory Nb-AlO/sub x/-Nb technology, and consists of gradiometer pick-up loops with an inductance of about 250 nH coupled to the SQUID by a flux transformer. The SQUID is designed as a highly balanced second order gradiometer consisting of four pairs of washers. It has an inductance of about 300 pH. The gradiometer chip has a size of 6 cm /spl times/2 cm. The baseline of the gradiometer is 4 cm and the effective volume is about 300 mm/sup 3/. We measured a white noise level of below 10 /spl mu//spl Phi//sub 0//Hz/sup 1/2 /, corresponding to a gradient field resolution smaller than 40 fT/m/spl middot/Hz/sup 1/2 /. The total flux noise in the frequency band from 0.01 Hz to 10 Hz is below 150 /spl mu//spl Phi//sub g/. The balance of the gradiometer was determined by moving the gradiometer in the Earth's magnetic field. Due to the high balance we can operate the gradiometer in field experiments.

High-Tc SQUID magnetometers for use in moderate magnetically-shielded room

We have fabricated and characterized high-T/sub c/ planar SQUID magnetometers and first derivative gradiometers with directly-coupled pickup loops. The devices were made from single layer of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film on LSAT bicrystal substrate with 30/spl deg/ or 24/spl deg/ misorientation angle. Magnetic properties were investigated by applying a magnetic field B/sub 0/ for the SQUID magnetometer patterned with holes to reduce the maximum structural width. We found an increasing low frequency noise with cooling fields B/sub 0/ larger than 1.5 /spl mu/T. This value consists with the threshold field estimated from the maximum structural width. A magnetic field noise level of 500 fT/Hz/sup 1/2/ at 10 Hz was observed by using FLL electronics with a bias current reversal in a moderate magnetically-shielded room consisting of only two 1 mm thick layers of permalloy. Measurements of magnetocardiograms demonstrate the suitability of this magnetometer for biomagnetic applications. On the other hand, the gradiometer with two symmetric pickup loops was operated without any shielding. The performance obtained was a field gradient resolution of about 1 pT/cmHz/sup 1/2/ at 1 kHz and 10 pT/cmHz/sup 1/2/ at 1 Hz, with a baseline of 4 mm. The imbalance of this gradiometer was around 0.7%, limited by the sensitivity to homogeneous field of the SQUID itself.

Improvement of spatial and field resolution in NDE systems using superconducting sensors

We present a NDE scanning system for industrial purposes in magnetically and electrically unshielded environment. In this scanning system, planar galvanically coupled dc SQUID gradiometers with a field gradient resolution of 300 fT.(cmHz/sup 1/2 /) in the white noise region are used. The spatial resolution is above 1 mm in this case. On the other hand many applications in NDE do not require the extraordinary sensitivity of SQUID based sensors. For those purposes hybrid sensors, in which Hall sensors are connected with high-T/sub c/ superconducting antennas, are an alternative sensor concept less sensitive than SQUID sensors but exhibiting better performance compared to other commercially available magnetic field sensors.

Inductance matching of high- TC dc-SQUIDs in planar gradiometers using a laser patterning technique

The inductance of the dc-SQUID in a planar galvanically coupled gradiometer of YBa 2Cu 3O 7− x was changed by lengthening the SQUID loop with a focused laser beam. The dc-SQUID inductance was increased from 50 pH to values close to 300 pH by this technique. This increased SQUID inductance changed the effective area of the gradiometer device as well as the SQUID parameter β L. We focused our investigations on the change of the peak-to-peak voltage of the SQUID modulation, on the change of the white noise properties as well as on the change in low frequency noise depending on an effective area and SQUID parameter β L. Different fixed critical currents of the dc-SQUID were considered in order to determine their influence on SQUID operation. For critical currents below 200 μA, a maximum of the white field gradient resolution could be achieved for corresponding SQUID parameters β L between 1.5 and 6 with 340 fT/(cm Hz 1/2) as the lowest noise level. Here, the necessity of a high I C R N product, an optimum critical current and an optimum dc-SQUID inductance was shown in order to realize a high device sensitivity. The influence of the grain boundary angle to achieve a high field gradient resolution even in unshielded environment as well as the combination of the increase of SQUID inductance with an additional ion beam etching technique for critical current reduction is considered.

Nonsurgical septal reduction therapy for hypertrophic obstructive cardiomyopathy: one-year follow-up

OBJECTIVEThe objective of this study is to evaluate the one-year outcome of the first 50 patients who underwent nonsurgical septal reduction for symptomatic hypertrophic obstructive cardiomyopathy at our institution.BACKGROUNDLeft ventricular outflow tract obstruction is an important determinant of clinical symptoms in patients with hypertrophic obstructive cardiomyopathy. Nonsurgical septal reduction is a new therapy that has been shown to result in left ventricular outflow tract gradient reduction and resolution of symptoms immediately after the procedure and on midterm follow-up.METHODSFifty patients with hypertrophic obstructive cardiomyopathy who underwent nonsurgical septal reduction at our institution and completed 1-year follow-up are described. Complete history, physical examination, two-dimensional echocardiography with Doppler and exercise treadmill testing have been analyzed.RESULTSThe mean age of the study group was 53 ± 17 years. All patients had refractory symptoms before enrollment. Ninety-four percent had class III or IV New York Heart Association class symptoms at baseline compared to none at 1 year (p < 0.001). The exercise duration increased by 136 s at 1 year (p < 0.021). Only 20% of patients were either receiving beta-blockers or calcium-channel blockers on follow-up. The resting left ventricular outflow tract gradient decreased from 74 ± 23 mm Hg to 6 ± 18 mm Hg (p < 0.01) and from 84 ± 28 mm Hg to 30 ± 33 mm Hg (p < 0.01) in patients with dobutamine-provoked gradient at one year. These changes are associated with decreased septal thickness and preserved systolic function.CONCLUSIONNonsurgical septal reduction therapy is an effective therapy for symptomatic patients with hypertrophic obstructive cardiomyopathy with persistence of the favorable outcome up to one year after the procedure.