Big Bang bifurcations and allee effect in Blumberg’s dynamics

This paper concerns dynamics and bifurcations properties of a class of continuous-defined one-dimensional maps, in a three-dimensional parameter space: Blumberg's functions. This family of functions naturally incorporates a major focus of ecological research: the Allee effect. We provide a necessary condition for the occurrence of this phenomenon, associated with the stability of a fixed point. A central point of our investigation is the study of bifurcations structure for this class of functions. We verified that under some sufficient conditions, Blumberg's functions have a particular bifurcations structure: the big bang bifurcations of the so-called "box-within-a-box" type, but for different kinds of boxes. Moreover, it is verified that these bifurcation cascades converge to different big bang bifurcation curves, where for the corresponding parameter values are associated distinct attractors. This work contributes to clarify the big bang bifurcation analysis for continuous maps. To support our results, we present fold and flip bifurcations curves and surfaces, and numerical simulations of several bifurcation diagrams.

Assessment of the ensembles regional climate models in the representation of precipitation variability and extremes over Portugal

A new data set of daily gridded observations of precipitation, computed from over 400 stations in Portugal, is used to assess the performance of 12 regional climate models at 25 km resolution, from the ENSEMBLES set, all forced by ERA-40 boundary conditions, for the 1961-2000 period. Standard point error statistics, calculated from grid point and basin aggregated data, and precipitation related climate indices are used to analyze the performance of the different models in representing the main spatial and temporal features of the regional climate, and its extreme events. As a whole, the ENSEMBLES models are found to achieve a good representation of those features, with good spatial correlations with observations. There is a small but relevant negative bias in precipitation, especially in the driest months, leading to systematic errors in related climate indices. The underprediction of precipitation occurs in most percentiles, although this deficiency is partially corrected at the basin level. Interestingly, some of the conclusions concerning the performance of the models are different of what has been found for the contiguous territory of Spain; in particular, ENSEMBLES models appear too dry over Portugal and too wet over Spain. Finally, models behave quite differently in the simulation of some important aspects of local climate, from the mean climatology to high precipitation regimes in localized mountain ranges and in the subsequent drier regions.

On chaos, transient chaos and ghosts in single populations models with allee effects

Density-dependent effects, both positive or negative, can have an important impact on the population dynamics of species by modifying their population per-capita growth rates. An important type of such density-dependent factors is given by the so-called Allee effects, widely studied in theoretical and field population biology. In this study, we analyze two discrete single population models with overcompensating density-dependence and Allee effects due to predator saturation and mating limitation using symbolic dynamics theory. We focus on the scenarios of persistence and bistability, in which the species dynamics can be chaotic. For the chaotic regimes, we compute the topological entropy as well as the Lyapunov exponent under ecological key parameters and different initial conditions. We also provide co-dimension two bifurcation diagrams for both systems computing the periods of the orbits, also characterizing the period-ordering routes toward the boundary crisis responsible for species extinction via transient chaos. Our results show that the topological entropy increases as we approach to the parametric regions involving transient chaos, being maximum when the full shift R(L)(infinity) occurs, and the system enters into the essential extinction regime. Finally, we characterize analytically, using a complex variable approach, and numerically the inverse square-root scaling law arising in the vicinity of a saddle-node bifurcation responsible for the extinction scenario in the two studied models. The results are discussed in the context of species fragility under differential Allee effects. (C) 2011 Elsevier Ltd. All rights reserved.

Scaling law in Saddle-node bifurcations for one-dimensional maps: A complex variable approach

The study of transient dynamical phenomena near bifurcation thresholds has attracted the interest of many researchers due to the relevance of bifurcations in different physical or biological systems. In the context of saddle-node bifurcations, where two or more fixed points collide annihilating each other, it is known that the dynamics can suffer the so-called delayed transition. This phenomenon emerges when the system spends a lot of time before reaching the remaining stable equilibrium, found after the bifurcation, because of the presence of a saddle-remnant in phase space. Some works have analytically tackled this phenomenon, especially in time-continuous dynamical systems, showing that the time delay, tau, scales according to an inverse square-root power law, tau similar to (mu-mu (c) )(-1/2), as the bifurcation parameter mu, is driven further away from its critical value, mu (c) . In this work, we first characterize analytically this scaling law using complex variable techniques for a family of one-dimensional maps, called the normal form for the saddle-node bifurcation. We then apply our general analytic results to a single-species ecological model with harvesting given by a unimodal map, characterizing the delayed transition and the scaling law arising due to the constant of harvesting. For both analyzed systems, we show that the numerical results are in perfect agreement with the analytical solutions we are providing. The procedure presented in this work can be used to characterize the scaling laws of one-dimensional discrete dynamical systems with saddle-node bifurcations.

On the initial conditions in continuous-time fractional linear systems

Signal Processing, Vol. 83, nº 11

The comb signal and its Fourier transform

SignalProcessing, Vol. 81, nº 3

The influence of the boundary conditions on low-velocity impact composite damage

The objective of this work was to study the influence of the boundary conditions on low-velocity impact behaviour of carbon-epoxy composite plates. Experimental work and numerical analysis were performed on [04,904]s laminates. The influence of different boundary conditions on the impacted plates was analysed considering rectangular and square plates. The X-radiography was used as a non-destructive technique to evaluate the internal damage caused by impact loading. A three-dimensional numerical analysis was also performed considering progressive damage modelling. The model includes three-dimensional solid elements and interface finite elements including a cohesive mixed-mode damage model, which allows simulating delamination between different oriented layers. It was verified that plate’s boundary conditions have influence on the delaminated area. Good agreement between experimental and numerical analysis for shape, orientation and size of the delamination was obtained.

Evaluation of tools and cutting conditions on carbon fibre reinforced laminates

The distinctive characteristics of carbon fibre reinforced plastics, like low weight or high specific strength, had broadened their use to new fields. Due to the need of assembly to structures, machining operations like drilling are frequent. In result of composites inhomogeneity, this operation can lead to different damages that reduce mechanical strength of the parts in the connection area. From these damages, delamination is the most severe. A proper choice of tool and cutting parameters can reduce delamination substantially. In this work the results obtained with five different tool geometries are compared. Conclusions show that the choice of an adequate drill can reduce thrust forces, thus delamination damage.

High-performance flexible hybrid field-effect transistors based on cellulose fiber paper

IEEE Electron Device Letters, VOL. 29, NO. 9,

Novos processos e produtos para o couro de base vegetal

Mestrado em Engenharia Química - Ramo Optimização Energética na Indústria Química

The Neogene of the Lower Tagus Basin (Portugal)

Revista Española de Paleontologia 19 (2), 229-242

Simulation and dynamics of freeway traffic

This paper presents the new package entitled Simulator of Intelligent Transportation Systems (SITS) and a computational oriented analysis of traffic dynamics. The SITS adopts a microscopic simulation approach to reproduce real traffic conditions considering different types of vehicles, drivers and roads. A set of experiments with the SITS reveal the dynamic phenomena exhibited by this kind of system. For this purpose a modelling formalism is developed that embeds the statistics and the Laplace transform. The results make possible the adoption of classical system theory tools and point out that it is possible to study traffic systems taking advantage of the knowledge gathered with automatic control algorithms. A complementary perspective for the analysis of the traffic flow is also quantified through the entropy measure.

Auditoria à qualidade do ar interior de um edifício de serviços - RSECE QAI

O objectivo principal deste trabalho é a realização de uma auditoria, à qualidade do ar interior (QAI), a um edifício de serviços – COCIGA, SA, tendo como base o Regulamento dos Sistemas Energéticos de Climatização dos Edifícios (RSECE). A auditoria QAI implica a medição de vários parâmetros físicos, químicos, microbiológicos e também a inspecção aos componentes do sistema de climatização com a finalidade de averiguar o seu estado de limpeza e manutenção. Assim, foram seleccionados 3 espaços, para a realização de amostragens designados por Comercial - Produtos, AVAC e Mezaninne das oficinas, nos quais foi efectuada a medição de diversos parâmetros, de acordo com as imposições do RSECE, utilizando medidores portáteis ou recorrendo a métodos analíticos. Relativamente aos parâmetros físicos, registaram-se valores de temperatura, para os três espaços estudados, entre os 21 e os 24 ºC e valores médios de humidade relativa de cerca de 50 %. Outro parâmetro medido, e de grande importância para garantir o conforto dos ocupantes, foi a velocidade do ar nos postos de trabalho. De acordo com o RSECE este valor não deve ser superior a 0,2 m/s, o que se verificou em todos os pontos medidos. O último parâmetro físico medido foi a concentração de partículas (PM10) tendo-se obtido valores de cerca de 23 μg/m3ar, valor bastante inferior ao máximo permitido pelo RSECE (150 μg/m3ar). Também no que diz respeito aos parâmetros químicos, ou seja, CO2, CO, formaldeído e ozono, não se verificaram valores superiores aos regulamentares. No caso do CO2, o valor máximo encontrado, nestes três espaços, foi de 745 ppm na Mezaninne das Oficinas e para o CO, na zona AVAC com uma concentração de 0,73 ppm. A medição do formaldeído registou valores perto dos 45 μg/m3ar e o ozono apenas foi detectado, em concentração muito reduzida, na zona Comercial – Produtos. Por fim, as concentrações de bactérias e fungos, de acordo com o RSECE, não devem ultrapassar as 500 UFC/m3ar (parâmetros microbiológicos). Em qualquer dos espaços, os valores medidos foram inferiores ao máximo legal, não ultrapassando as 50 UFC/m3ar. Da avaliação do projecto AVAC, e através da medição dos caudais de insuflação/ extracção em cada zona, concluiu-se que os seus valores não estão de acordo com os valores do projecto inicial que poderá ser imputada a uma insuficiência no funcionamento do sistema detectada na altura das medições. No que diz respeito ao estado de limpeza do sistema AVAC, apenas foi possível inspeccionar as unidades de tratamento de ar, tendo-se constatado que se encontram em boas condições. Ou seja, do ponto de vista do RSECE, e referindo-nos apenas à vertente da Qualidade do Ar Interior, o edifício em causa, cumpre todos os limites impostos para as concentrações de poluentes mas, apresenta algumas deficiências no que respeita aos caudais de ar novo insuflados em cada espaço.

Primary bioreceptivity of limestones from the mediterranean basin to phototrophic microorganisms

Dissertação apresentada para a obtenção do grau de Doutor em Conservação e Restauro pela Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia

Numerical solution of a PDE system with non-linear steady state conditions that translates the air stripping pollutants removal

This work deals with the numerical simulation of air stripping process for the pre-treatment of groundwater used in human consumption. The model established in steady state presents an exponential solution that is used, together with the Tau Method, to get a spectral approach of the solution of the system of partial differential equations associated to the model in transient state.