Showing papers by "Georgia Institute of Technology published in 1985"

Lithium niobate: Summary of physical properties and crystal structure

Abstract: Ferroelectric lithium niobate (LiNbO3) is widely used in integrated and guided-wave optics because of its favorable optical, piezoelectric, electro-optic, elastic, photoelastic, and photorefractive properties. However, detailed summaries of its pertinent physical properties and crystal structure are not readily available. In this tutorial paper, the important tensor physical properties and their mathematical descriptions are compiled and presented. The essential features of the structure of lithium niobate, including its hexagonal and rhombohedral unit cells, are illustrated and the principal (Cartesian) axes used in the description of the anisotropic properties are specified relative to the crystal structure. Errors in property coefficient values and structure information that have been propagated in the literature are corrected.

Analysis and applications of optical diffraction by gratings

Abstract: Diffraction characteristics of general dielectric planar (slab) gratings and surface-relief (corrugated) gratings are reviewed. Applications to laser-beam deflection, guidance, modulation, coupling, filtering, wavefront reconstruction, and distributed feedback in the fields of acoustooptics, integrated optics, holography, and spectral analysis are discussed. An exact formulation of the grating diffraction problem without approximations (rigorous coupled-wave theory developed by the authors) is presented. The method of solution is in terms of state variables and this is presented in detail. Then, using a series of fundamental assumptions, this rigorous theory is shown to reduce to the various existing approximate theories in the appropriate limits. The effects of these fundamental assumptions in the approximate theories are quantified and discussed.

Iterated function systems and the global construction of fractals

Abstract: Iterated function systems (i. f. ss) are introduced as a unified way of generating a broad class of fractals. These fractals are often attractors for i. f. ss and occur as the supports of probability measures associated with functional equations. The existence of certain ‘ p -balanced’ measures for i. f. ss is established, and these measures are uniquely characterized for hyperbolic i. f. ss. The Hausdorff—Besicovitch dimension for some attrac­tors of hyperbolic i. f. ss is estimated with the aid of p -balanced measures. What appears to be the broadest framework for the exactly computable moment theory of p -balanced measures — that of linear i. f. ss and of probabilistic mixtures of iterated Riemann surfaces — is presented. This extensively generalizes earlier work on orthogonal polynomials on Julia sets. An example is given of fractal reconstruction with the use of linear i. f. ss and moment theory.

The polynomial hierarchy and a simple model for competitive analysis

Abstract: The multi-level linear programs of Candler, Norton and Townsley are a simple class of sequenced-move games, in which players are restricted in their moves only by common linear constraints, and each seeks to optimize a fixed linear criterion function in his/her own continuous variables and those of other players. All data of the game and earlier moves are known to a player when he/she is to move. The one-player case is just linear programming. We show that questions concerning only the value of these games exhibit complexity which goes up all levels of the polynomial hierarchy and appears to increase with the number of players. For three players, the games allow reduction of theΣ 2 andΠ 2 levels of the hierarchy. These levels essentially include computations done with branch-and-bound, in which one is given an oracle which can instantaneously solve NP-complete problems (e.g., integer linear programs). More generally, games with (p + 1) players allow reductions ofΣ p andΠ p in the hierarchy. An easy corollary of these results is that value questions for two-player (bi-level) games of this type is NP-hard.

Effect of delamination of axially loaded homogeneous laminated plates

Abstract: A simple one-dimensional model has been developed and analyzed for predicting delamination buckling loads. The model is employed to predict critical loads for delaminated homogeneous plates with both simply supported and clamped ends. The effects of delamination position, size, and thickness on the critical loads are studied in detail for both sets of boundary conditions. The results reveal that for certain geometries the buckling load can serve as a measure of the load carrying capacity of the delaminated configuration. In other cases, the buckling load is very small and delamination growth is a strong possibility, depending on the toughness of the material.

Infrared determination of the degree of substitution of hydroxyl by carbonate ions in human dental enamel

Abstract: About 11 +/- 1% of the carbonate ions in a human tooth enamel specimen (dense fraction, sp. g. greater than 2.95) were found to be in the A-sites, replacing hydroxyl ions. The determination was made with a difference infrared spectrometry technique utilizing both tooth enamel and a reconstituted biological apatite with a known amount of replacement of hydroxyl by carbonate ions.

Numerical simulation of the turbulent Rayleigh–Bénard problem using subgrid modelling

Abstract: A numerical simulation of turbulent natural convection (the Rayleigh–Benard problem) has been conducted using large-eddy-simulation (LES) methods and the results compared with several experiments. The development of the LES equation is outlined and discussed. The modelling of the small-scale turbulent motion (called subgrid modelling) is also discussed. The resulting LES equations are solved and data collected over a short period of time in a similar manner to the direct simulation of the governing conservation equations. An explicit, second-order accurate, finite-difference scheme is used to solve the equations. Various average properties of the resulting flow field are calculated from the data and compared with experimental data in the literature. The use of a subgrid model allows a higher value of Ra to be simulated than was previously possible with a direct simulation. The highest Ra successfully simulated was 2.5 × 106. The problems at higher values of Ra are discussed and suggestions for improvements made.

A process model of cased-based reasoning in problem solving

Abstract: Much of the problem solving done by both novices and experts uses "case-based" reasoning, or reasoning by analogy to previous similar cases. We explore the ways in which case-based reasoning can help in problem solving. According to our model, transfer of knowledge between cases is guided largely by the problem solving process itself. Our model shows the interactions between problem solving processes and memory for experience. Our computer program, called the MEDIATOR, illustrates case-based reasoning in interpreting and resolving common sense disputes.

A Two Surface Model for Transient Nonproportional Cyclic Plasticity, Part 1: Development of Appropriate Equations

Abstract: A two surface stress space model is introduced with internal state variable repositories for fading memory of maximum plastic strain range and non-proportionality of loading Evolution equations for isotropic hardening variables are prescribed as a function of these internal variables and accumulated plastic strain, and reflect dislocation interactions that occur in real materials The hardening modulus is made a function of prior plastic deformation and the distance of the current stress point from the limit surface The kinematic hardening rules of Mroz and Prager are used for the yield and limit surfaces, respectively The structure of the model is capable of representing essential aspects of complex nonproportional deformation behavior, including direction of the plastic strain rate vector, memory of plastic strain range, cross-hardening effects, variation of hardening modulus, cyclic hardening or softening, cyclic racheting, and mean stress relaxation

Fractals in fractography

Abstract: The fractal characteristics of a series of fractured AISI 4340 steel specimens were studied experimentally by means of vertical sections through the fracture surface. Extensive fractal data have been generated for both the fracture surfaces and their profiles. Fractal plots for the irregular profile curves differ significantly from those of the infinitely subdivisible curves with “self-similarity” as postulated by Mandelbrot. Instead of a linear fractal curve and constant fractal dimension D , we find a reversed sigmoidal curve (RSC) and variable D . Because of the inadequacies of the linear Richardson fractal equation, we have developed an alternative procedure whereby constant slopes β and γ are obtained from the RSCs for both profiles and surfaces respectively. Since a fractal equation for surfaces apparently does not exist, we propose one that parallels the Richardson equation for profiles. Slopes from the RSCs are related to the new constant fractal dimensions D β and D γ in the modified fractal equations for profiles and fracture surfaces. Some insight into the physical nature of the fractal dimension is afforded by its close similarity to fracture roughness parameters that have simple physical meanings. It is believed that the results of the extensive study may have general validity for the irregular complex curves and surfaces of nature.

Mammalian chymotrypsin-like enzymes: comparative reactivities of rat mast cell proteases, human and dog skin chymases, and human cathepsin G with peptide 4-nitroanilide substrates and with peptide chloromethyl ketone and sulfonyl fluoride inhibitors

Abstract: The extended substrate binding sites of several chymotrypsin-like serine proteases, including rat mast cell proteases I and II (RMCP I and II, respectively) and human and dog skin chymases, have been investigated by using peptide 4-nitroanilide substrates. In general, these enzymes preferred a P1 Phe residue and hydrophobic amino acid residues in P2 and P3. A P2 Pro residue was also found to be quite acceptable. The S4 subsites of these enzymes are less restrictive than the other subsites investigated. The substrate specificity of these enzymes was also investigated by using substrates which contain model desmosine residues and peptides with amino acid sequences of the physiologically important substrates angiotensin I and angiotensinogen and alpha 1-antichymotrypsin, the major plasma inhibitor for chymotrypsin-like enzymes. These substrates were less reactive than the most reactive tripeptide reported here, Suc-Val-Pro-Phe-NA. The thiobenzyl ester Suc-Val-Pro-Phe-SBzl was found to be an extremely reactive substrate for the enzymes tested and was 6-171-fold more reactive than the 4-nitroanilide substrate. The four chymotrypsin-like enzymes were inhibited by chymostatin and N-substituted saccharin derivatives which had KI values in the micromolar range. In addition, several potent peptide chloromethyl ketone and substituted benzenesulfonyl fluoride irreversible inhibitors for these enzymes were discovered. The most potent sulfonyl fluoride inhibitor for RMCP I, RMCP II, and human skin chymase, 2-(Z-NHCH2CONH)C6H4SO2F, had kobsd/[I] values of 2500, 270, and 1800 M-1 s-1, respectively. The substrates and inhibitors reported here should be extremely useful in elucidating the physiological roles of these proteases.

Hemodynamics of the normal human carotid bifurcation: in vitro and in vivo studies.

Abstract: The spatial and temporal characteristics of blood flow in the normal adult human carotid bifurcation are investigated by two different methods: in vitro pulsatile flow model experiments using laser Doppler anemometry and in vivo studies employing pulsed Doppler velocity measurements obtained with an ultrasound duplex scanner. Glass and Plexiglas models based upon arteriographic measurements were evaluated with laser Doppler anemometer methods for pulsatile flow. A similarity approach permits the model study to be geometrically and hydrodynamically accurate with respect to the human carotid bifurcation. These parallel but separate approaches were originally performed by the principal authors without knowledge of each others' work. Normal flow patterns in the proximal internal carotid artery are demonstrated to include: unidirectional, helical, transient reversal, and low velocity regions of flows. The characterization of these complex temporal and spatially variant flow fields required the high sample volume resolution afforded by the model study. Pulsed Doppler ultrasound and a novel method of positioning the sample volume permitted a qualitative description of the complex flow velocity fields in the normal human bifurcation. Results of the two methods are compared and a striking similarity between the two methods is observed for the primary and secondary flow features. The problem of associating blood flow velocity disturbances with the presence of intralumenal disease is addressed in the discussion. It is suggested that the flow disturbances associated with the normal carotid bifurcation are different from those associated with intraluminal disease and further, that the secondary flow structures can be usefully employed to establish normalcy.

An indicator of cross-disciplinary research

Abstract: Study of interdisciplinary research processes and performance is hampered by a lack of data. This project investigated possible indicators based in the open scientific literature to measure such processes. Focusing on theJournal Citation Reports as a suitable data base, alternative indicators were validated on a sample of 383 articles drawn from 19 journals. The results support the use ofCitations Outside Category as an indicator of cross-disciplinary research activity. An estimated version of this indicator is used to examine three research categories — Demography, Operations Research/Management Science, and Toxicology — as to the extent of cross-disciplinary citation occurring by the journals in these categories and to them. Results suggest thatCitations Outside Category can be a quite informative bibliometric measure. A key substantive finding is that citation across broad field categories (engineering, life sciences, physical sciences, and social sciences) is extremely infrequent.

Applications of the method of partial inverses to convex programming: Decomposition

Abstract: A primal–dual decomposition method is presented to solve the separable convex programming problem Convergence to a solution and Lagrange multiplier vector occurs from an arbitrary starting point The method is equivalent to the proximal point algorithm applied to a certain maximal monotone multifunction In the nonseparable case, it specializes to a known method, the proximal method of multipliers Conditions are provided which guarantee linear convergence

Construction of fractal objects with iterated function systems

Abstract: In computer graphics, geometric modeling of complex objects is a difficult process. An important class of complex objects arise from natural phenomena: trees, plants, clouds, mountains, etc. Researchers are at present investigating a variety of techniques for extending modeling capabilities to include these as well as other classes. One mathematical concept that appears to have significant potential for this is fractals. Much interest currently exists in the general scientific community in using fractals as a model of complex natural phenomena. However, only a few methods for generating fractal sets are known. We have been involved in the development of a new approach to computing fractals. Any set of linear maps (affine transformations) and an associated set of probabilities determines an Iterated Function System (IFS). Each IFS has a unique "attractor" which is typically a fractal set (object). Specification of only a few maps can produce very complicated objects. Design of fractal objects is made relatively simple and intuitive by the discovery of an important mathematical property relating the fractal sets to the IFS. The method also provides the possibility of solving the inverse problem. given the geometry of an object, determine an IFS that will (approximately) generate that geometry. This paper presents the application of the theory of IFS to geometric modeling.

A Model for Solar Spectral Irradiance and Radiance at the Bottom and Top of a Cloudless Atmosphere

Abstract: A simple model is presented that, in a cloud-free atmosphere, calculates solar spectral direct and diffuse irradiance and directional radiance at the surface, spectral absorption within the atmosphere and the upward reflected spectral irradiance or directional radiance at the top of the atmosphere. The irradiance model, based on similar approaches by Brine and Iqbal and others, evaluates the spectral irradiances between 0.29 and 4.0 μm, with a resolution that varies from 0.005 to 0.1 μm. Absorption by water vapor, ozone and the uniformly mixed gases is included, as are both scattering and absorption by atmospheric aerosols, which are modeled with simple wavelength-dependent optical depth, single scattering albedo and asymmetry parameter functions. Comparisons are presented of the model results with spectral irradiance and radiance computed by other more sophisticated models and with measurements from both ground-based and satellite instruments. The reasonable accuracy and simplicity of the model ...

Monoclinic Crystal Structures of ZrO2 and HfO2 Refined from X-ray Powder Diffraction Data

Abstract: The crystal structures of ZrO2 and HfO2 (P21/c) were refined in detail from X-ray powder diffraction data. The precision obtained for the atom positions for both compounds is comparable to that of previous single-crystal work.

A progression of high-frequency RCS prediction techniques

Abstract: Since the widespread use of radar in World War II, engineers have grappled with the problem of calculating the radar echo characteristics of a wide variety of bodies, and over the years methods have steadily improved. This paper surveys a progression of methods, not necessarily in chronological order, each of which offers its own peculiar advantage. We begin with the caveat that large, complex targets are of prime current interest, hence the scattering calculations are for the "high-frequency" regime. As such, the method of moments, although an extremely powerful tool, is not suitable for routine computations for large bodies. Geometric optics, perhaps the oldest method in use, is a simple prescription, but gives the wrong answer for flat and singly curved surfaces, and no answer at all if the specular point is not on the surface. Physical optics does yield results in those cases, but fails by progressively wider margins as the scattering direction swings farther from the specular direction. The theories of Keller and Ufimtsev introduce edge-diffraction terms that improve the prescription, but the diffraction coefficients are poorly behaved in the transition regions of the shadow and reflection boundaries. The uniform theory of Kouyoumjian and Pathak provides the proper behavior of the transition regions, but the scattering direction is constrained to lie on the Keller cone. A method of equivalent currents suggested by Ryan and Peters, extended by Knott and Senior, and refined by Michaeli allows the scattering direction to be arbitrary, but the equivalent currents are nonphysical because they depend on the direction of the observation. Mitzner developed an "incremental length diffraction coefficient" which extends Ufimtsev's theory much as Michaeli's equivalent currents extend Keller's theory. None of these methods can handle the surface traveling wave, a significant echo mechanism for long, smooth bodies, because the methods treat localized scattering phenomena, while the surface traveling wave involves the entire surface. However, as demonstrated by Ross, the repeated application of edge-diffraction theory to account for multiple interactions between pairs of parallel edges comes close to giving the correct result. Unfortunately, there has been no extension of this application to the routine calculation of the surface traveling wave contribution for arbitrary surface geometries. Moreover, practical schemes developed for re-entrant structures and for the interaction between scattering elements consume a great deal of computer time. Thus while our repertory of useful computation methods has greatly expanded in the last 40 years, there remain some scattering mechanisms we still cannot treat in a routine fashion.

A low noise receiver for millimeter and submillimeter wavelengths

Abstract: A broadband, low noise heterodyne receiver, suitable for astronomical use, has been built using a Pb alloy superconducting tunnel junction (SIS). The RF coupling is quasioptical via a bowtie antenna on a quartz lens and is accomplished without any tuning elements. In this preliminary version the double sideband receiver noise temperature rises from 205 K at 116 GHz to 375 K at 349 Ghz, and to 815 K at 466 GHz. This is the most versatile and sensitive receiver yet reported for sub-mm wavelengths.

Reactions to Feedback: The Role of Attributions

Abstract: This study tested reactions to feedback in an experimental simulation. Feedback specificity and information that could be used to make causal inferences were manipulated through varying information...

The relationship between Mitzner's ILDC and Michaeli's equivalent currents

Abstract: Michaeli recently derived a set of equivalent edge currents for scattering directions not on the Keller cone. Some years ago, Mitzner developed his incremental length diffraction coefficient (ILDC) for the same purpose. It is shown that Michaeli's results relate to Mitzner's for arbitrary directions in identically the way Keller's relate to Ufimtsev's for directions on the Keller cone.

A large scale machine loading problem in flexible assembly

Abstract: Flexible manufacturing is characterized by versatile work stations with minimum change over times and a versatile material handling system. The loading problem in flexible manufacturing is to assign tools, material, operations and jobs to work stations in order to minimize the total number of job-to-work station assignments. In this paper, we describe a special case of the general loading problem applied to flexible assembly and develop a discrete optimization model. We then discuss approaches for obtaining good heuristic solutions and present results for a large scale study.

Application of a reciprocal transformation to a two-phase Stefan problem

Abstract: A reciprocal transformation is employed to reduce a two-phase Stefan problem in nonlinear heat conduction to a form which admits a class of exact solutions analogous to the classical Newmann solution.

Viscous effects in a collisional tokamak plasma with strong rotation

Abstract: The full viscosity tensor for an axisymmetric toroidal plasma in the collisional regime (with strong rotation) is calculated, including gyroviscosity and O(e) poloidal variations over the flux surface. It is shown that the resulting viscous force is of sufficient magnitude to account for the radial transfer of toroidal momentum that must be inferred in order to explain the rotation measurements in tokamak experiments. The consequences of a viscous force of this form and magnitude on particle transport and on the evolution of toroidal and poloidal rotation velocities are discussed.