Boundary conditions for the diffusion equation in radiative transfer

Using the method of images, we examine the three boundary conditions commonly applied to the surface of a semi-infinite turbid medium. We find that the image-charge configurations of the partial-current and extrapolated-boundary conditions have the same dipole and quadrupole moments and that the two corresponding solutions to the diffusion equation are approximately equal. In the application of diffusion theory to frequency-domain photon-migration (FDPM) data, these two approaches yield values for the scattering and absorption coefficients that are equal to within 3%. Moreover, the two boundary conditions can be combined to yield a remarkably simple, accurate, and computationally fast method for extracting values for optical parameters from FDPM data. FDPM data were taken both at the surface and deep inside tissue phantoms, and the difference in data between the two geometries is striking. If one analyzes the surface data without accounting for the boundary, values deduced for the optical coefficients are in error by 50% or more. As expected, when aluminum foil was placed on the surface of a tissue phantom, phase and modulation data were closer to the results for an infinite-medium geometry. Raising the reflectivity of a tissue surface can, in principle, eliminate the effect of the boundary. However, we find that phase and modulation data are highly sensitive to the reflectivity in the range of 80-100%, and a minimum value of 98% is needed to mimic an infinite-medium geometry reliably. We conclude that noninvasive measurements of optically thick tissue require a rigorous treatment of the tissue boundary, and we suggest a unified partial-current--extrapolated boundary approach.     

Soft-switching DC-DC converter with parallel-connected full-wave rectifiers

A soft-switching converter with parallel-connected full-wave rectifiers is presented. In the proposed converter, the primary windings of two transformers are connected in series. Two full-wave rectifiers with ripple current cancellation are connected in parallel at the output side to reduce the current stress of the secondary winding of the transformer. The clamp circuit, based on an auxiliary switch and a clamp capacitor, is connected in parallel with the primary side of the transformer to recycle the energy stored in the leakage inductance. The leakage inductance of transformers, the magnetising inductance and the clamp capacitance are resonant to achieve zero-voltage switching (ZVS) of the auxiliary switch. The resonance between the leakage inductance of the transformer and the output capacitance of the switch will achieve ZVS operation for the main switch in the proposed converter. The pulse-width modulation technique is adopted to regulate the output voltage. The operation principle and system analysis of the proposed converter are provided. Some experimental results for a 200 W (5V/40 A) prototype are given to demonstrate the effectiveness of the proposed converter.     

The effect of short time post-weld heat treatment on the fatigue crack growth of 2205 duplex stainless steel welds

The influence of γ content and its morphology on the impact and fatigue crack growth behavior of 2205 duplex stainless steel (DSS) welds were studied in this work. Short time post-heating was able to effectively raise the γ content and the impact toughness of the weld. The variation in microstructures showed less influence on the fatigue crack growth rate (FCGR) of the steel plate and weld except in the low ΔK regime. In contrast, residual welding stresses played a more significant affection on the FCGR of the DSS weld than microstructural factors did. Plastic deformation induced martensitic transformation within a definitely thin layer was responsible for the difference in crack growth behavior between specimens in the low ΔK range. Coarse columnar structure was more likely to have tortuous crack path in comparison with the steel plate.     

Efferent enhancement of a methylcholanthrene-induced sarcoma transplantable in strain 13 guinea pigs

The immunobiology of an antigenic methylcholanthrene-induced sarcoma (MC-D) of inbred strain 13 guinea pigs has been investigated. The induction of concomitant immunity by growing MC-D tumors was indicated by the suppression of small tumor inocula in the presence of a large tumor cell dose and by the regression of intradermal tumor nodules. Furthermore, it was demonstrated that this tumor was coated with antibody in vivo. Previous studies showed that MC-D tumors were infiltrated with killer T cells which were capable of complete tumor destruction in vitro, but could never induce spontaneous regression in vivo. On the basis of all these facts, antibody-mediated efferent enhancement is proposed to be the major escape mechanism of this tumor.     

Structures and magnetic properties of Co and CoFe films prepared by magnetron sputtering

Structures and magnetic properties of Co and CoFe films on Si(100) have been investigated by employing scanning tunneling microscopy, atomic force microscopy, and magneto-optic Kerr effect techniques. As the film thickness increases, Co or CoFe clusters with different sizes are observed. As the film thickness increases below 20 nm, the size of the metal clusters decreases. For thicker films, the surface roughness increases monotonously by increasing the thickness. The easy axis of magnetization for both Co/Si(100) and CoFe/Si(100) prefers to be in the surface plane. By deposition of the Co or CoFe overlayers, the evolution of the longitudinal coercive force shows similar trend to the surface roughness. Minimum coercive force coincides with the smallest roughness of the film. For a film with greater roughness, the observation of larger coercive force could be explained by the impediment of the propagation of domain wall motion by defects of the films. At a higher deposition rate, Co islands in triangle shapes with an edge length around 100 nm are observed. This nanostructure shows an hcp-Co with the c axis parallel to the surface plane and is observed to be able to stabilize the coercive force for Co/Si(100) films.     

Laminar convection to rotating disks in non-Newtonian power-law fluids

A theoretical result has been obtained for heat transfer to a rotating disk in a non-Newtonian fluid which includes the convective transport in the radial direction. The transient heat transfer problem with step change in disk temperature is studied by instant-local similarity method and the approximate solution is very accurate for small time.     

Aerosol properties over bright-reflecting source regions

Retrieving aerosol properties from satellite remote sensing over a bright surface is a challenging problem in the research of atmospheric and land applications. In this paper we propose a new approach to retrieve aerosol properties over surfaces such as arid, semiarid, and urban areas, where the surface reflectance is usually very bright in the red part of visible spectrum and in the near infrared, but is much darker in the blue spectral region (i.e., wavelength <500 nm). In order to infer atmospheric properties from these data, a global surface reflectance database of 0.1/spl deg/ latitude by 0.1/spl deg/ longitude resolution was constructed over bright surfaces for visible wavelengths using the minimum reflectivity technique (e.g., finding the clearest scene during each season for a given location). The aerosol optical thickness and aerosol type are then determined simultaneously in the algorithm using lookup tables to match the satellite observed spectral radiances. Examples of aerosol optical thickness derived using this algorithm over the Sahara Desert and Arabian Peninsula reveal various dust sources, which are important contributors to airborne dust transported over long distances. Comparisons of the satellite inferred aerosol optical thickness and the values from ground-based Aerosol Robotic Network (AERONET) sun/sky radiometer measurements indicate good agreement (i.e., within 30%) over the sites in Nigeria and Saudi Arabia. This new algorithm, when applied to Moderate Resolution Imaging Spectroradiometer (MODIS), Sea-viewing Wide Field of view Sensor (SeaWiFS), and Global Imager (GLI) satellite data, will provide high spatial resolution (/spl sim/1 km) global information of aerosol optical thickness over bright surfaces on a daily basis.     

Effect of hydride orientation on fracture toughness of Zircaloy-4 cladding

Hydrogen embrittlement is one of the major degradation mechanisms for high burnup fuel cladding during reactor service and spent fuel dry storage, which is related to the hydrogen concentration, morphology and orientation of zirconium hydrides. In this work, the J-integral values for X-specimens with different hydride orientations are measured to evaluate the fracture toughness of Zircaloy-4 (Zry-4) cladding. The toughness values for Zry-4 cladding with various percentages of radial hydrides are much smaller than those with circumferential hydrides only in the same hydrogen content level at 25 °C. The fractograghic features reveal that the crack path is influenced by the orientation of zirconium hydride. Moreover, the fracture toughness measurements for X-specimens at 300 °C are not sensitive to a variation in hydride orientation but to hydrogen concentration.     

On the parallel-decomposability of geometric problems

There is a large and growing body of literature concerning the solutions of geometric problems on mesh-connected arrays of processors. Most of these algorithms are optimal (i.e., run in timeO(n ^1/d ) on ad-dimensionaln-processor array), and they all assume that the parallel machine is trying to solve a problem of sizen on ann-processor array. Here we investigate the situation where we have a mesh of sizep and we are interested in using it to solve a problem of sizen >p. The goal we seek is to achieve, when solving a problem of sizen >p, the same speed up as when solving a problem of sizep. We show that for many geometric problems, the same speedup can be achieved when solving a problem of sizen >p as when solving a problem of sizep.The research of M. J. Atallah was supported by the Office of Naval Research under Contracts N00014-84-K-0502 and N00014-86-K-0689, the Air Force Office of Scientific Research under Grant AFOSR-90-0107, the National Science Foundation under Grant DCR-8451393, and the National Library of Medicine under Grant R01-LM05118. Jyh-Jong Tsay's research was partially supported by the Office of Naval Research under Contract N00014-84-K-0502, the Air Force Office of Scientific Research under Grant AFOSR-90-0107, and the National Science Foundation under Grant DCR-8451393.