Importance Functions for RESTART Simulation of General Jackson Networks

RESTART is an accelerated simulation technique that allows the evaluation of extremely low probabilities. In this method a number of simulation retrials are performed when the process enters regions of the state space where the chance of occurrence of the rare event is higher. These regions are defined by means of a function of the system state called the importance function. Guidelines for obtaining suitable importance functions and formulas for the importance function of two-stage networks were provided in previous papers. In this paper, we obtain effective importance functions for RESTART simulation of Jackson networks where the rare set is defined as the number of customers in a particular (‘target’) node exceeding a predefined threshold. Although some rough approximations and assumptions are used to derive the formulas of the importance functions, they are good enough to estimate accurately very low probabilities for different network topologies within short computational time

RESTART Simulation of Non-Markov Consecutive-K-Out-of-N: F Repairable Systems

The reliability of consecutive-k-out-of-n: F repairable systems and (k−1)-step Markov dependence is studied. The model analyzed in this paper is more general than those of previous studies given that repair time and component lifetimes are random variables that follow a general distribution. The system has one repair service which adopts a priority repair rule based on system failure risk. Since crude simulation has proved to be inefficient for highly dependable systems, the RESTART method was used for the estimation of steady-state unavailability, MTBF and unreliability. Probabilities up to the order of 10−16 have been accurately estimated with little computational effort. In this method, a number of simulation retrials are performed when the process enters regions of the state space where the chance of occurrence of a rare event (e.g., a system failure) is higher. The main difficulty for the application of this method is to find a suitable function, called the importance function, to define the regions. Given the simplicity involved in changing some model assumptions with RESTART, the importance function used in this paper could be useful for dependability estimation of many systems

Improving the BADA 3 aerodynamic database for trajectory optimization

Although commercial aviation has developed strongly during recent years, the optimization of trajectories is still a challenge due to security, environmental and procedural restrictions. In order optimize an aircraft trajectory, and therefore, to model it, first of all, it is required to study the aircraft performance. The aircraft performance model is given by the basic kinematic and dynamic equations, although it is also necessary to know the aircraft aerodynamic model, i.e. its aerodynamic coefficients. Aware of this challenge, EUROCONTROL has developed an aircraft performance model called BADA (Base of Aircraft Data) that contains the aircraft performance model for a large percentage of today's commercial aircraft. This tool has been designed by EUROCONTROL for their own research projects but has finally made available for the R& D collective. The BADA model consists on a set of theoretical concepts in the form of generic polynomials used to calculate the aircraft performance. It also comes with a set of individual data sets for each plane to particularize these polynomials. But still, it remains a generic model, and therefore, it is not really accurate for what the trajectory optimization processes require. This thesis proposes a methodology based on the kinetic approximation of the aircraft performance model and an improvement of BADA by using a software called United States Air Force and Stability Control Digital DATCOM. DATCOM is a software that implements calculation methods of aerodynamic stability and control developed in 1960 by the US Air Force. After developing the model of two common long-haul aircraft, it will be compared with BADA model and a model developed by Ms. Caroline Dietrich, master student who worked as a researcher at Ecole Polytechnique de Montréal. This comparison is performed by modeling the trajectory of a Boeing 767-300ER flight from Toronto to Los Angeles.Tot i que l’aviació comercial s’ha desenvolupat intensament durant els últims anys, la optimització de trajectòries encara és un repte degut a les restriccions de seguretat, ambientals i de procediments. Per poder estudiar una trajectòria, i per tant optimitzar-la, primer de tot, s’ha de poder estudiar el comportament de l’aeronau. El comportament d’un avió ve donat per les equacions cinemàtiques i dinàmiques bàsiques, però evidentment, també és necessari conèixer el model aerodinàmic de l’aeronau. Conscients d’aquest repte, EUROCONTROL ha desenvolupat un model de comportament d’aeronaus anomenat BADA (Base of Aircraft DAta) que conté el model de comportament, i per tant, aerodinàmic, d’un gran percentatge d’avions comercials actuals. Aquesta eina ha estat dissenyada per EUROCONTROL pels seus propis projectes de recerca però finalment ha estat posada a disposició del colectiu R+D. El model BADA està format per un conjunt de fonaments teòrics en forma de polinomis genèrics utilitzat per calcular diferents paràmetres del comportament de les aeronaus. A més, ve acompanyat per un conjunt de sets de dades particulars per cada avió o per poder particularitzar aquests polinomis a cada avió. Encara així, segueix sent un model genèric, i per tant, es pot entendre que no es del tot precís pel que la modelització de trajectòries requereix. En aquest document es proposa una metodologia basada en la aproximació o cinètica del model de comportaments d’aeronaus BADA i es millora fent servir un software anomenat United States Air Force Stability and Control Digital DATCOM. DATCOM és un software que implementa uns mètodes de càlcul d’estabilitat i control aerodinàmic desenvolupats al 1960 per la força aèria dels Estats Units. Desprès de desenvolupar el model de dues de les aeronaus de llarg abast més comunes dins l’aviació comercial, aquest serà comparat amb el mateix model BADA i el model desenvolupat per Caroline Dietrich, estudiant de màster amb qui es col·labora en aquest projecte. La comparació es porta a terme amb un Boeing 767-300ER fent la modelització del creuer d’una trajectòria d’un vol Toronto - Los Angeles

Dependability Estimation For Non-Markov Cosecutive-K-Out-Of-N: F Repairable Systems By Restart Simulation

The reliability of consecutive-k-out-of-n: F system (or C (k, n: F) system) has aroused great interest since it was first studied by Kontoleon in 1980 [1]. The system consists of a sequence of n ordered components along a line such that the system fails if and only if at least k consecutive components in the system have failed. A list of typical applications of C (k, n: F) system was given by Yam et al. [2]. A research book by Chang et al. [3] provide rich information about C (k,n: F) system

Global warming will affect the maximum potential abundance of boreal plant species

Forecasting the impact of future global warming on biodiversity requires understanding how temperature limits the distribution of species. Here we rely on Liebig's Law of Minimum to estimate the effect of temperature on the maximum potential abundance that a species can attain at a certain location. We develop 95%-quantile regressions to model the influence of effective temperature sum on the maximum potential abundance of 25 common understory plant species of Finland, along 868 nationwide plots sampled in 1985. Fifteen of these species showed a significant response to temperature sum that was consistent in temperatura-only models and in all-predictors models, which also included cumulative precipitation, soil texture, soil fertility, tree species and stand maturity as predictors. For species with significant and consistent responses to temperature, we forecasted potential shifts in abundance for the period 2041-2070 under the IPCC A1B emission scenario using temperatura-only models. We predict major potential changes in abundance and average northward distribution shifts of 6-8 km y r-1. Our results emphasize inter-specific differences in the impact of global warming on the understory layer of boreal forests. Species in all functional groups from dwarf shrubs, herbs and grasses to bryophytes and lichens showed significant responses to temperature, while temperature did not limit the abundance of 10 species. We discuss the interest of modelling the 'maximum potential abundance' to deal with the uncertainty in the predictions of realized abundances associated to the effect of environmental factors not accounted for and to dispersal limitations of species, among others. We believe this concept has a promising and unexplored potential to forecast the impact of specific drivers of global change under future scenarios.Comunidad de MadridUniversidad de AlcaláAcademy of Finlan

Fission yeast SWI/SNF and RSC complexes show compositional and functional differences from budding yeast.

SWI/SNF chromatin-remodeling complexes have crucial roles in transcription and other chromatin-related processes. The analysis of the two members of this class in Saccharomyces cerevisiae, SWI/SNF and RSC, has heavily contributed to our understanding of these complexes. To understand the in vivo functions of SWI/SNF and RSC in an evolutionarily distant organism, we have characterized these complexes in Schizosaccharomyces pombe. Although core components are conserved between the two yeasts, the compositions of S. pombe SWI/SNF and RSC differ from their S. cerevisiae counterparts and in some ways are more similar to metazoan complexes. Furthermore, several of the conserved proteins, including actin-like proteins, are markedly different between the two yeasts with respect to their requirement for viability. Finally, phenotypic and microarray analyses identified widespread requirements for SWI/SNF and RSC on transcription including strong evidence that SWI/SNF directly represses iron-transport genes

Query Expansion of Zero-Hit Subject Searches: Using a Thesaurus in Conjunction with NLP Techniques

The focus of our study is zero-hit queries in keyword subject searches and the effort of increasing recall in these cases by reformulating and, then, expanding the initial queries using an external source of knowledge, namely a thesaurus. To this end, the objectives of this study are twofold. First, we perform the mapping of query terms to the thesaurus terms. Second, we use the matched terms to expand the user’s initial query by taking advantage of the thesaurus relations and implementing natural language processing (NLP) techniques. We report on the overall procedure and elaborate on key points and considerations of each step of the process

Forward Flux Sampling for rare event simulations

Rare events are ubiquitous in many different fields, yet they are notoriously difficult to simulate because few, if any, events are observed in a conventiona l simulation run. Over the past several decades, specialised simulation methods have been developed to overcome this problem. We review one recently-developed class of such methods, known as Forward Flux Sampling. Forward Flux Sampling uses a series of interfaces between the initial and final states to calculate rate constants and generate transition paths, for rare events in equilibrium or nonequilibrium systems with stochastic dynamics. This review draws together a number of recent advances, summarizes several applications of the method and highlights challenges that remain to be overcome.Comment: minor typos in the manuscript. J.Phys.:Condensed Matter (accepted for publication