Generic Scenario Research Articles

Pyramid growth without deposition noise

Several models of molecular-beam epitaxy, both atomistic and ones based on Langevin equations, have as one of their generic growth scenarios the formation of three-dimensional structures such as mounds or pyramids. The characteristic size R of these structures increases as a function of deposition time with a power law $R\ensuremath{\sim}{t}^{n}$. In order to investigate the dependence of the growth exponent n on the characteristics of the fluctuations of the deposition flux we compare results of Monte-Carlo simulations for random deposition and for deposition on an artificially constructed deterministic sequence of sites. Although the latter algorithm leads to much smaller height fluctuations on each site, the growth exponent in both cases is found to be close to 0.25.

Theory of ocular dominance pattern formation.

We investigate a general and analytically tractable model for the activity-dependent formation of neuronal connectivity patterns. Previous models are contained as limiting cases. As an important example we analyze the formation of ocular dominance patterns in the visual cortex. A linear stability analysis reveals that the model undergoes a Turing-type instability as a function of interaction range and receptive field size. The phase transitions is of second order. After the linear instability the patterns may reorganize which we analyze in terms of a potential for the dynamics. Our analysis demonstrates that the experimentally observed dependency of ocular dominance patterns on interocular correlations of visual experience during development can emerge according to two generic scenarios: either the system is driven through the phase transition during development thereby selecting and stabilizing the first unstable mode or a primary pattern reorganizes towards larger wavelength according their lower energy. Experimentally observing the time course of ocular dominance pattern formation will decide which scenario is realized in the brain.

Facilities development and planning model - a management tool for the strategic planning process and application thereof

The comprehensive content of the planning process for wastewater conveyance and treatment facilities, and the optimization thereof, resulted in the development of the Facilities Development and Planning Model (FDP). The FDP model was developed for the operators of wastewater care facilities in order to obtain an efficient and dynamic means of assessing the regional inputs, evaluating the existing system and situation, and for considering various generic scenarios. The FDP model can be adapted to be utilized for the planning of facilities for other bulk services infrastructure. The paper describes the application of the FDP model in an wastewater treatment environment.

Berry’s phase in the presence of a stochastically evolving environment: A geometric mechanism for energy-level broadening

The generic Berry phase scenario in which a two-level system is coupled to a second system whose dynamical coordinate is slowly-varying is generalized to allow for stochastic evolution of the slow system. The stochastic behavior is produced by coupling the slow system to a heat resevoir which is modeled by a bath of harmonic oscillators initially in equilibrium at temperature T, and whose spectral density has a bandwidth which is small compared to the energy-level spacing of the fast system. The well-known energy-level shifts produced by Berry's phase in the fast system, in conjunction with the stochastic motion of the slow system, leads to a broadening of the fast system energy-levels. In the limit of strong damping and sufficiently low temperature, we determine the degree of level-broadening analytically, and show that the slow system dynamics satisfies a Langevin equation in which Lorentz-like and electric-like forces appear as a consequence of geometrical effects. We also determine the average energy-level shift produced in the fast system by this mechanism.

Pecking order as a dynamic leverage theory

Static tradeoff theories, which do not explicitly treat the impact of transaction costs, do not explain the policy of asymmetry between frequent small debt transactions and infrequent large equity transactions. Nor do these theories explain why the debt ratio is allowed to wander a considerable distance from its alleged static optimum, or how much of a distance should be tolerated. We offer a class of diffusion models that mimic this behaviour in a stochastic-dynamic framework and are designed to optimize a financing strategy using any static tradeoff theory as input. The models developed reveal the determinants of the size and frequency of equity transactions and the range of values over which leverage variations are tolerated in four generic scenarios. They also yield a new formulation of the cost of capital that recognizes stochastic transaction costs and a penalty for deviation from any static-optimal leverage. Our class of models augments the pecking order theory, provides a flexible quantitative framework for its implementation as a decision tool, and facilitates the formulation of additional hypotheses for its empirical validation. Symmetrically, our results show the importance of dynamic factors in designing and interpreting empirical tests of static tradeoff theories. The results presented have important implications for the role played by static tradeoff theories in a stochastic-dynamic framework. One such implication is that the static-optimal leverage has no direct effect on the firm's leverage policy in this setting. The target leverage for refinancing transactions is different from the static-optimal leverage, and the mean leverage is generally different from both. As a consequence, the latter cannot be used to estimate the former. Another implication is that even when the mean leverage equals the static optimum, mean reversion is not an optimal behaviour and therefore not a legitimate test for the existence of a static tradeoff in a dynamic context. Still another implication is that wide variations in leverage ratios cannot be interpreted as evidence of leverage indifference. It follows that the pecking order theory is consistent with static tradeoff theories and does not require the assumption of leverage indifference.

Facilities development and planning model — a management tool for the strategic planning process and application thereof

The comprehensive content of the planning process for wastewater conveyance and treatment facilities, and the optimization thereof, resulted in the development of the Facilities Development and Planning Model (FDP). The FDP model was developed for the operators of wastewater care facilities in order to obtain an efficient and dynamic means of assessing the regional inputs, evaluating the existing system and situation, and for considering various generic scenarios. The FDP model can be adapted to be utilized for the planning of facilities for other bulk services infrastructure. The paper describes the application of the FDP model in an wastewater treatment environment.

Delay Claims in U.K. Contracts

Contract conditions in both the United Kingdom and the United States are sufficiently similar to allow common generic delay claim scenarios to be recognized, but there are important differences in the extent of legal precedent for assessing such claims. In the United States recognized procedures have been developed to allow critical path method (CPM) to be adopted in this area, while in the United Kingdom the situation is much less clear. It is true that U.K. professionals will generally recognize that CPM will be useful in dealing with delay claims, but there is little evidence that a standard approach has been accepted. A survey was conducted by the writer to discover how U.K. claims for delay were being prepared (by the contractors) and how they were being assessed (by the supervisors). Because of the complexity of this area, the study was approached in a number of ways. This paper presents conclusions concerning attitudes towards such claims and information about the mechanisms that are being adopted to prepare and evaluate them.

Aquatic hazard and risk assessment of two selected substances produced in high volumes in the Slovak Republic

In the EU, the work on the development of a standardised guidance for risk assessment of chemical substances has been going on for the latest few years. The principles in this guidance were used for assessing the aquatic risk of two substances: caprolactamte [105-60-2] and diphenylamine [122-39-4], which are produced and used in large quantities in the Slovak Republic. Standard laboratory tests on toxicity to the algae Kirchneria subcapitata and to Daphnia magna were conducted for both substances, and for diphenylamine also tests on sludge respiration activity and ready (Closed Bottle test) and inherent (Modified MITI II test) biodegradability. Based on the generic European scenario for release and environmental distribution defined in the EU risk assessment guidance, predicted aquatic concentrations for a local environment (PEC local) were estimated and compared with the predicted no-effect-concentration for the aquatic environment (PNEC aquatic). The ratio, defined as the risk quotient, showed that both substances should be considered of high risk for the aquatic environment (risk quotient of 290 and 1300 for caprolactame and diphenylamine, respectively). However, modifying the calculation of PEC local to local Slovak conditions reduced the risk quotients to 0.1 and 5.3 for the two substances, respectively. Thus, the generic scenario defined in the EU risk assessment guidance may be considered conservative compared to the realistic local aquatic environments in the Slovak Republic.

The identification of thresholds of acceptability and danger: the chemical presence route.

European Union chemical legislation requires the calculation of local and regional Predicted Environmental Concentrations (PEC) for the assessment of the exposure of new and existing chemicals to aquatic and terrestrial ecosystems. Current methods use local models for air, water and soil to estimate chemical concentrations close to the source and a generic multimedia 'unit world' approach to estimate regional PECs. These models assume generic environmental scenarios representing typical situations in European countries and do not account for the spatial heterogeneity and temporal variability in ecosystem characteristics, soil properties, river flow rates, chemical emissions, etc. The environmental and ecological complexity can best be represented in a Geographic Information System (GIS). By coupling a GIS with a fate simulation model the concentrations of substances in a specific environment are predicted more realistically. The GREAT-ER project (Geography-referenced Regional Exposure Assessment Tool for European Rivers) was launched to refine regional and local exposure assessments for down-the-drain chemicals by applying real, spatial-referenced datasets instead of generic or average values. A modular approach was developed consisting of a hydrological model and a waste-flow, river quality and fate model which are linked to a regional GIS-database. For the calibration and validation in two European study areas representative detergent chemicals (LAS, boron) are used. In a parallel study, high-volume intermediates discharged into the river Rhine are simulated.

Reorientation transitions in ultrathin ferromagnetic films by thickness- and temperature-driven anisotropy flows.

A unified thermodynamic treatment is presented for reorientation transitions in ultrathin ferromagnetic films, driven by temperature or thickness variations. Since the physical mechanism underlying such transitions is the competition between surface, bulk, and shape anisotropy, the natural and proper scenery to examine the problem is the anisotropy space of the corresponding system. The recently developed anisotropy-flow concept is then applied to detect and characterize the possible thickness- and temperature-driven reorientation transitions, consistent with the standard, though not universally valid, assumption for the additive separation of the bulk and surface contributions. Three generic types of transition are easily identified with a number of subcases each. Each generic scenario is seen to come about as a simple consequence of the route which the system follows in the anisotropy space under variations of the driving parameter. The relevant characteristic thicknesses are easily identified. We find that, as a rule, there exist two borderlines in the ferromagnetic part of the T-d diagram of the system; these should lead in principle to a much richer structure of the diagram in question than has been reported so far. \textcopyright{} 1996 The American Physical Society.

Dose and Risk Assessment for Intrusion into Mixed-Waste Disposal Sites

Abstract Sites previously used for disposal of radioactive and hazardous chemical materials have resulted in situations that pose a potential threat to humans from inadvertent intrusion. An example of a generic scenario analysis was developed to demonstrate the evaluation of potential exposure to cleanup workers or members of the public who intrude into buried waste containing both radioactive and hazardous chemical contaminants. The example scenarios consist of a collection of exposure routes (or pathways) with specific modeling assumptions for well drilling and for excavation for building construction. These scenarios are used to conceptually describe potential patterns of human activity during intrusion into mixed-waste disposal sites, assuming that no protective measures are taken. Radiation doses are calculated using the GENII software system and converted to risk using factors from the International Commission on Radiological Protection in Publication 60. The hazard assessment for nonradioactive mat...

The concept of anisotropy flows in thickness- and temperature-driven reorientation transitions in ultrathin ferromagnetic films

The anisotropy-flow concept is applied to analyze in detail the reorientation transitions, driven by either thickness or temperature variations. The analysis brings to the fore the common underlying features in both cases. Three generic scenarios for the occurrence of the reorientation are predicted and described in both cases on general thermodynamic grounds. The method is especially effective when underpinned by the usual assumption about the explicit thickness dependence of the first two anisotropy constants.

Formation of pyramids and mounds in molecular beam epitaxy.

One of the generic scenarios in molecular beam epitaxy is the growth of three-dimensional structures such as mounds and pyramids. These structures are a nonequilibrium effect thought to be due to a combination of the microscopic Ehrlich-Schwoebel barriers and the breaking of detailed balance by the deposition process. We propose and investigate by computer simulation a simple microscopic model that displays (i) slope selection, (ii) pyramid and moundlike structures, and (iii) coarsening. The characteristic length scale of our three-dimensional features grows as $R(t)\ensuremath{\sim}{t}^{n}$ with $n$ between 0.17 and 0.26. We discuss these results in the light of recent experiments and continuum models of molecular beam epitaxy.

Does fully developed turbulence exist? Reynolds number independence versus asymptotic covariance

By analogy with recent arguments concerning the mean velocity profile of wall-bounded turbulent shear flows, we suggest that there may exist corrections to the 2/3 law of Kolmogorov, which are proportional to (ln Re)−1 at large Re. Such corrections to K41 are the only ones permitted if one insists that the functional form of statistical averages at large Re be invariant under a natural redefinition of Re. The family of curves of the observed longitudinal structure function DLL(r,Re) for different values of Re is bounded by an envelope. In one generic scenario, close to the envelope, DLL(r,Re) is of the form assumed by Kolmogorov, with corrections of O[(ln Re)−2]. In an alternative generic scenario, both the Kolmogorov constant CK and corrections to Kolmogorov’s linear relation for the third-order structure function DLLL(r) are proportional to (ln Re)−1. Recent experimental data of Praskovsky and Oncley appear to show a definite dependence of CK on Re, which, if confirmed, would be consistent with the arguments given here.

Approaches for exposure characterization and data needs for hazardous waste site assessment.

This article provides an understanding of the approaches for determining exposure and dose to populations in the vicinity of hazardous waste sites. A review of the federal legislation and jurisdiction for assessments is provided, and the approaches of the U.S. Environmental Protection Agency and the Agency for Toxic Substances and Disease Registry are compared. These methods strive to aid in the evaluation of public health impacts of contaminants that were, are, or may be released to the community, and they are concerned with various aspects of the contaminant fate, human contact, and toxic response for chemicals of concern. Such approaches have been designed for generic contamination scenarios, but they aim to be applicable to a wide range of chemicals and sites in the real world. Along with any modeling framework for exposure and dose characterization, detailed information or real data are requisite for the completion of any site-specific assessment. What kinds of data are needed and where they may be found are also discussed. A comprehensive framework for exposure characterization, recently proposed by Georgopoulos and Lioy, is outlined. The framework is one employing the following elements: chemodynamic analyses of sources and receptors; characterization of the target population; toxicokinetic/toxicodynamic analyses; uncertainty/error analyses; and evaluation of the characterization performance.

Fermion mass predictions in a generalized extended technicolor scenario.

One family extended technicolor (ETC) models with technifermion mass spectra compatible with the experimental data for the precision parameters S, T, U, V, W, and X, and ETC interactions compatible with the CERN LEP measurements of the Z\ensuremath{\rightarrow}bb\ifmmode\bar\else\textasciimacron\fi{} vertex have recently been proposed. To investigate whether these scenarios are consistent with the third family fermion masses we develop a generalized ETC model in which ETC interactions are represented by four-Fermi interactions. We discuss the reliability of the gap equation approximation to the nonperturbative dynamics. Two generic scenarios of couplings fit the precision data and third family masses: one is an unpredictive existence proof, the other, which generates the large top mass by direct top quark condensation, has a minimal number of interactions that break the global symmetry of the light fermions in the observed manner. This latter scenario makes surprisingly good predictions of the charm, strange, and up quark masses.

Pyramid Formation and Coarsening During Homoepitaxial Growth

ABSTRACTOne of the generic scenarios in molecular beam epitaxy is the growth of large scale pyramid-like three-dimensional structures. These structures are a consequence of non-equilibrium surface diffusion currents that result from a transport of adatoms in the uphill direction in regions of the surfaces that are tilted away from a high symmetry orientation. The microscopic origin of these currents are, e.g., diffusion barriers at step edges that suppress the diffusion of adatoms to lower terraces. The temporal evolution of the resulting instabilities can be described by simple continuum equations that display slope-selection, pyramid-like structures, and coarsening. We show that similar scenarios can be found in microscopic models investigated by Monte-Carlo simulations. However, crossover phenomena can complicate the comparison of the asymptotic theory with computer simulations or experiments. We therefore discuss criteria that allow to distinguish experimentally between kinetic roughening with exponents ζ ≃ 1 and unstable pyramid-like growth.

The effect of changing disease risk on clinical reasoning.

To assess the ability of health care professionals to evaluate the effect of clinical test results in different settings. Subjects were presented with a series of generic clinical scenarios in which information about the test performance and the pretest probability of disease was varied. The subject estimates of posttest probability were compared with those calculated on the basis of Bayes' theorem. Fifty health care professionals, including 31 physicians and 19 nonphysicians, associated with a university teaching hospital. Under a variety of testing conditions, both the physicians and the nonphysicians inaccurately estimated the posttest probability of disease. Based on a logarithmic transformation, the error in probability estimation was divided into a portion related to the pretest probability of disease and a portion related to the test performance. Most of the error in posttest probability estimation was associated with the incorrect use of pretest probabilities. The subjects consistently overestimated the posttest probability of disease expected under Bayes' theorem, with increasing error associated with decreasing pretest probability. Physician estimates of posttest probability increased with increasing likelihood ratios for each scenario. Nonphysician estimates of posttest probabilities increased with increasing likelihood ratios for a positive test, but the estimates associated with a negative test result were inconsistent. Physicians and nonphysicians overestimate posttest probabilities with increasing error associated with decreasing disease risk. Some nonphysicians may not fully understand the effect of test performance on risk estimation, particularly in the setting of a negative test. Health care professionals should receive training in the proper evaluation of test information, with particular emphasis on the influence of pretest disease risk on the posttest probability of disease.

Propagation of measurement errors in pesticide application computations

Abstract The propagation of non‐statistical measurement error was studied in four simple generic pesticide calibration scenarios. Measurement error was represented using intervals, and error propagation was studied by computing with these intervals and measuring the distance between the interval result and result under no uncertainty. Intervals were generated either by assuming loss of significant digits, or by estimating reasonable limits on the precision with which certain variables could be measured. Measurement error was found to perpetuate and propagate. The resultant error depended on the magnitude of the uncertainty in input variables, and the number of arithmetic operations. In a computation with one numerical division, error arising from the loss of significant digits can be best controlled by increasing the precision of all variables proportionally and simultaneously. In a computation containing only multiplications, a decrease in the uncertainty of any variable is directly realized in the result. Including numerical division in calibration equations can reduce the width of resultant intervals, but relative uncertainty never decreases. In an example with multiple multiplications and divisions, uncertainty was propagated non‐linearly. Small measurement errors, therefore, can propagate quickly and increase the likelihood that applicators apply incorrect amounts of pesticide.

B → sγ and Bs → μ+μ− in extended technicolor models

The rates of the rare flavor-changing processes, b → sγ and B s → μ + μ − are estimated in extended technicolor models with and without a GIM mechanism. We find the b → sγ rate in ETC models with a GIM mechanism to be at most slightly larger than the standard model rate, whereas there is no significant extra model-independent contribution in other ETC scenarios. In the case of B s → μ + μ −, ETC models with a GIM mechanism can yield a rate up to two order of magnitude bigger than that of the standard model, whereas generic ETC scenarios are likely to give a rate which is about an order of magnitude bigger than that of the standard model.