Self-contained Model Research Articles

Deliberation or Conversation: Michael Oakeshott on the Ethos and Conduct of Democracy

In recent years, scholars have begun to explore the democratic valences of Michael Oakeshott's political thought. Commentators generally present him either as a skeptical and reluctant democrat or as an advocate of deliberative democracy. This article contends that Oakeshott criticizes some epistemological and substantive commitments that are characteristic of deliberative theories, yet his vision of democracy is more robust than skeptical readers admit. Using the themes of consensus and epistemic politics as points for comparison with the ideas of Oakeshott and deliberative democrats, I consider how his theory of civil association, critique of rationalism, and characterization of conversation as “the gist and meaning of democracy” speak to contemporary democratic theory. I conclude that he offers a pluralistic ethos, rather than a self-contained model, for democratic politics that has affinities with agonistic theories of democracy.

On the role of turbulence and compositional fluctuations in rapid compression machines: Autoignition of syngas mixtures

With the increasing interest in utilizing syngas in gas turbine applications, the characterization of H2/CO combustion and reaction chemistry under high-pressure and moderate-temperature operating conditions has been the focus of recent investigations. Different chemical-kinetics and hydrodynamic processes have been identified as being responsible for the discrepancies between experimental measurements and kinetic predictions of syngas ignition delay times. This contribution complements previous studies, and provides improved understanding about the role of turbulence and fluctuations in temperature and mixture composition on the syngas combustion process in rapid compression machines (RCMs). To this end, a self-contained model is developed that describes the ignition and combustion process by considering the interaction between turbulence, detailed reaction chemistry, and wall heat loss effects. Different mechanisms can be identified as being responsible for the generation of inhomogeneities in flow-field, temperature, and mixture composition, including turbulence-generation during the filling process, corner vortices, wall-generated turbulence, and mixing between fluid in the core region and the boundary layer. In the present model, these contributions are parametrically represented in terms of initial turbulence levels, and the mean-strain amplification of these perturbations during the compression phase is described using rapid distortion theory. The model is applied to different syngas mixtures and operating conditions, including pressures up to 20atm and temperatures between 600 and 1300K. Parametric studies show that the model captures experimentally observed trends of reduced ignition delay and prolonged reaction progress during the ignition phase. A Damköhler criterion is proposed in order to characterize the sensitivity of the induction chemistry to turbulence fluctuations. Results suggest that syngas mixtures with Damköhler numbers below 50 exhibit increasing sensitivity to turbulence and mixture fluctuations. This parametric study indicates that the turbulence/chemistry interaction can play an equally important role in affecting the syngas ignition chemistry, and requires consideration in addition to chemical-kinetics and hydrodynamic processes previously identified as leading mechanisms for the observed discrepancy in syngas ignition.

The evolution of the stellar mass function in star clusters

<i>Context. <i/>The dynamical escape of stars from star clusters affects the shape of the stellar mass function (MF) in these clusters, because the escape probability of a star depends on its mass. This is found in <i>N<i/>-body simulations and has been approximated in analytical cluster models by fitting the evolution of the MF. Both approaches are naturally restricted to the set of boundary conditions for which the simulations were performed.<i>Aims. <i/>The objective of this paper is to provide and to apply a simple physical model for the evolution of the MF in star clusters for a large range of the parameter space. It should also offer a new perspective on the results from <i>N<i/>-body simulations.<i>Methods. <i/>A simple, physically self-contained model for the evolution of the stellar MF in star clusters is derived from the basic principles of two-body encounters and energy considerations. It is independent of the adopted mass loss rate or initial mass function (IMF), and contains stellar evolution, stellar remnant retention, dynamical dissolution in a tidal field, and mass segregation.<i>Results. <i/>The MF evolution in star clusters depends on the disruption time, remnant retention fraction, initial-final stellar mass relation, and IMF. Low-mass stars are preferentially ejected after <i>t<i/> ~ 400 Myr. Before that time, masses around 15–20% of the maximum stellar mass are lost due to their rapid two-body relaxation with the massive stars that still exist at young ages. The degree of low-mass star depletion grows for increasing disruption times, but can be quenched when the retained fraction of massive remnants is large. The highly depleted MFs of certain Galactic globular clusters are explained by the enhanced low-mass star depletion that occurs for low remnant retention fractions. Unless the retention fraction is exceptionally large, dynamical evolution always decreases the mass-to-light ratio. The retention of black holes reduces the fraction of the cluster mass in remnants because white dwarfs and neutron stars have masses that are efficiently ejected by black holes.<i>Conclusions. <i/>The modeled evolution of the MF is consistent with <i>N<i/>-body simulations when adopting identical boundary conditions. However, it is found that the results from <i>N<i/>-body simulations only hold for their specific boundary conditions and should not be generalised to all clusters. It is concluded that the model provides an efficient method to understand the evolution of the stellar MF in star clusters under widely varying conditions.

Analytical modeling of the input admittance of an electric drive for stability analysis purposes

Embedded electric HVDC distribution network are facing difficult issues on quality and stability concerns. In order to help to resolve those problems, this paper proposes to develop an analytical model of an electric drive. This self-contained model includes an inverter, its regulation loops and the PMSM. After comparing the model with its equivalent (abc) full model, the study focuses on frequency analysis. The association with an input filter helps in expressing stability of the whole assembly by means of Routh-Hurtwitz criterion.

Canonical formalism for a2n-dimensional model with topological mass generation

The four-dimensional model with topological mass generation that was found by Dvali, Jackiw, and Pi has recently been generalized to any even number of dimensions ($2n$ dimensions) in a nontrivial manner in which a St\"uckelberg-type mass term is introduced [S. Deguchi and S. Hayakawa, Phys. Rev. D 77, 045003 (2008)]. The present paper deals with a self-contained model, called here a modified hybrid model, proposed in this $2n$-dimensional generalization and considers the canonical formalism for this model. For the sake of convenience, the canonical formalism itself is studied for a model equivalent to the modified hybrid model by following the recipe for treating constrained Hamiltonian systems. This formalism is applied to the canonical quantization of the equivalent model in order to clarify observable and unobservable particles in the model. The equivalent model (with a gauge-fixing term) is converted to the modified hybrid model (with a corresponding gauge-fixing term) in a Becchi-Rouet-Stora-Tyutin-invariant manner. Thereby it is shown that the Chern-Pontryagin density behaves as an observable massive particle (or field). The topological mass generation is thus verified at the quantum-theoretical level.

Colluthus' Pastoral Traditions: Narrative Strategies and Bucolic Criticism in the Abduction of Helen

It is nowadays a commonplace to state that every literary genre is a highly selective segment of a broader world of potential representations, and presents itself to the reader as a complete, self-contained model of interpretable mimesis of that particular aspect of reality. Yet this is especially true of bucolic poetry, whose very act of foundation rests on a joint effort, on the part both of the poets and their readers, to ‘conjure up a pre-existing “bucolic” tradition’ in the very same act of ‘founding such a tradition’. Theocritus' pastoral universe has its own bucolic hallmarks: landscape, gods and ‘professional’ accessories such as those required of a rustic life (milk-pails, shepherd's staffs, goatskin-coats and the like) are appropriately paraded and customised, and these hyper-‘realist’ markers are casually made to exist on the same level as the most unrealistic aspects of bucolic life (Theocritus' shepherds sing their time away while occasionally looking after their flocks). But it is especially in later imitators and interpreters that the possibilities of Theocritus' pastoral microcosm become necessities: generic consistency and recognisability are constantly pointed out and alluded to by obsessive repetition and normalisation of Theocritus ‘open’ pastoral world. The aim of the present paper is to read Colluthus' exploitation and, I would say, mobilisation of such a crystallised pastoral world against the background of ancient exegesis on the ‘bucolic problem’. In particular, it will be shown how bucolic criticism and Homererklärung (together with some important Hesiodic elements) are indissolubly intertwined in Colluthus' interpretation and reception of Theocritus' pastoral world. Comprehensiveness in charting Colluthus' critical response to such reading practices will not be attempted here: instead attention will be focused on those passages where Colluthus' scholarly engagement with bucolic generic conventions and their later accretions has a more direct impact on his narrative strategy.

Slide-and-Cluster Models for Spindle Assembly

Mitotic and meiotic spindles are assemblies of microtubules (MTs) that form during cell division to physically separate sister chromosomes. How the various components of spindles act together to establish and maintain the dynamic bipolar structure of spindles is not understood. Interactions between MTs and motors have been studied both experimentally and theoretically in many contexts, including the self-organization of arrays of MTs by motors and the competition between different classes of motors to move a single load. This work demonstrates how the interplay between two types of motors together with continual nucleation of MTs by chromosomes could organize the MTs into spindles. We propose a slide-and-cluster model based on four known molecular activities: MT nucleation near chromosomes, the sliding of MTs by a plus-end-directed motor, the clustering of their minus ends by a minus-end-directed motor, and the loss of MTs by dynamic instability. Our model applies to overlapping, nonkinetochore MTs in anastral spindles, and perhaps also to interpolar MTs in astral spindles. We show mathematically that the slide-and-cluster mechanism robustly forms bipolar spindles with sharp poles and a stable steady-state length. This model accounts for several experimental observations that were difficult to explain with existing models. Three new predictions of the model were tested and verified in Xenopus egg extracts. We show that a simple two-motor model could create stable, bipolar spindles under a wide range of physical parameters. Our model is the first self-contained model for anastral spindle assembly and MT sliding (known as poleward flux). Our experimental results support the slide-and-cluster scenario; most significantly, we find that MT sliding slows near spindle poles, confirming the model's primary prediction.

The Standard Scheme of the Moon and Its Mean Quantities

The Papyrus Rylands 27, of the John Rylands Library, Manchester, is a portion of a Greek treatise or tract of the third century A.D., providing rules for the longitude and node of the Moon. Apart from the illuminating analysis provided by Hunt, it has been the subject of a number of profound studies by Neugebauer, van der Waerden and Jones.1 These have culminated in the presentation by Jones of what he calls the Standard Scheme of lunar motion, illustrated by its application in various parts of the Oxyrhynchus Papyri, and elsewhere.2 It is clear from all this that we have here a scheme for the computation of the true longitude and node of the Moon based on a Babylonian type of 'zigzag' model. Jones has provided all that one needs for the practical derivation of the lunar coordinates, including lengthy tables to facilitate the computation of the summation of values from the zigzag functions that determine the coordinates. That much completes the subject as a self-contained model within the context of 'arithmetical' schemes, but leaves untouched the question of the relation between the Standard Scheme and the trigonometrical models, that is, those based on the use of one or more epicycles, etc. The purpose of this article is to develop this relation, one that situates the model in the interface between Babylonian and Greek astronomy.

Role of Charge Interaction in the Behavior of Organic Thin Film Transistors

AbstractStrong hysteresis in the I-V characteristics of organic thin film transistors are a severe obstacle for the implementation of large circuits. It therefore is a key success factor for the optimization and widespread application of organic electronics to understand the underlying principles. We report the fabrication of two types of pentacene transistors with either polyvinyl alcohol (PVA) or SiO2 as gate dielectric. These devices respond to transient measurement sweeps with a fundamentally different I-V hysteresis. A self-contained model is presented, which associates this behavior with the influence of traps at the SiO2/pentacene interface and polarization in the PVA layer. Simulations employing the commercial drift-diffusion tool SENTAURUSTM are performed to verify our models.

Dissociation in a Polymerization Model of Homochirality

A fully self-contained model of homochirality is presented that contains the effects of both polymerization and dissociation. The dissociation fragments are assumed to replenish the substrate from which new monomers can grow and undergo new polymerization. The mean length of isotactic polymers is found to grow slowly with the normalized total number of corresponding building blocks. Alternatively, if one assumes that the dissociation fragments themselves can polymerize further, then this corresponds to a strong source of short polymers, and an unrealistically short average length of only 3. By contrast, without dissociation, isotactic polymers becomes infinitely long.

Evaluation of retropulsion caused by holmium:YAG laser with various power settings and fibers.

The ideal intracorporeal lithotriper would comminute all types of calculi into small readily excreted particles. It would be small and flexible with an energy source safe for the uroepithelium. It should not break, should be inexpensive, and should not retropulse the stone up the urinary tract. This investigation was designed to quantify the last quality for the holmium:YAG laser. The mechanism of action of the pulsed Ho:YAG laser (wavelength 2100 nm) is the generation of a gas plasma at the stone-fluid interface causing a shockwave. The holmium laser was employed for lithotripsy of model stones composed of silicate with a ferrous coating. Stones were selected with a mass of 2 mg +/- 0.1 mg. We sequentially investigated three variables: energy (0.6, 0.8, and 1.0 J), frequency (10, 16, and 20 Hz), and fiber diameter (200, 365, 550, and 1000 microm). Ten stone trials were performed with each of the 36 possible combinations of energy, pulse frequency, and fiber diameter. Our model ureter consisted of a clear rigid polymer tube filled with 0.9% saline. The system was closed and permitted intertrial flushing of generated air bubbles. The laser fiber was maintained at constant extension from the ureteroscope, with stones positioned at the fiber tip before each trial. Laser energy was applied for 2 seconds, with maximum and net retropulsion recorded in millimeters. Each measurement series was recorded in a database for paired Student t-tests. Net retropulsion was then compared by statistically holding each of the three variables constant (fiber size constant with power and frequency varying; frequency constant with power and fiber size varying; and power constant with fiber size and frequency varying). Most retropulsion occurred with the 365-microm and 550-microm fibers. Most comminution was also noted with these fiber sizes. There was no statistical correlation between observed retropulsion and efficiency of comminution. This self-contained model for laser lithotripsy allowed us to measure retropulsion accurately. Silicate stones are not chemically similar to human uroliths but are of uniform composition. The irregular surface characteristics are similar to human stones, making them ideal for retropulsion investigations.

Sequential tunneling in doped superlattices: Fingerprints of impurity bands and photon-assisted tunneling

A theoretical and experimental study of transport in a low doped multiple quantum well structure shows that impurity bands are essential in understanding the electronic transport both with and without terahertz irradiation. A full self-contained model, which involves only the nominal parameters of the sample, agrees quantitatively with a wide range of experiments.

Carrier heating effects in dynamic-single-frequency GaInAsP-InP laser diodes

The fully self-contained model of a laser diode (LD) under the carrier heating conditions based on the description of a nonequilibrium carrier-phonon-photon system excited by an injection in a p-i-n double heterostructure (DHS) is presented. It combines the microscopic approach to the light-carrier interaction with the macroscopic treatment of the device characteristics and does not for example involve any empirical formulation of nonlinear gain. This model is used to investigate the carrier heating effects in the CW operation, small-signal modulation response and nonlinear picosecond (ps) dynamics of a single-frequency GaInAsP-InP laser. The carrier heating induced suppression of the material gain and enhancement of the intracavity losses are shown to be important for all operational modes. In the CW performance, these effects are found to cause the saturation of lasing and blue shift of the generation wavelength. In the high-frequency response, they are established to ensure an additional mechanism of dynamical carrier-photon coupling and therefore modify the modulation behavior of an LD. For nonlinear dynamics, the carrier heating induced perturbation of the gain and losses are shown to lead to deep pulse modulation on a ps time-scale. All the numerical estimations and modeling examples are given for 1.55 /spl mu/m distributed feedback (DFB) laser operating at room temperature. It is concluded that carrier heating effects are unwelcome phenomena for CW operation, but they can be engaged to improve the modulation behavior of an LD. >

Self-Contained versus Team Teaching: An Analysis of a Physical Education Intervention by Classroom Teachers

A primary purpose of this study was to describe differences between self-contained and team teaching approaches when two groups of fourth- and fifth-grade classroom teachers attempted to implement a physical education curriculum during a 4-month in-service program. One school featured team teaching in pairs during physical education classes; the other used a self-contained teaching approach. The program required a minimum of three 30-min physical education classes weekly. All teachers participated in an extensive in-service training program that included weekly on-site assistance. Data collection included teachers’ lesson-completion forms, specialist’s reports, SOFIT PE class observations, teacher-completed Stages of Concern questionnaires, and teachers’ formal interviews. Results indicated that classroom teachers who used the self-contained model more consistently implemented the curriculum and more frequently expressed positive responses. Participants who used the team model for the physical education curriculum frequently strayed from the assigned pedagogical approach, ignored major portions of the program, and experienced extreme management concerns.

First-principles simulation of a charged electrode: the electronic structure of a Ag(001) surface under the influence of an external electrostatic field

Results of local density full-potential linearized-augmented-plane-wave (FLAPW) calculations for an 11 layer slab of a Ag(001) surface under the influence of an external electrostatic field are presented. For the simulation of a charged electrode a self-contained model is designed by calculating the external electrostatic potential acting on the surface arising from an external charge distribution which should represent the electrolyte. In order to describe this external charge density distribution we make use of the Gouy-Chapman model (which is well known in electrochemistry) and calculate the corresponding potential by separating the electrolyte into two parts with different dielectric constants, inside and outside the outer Helmholtz plane. This calculated potential is incorporated as an external potential into the local density equations for solving the quantum mechanical equations of the Ag(001) surface. From the self-consistent solutions obtained by the FLAPW method, we evaluate the electronic charge distribution in the region of the metal surface, determine the mean electrostatic potential inside the electrode and the work function dependence on the applied field. Furthermore, we study the influence of the external field on some characteristic surface states (Stark shift). We conclude that our first-principles model for treating charged electrodes seems to represent the available experimental data in a reliable manner.

Strategies for Achieving an Integrated Education for Middle School Students with Severe Disabilities

There is much discussion about and emerging demonstrations of children with severe disabilities being included to greater degrees in the mainstream of school life, including general education classes and extracurricular activities. Educational service providers are struggling with how to change from a self-contained special education classroom model of service delivery to a more integrated approach in which special education is a service and support provided in general education environments. Presented in this paper are general and specific strategies that have been useful in facilitating such a change. Specific examples relate to middle and high school students, although the principles and strategies have applications for the integration of younger children as well.

Estimating the impacts of L.P.G. spills during transportation accidents

Transportation accidents involving releases of liquefied petroleum gases can cause substantial damage to the population and property adjacent to an accident scene. While some researchers have analyzed specific aspects of L.P.G. releases in detail, to date no single self-contained model could estimate all significant impacts of transporation-related releases at once. In response to this need, this paper describes a model which was designed to perform such an assessment. Both interactive and batch versions of the LPG-1 model have been Implemented, which can be applied either in isolation or as modules within a larger risk assessment model. The model's first module contains a representation of the spill mechanism following a transportation accident. It determines the amount of vapour flash-off and the size of any remaining pool based on the atmospheric conditions at the time of the accident, the amount of pre-heating of the container and the thermodynamic properties of the material shipped. Subsequently, simplified fireball, vapour cloud explosion and BLEVE models are used to compute the extent to which pre-speclfied levels of shockwave and heat radiation damage will be reached and the radius within which direct flame exposure will be experienced. During each phase of the analysis, the model considers relevant site-specific mitigating factors, such as the wind speed, air stability, combustion characteristics and the probability of encountering sparks.

Estimating the impacts of L.P.G. spills during transportation accidents

Transportation accidents involving releases of liquefied petroleum gases can cause substantial damage to the population and property adjacent to an accident scene. While some researchers have analyzed specific aspects of L.P.G. releases in detail, to date no single self-contained model could estimate all significant impacts of transporation-related releases at once. In response to this need, this paper describes a model which was designed to perform such an assessment. Both interactive and batch versions of the LPG-1 model have been Implemented, which can be applied either in isolation or as modules within a larger risk assessment model. The model's first module contains a representation of the spill mechanism following a transportation accident. It determines the amount of vapour flash-off and the size of any remaining pool based on the atmospheric conditions at the time of the accident, the amount of pre-heating of the container and the thermodynamic properties of the material shipped. Subsequently, simplified fireball, vapour cloud explosion and BLEVE models are used to compute the extent to which pre-speclfied levels of shockwave and heat radiation damage will be reached and the radius within which direct flame exposure will be experienced. During each phase of the analysis, the model considers relevant site-specific mitigating factors, such as the wind speed, air stability, combustion characteristics and the probability of encountering sparks.

Electronic and vibronic states of the acceptor-bound-exciton complex (A0,X) in CdS. II. Determination of the fine structure of the (A0,XB) electronic states by high-resolution excitation spectroscopy.

In a previous paper [R. Baumert, I. Broser, J. Gutowski, and A. Hoffman, Phys. Rev. B 27, 6263 (1983)] it has been shown that high-density, high-resolution excitation spectroscopy gives new information on the electronic and vibronic excited states of the acceptor--bound-exciton complex (${A}^{0}$,${X}_{A}$) with two holes from the A valence band in CdS. We now report on corresponding results for the (${A}^{0}$,${X}_{B}$) configuration which includes one hole from the second B valence band. This complex is unstable for a very fast B\ensuremath{\rightarrow}A hole conversion, and therefore gives rise to a set of excitation resonances of the ${I}_{1}$ luminescence arising from the (${A}^{0}$,${X}_{A}$) recombination. A detailed theoretical analysis of the energetic structure of the (${A}^{0}$,${X}_{B}$) complex including the dependence on the excitation intensity and on an applied magnetic field allows the correct assignment of the excitation resonances to the (${A}^{0}$,${X}_{B}$) fine-structure levels originating from the interparticle-exchange interactions. It is shown that the magnetic field is a suitable means of distinguishing the different (${A}^{0}$,${X}_{B}$) ground-state levels. The magnetic field also creates allowed transitions which are dipole forbidden in the zero-field case. A self-contained model of the (${A}^{0}$,${X}_{B}$) complex thus can be developed, including all symmetry states and yielding adequate values for the exchange energies within the complex.

A Neoclassical Model of the Balance of Payments

This paper analyses the transitory and long run effects of devaluation and tariffs, as well as the effects of alterations in the domestic money supply and the terms of trade, for a small country on fixed exchange rates. These issues are investigated in a self-contained dynamic optimization model of the sort popularized by Brock (1974, 1975). Familiar results in the trade literature are thereby explicated for the nonspecialist who is nonetheless familiar with Brock's work. There are some new results regarding the role played by income and substitution effects in determining the consequences of terms-of-trade shifts. This paper introduces a perfect foresight optimizing model of the monetary approach to the balance of payments for a small country on fixed exchange rates. The model is then used to analyse the effects on the balance of payments of devaluation, of a tariff, and of changes in the terms of trade. The effects of devaluation, tariffs, and changes in the terms of trade have been analysed previously within the framework of nonoptimizing models: Dornbusch (1973, 1980) and Anderson and Takayama (1977) have studied the effect of devaluation, Mussa (1974, 1976), Anderson and Takayama (1978, 1981) and Batra and Ramachandran (1980) have studied the effect of tariffs, and Rodriguez (1976), Batra and Ramachandran (1980), and Anderson and Takayama (1981) have studied the effect of changes in the terms of trade. Optimizing models in which real balances appear as an argument in the utility function are increasingly popular in international economics. The general approach was inspired by Sidrauski (1967) and Brock (1974, 1975). Recent contributions to the trade literature using this framework include Dornbusch and Mussa (1975), Calvo (1980, 1981), Liviatan (1981), and Obstfeld (1981 a, 1981 b, 1982). The purpose of this paper is to translate the conventional small country/fixed exchange rate model into this framework. The results are generally congruent with those in the nonoptimizing literature, which suggests that the money-in-the-utility-function approach works well here. Compared to the nonoptimizing literature, the real advance is that price adjustment and price expectations are endogenous and rational. Among the benefits are that one need not postulate the ad hoc price adjustment mechanism used by Anderson and Takayama, nor the backward-looking (and hence irrational) permanent income formulation used by Rodriguez.' Furthermore, there has been controversy over the balance of payments effects of a shift in the terms of trade. While the model's predictions on this issue are theoretically ambiguous, the model shows that the answer depends crucially on the magnitude of income effects. This clarifies the conditions required for a terms of trade shift to improve or worsen the balance of payments.