Showing papers by "University of Texas at Arlington published in 1993"

Theory and applications of guided-mode resonance filters

Abstract: The guided-mode resonance properties of planar dielectric waveguide gratings are presented and explained. It is shown that these structures function as filters that produce complete exchange of energy between forward- and backward-propagating diffracted waves with smooth line shapes and arbitrarily narrow filter linewidths. Simple expressions based on rigorous coupled-wave theory and on classical slab waveguide theory give a clear view and quantification of the inherent TE/TM polarization separation and the free spectral ranges of the filters. Furthermore, the resonance regimes, defining the parametric regions of the guided-mode resonances, can be directly visualized. It is shown that the linewidths of the resonances can be controlled by the grating modulation amplitude and by the degree of mode confinement (refractive-index difference at the boundaries). Examples presented of potential uses for these elements include a narrow-line polarized laser, a tunable polarized laser, a photorefractive tunable filter, and an electro-optic switch. The guided-mode resonance filter represents a basic new optical element with significant potential for practical applications.

Substructure discovery using minimum description length and background knowledge

Abstract: The ability to identify interesting and repetitive substructures is an essential component to discovering knowledge in structural data. We describe a new version of our SUBDUE substructure discovery system based on the minimum description length principle. The SUBDUE system discovers substructures that compress the original data and represent structural concepts in the data. By replacing previously-discovered substructures in the data, multiple passes of SUBDUE produce a hierarchical description of the structural regularities in the data. SUBDUE uses a computationally-bounded inexact graph match that identifies similar, but not identical, instances of a substructure and finds an approximate measure of closeness of two substructures when under computational constraints. In addition to the minimumdescription length principle, other background knowledge can be used by SUBDUE to guide the search towards more appropriate substructures. Experiments in a variety of domains demonstrate SUBDUE's ability to find substructures capable of compressing the original data and to discover structural concepts important to the domain.

Divergence between Archival and Perceptual Measures of the Environment: Causes and Consequences

Abstract: Based on a review of previous theoretical and empirical research on environmental measurement, we develop a framework to explain both causes and consequences of divergence between archival and perceptual measures of the environment. Through this framework, we identity two central factors that account for this divergence: level of analysis and mediating filters. Research implications of this divergence between archival and perceptual characterizations are presented. Theoretical and methodological suggestions are advanced to improve the use of environmental measures.

The Laki (Skaftár Fires) and Grímsvötn eruptions in 1783–1785

Abstract: The Laki (Skaftar Fires) fissure eruption in southern Iceland lasted for eight months during 1783 to 1784, and produced one of the largest basaltic lava flows in historic times (14.7±1.0 km3). In addition, neighboring Grimsvotn central volcano was frequently active during the period from May 1783 to May 1785. The combined activity is interpreted as having been the result of a two-year-long volcano-tectonic episode on the Grimsvotn volcanic system. Contemporary descriptions of the explosive activity make it possible to relate the tephra stratigraphy to the progress of the eruption on a weekly basis and show that activity on the fissures propagated to the NE with time, towards Grimsvotn. The eruption at Laki began on 8 June with a brief explosive event on a short fissure, and lava rapidly began to flow into the Skafta river gorge. It reached the lowlands, 35 km away, four days later and continued to flow, with variable discharge, until 7 February 1784. Approximately 90% of the lava was emplaced in the first five months of activity. The 27-km-long vent complex is composed of tenen echelon fissures distributed on both sides of the much older Laki hyaloclastite mountain. The surface expression of each fissure is a continuous row of vents consisting of scoria cones, spatter cones, and tuff cones. Six tephra fall units are positively identified; two units are completely compsed of phreatomagmatic tephra derived from two tuff cones and the others are Strombolian deposits. The volume of tephra, including ash fall that extended to mainland Europe, is 0.4 km3 dense rock equivalent volume, or 2.6% of the total erupted volume. Interpretation of contemporary descriptions of tephra falls, combined with the preserved stratigraphy, allow the identification of ten eruptive episodes during the eight months of activity on the Laki fissures. These eruptive episodes are inferred to have resulted from the unsteady flow of magma in the feeder system. In addition, at least eight eruption episodes occurred at Grimsvotn in 1783 to 1785, five in 1783, two in 1784, and one in 1785. Each episode at Laki began with a seismic swarm of increasing intensity that led to the formation of a new fissure, the opening of which was followed by short-lived phreatomagmatic activity caused by the high water table around the eruption site. Activity usually changed to violent Strombolian or sub-Plinian, followed by Hawaiian fire fountaining and effusive activity as the availability of groundwater dwindled. Thus, the explosive activity associated with the opening of each fissure was largely controlled by external watermagma interactions. Maximum effusion rates, occurring in the first two episodes, are estimated to have been 8.5x103 and 8.7x103 m3 s-1 from fissures totaling 2.2 and 2.8 km in length, respectively, and, in general, discharge gradually decreased over time. The highest rates are equivalent to 5.6x103 and 4.5x103 kg s-1 per meter length of fissure, values that could conceivably be similar to those that produced some flood basalt lava flows. Maximum fire fountain heights are estimated to have varied from 800 m to 1400 m and convecting eruption columns above the vents rose to a maximum altitude of about 15 km. The release of sulfur gases during fountaining produced an acid haze (aerosol) which spread widely and had a considerable environmental, and possibly climatic, impact on the Northern Hemisphere.

Assessing porter's (1980) model in terms of its generalizability, accuracy and simplicity

Abstract: The Porter (1980) model of generic strategies is evaluated in terms of simplicity, accuracy (i.e., predictive and explanatory power), and generalizability, through an empirical analysis of PIMS data. Results lead to the following conclusions: (1) the simplicity of Porter's model captures much of the complexity inherent in strategic gestalts; (2) Porter's framework could be improved by viewing it as providing three important dimensions of strategic positioning rather than three (or four) distinct strategies; (3) performance norms vary significantly across strategic types, though Porter's predictions of performance are not entirely accurate; (4) strategies such as those described by Porter are possibly more contingent than generic, thus limiting the generalizability of some prescriptions implicit in the model.

Dynamic analysis and control of a Stewart platform manipulator

Abstract: The Stewart platform is a six-axis parallel robot manipulator with a force-to-weight ratio and positioning accuracy far exceeding that of a conventional serial-link arm. Its stiffness and accuracy approach that of a machine tool yet its workspace dexterity approaches that of a conventional manipulator. In this article, we study the dynamic equations of the Stewart platform manipulator. Our derivation is closed to that of Nguyen and Pooran because the dynamics are not explicitly given but are in a step-bystep algorithm. However, we give some insight into the structure and properties of these equations: We obtain compact expressions of some coefficients. These expressions should be interesting from a control point of view. A stiffness control scheme is designed for milling application. Some path-planning notions are discussed that take into account singularity positions and the required task. The objective is to make the milling station into a semiautonomous robotic tool needing some operator interaction but having some intelligence of its own. It should interface naturally with part delivery and other higher-level tasks. 0 1993 John Wiley & Sons, Inc.

Phytoplankton nutrient limitation and food quality for Daphnia

Abstract: The influence of nutrient limitation on the quality of Scenedesmus acutus as food for Daphnia obtusa is examined. The nature and degree of nutrient limitation greatly influences the rate at which Daphnia converts Scenedesmus biomass into herbivore biomass. From high to low quality, Scenedesmus food is ranked moderately N limited, severely N limited, and severely P limited. Even a very high concentration (3 mg DW.liter-I) of low quality food yields slow Daphnia growth, and it appears that no amount of low quality food would support rapid Daphnia growth. Food-limited animals display lowered intercepts of length-weight regressions (prereproductive females), reduced rates of biomass gain (both males and females), increased ages at first reproduction, lowered clutch sizes, increased mortality, and lowered reproductive rate. The N and P contents and the N : P ratio of Scenedesmus all vary considerably under N and P limitation, while the N content (and possibly P content) of Daphnia is less variable and the N: P ratio of Daphnia is essentially constant. Clearance and feeding rates are lower on severely P-limited cells than on severely N-limited cells. These results indicate that the mineral nutrition of their algal food may influence the demographics of zooplankton herbivores to a degree not before realized.

Perception of Performance in Group Brainstorming: The Illusion of Group Productivity

Abstract: Research has shown that individuals produce fewer ideas in interactive brainstorming groups than when brainstorming alone. However, group brainstorming remains a popular technique in organizations and industry. One basis for this popularity may be the perceived productivity of group brainstorming. A survey of expected performance in group brainstorming revealed that most individuals believed they would generate more ideas in groups than alone. Individuals who, in a second experiment, actually performed in brainstorming groups also perceived their performance more favorably than individuals who brainstormed alone. The results of a third experiment indicate that the illusion of group productivity may derive in part from the opportunity for social comparison that is available in group brainstorming. It also appears that individuals tend to take credit for a disproportionate amount of the brainstorming activity in groups.

Daphnia growth on varying quality of Scenedesmus: Mineral limitation of zooplankton

Abstract: Because the biochemical content of primary producers varies with their growth rate, herbivores feeding on foods of high growth rate have different diets than herbivores feeding on foods of low growth rate. Herbivores' food quality is thus related to their resource's growth rate. Here, the growth in body mass of individuals of the herbivore Daphnia obtusa was measured for a variety of foods, which consisted of the chlorophyte Scenedesmus acutus cultured under a suite of N- and P-limited growth rates. Food quantity (measured as carbon) was held constant at 0.5 mg/L. In general, low growth rate algae were poorer in quality than high growth rate algae. The logarithm of Daphnia mass was well predicted by the logarithm of algal growth rate. Algal growth rates at 1% P were high in quality, while foods lower than this exhibited reduced quality. These results are consistent with P limitation of Daph- nia growth when feeding on foods of low P content.

Photocatalytic Reduction and Immobilization of Hexavalent Chromium at Titanium Dioxide in Aqueous Basic Media

Abstract: The thermodynamic aspects of the photocatalytic reduction of Cr(VI) at TiO[sub 2] are first discussed and contrasted for pH 0 and pH 10 aqueous media. The photocorrosion of TiO[sub 2] is shown to be thermodynamically less favorable in basic media. A scheme for the one-step removal of Cr(VI) at TiO[sub 2] via its reduction and subsequent immobilization [as Cr(OH)[sub 3]] is demonstrated for pH 10 chromate solutions. Dissolved oxygen is shown to inhibit the reduction of Cr(VI). A preliminary kinetics analysis reveals adherence of the initial rate of Cr(VI) reduction to the simple Langmuir model.

Franchising, liquidity constraints and entry

Abstract: The short- and long-run incentives to franchise are considered. While monitoring problems due to geographic dispersion ensure that franchising is an efficient organizational form in the long run, entry costs suggest that franchising is an efficient solution to the formidable problems posed by entry in the short run. This explains the intensive use of franchising in the early growth stages, followed by a greater proportion of company-owned outlets as the system matures.

Traditional Gender Roles: Do They Make, and Then Break, our Relationships?

Abstract: Despite societal pressure for change in traditional gender roles, the coevolution of genes and culture may still lead us to be attracted to potential mates whose appearance and behavior is stereotypically masculine or feminine. This attraction is ironic in light of a growing body of research evidence indicating that the relationships of men and women with traditional gender roles are far from optimal—and are generally worse than those of androgynous men and women. These seemingly paradoxical findings may reflect the conflict between what our genes and past culture dispose us to do and what our present culture prescribes.

Political Ambition's Effect on Legislative Behavior: Schlesinger's Typology Reconsidered and Revisited

Abstract: According to Schlesinger's ambition theory, legislators seeking higher office behave differently from their less ambitious colleagues. In this note we examine Schlesinger's proposition that political ambition affects legislators' behavior. In the process, we argue that Schlesinger's typology of political ambition should be expanded to include intrainstitutional ambition--the members' desire for leadership positions within their present institution. Using data on U.S. House members we conclude that intrainstitutional ambition is distinct from both progressive and static ambition with each type of ambition exhibiting unique effects on various measures of legislative behavior.

Output feedback eigenstructure assignment using two Sylvester equations

Abstract: The eigenstructure assignment problem with output feedback is studied for systems satisfying the condition p+m>n. The main tool used is the concept of (C, A, B)-invariance and two coupled Sylvester equations, the solution of which leads to the computation of an output stabilizing feedback. A computationally efficient algorithm for the solution of these two coupled equations, which leads to the computation of a desired output feedback, is presented. >

Chiral host-guest recognition in an ion-molecule reaction

Abstract: Chiral rccognition is observed for a gas-phase host-guest system by using Fourier transform ion cyclotron resonance mass spectrometry to determine equilibrium constants for the transfer of (R)- or (S)-(1-naphthyl)-ethylammonium (NapEt + ) cation from (S,S)-dimethyldiketopyridino-18-crown-6 to unsubstituted 18-crown-6. The unsubstituted crown has substantially higher affinity for both guests than the chiral crown. The measured equilibrium constants are 130±15 for R-NapEt + , and 567±68 for S-NapEt + , yielding a difference of 4.2±0.4 kJ mol -1 between the stabilities of the complexes of the two enantiomeric guests with the chiral host

Reduction of Hexavalent Chromium in Aqueous Solutions by Polypyrrole

Abstract: Spontaneous electron transfer, along with anion uptake by doping, form the basis of a new concept for the reduction of toxic Cr(VI) using an electronically conductive polymer, e.g., polypyrrole. Proof of concept experiments are described which show that aqueous sulfuric acid solutions containing 5–100 ppm Cr(VI) can be thus converted at ~100% efficiency to the environmentally more tractable Cr(III) species at time intervals spanning several minutes. The reversibility of the polymer redox process enables the electrochemical recycling of polypyrrole for repeated treatment of Cr(VI). The key advantages relative to the chemical and electrochemical cleanup strategies currently employed include those related to material recyclability, selectivity, and efficiency at low (ppm) Cr(VI) levels.

A matter of degree: job skills for human service administrators.

Abstract: A small but important literature discusses which degrees and which skills are "best" for human service administrators. This research surveys current human service administrators to determine their views. The MSW administration degree, while seen as useful at the entry and middle levels of management, falls behind both the MBA and the MPA at top levels. Skills considered important are very similar at all levels of management, although managers of nonprofits had higher expectations for skill level in many areas than did managers at government agencies. Implications for schools of social work are discussed.

The London Hanged: Crime and Civil Society in the Eighteenth Century

Abstract: In eighteenth-century London the gallows at Tyburn was the dramatic focus of a struggle between the rich and the poor. Most of the London hanged were executed for property crimes, and the chief lesson that the gallows had to teach was: 'Respect private property'. The executions took place amid a London populace that knew the same poverty and hunger as the condemned. Indeed, in this stimulating account Peter Linebaugh shows how there was little distinction between a 'criminal' population and the poor population of London as a whole. Necessity drove the city's poor into inevitable conflict with the laws of a privileged ruling class. Peter Linebaugh examines how the meaning of 'property' changed substantially during a century of unparalleled growth in trade and commerce, analyses the increasing attempts of the propertied classes to criminalize 'customary rights'--perquisites of employment that the labouring poor depended upon for survival--and suggests that property-owners, by their exploitation of the emergent working class, substantially determined the nature of crime, and that crime, in turn, shaped the development of the economic system. Peter Linebaugh's account not only pinpoints critical themes in the formation of the working class, but also presents the plight of the individuals who made up that class. Contemporary documents of the period are skilfully used to recreate the predicament of men and women who, in the pursuit of a bare subsistence, had good reason to fear the example of Tyburn's 'triple tree'.

An Analysis of the Determinants of Industrial Property Valuation

Abstract: Recent research investigated the relationship between physical factors and the asking price of industrial buildings. We extend prior research by including physical, locational, financial and econom...

Propagation of shock waves in elastic solids caused by cavitation microjet impact. II: Application in extracorporeal shock wave lithotripsy

Abstract: To better understand the mechanism of stone fragmentation during extracorporeal shock wave lithotripsy (ESWL), the model developed in Part I [P. Zhong and C. J. Chuong, J. Acoust. Soc. Am. 94, 19–28 (1993)] is applied to study cavitation microjet impingement and its resultant shock wave propagation in renal calculi. Impact pressure at the stone boundary and stress, strain at the propagating shock fronts in the stone were calculated for typical ESWL loading conditions. At the anterior surface of the stone, the jet induced compressive stress can vary from 0.82∼4 times that of the water hammer pressure depending on the contact angles; whereas the jet‐induced shear stress can achieve its maximum, with a magnitude of 30%∼54% of the water hammer pressure, near the detachment of the longitudinal (or P) wave in the solid. Comparison of model predictions with material failure strengths of renal calculi suggests that jet impact can lead to stone surface erosion by combined compressive and shear loadings at the jet ...

The Application of the Theory of Reasoned Action to Senior Management and Strategic Information Systems

Abstract: In recent years MIS research has been concerned with the development and use of information systems to achieve a competitive advantage One of the many encompassing issues related to this topic is the need to gain the support and commitment for such systems from senior management Although previous research has discussed some of the difficulties with and reasons for senior management's reluctance to adopt strategic information systems, there does not appear to be the application of any integrated theory or framework to this problem In this paper, we discuss an established and well-researched theory, the Theory of Reasoned Action, and we provide a basis for how it can be applied to the acceptance of strategic information systems by senior management Initially, a general discussion of competitive advantage and strategic information systems is provided as foundation Next, the Theory of Reasoned Action is discussed in detail Finally, the theory is applied to senior organizational executives, with an emphasis on a methodology, to uncover some of the underlying reasons for resistance to adopting strategic information systems

The singularities and dynamics of a Stewart platform manipulator

Abstract: The Stewart platform manipulator is a fully parallel kinematic linkage system that has major mechanical differences over typical serial link robots. Its closed kinematic chain and parallel linkage structure give it great rigidity and a high force-to-weight ratio. In this paper, based on the forward and inverse kinematic analysis, the Jacobian matrix and the dynamic equations of the six-degree-of-freedom Stewart platform are derived. The singularities of the Stewart platform are also studied. Four singular positions are proved and some other conditions under which the possible singular positions may occur are given. These results provide us with the necessary information to avoid passing through singular points. The dynamic equations in Cartesian space appear in a very simple form. Especially in some applications if there is no rotation about the fixed X-axis, then the ‘inertia matrix’ reduces to a constant, diagonal matrix and the ‘Coriolis and centrifugal matrix’ goes to zero, which makes the Stewart platform become a decoupled, linear system in Cartesian space.

Adoptive parents' perceptions of, and comfort with, open adoption.

Abstract: Open adoption is becoming a common element of preparation for and ongoing services in infant and special-needs adoptions. This article discusses the results of a recent survey of 1,268 adoptive parents (1,396 adoptions) in California, in which questions were asked concerning openness and ongoing contact with biological parents. Postplacement contact with biological parents was common in this sample, and adoptive parents seemed cautiously comfortable with contact, with some exceptions.

The New Creative Thinking Skills Needed for Total Quality Management to Become Fact, Not Just Philosophy:

Abstract: This article models the change-making process for organizations, summarizes research on the model, and shows the fundamental connection between change-making skills and the concept called total qua...