Stability Arguments Research Articles

Immigration and the Democratic Stability Argument

AbstractAccording to the Democratic Stability Argument, since currently existing democratic states are overwhelmingly likely to play a leading role in the establishment of a just cosmopolitan order, should one ever be established, there are moral reasons to take measures that are necessary to preserve the adequate functioning of these states. If large-scale immigration can undermine their adequate functioning, then immigration restrictions are justified even from a cosmopolitan perspective, under non-ideal conditions. This paper argues that this argument may not succeed in justifying immigration restrictions under current conditions. Properly understood, the problem involves the competing claims of current poor admission-seekers and of the global poorest at some point in the future, i. e. the earliest feasible date of the establishment of the cosmopolitan order. The paper invokes normative arguments suggesting that the claims of the poorest in the distant future do not have lexical priority as a matter of principle. It also argues that available empirical estimates imply that the claims of admission-seekers in the present and near future potentially outcompete the claims of the poorest in the distant future. These considerations point towards a specification of the Democratic Stability Argument rather than to its complete rejection.

Revisiting the mechanistic origins of Thiele's ester dimerization: probing the reliability of predictive models for cycloadditions.

The Diels-Alder dimerization of cyclopentadiene carboxylic acid (or ester) affords only three products out of a total of 36 possible regioisomers. In this Communication, we compare two competing conceptual models for their ability to rationalize the selective formation of these three compounds. We find that calculated ΔHf values and orbital energy levels are sufficient to describe gross reactivity for the system, but that orbital coefficient arguments (and frontier molecular orbital theory in general) do not reliably predict the regioisomeric outcome for the reaction. By contrast, Deslongchamps' radical stabilization logic (a consequence of the bent bond model of reactivity) correctly predicts all three Diels-Alder products. We anticipate that these data will stimulate broader consideration of radical stabilization effects as a predictive tool for cycloaddition chemistry.

A closed-form criterion for dislocation emission in nano-porous materials under arbitrary thermomechanical loading

Our traditional view of void nucleation is associated with interface debonding at second-phase particles. However, under extreme dynamic loading conditions second-phase particles may not necessarily be the dominant source of void nucleation sites. A few key experimental observations of laser spall surfaces support this assertion. Here, we describe an alternative mechanism to the traditional view, namely shock-induced vacancy generation and clustering followed by nanovoid growth mediated by dislocation emission. This mechanism only becomes active at very large stresses. It is therefore desirable to establish a closed-form criterion for the macroscopic stress required to activate dislocation emission in porous materials. Following an approach similar to Lubarda and co-workers, we derive the desired criterion by making use of stability arguments applied to the analytic solutions for the elastic interactions of dislocations and voids. Our analysis significantly extends that of Lubarda and co-workers by accounting for a more general stress state, finite porosity, surface tension, as well as temperature and pressure dependence. The resulting simple stress-based criterion is validated against a number of molecular dynamics simulations with favorable agreement.

Role of Site Stability in Methane Activation on PdxCe1–xOδ Surfaces

Doped metal oxide catalysts can be optimized by identifying dopant metal/host oxide combinations that exhibit synergistic interactions not present in the parent systems. This is exemplified by PdxCe1–xOδ mixed oxides that yield methane oxidation rates unobtainable by the separate systems. Here we demonstrate that rapid C–H activation on PdxCe1–xOδ catalysts can be attributed to emergent behavior of the doped oxide enabling Pd4+ ↔ Pd2+ transitions not evident in catalysts featuring a PdOx active phase. PdxCe1–xOδ surfaces activate methane through hydrogen abstraction over Pd4+ surface states, in contrast to the σ-complex activation route favored over PdOx surfaces. The stability of the active Pd4+ state is dependent on temperature and oxygen pressure during catalytic operation, and as such we combine reaction kinetics and thermodynamic stability arguments from density functional theory (DFT) calculations to derive the apparent methane activation barrier. This accounts for varying conditions affecting the s...

A parallel prefiltering approach for the identification of a biased sinusoidal signal: Theory and experiments

SummaryThe problem of estimating the amplitude, frequency, and phase of an unknown sinusoidal signal from a noisy‐biased measurement is addressed in this paper by a family of parallel prefiltering schemes. The proposed methodology consists in using a pair of linear filters of specified order to generate a suitable number of auxiliary signals that are used to estimate—in an adaptive way—the frequency, the amplitude, and the phase of the sinusoid. Increasing the order of the prefilters improves the noise immunity of the estimator, at the cost of an increase of the computational complexity. Among the whole family of estimators realizable by varying the order of the filters, the simple parallel prefilters of orders 2 + 2 and 3 + 3 are discussed in detail, being the most attractive from the implementability point of view. The behavior of the two algorithms with respect to bounded external disturbances is characterized by input‐to‐state stability arguments. Finally, the effectiveness of the proposed technique is shown both by comparative numerical simulations and by a real experiment addressing the estimation of the frequency of the electrical mains from a noisy voltage measurement. Copyright © 2015 John Wiley & Sons, Ltd.

DYNAMICAL STABILITY OF IMAGED PLANETARY SYSTEMS IN FORMATION: APPLICATION TO HL TAU

A recent ALMA image revealed several concentric gaps in the protoplanetary disk surrounding the young star HL Tau. We consider the hypothesis that these gaps are carved by planets, and present a general framework for understanding the dynamical stability of such systems over typical disk lifetimes, providing estimates for the maximum planetary masses. We collect these easily evaluated constraints into a workflow that can help guide the design and interpretation of new observational campaigns and numerical simulations of gap opening in such systems. We argue that the locations of resonances should be significantly shifted in massive disks like HL Tau, and that theoretical uncertainties in the exact offset, together with observational errors, imply a large uncertainty in the dynamical state and stability in such disks. This presents an important barrier to using systems like HL Tau as a proxy for the initial conditions following planet formation. An important observational avenue to breaking this degeneracy is to search for eccentric gaps, which could implicate resonantly interacting planets. Unfortunately, massive disks like HL Tau should induce swift pericenter precession that would smear out any such eccentric features of planetary origin. This motivates pushing toward more typical, less massive disks. For a nominal non-resonant model of the HL Tau system with five planets, we find a maximum mass for the outer three bodies of approximately 2 Neptune masses. In a resonant configuration, these planets can reach at least the mass of Saturn. The inner two planets' masses are unconstrained by dynamical stability arguments.

Bayesian nonparametric adaptive control using Gaussian processes.

Most current model reference adaptive control (MRAC) methods rely on parametric adaptive elements, in which the number of parameters of the adaptive element are fixed a priori, often through expert judgment. An example of such an adaptive element is radial basis function networks (RBFNs), with RBF centers preallocated based on the expected operating domain. If the system operates outside of the expected operating domain, this adaptive element can become noneffective in capturing and canceling the uncertainty, thus rendering the adaptive controller only semiglobal in nature. This paper investigates a Gaussian process-based Bayesian MRAC architecture (GP-MRAC), which leverages the power and flexibility of GP Bayesian nonparametric models of uncertainty. The GP-MRAC does not require the centers to be preallocated, can inherently handle measurement noise, and enables MRAC to handle a broader set of uncertainties, including those that are defined as distributions over functions. We use stochastic stability arguments to show that GP-MRAC guarantees good closed-loop performance with no prior domain knowledge of the uncertainty. Online implementable GP inference methods are compared in numerical simulations against RBFN-MRAC with preallocated centers and are shown to provide better tracking and improved long-term learning.

Design of APOD-based switching dynamic observers and output feedback control for a class of nonlinear distributed parameter systems

The output feedback control problem for a class of nonlinear distributed parameter systems with limited number of continuous measurement sensors that describes a wide range of physico-chemical systems is investigated using adaptive proper orthogonal decomposition (APOD) method. Specifically, APOD is used to initiate and recursively revise locally valid reduced order models (ROMs) that approximate the dominant dynamic behavior of such physico-chemical systems. The controller is designed based on ROMs by combining a robust state controller with an APOD-based nonlinear Luenberger-type switching dynamic observer of the system states to reduce measurement sensor requirements. The important static observer requirements on the number of measurement sensors (that they must be supernumerary to the ROM dimension) and their location are circumvented by synthesizing dynamic observers. Three different approaches are introduced to recursively compute the dynamic observer gains at the ROM revisions. The stability of the closed-loop system is proven via Lyapunov and hybrid system stability arguments without invoking the separation principle between control and observation. The proposed method is successfully used to regulate a physico-chemical system that can be described in the form of the Kuramoto–Sivashinsky equation when the process exhibits significant nonlinear behavior.

Maximum kinetic energy dissipation and the stability of turbulent Poiseuille flow

AbstractFollowing Malkus’s (J. Fluid Mech., vol. 1, 1956, pp. 521–539) proposal that turbulent Poiseuille channel flow maximises total viscous dissipation $D$, a variety of variational procedures have been explored involving the maximisation of different flow quantities under different constraints. However, the physical justification for these variational procedures has remained unclear. Here we address more recent claims that mean flow viscous dissipation $D_{m}$ should be maximised on the basis of a statistical stability argument, and that maximising $D_{m}$ yields realistic mean velocity profiles (Malkus, J. Fluid Mech., vol. 489, 2003, pp. 185–198). We clarify the connection between maximising $D_{m}$ and other flow quantities, verify Malkus & Smith’s, (J. Fluid Mech., vol. 208, 1989, pp. 479–507) claim that maximising the ‘efficiency’ yields realistic profiles and show that, in contrast, maximising $D_{m}$ does not yield realistic mean velocity profiles as recently claimed. This leads us to revisit Malkus’s statistical stability argument for maximising $D_{m}$ and to address some of its limitations. We propose an alternative statistical stability argument leading to a principle of minimum kinetic energy for fixed pressure gradient, which suggests a principle of maximum $D$ for fixed Reynolds number under certain conditions. We discuss possible ways to reconcile these conflicting results, focusing on the choice of constraints.

Minimum Number of Monotone Subsequences of Length 4 in Permutations

We show that for every sufficiently largen, the number of monotone subsequences of length four in a permutation onnpoints is at least\begin{equation*} \binom{\lfloor{n/3}\rfloor}{4} + \binom{\lfloor{(n+1)/3}\rfloor}{4} + \binom{\lfloor{(n+2)/3}\rfloor}{4}. \end{equation*}Furthermore, we characterize all permutations on [n] that attain this lower bound. The proof uses the flag algebra framework together with some additional stability arguments. This problem is equivalent to some specific type of edge colourings of complete graphs with two colours, where the number of monochromaticK4is minimized. We show that all the extremal colourings must contain monochromaticK4only in one of the two colours. This translates back to permutations, where all the monotone subsequences of length four are all either increasing, or decreasing only.

An Adaptive Observer-Based Switched Methodology for the Identification of a Perturbed Sinusoidal Signal: Theory and Experiments

This paper deals with a novel adaptive observer-based technique for estimating the amplitude, frequency, and phase of a single sinusoidal signal from a measurement affected by structured and unstructured disturbances. The structured disturbances are modeled as a time-polynomial so as to represent bias and drift phenomena typically present in applications, whereas the unstructured disturbances are modelled as bounded noise signals. The proposed estimation technique exploits a specific adaptive observer scheme equipped with a switching criterion allowing to properly address in a stable way poor excitation scenarios. The estimator's stability properties are analyzed by input-to-state stability arguments. The practical characteristics of the proposed estimation approach are evaluated and compared with other existing tools by extensive simulation trials. Real experimental results are provided as well.

Final case for a stainless steel diagnostic first wall on ITER

In 2010 the ITER Organization (IO) proposed to eliminate the beryllium armour on the plasma-facing surface of the diagnostic port plugs and instead to use bare stainless steel (SS), simplifying the design and providing significant cost reduction. Transport simulations at the IO confirmed that charge-exchange sputtering of the SS surfaces would not affect burning plasma operation through core impurity contamination, but a second key issue is the potential melt damage/material loss inflicted by the intense photon radiation flashes expected at the thermal quench of disruptions mitigated by massive gas injection. This paper addresses this second issue through a combination of ITER relevant experimental heat load tests and qualitative theoretical arguments of melt layer stability. It demonstrates that SS can be employed as material for the port plug plasma-facing surface and this has now been adopted into the ITER baseline.

Here today, gone tomorrow? Revisiting the stability of teachers' achievement goals

Teachers' achievement goals are typically considered to be stable characteristics although there are arguments for both stability and instability. Empirical investigations regarding the stability of teachers' achievement goals are rare. In this study, we investigated the stability of teachers' achievement goals (i.e., learning, performance approach, performance avoidance, and work avoidance goals) using generalizability theory. The sample comprised 166 German mathematics teachers in academic-track secondary schools who completed self-report questionnaires three times over the course of one school year. The ratio of stable to unstable aspects of teachers' achievement goals varied between 2:1 and 4:1. The number of measurement points needed for a reliable measure of the trait aspects of achievement goals varied between one and three. The results underline the importance of advancing research on teachers' achievement goals both theoretically and methodologically.

Local Ramp Metering in the Presence of a Distant Downstream Bottleneck: Theoretical Analysis and Simulation Study

The well-known feedback ramp metering algorithm ALINEA can be applied for local ramp metering or used as a key component in a coordinated ramp metering system. ALINEA uses real-time occupancy measurements from the ramp-flow merging area that may be at most few hundred meters downstream of the metered on-ramp nose. In many practical cases, however, bottlenecks with smaller capacity than the merging area may exist further downstream for various reasons, which suggests using measurements from those further downstream bottlenecks rather than from the merging area. This paper addresses the local ramp metering problem in such a downstream bottleneck case. Theoretical analysis indicates that ALINEA may lead to a poorly damped closed-loop behavior in this case, but PI-ALINEA, which is a suitable proportional-integral (PI) extension of ALINEA, can lead to satisfactory control performance. The stability of the closed-loop ramp metering system with PI-ALINEA is rigorously proved by Lyapunov stability arguments. The root locus method is also employed to analyze the linearized closed-loop system performance of ALINEA and PI-ALINEA with and without a downstream bottleneck to provide insights on both controllers' performance. Simulation studies are conducted using a macroscopic traffic flow model to demonstrate that the ramp metering performance of ALINEA indeed deteriorates in the distant downstream bottleneck case, whereas a significant improvement is obtained using PI-ALINEA. Moreover, with its control parameters appropriately tuned, PI-ALINEA is found to be universally applicable to a range of distances between the on-ramp and downstream bottlenecks. This indicates that little fine-tuning would be necessary in field applications.

Revisiting Surface Heat-Flux and Temperature Boundary Conditions in Models of Stably Stratified Boundary-Layer Flows

Two formulations of the surface thermal boundary condition commonly employed in numerical modelling of atmospheric stably stratified surface-layer flows are evaluated using analytical considerations and observational data from the Cabauw site in the Netherlands. The first condition is stated in terms of the surface heat flux and the second is stated in terms of the vertical potential temperature difference. The similarity relationships used to relate the flux and the difference are based on conventional log-linear expressions for vertical profiles of wind velocity and potential temperature. The heat-flux formulation results in two physically meaningful values for the friction velocity with no obvious criteria available to choose between solutions. Both solutions can be obtained numerically, which casts doubt on discarding one of the solutions as was previously suggested based on stability arguments. This solution ambiguity problem is identified as the key issue of the heat-flux condition formulation. In addition, the agreement between the temperature difference evaluated from similarity solutions and their measurement-derived counterparts from the Cabauw dataset appears to be very poor. Extra caution should be paid to the iterative procedures used in the model algorithms realizing the heat-flux condition as they could often provide only partial solutions for the friction velocity and associated temperature difference. Using temperature difference as the lower boundary condition bypasses the ambiguity problem and provides physically meaningful values of heat flux for a broader range of stability condition in terms of the flux Richardson number. However, the agreement between solutions and observations of the heat flux is again rather poor. In general, there is a great need for practicable similarity relationships capable of treating the vertical turbulent transport of momentum and heat under conditions of strong stratification in the surface layer.

Age-structured dengue epidemic model

This paper deals with a new age-structured epidemic model for dengue. The proposed model consists of an age-structured, coupled set of partial differential equations describing both the vector life cycle and the host life cycle. We provide a numerical discretization with stability arguments and show that the model emits reasonable solutions.

Robust Sinusoid Identification With Structured and Unstructured Measurement Uncertainties

In this note a globally stable methodology is proposed to estimate the frequency, phase, and amplitude of a sinusoidal signal affected by additive structured and bounded unstructured disturbances. The structured disturbances belong to the class of time-polynomial signals incorporating both bias and drift phenomena. Stability and robustness results are given by resorting to Input-to-State stability arguments. Simulation comparative results show the effectiveness of the proposed technique.

Black Holes Without Spacelike Singularities

It is shown that for small, spherically symmetric perturbations of asymptotically flat two-ended Reissner–Nordström data for the Einstein–Maxwell-real scalar field system, the boundary of the dynamic spacetime which evolves is globally represented by a bifurcate null hypersurface across which the metric extends continuously. Under additional assumptions, it is shown that the Hawking mass blows up identically along this bifurcate null hypersurface, and thus the metric cannot be extended twice differentiably; in fact, it cannot be extended in a weaker sense characterized at the level of the Christoffel symbols. The proof combines estimates obtained in previous work with an elementary Cauchy stability argument. There are no restrictions on the size of the support of the scalar field, and the result applies to both the future and past boundary of spacetime. In particular, it follows that for an open set in the moduli space of solutions around Reissner–Nordström, there is no spacelike component of either the future or the past singularity.

Design Modified Sliding Mode Controller with Parallel Fuzzy Inference System Compensator to Control of Spherical Motor

The increasing demand for multi-degree-of- freedom (DOF) actuators in a number of industries has motivated a flurry of research in the development of non-conventional actuators, spherical motor. This motor is capable of providing smooth and isotropic three- dimensional motion in a single joint. Not only can the spherical motor combine 3-DOF motion in a single joint, it has a large range of motion with no singularities in its workspace. The spherical motor, however, exhibits coupled, nonlinear and very complex dynamics that make the design and implementation of feedback controllers very challenging. The orientation- varying torque generated by the spherical motor also contributes to the challenges in controller design. This paper contributes to the on-going research effort by exploring alternate methods for nonlinear and robust controlling the motor. The robust sliding mode controller proposed in this paper is used to further demonstrate the appealing features exhibited by the spherical motor. In opposition, sliding mode controller is used in many applications especially to control of highly uncertain systems; it has two significant drawbacks namely; chattering phenomenon and nonlinear equivalent dynamic formulation in uncertain dynamic parameter. The nonlinear equivalent dynamic formulation problem and chattering phenomenon in uncertain system (e.g., spherical motor) can be solved by using artificial intelligence theorem and applied a modified linear controller to switching part of sliding mode controller. Using Lyapunov-type stability arguments, a robust modified linear fuzzy sliding mode controller is designed to achieve this objective. The controller developed in this paper is designed in a robust stabilizing torque is designed for the nominal spherical motor dynamics derived using the constrained Lagrangian formulation. The eventual stability of the controller depends on the torque generating capabilities of the spherical motor.

Design Intelligent Robust Back stepping Controller

The increasing demand for multi-degree-of- freedom (DOF) continuum robot in presence of highly nonlinear dynamic parameters in a number of industries has motivated a flurry of research in the development of soft computing nonlinear methodology. The robust backstopping controller proposed in this research is used to further demonstrate the appealing features exhibited by the continuum robot. Robust feedback controller is used to position control of continuum robot in presence of uncertainties. Using Lyapunov type stability arguments, a robust backstopping controller is designed to achieve this objective. The controller developed in this research is designed into two steps. Firstly, a robust stabilizing torque is designed for the nominal continuum robot dynamics derived using the constrained Lagrangian formulation based on modified PD backstopping controller. Next, the fuzzy logic methodology applied to it to solution uncertainty problem. The fuzzy model free problem is formulated to estimate the nonlinear formulation of continuum robot. The eventual stability of the controller depends on the torque generating capabilities of the continuum robots.