Finite Periods Of Time Research Articles

Temperature field optimization for laser powder bed fusion as a traveling salesperson problem with history

AbstractLaser Powder Bed Fusion (LPBF) is a form of metal additive manufacturing in which a laser traces a path on a metal powder bed to heat powder particles and progressively melt and/or fuse them together to form a part. Outcomes of LPBF are functions of the metal's temperature history and are strongly affected by the path traced by the laser. Using temperature field optimization for LPBF as a motivating application, we focus on a class of combinatorial problems where admissible control policies are a permutation of a set of control actions. We abstract this class of problems as an optimal control problem with the objective of identifying an admissible control policy that minimizes a cost function of a dynamical system's state. While, in principle, the effect of the control actions on the cost function can last an infinitely long time, we will consider systems in which the effects of these control actions can be considered to last short (finite) periods of time. In this paper, we formalize this class of combinatorial problems as a Traveling Salesperson Problem with History and prove its equivalence to an Equality Generalized Traveling Salesperson Problem (E‐GTSP), enabling the use of well‐developed E‐GTSP solvers. We demonstrate this equivalence by computing the solutions obtained using an E‐GTSP solver for an LPBF‐inspired application.

Electron trapping in magnetic driven graphene quantum dots

In this paper we present an in-depth theoretical investigation of an electron scattering process on a graphene quantum dot (GQD) supposed to a perpendicular magnetic field. As it is well known, because of Klein tunneling, an electrostatic potential is not helpful to localize an electron inside a GQD. However, we report here that in the case of a magnetic driven GQD, there emerge scattering resonances characterized by trapped electronic states inside the dot for finite periods of time, otherwise known as quasi-bound states. Using a real space scattering analysis, we highlight in a very intuitive manner how the quasi-bound states are generated and, for a comprehensive investigation, we evaluate their corresponding lifetime (trapping time). As well, we explore the case of an electrostatically biased GQD. We show that this configuration may be very advantageous regarding the control of the trapping times which reach values much higher than in the unbiased case.

Coordinated Robotic Exploration of Dynamic Open Ocean Phenomena

The study of dynamic features of the ocean, in which complex physical, chemical, and biological interactions evolve on multiple time scales, poses significant sampling challenges because the required spatial and temporal resolutions are not possible by ship or satellite studies alone. Satellite remote sensing captures only surface effects while expensive research vessels can only make discrete observations in finite periods of time. Our work with networked marine robotics in the aerial, surface, and underwater domains is at the vanguard of a new approach to scientific exploration and observation, which brings together several technologies to enable oceanographic vessels and robots to work in tandem, thus expanding the observational footprint of these vessels. We describe a scientific cruise in the Spring of 2018 in the open waters of the Pacific where we deployed a fleet of autonomous robots to demonstrate this approach for the synoptic observation of mesoscale and sub-mesoscale features of a frontal zone. We articulate the elements and methods to multi-vehicle coordination and challenges that lie ahead in ocean observation.

Electron trapping in twisted light driven graphene quantum dots

In this paper, we present a theoretical perspective concerning the scattering of electrons on a twisted light (TL) driven graphene quantum dot (GQD). Relatively recently discovered, TL is a novel type of electromagnetic field which carries a finite orbital angular momentum oriented on the propagation direction, besides its spin. This striking property of TL is due to its spatial structure. It is well known that the localization of electrons in a GQD is forbidden by the Klein tunneling, an effect that manifests by the perfect transmission of electrons through a potential barrier, regardless of its magnitude. Here we demonstrate that, for a suitable choice of the scattering regimes, there emerge scattering resonances characterized by trapping states of the incident electron inside the GQD for finite periods of time. The most interesting result is the prediction regarding the possibility to control the trapping times using a TL irradiation. Also, we mention that the investigation was performed for a frequency of the TL within the infrared spectrum.

Analysis of Frequency and Polarization Scaling on Rain Attenuated Signal of a KU-Band Link in Jos, Nigeria.

This paper presents the analysis of frequency and polarization scaling on rain attenuated signal of a KU-Band link. The study was carried out in Jos, Plateau state, Nigeria (9.89650 N, 8.85830 E; 1192 meters) with Maximum, Average and Minimum Temperatures of 29.80C, 22.80C and 170C respectively. Data were obtained for the months of May, June, July, August, September and October 2020.Davis Vantage Vue weather station was used to measured and record one-minute rain-rates from a minimum of 0.8 mm/h up to a value of 460 mm/h, with an accuracy of 0.2 mm/h. The precipitation data, with date and time is captured on the micro-chip of the wireless electronic data logger, which, when calibrated, logs on data every minute. The down converted Ku-band signal was fed into the digital satellite meter and a spectrum analyzer for signal level analysis, logging and recording samples of viewed spectrum over finite periods of time on a computer system. Both satellite signal and precipitation measurements were done concurrently. Data were analyzed using Microsoft excel and results were obtained based on ITU-R model. Results obtained revealed that rain attenuated signal for vertical and horizontal polarization varies for different rain rate and months under reviewed. For the month of May 2020, rain attenuated signal is more severe from rain rate of 100mm/hr with the highest rain attenuation of 25.57dB, 30.37dB, and 39.84dB at frequencies of 12GHz, 15GHz and 18GHz respectively while the rain attenuated signal for vertical polarization is more severe from 80mm/hr with highest recorded rain attenuation of 27.96dB, 40.33dB and 45.71dB at frequencies of 12GHz, 15GHz and 18GHz respectively. For the month of July 2020, the results shows that rain attenuated signal for vertical polarization is more severe from rain rate of 80mm/hr with the highest rain attenuation of 60.57dB, 73.37dB, and 100.84dB for frequencies of 12GHz, 15GHz, and 18GHz respectively. At Horizontal polarization, signal losses are more severe from 60mm/hr. The results further proves that frequency and polarization scaling of rain attenuated signalare major factors to consider when designing a microwave link budget especially in the study area that experiences high amount of rainfall annually.

The movement investigation of an axisymmetric rotation body under the action of electromagnetic fields

Purpose: To ensure an adequate level of accuracy, it is rational to study the ponderomotor forces of the ring, which drive a hollow disk of variable thickness, hung on the ring. Design/methodology/approach: The solution of the motion problem of a hollow disk of variable thickness suspended on a force ring of rectangular cross section is based on the method of solving the equations of the theory of thermoelasticity. The stress-strain state, as well as the motion of the specified body of rotation, the disk, in studies in a cylindrical coordinate system, under the action of ponderomotor forces. Findings: The motion equation of a hollow disk hung on a force ring-torus is made, exact solutions of the motion equations of a ring in the torus form of rectangular cross section are found. New component expressions of ponderomotor forces, which appear from the action of the ring's own electromagnetic field and cause the motion of a hollow disk, have been found on the basis of Maxwell's equations. It is proved that at high speeds and low natural accelerations the stress - strain state of the disk material does not cause the destruction of the structure. Research limitations/implications: Calculations of ponderomorphic forces are valid for the ring, which drives a hollow disk of variable thickness, hung on the ring. Practical implications: It is proved that at high velocities and small natural accelerations the stress-strain state of the disk medium does not cause structural damage. It is determined that the rotation in the direction of movement at an angle of 90 degrees changes only the direction of the acceleration vector without increasing its value. Originality/value: The dependences between own time and coordinate time are formulated. It is proved that a small change in the natural time for the studied disk can significantly change the coordinate time, and the pulsed electromagnetic field provides the ability to cover infinitely large distances over finite periods of time.

DYNAMICS OF CHOLERA EPIDEMIC MODELS IN FLUCTUATING ENVIRONMENTS.

Based on our deterministic models for cholera epidemics, we propose a stochastic model for cholera epidemics to incorporate environmental fluctuations which is a nonlinear system of Itô stochastic differential equations. We conduct an asymptotical analysis of dynamical behaviors for the model. The basic stochastic reproduction value is defined in terms of the basic reproduction number R 0 for the corresponding deterministic model and noise intensities. The basic stochastic reproduction value determines the dynamical patterns of the stochastic model. When , the cholera infection will extinct within finite periods of time almost surely. When , the cholera infection will persist most of time, and there exists a unique stationary ergodic distribution to which all solutions of the stochastic model will approach almost surely as noise intensities are bounded. When the basic reproduction number R 0 for the corresponding deterministic model is greater than 1, and the noise intensities are large enough such that , the cholera infection is suppressed by environmental noises. We carry out numerical simulations to illustrate our analysis, and to compare with the corresponding deterministic model. Biological implications are pointed out.

Mimicking transient activation of protein kinases in living cells

Physiological stimuli activate protein kinases for finite periods of time, which is critical for specific biological outcomes. Mimicking this transient biological activity of kinases is challenging due to the limitations of existing methods. Here, we report a strategy enabling transient kinase activation in living cells. Using two protein-engineering approaches, we achieve independent control of kinase activation and inactivation. We show successful regulation of tyrosine kinase c-Src (Src) and Ser/Thr kinase p38α (p38), demonstrating broad applicability of the method. By activating Src for finite periods of time, we reveal how the duration of kinase activation affects secondary morphological changes that follow transient Src activation. This approach highlights distinct roles for sequential Src-Rac1- and Src-PI3K-signaling pathways at different stages during transient Src activation. Finally, we demonstrate that this method enables transient activation of Src and p38 in a specific signaling complex, providing a tool for targeted regulation of individual signaling pathways.

Built to Last or Meant to End: Intertemporal Choice in Strategic Alliance Portfolios

Asalient but rarely explicitly studied characteristic of interfirm relationships is that they can intentionally be formed for finite periods of time. What determines firms’ intertemporal choices between different alliance time horizons? Shadow of the future theorists suggest that when an alliance has an explicitly set short-term time frame, there is an increased risk that partners may behave opportunistically. This does not readily explain the high incidence of time-bound alliances being formed. Reconciling insights from the shadow of the future perspective with nascent research on the flexibility of temporary organizations, and shifting the focus from the level of individual transactions to that of strategic alliance portfolios, we argue that firms may be willing to accept a higher risk of opportunism when there are offsetting gains in strategic flexibility in managing their strategic alliance portfolio. Consequently, we hypothesize that environmental factors that increase the need for strategic flexibility—namely, dynamism and complexity in the environment—are likely to increase the relative share of time-bound alliances in strategic alliance portfolios. Our analysis of longitudinal data on the intertemporal alliance choices of a large sample of small and medium-sized enterprises provides support for this argument. Our findings fill an important gap in theory about time horizons in interfirm relationships and temporary organizations and show the importance of separating planned terminations from duration-based performance measures.

On Chattering Solutions for the Maximum Principle Boundary-Value Problem in the Optimal Control Problem in Microeconomics

We consider the problem of optimal control of the maximization of income for the nonlinear mathematical model of microeconomical system used for the description of production, storage, and selling of consumer goods. For this model, we formulate the boundary-value problem of the maximum principle giving the necessary conditions of optimality. We consider conditions for the parameters characterizing the original system under which the solution of the boundary-value problem of maximum principle possesses a section of singular mode. It is shown that this section of singular mode neighbors with nonsingular sections containing containing infinitely many switchings on finite periods of time. For the investigation of this phenomenon, we use the theory developed by M. Zelikin and V. Borisov. It is shown that the solution of the boundary-value problem of maximum principle is locally optimal in the analyzed problem of optimal control. Hence, the corresponding optimal trajectory has three sections. The first part is a nonsingular section in which the original system with infinitely many switchings passes to the singular section for a finite period of time. The second part is the section of singular mode and the last (third) part is a section of the trajectory in which the analyzed system leaves the singular mode with more and more frequent switchings in the inverse time.

A switching multi-scale dynamical network model of EEG/MEG

We introduce a new generative model of the Encephalography (EEG/MEG) data, the inversion of which allows for inferring the locations and temporal evolution of the underlying sources as well as their dynamical interactions. The proposed Switching Mesostate Space Model (SMSM) builds on the multi-scale generative model for EEG/MEG by Daunizeau and Friston (2007). SMSM inherits the assumptions that (1) bioelectromagnetic activity is generated by a set of distributed sources, (2) the dynamics of these sources can be modelled as random fluctuations about a small number of mesostates, and (3) the number of mesostates engaged by a cognitive task is small. Additionally, four generalising assumptions are now included: (4) the mesostates interact according to a full Dynamical Causal Network (DCN) that can be estimated; (5) the dynamics of the mesostates can switch between multiple approximately linear operating regimes; (6) each operating regime remains stable over finite periods of time (temporal clusters); and (7) the total number of times the mesostates' dynamics can switch is small. The proposed model adds, therefore, a level of flexibility by accommodating complex brain processes that cannot be characterised by purely linear and stationary Gaussian dynamics. Importantly, the SMSM furnishes a new interpretation of the EEG/MEG data in which the source activity may have multiple discrete modes of behaviour, each with approximately linear dynamics. This is modelled by assuming that the connection strengths of the underlying mesoscopic DCN are time-dependent but piecewise constant, i.e. they can undergo discrete changes over time. A Variational Bayes inversion scheme is derived to estimate all the parameters of the model by maximising a (Negative Free Energy) lower bound on the model evidence. This bound is used to select among different model choices that are defined by the number of mesostates as well as by the number of stationary linear regimes. The full model is compared to a simplified version that uses no dynamical assumptions as well as to a standard EEG inversion technique. The comparison is carried out using an extensive set of simulations, and the application of SMSM to a real data set is also demonstrated. Our results show that for experimental situations in which we have some a priori belief that there are multiple approximately linear dynamical regimes, the proposed SMSM provides a natural modelling tool.

Effects of Participation vs. Observation of a Simulation Experience on Testing Outcomes: Implications for Logistical Planning for a School of Nursing

The simulation program within our nursing curriculum covers a wide array of content areas and has been rated very highly by students and faculty. Due to this success, requests for increased simulation experiences have grown dramatically. The challenge lies, however, in logistically accommodating large student groups in finite periods of time. To overcome these constraints, an alternative plan was devised where half of the students actively participate in the simulation while their cohort observes. Both groups participate in the debriefing process. Findings revealed no significant differences (p=.97) between the simulation and observational groups on scoring of the test items related to this content (n=92). Over 70% reported the simulation experience as enjoyable, well-organized, clarified issues, increased knowledge and prepared them to work in a hospital. These findings indicate that the use of this alternate plan was effective for student learning and could therefore be incorporated into the simulation program design.

Transport in time-dependent dynamical systems: Finite-time coherent sets

We study the transport properties of nonautonomous chaotic dynamical systems over a finite-time duration. We are particularly interested in those regions that remain coherent and relatively nondispersive over finite periods of time, despite the chaotic nature of the system. We develop a novel probabilistic methodology based upon transfer operators that automatically detect maximally coherent sets. The approach is very simple to implement, requiring only singular vector computations of a matrix of transitions induced by the dynamics. We illustrate our new methodology on an idealized stratospheric flow and in two and three-dimensional analyses of European Centre for Medium Range Weather Forecasting (ECMWF) reanalysis data.

Strong Scalar QED in Inhomogeneous Electromagnetic Fields

Strong QED has attracted attention recently partly because many astrophysical phenomena have been observed to involve electromagnetic fields beyond the critical strength for electron-positron pair production and partly because terrestrial experiments will generate electromagnetic fields above or near the critical strength in the near future. In this talk we critically review QED phenomena involving strong external electromagnetic fields. Strong QED is characterized by vacuum polarization due to quantum fluctuations and pair production due to the vacuum instability. A canonical method is elaborated for pair production at zero or finite temperature by inhomogeneous electric fields. An algorithm is advanced to calculate pair production rate for electric fields acting for finite periods of time or localized in space or oscillating electric fields. Finally, strong QED is discussed in astrophysics, in particular, strange stars.

Free energy wells for small gas bubbles in soft deformable materials

Thermodynamic expressions are derived for the system relative Gibbs free energy, and the relative Gibbs free energy per bubble, for all possible equilibrium bubble states that can form in a soft slightly rigid material, initially supersaturated with a dissolved inert gas (N(2)). While the thermodynamic manipulations are exact, the final expressions are approximate, due to an approximation made in deriving the expression for the elastic free energy of a soft material containing more than a single bubble. The expressions predict that provided the shear modulus of the soft material is not negligibly small, free energy wells which stabilize small gas bubbles for finite periods of time exist in such materials. This is consistent with a previous calculation, based solely on the bubble pressure equation, which resulted in the conjecture that bubbles found in soft materials with some rigidity (or shear resistance) are likely to be small. The possible relevance of this to the field of decompression sickness is outlined.

Denying autonomy in order to create it: the paradox of forcing treatment upon addicts

Denying autonomy in order to create it: the paradox of forcing treatment upon addicts

Pseudoresonant interaction between flame and upstream velocity fluctuations

This work is dedicated to the analysis of the delicate details of the effect of upstream velocity fluctuations on the flame propagation speed. The investigation was carried out using the Sivashinsky model of cellularization of hydrodynamically unstable flame fronts. We identified the perturbations of the steadily propagating flames which can be significantly amplified over finite periods of time. These perturbations were used to model the effect of upstream velocity fluctuations on the flame front dynamics and to study a possibility to control the flame propagation speed.

Finite hedging in field theory models of interest rates.

We use path integrals to calculate hedge parameters and efficacy of hedging in a quantum field theory generalization of the Heath, Jarrow, and Morton [Robert Jarrow, David Heath, and Andrew Morton, Econometrica 60, 77 (1992)] term structure model, which parsimoniously describes the evolution of imperfectly correlated forward rates. We calculate, within the model specification, the effectiveness of hedging over finite periods of time, and obtain the limiting case of instantaneous hedging. We use empirical estimates for the parameters of the model to show that a low-dimensional hedge portfolio is quite effective.

Pulling together and pushing apart: tides of tension in the ICU team.

Pulling together and pushing apart: tides of tension in the ICU team.

Models for maintenance optimization: a study for repairable systems and finite time periods

Abstract The problem of selecting a suitable maintenance policy for repairable systems and for a finite time period is presented. Since the late seventies, examples of models assessing corrective and preventive maintenance policies over an equipment life cycle exist in the literature. However, there are not too many contributions regarding real implementation of these models in the industry, considering realistic timeframes and for repairable systems. Modeling this problem requires normally the representation of different corrective and/or preventive actions that could take place at different moments, driving the equipment to different states with different hazard rates. An approach to pattern the system under finite periods of time has been the utilization of semi-Markovian probabilistic models, allowing later a maintenance policy optimization using dynamic programming. These models are very flexible to represent a given system, but they are also complex and therefore very difficult to handle when the number of the system possible states increases. This paper explores the trade-off between flexibility and complexity of these models, and presents a comparison in terms of model data requirements versus potential benefits obtained with the model.