Challenges in the Application of Stochastic Modal Identification Methods to a Cable-Stayed Bridge

This paper presents an analysis of the data collected in the ambient vibration test of the International Guadiana cable-stayed Bridge, which links Portugal and Spain, based on different output-only identification techniques: peak-picking, frequency domain decomposition, covariance-driven stochastic subspace identification, and data-driven stochastic subspace identification. The purpose of the analysis is to compare the performance of the four techniques and evaluate their efficiency in dealing with specific challenges involved in the modal identification of the tested cable-stayed bridge, namely the existence of closely spaced modes, the perturbation produced by the local vibration of stay-cables, and the variation of modal damping coefficients with wind velocity. The identified natural frequencies and mode shapes are compared with the corresponding modal parameters provided by a previously developed numerical model. Additionally, the variability of some modal damping coefficients is related with the variation of the wind characteristics and associated with a component of aerodynamic damping.     

Functions for relative maximization

We introduce functions for relative maximization in a general context: the beta and alpha applications. After a systematic study of different kinds of regularities, we investigate how to approximate certain values of these functions using periodic orbits. We also show that the differential of an alpha application determines the asymptotic behavior of the optimal trajectories.     

Creep of PVDF monofilament sutures: service performance prediction from short-term tests

Isothermal short-term creep of poly (vinylidene fluoride) (PVDF) monofilament sutures was determined at several temperatures between 10 and 90 °C under the stress of 10 MPa. Long term service performance was predicted for 10 decades of time. The compliance master curve as a function of time fits a hyperbolic sine equation. The temperature shift factor as a function of the temperature a_T (T) is accurately represented by a general equation based on free volume. A simple relationship between the two parameters of the equation is explored. The viscoelasticity of PVDF is also seen in dynamic mechanical analysis performed at the frequency of 1 Hz. The origin of the viscoelastic character well present in the deformability of the PVDF in service is due to the occurrence of the α_c relaxation that is active at ∼50 °C (E″ peak at 1 Hz).     

XPS studies of directly fluorinated HDPE: problems and solutions

X-ray photoelectron spectroscopy (XPS) applied to the study of fluorinated polymer surfaces presents several problems related both to peak assignment and to degradation. In this work, we analyse extensively the question of XPS peak assignments in this kind of surfaces. We conclude that in this kind of surfaces using binding energy differences between fluorine and carbon is better than using absolute binding energies. Also a useful relation between fluorine photoelectron energy vs. polymer composition expressed through the atomic ratio fluorine/carbon (F/C) was found. A protocol for data treatment is proposed and applied to a XPS study of the degradation induced by X-ray on high-density polyethylene surfaces modified by direct fluorination. Results obtained for the degradation, namely the atomic ratio F/C obtained by two different methods, combined with angle resolved X-ray photoelectron spectroscopy (ARXPS) were used to study the fluorine concentration profile in depth, producing self-consistent results.     

Bedload Transport in Alluvial Channels

Hydraulic, sediment, land-use, and rock-erosivity data of 22 alluvial streams were used to evaluate conditions of bedload transport and the performance of selected bedload-transport equations. Transport categories of transport-limited (TL), partially transport-limited (PTL), and supply-limited (SL) were identified by a semiquantitative approach that considers hydraulic constraints on sediment movement and the processes that control sediment availability at the basin scale. Equations by Parker et al. in 1982, Schoklitsch in 1962, and Meyer-Peter and Muller in 1948 adequately predicted sediment transport in channels with TL condition, whereas the equations of Bagnold in 1980, and Schoklitsch, in 1962, performed well for PTL and SL conditions. Overall, the equation of Schoklitsch predicted well the measured bedload data for eight of 22 streams, and the Bagnold equation predicted the measured data in seven streams.     

Lexically scoped distribution: what you see is what you get

We define a lexically scoped, asynchronous and distributed π-calculus, with local communication and process migration. This calculus adopts the network-awareness principle for distributed programming and follows a simple model of distribution for mobile calculi: a lexical scope discipline combines static scoping with dynamic linking, associating channels to a fixed site throughout computation. This discipline provides for both remote invocation and process migration. A simple type system is a straightforward extension of that of the π-calculus, adapted to take into account the lexical scope of channels. An equivalence law captures the essence of this model: a process behavior depends on the channels it uses, not on where it runs.     

Fatigue behaviour of a phenolic matrix composite

This paper presents results of a fatigue life investigations carried out in plate specimens of a fibre-glass-reinforced phenolic matrix composite. Tensile and Young's modulus data were obtained at four different testing temperatures (room temperature, 100, 150 and 200 °C). The fatigue S−N data were obtained at room temperature only and for two stress ratio values (R=0 and 0.4). Fatigue and tensile behaviour was assesesed in the composite with the fibres aligned in the longitudinal loading direction. The results were obtained for two values of volume fraction (0.28 and 0.42) and three different glass surface treatments. A detailed comparison of fatigue results is given taking into account several fatigue parameters and also the testing variables. Results of observations of SEM fracture surfaces are also presented.     

Simulation-based sequential analysis of Markov switching stochastic volatility models

We propose a simulation-based algorithm for inference in stochastic volatility models with possible regime switching in which the regime state is governed by a first-order Markov process. Using auxiliary particle filters we developed a strategy to sequentially learn about states and parameters of the model. The methodology is tested against a synthetic time series and validated with a real financial time series: the IBOVESPA stock index (São Paulo Stock Exchange).     

A fuzzy queue-aware routing approach for wireless mesh networks

Recent advances in Wireless Mesh Networks (WMNs) have overcome the drawbacks of traditional wired and ad-hoc networks and now they are seen as a means of allowing last mile communications with quality level assurance in Future Multimedia Systems. However, new routing schemes are needed to provide end-to-end Quality of Service (QoS) and Quality of Experience (QoE) support for delay/loss/jitter-sensitive multimedia applications. The well-known OLSR (Optimized Link State Routing) protocol with ETX (Expected Transmission Count) metric brings many benefits to the path selection process, but has a drawback with regard to queue availability management, which reduces the system performance. This problem is caused when OLSR-EXT control messages are exchanged and the queues of mesh routers along the end-to-end communication path are overloaded. As a result, multimedia-related packets will suffer from loss/delay/jitter and the overall system performance will decrease. This paper proposes the Optimized Link State Routing-Fuzzy ETX Queue (OLSR-FEQ) protocol to overcome the limitations of OLSR-ETX regarding queue availability, QoS and QoE assurance. OLSR-FEQ optimizes network and user-based parameters by coordinating queue availability, QoS and fuzzy issues in the routing decision process as a way of allocating the best paths for multimedia applications. Performance evaluations were carried out with the Network Simulator (NS-2.34) to show the benefits of the proposed solution when compared with existing routing schemes, namely OLSR-ETX, OLSR-FLC, OLSR-MD and HWMP (IEEE 802.11s standard), regarding QoS (unsuccessful packet delivery and throughput) and QoE (PSNR, SSIM, VQM and MOS) parameters.     

A random dipole model for spontaneous brain activity

The statistical properties of the EEG and the MEG are described mathematically as the result of randomly distributed dipoles. These dipoles represent the interactions of cortical neurons. For certain dipole distributions, the first- and second-order moments of the electric and magnetic fields are derived analytically. If the dipoles are in a spherical volume conductor and have no preference for any direction, the variance of a differentially measured EEG-signal is only a function of the electrode distance. In this paper, the theoretically derived variance function will be compared with EEG- and MEG-measurements. It is shown that a dipole with a fixed position and a randomly fluctuating amplitude is an adequate model for the alpha-rhythm. An expression for the covariance between the magnetic field and a differentially measured EEG-signal is derived. This covariance is considered as a function of the magnetometer position, and is compared with the measurements of Chapman et al. [23]. The theory can be used to obtain a (spatial) covariance matrix of the background noise, which occurs in evoked potential measurements. Such a covariance matrix can be used to obtain a maximum likelihood estimator of the dipole parameters in evoked potential studies, to evaluate the merits of the so-called "Laplacian derivation," and for the interpolation of electromagnetic data.