Hysteresis phenomena in transverse galloping of triangular cross-section bodies

Transverse galloping is a type of aeroelastic instability characterised by large amplitude, low frequency oscillation of a structure in the direction normal to the mean wind direction. It normally appears in bodies with small stiffness and structural damping, provided the incident flow velocity is high enough. In the simplest approach transverse galloping can be considered as a one-degree-of-freedom oscillator subjected to aerodynamic forces, which in turn can be described by using a quasi-steady description. In this frame it has been demonstrated that hysteresis phenomena in transverse galloping is related to the existence of inflection points in the curve giving the dependence with the angle of attack of the aerodynamic coefficient normal to the incident flow. Aiming at experimentally checking such a relationship between these inflection points and hysteresis, wind tunnel experiments have been conducted. Experiments have been restricted to isosceles triangular cross-section bodies, whose galloping behaviour is well documented. Experimental results show that, according to theoretical predictions, hysteresis takes place at the angles of attack where there are inflection points in the lift coefficient curve, provided that the body is prone to gallop at these angles of attack.

Empiric k·p Hamiltonian calculation of the band-to-band photon absorption in semiconductors

The Empiric k·p Hamiltonian method is usually applied to nanostructured semiconductors. In this paper, it is applied to a homogeneous semiconductor in order to check the adequacy of the method. In this case, the solutions of the diagonalized Hamiltonian, as well as the envelope functions, are plane waves. The procedure is applied to the GaAs and the interband absorption coefficients are calculated. They result in reasonable agreement with the measured values, further supporting the adequacy of the Empiric k·p Hamiltonian method.

Comparative analysis of passenger transport sustainability in European cities

Sustainable development in its three dimensions – economic, social and environmental – has become a major concern on an international scale. The problem is global, but must be solved locally. Most of the world’s population lives in cities that act as centres of economic growth and productivity, but which – if they develop in the wrong direction – can cause social inequalities, or irreversibly harm the environment. Urban transport causes a number of negative impacts that can affect sustainability targets. The objective of this study is to propose an analysis of sustainability of urban passenger transport systems based on available indicators in most cities. This will serve to benchmark the practices of different cities and manage their transport systems. This work involves the creation of composite indicators (CI) to measure the sustainability of urban passenger transport systems. The methodology is applied to 23 European cities. The indicators are based on a benchmarking approach, and the evaluation of each aspect in each case therefore depends on the performance of the whole sample. The CI enabled us to identify which characteristics have the greatest influence on the sustainability of a city’s transport system, and to establish transport policies that could potentially improve its shortcomings. Finally, the cities are clustered according to the values obtained from the CIs, and thus according to the weaknesses and strengths of their transport systems.

Time-multiscale methods for the simulation of slow transport problems in atomistic systems

En esta tesis presentamos una teoría adaptada a la simulación de fenómenos lentos de transporte en sistemas atomísticos. En primer lugar, desarrollamos el marco teórico para modelizar colectividades estadísticas de equilibrio. A continuación, lo adaptamos para construir modelos de colectividades estadísticas fuera de equilibrio. Esta teoría reposa sobre los principios de la mecánica estadística, en particular el principio de máxima entropía de Jaynes, utilizado tanto para sistemas en equilibrio como fuera de equilibrio, y la teoría de las aproximaciones del campo medio. Expresamos matemáticamente el problema como un principio variacional en el que maximizamos una entropía libre, en lugar de una energía libre. La formulación propuesta permite definir equivalentes atomísticos de variables macroscópicas como la temperatura y la fracción molar. De esta forma podemos considerar campos macroscópicos no uniformes. Completamos el marco teórico con reglas de cuadratura de Monte Carlo, gracias a las cuales obtenemos modelos computables. A continuación, desarrollamos el conjunto completo de ecuaciones que gobiernan procesos de transporte. Deducimos la desigualdad de disipación entrópica a partir de fuerzas y flujos termodinámicos discretos. Esta desigualdad nos permite identificar la estructura que deben cumplir los potenciales cinéticos discretos. Dichos potenciales acoplan las tasas de variación en el tiempo de las variables microscópicas con las fuerzas correspondientes. Estos potenciales cinéticos deben ser completados con una relación fenomenológica, del tipo definido por la teoría de Onsanger. Por último, aportamos validaciones numéricas. Con ellas ilustramos la capacidad de la teoría presentada para simular propiedades de equilibrio y segregación superficial en aleaciones metálicas. Primero, simulamos propiedades termodinámicas de equilibrio en el sistema atomístico. A continuación evaluamos la habilidad del modelo para reproducir procesos de transporte en sistemas complejos que duran tiempos largos con respecto a los tiempos característicos a escala atómica. ABSTRACT In this work, we formulate a theory to address simulations of slow time transport effects in atomic systems. We first develop this theoretical framework in the context of equilibrium of atomic ensembles, based on statistical mechanics. We then adapt it to model ensembles away from equilibrium. The theory stands on Jaynes' maximum entropy principle, valid for the treatment of both, systems in equilibrium and away from equilibrium and on meanfield approximation theory. It is expressed in the entropy formulation as a variational principle. We interpret atomistic equivalents of macroscopic variables such as the temperature and the molar fractions, wich are not required to be uniform, but can vary from particle to particle. We complement this theory with Monte Carlo summation rules for further approximation. In addition, we provide a framework for studying transport processes with the full set of equations driving the evolution of the system. We first derive a dissipation inequality for the entropic production involving discrete thermodynamic forces and fluxes. This discrete dissipation inequality identifies the adequate structure for discrete kinetic potentials which couple the microscopic field rates to the corresponding driving forces. Those kinetic potentials must finally be expressed as a phenomenological rule of the Onsanger Type. We present several validation cases, illustrating equilibrium properties and surface segregation of metallic alloys. We first assess the ability of a simple meanfield model to reproduce thermodynamic equilibrium properties in systems with atomic resolution. Then, we evaluate the ability of the model to reproduce a long-term transport process in complex systems.

Trapping phenomena in AlGaN and InAlN barrier HEMTs with different geometries

PAPER Trapping phenomena in AlGaN and InAlN barrier HEMTs with different geometries S Martin-Horcajo1, A Wang1, A Bosca1, M F Romero1, M J Tadjer1,2, A D Koehler2, T J Anderson2 and F Calle1 Published 11 February 2015 • © 2015 IOP Publishing Ltd Semiconductor Science and Technology, Volume 30, Number 3 Article PDF Figures References Citations Metrics 350 Total downloads Cited by 1 articles Export citation and abstract BibTeX RIS Turn on MathJax Share this article Article information Abstract Trapping effects were evaluated by means of pulsed measurements under different quiescent biases for GaN/AlGaN/GaN and GaN/InAlN/GaN. It was found that devices with an AlGaN barrier underwent an increase in the on-resistance, and a drain current and transconductance reduction without measurable threshold voltage change, suggesting the location of the traps in the gate-drain access region. In contrast, devices with an InAlN barrier showed a transconductance and a decrease in drain associated with a significant positive shift of threshold voltage, indicating that the traps were likely located under the gate region; as well as an on-resistance degradation probably associated with the presence of surface traps in the gate-drain access region. Furthermore, measurements of drain current transients at different ambient temperatures revealed that the activation energy of electron traps was 0.43 eV and 0.38 eV for AlGaN and InAlN barrier devices, respectively. Experimental and simulation results demonstrated the influence of device geometry on the observed trapping effects, since devices with larger gate lengths and gate-to-drain distance values exhibited less noticeable charge trapping effects.

Morphological Functions with Parallel Sets for the Pore Space of X-ray CT Images of Soil Columns

During the last few decades, new imaging techniques like X-ray computed tomography have made available rich and detailed information of the spatial arrangement of soil constituents, usually referred to as soil structure. Mathematical morphology provides a plethora of mathematical techniques to analyze and parameterize the geometry of soil structure. They provide a guide to design the process from image analysis to the generation of synthetic models of soil structure in order to investigate key features of flow and transport phenomena in soil. In this work, we explore the ability of morphological functions built over Minkowski functionals with parallel sets of the pore space to characterize and quantify pore space geometry of columns of intact soil. These morphological functions seem to discriminate the effects on soil pore space geometry of contrasting management practices in a Mediterranean vineyard, and they provide the first step toward identifying the statistical significance of the observed differences.

WCSPH viscosity diffusion processes in vortex flows

The aim of this paper is to clarify the role played by the most commonly used viscous terms in simulating viscous laminar flows using the weakly compressible approach in the context of smooth particle hydrodynamics (WCSPH). To achieve this, Takeda et al. (Prog. Theor. Phys. 1994; 92(5):939–960), Morris et al. (J. Comput. Phys. 1997; 136:214–226) and Monaghan–Cleary–Gingold's (Appl. Math. Model. 1998; 22(12):981–993; Monthly Notices of the Royal Astronomical Society 2005; 365:199–213) viscous terms will be analysed, discussing their origins, structures and conservation properties. Their performance will be monitored with canonical flows of which related viscosity phenomena are well understood, and in which boundary effects are not relevant. Following the validation process of three previously published examples, two vortex flows of engineering importance have been studied. First, an isolated Lamb–Oseen vortex evolution where viscous effects are dominant and second, a pair of co-rotating vortices in which viscous effects are combined with transport phenomena. The corresponding SPH solutions have been compared to finite-element numerical solutions. The SPH viscosity model's behaviour in modelling the viscosity related effects for these canonical flows is adequate

Community Structure in Soil Porous System

Diffusion controls the gaseous transport process in soils when advective transport is almost null. Knowledge of the soil structure and pore connectivity are critical issues to understand and modelling soil aeration, sequestration or emission of greenhouse gasses, volatilization of volatile organic chemicals among other phenomena. In the last decades these issues increased our attention as scientist have realize that soil is one of the most complex materials on the earth, within which many biological, physical and chemical processes that support life and affect climate change take place. A quantitative and explicit characterization of soil structure is difficult because of the complexity of the pore space. This is the main reason why most theoretical approaches to soil porosity are idealizations to simplify this system. In this work, we proposed a more realistic attempt to capture the complexity of the system developing a model that considers the size and location of pores in order to relate them into a network. In the model we interpret porous soils as heterogeneous networks where pores are represented by nodes, characterized by their size and spatial location, and the links representing flows between them. In this work we perform an analysis of the community structure of porous media of soils represented as networks. For different real soils samples, modelled as heterogeneous complex networks, spatial communities of pores have been detected depending on the values of the parameters of the porous soil model used. These types of models are named as Heterogeneous Preferential Attachment (HPA). Developing an exhaustive analysis of the model, analytical solutions are obtained for the degree densities and degree distribution of the pore networks generated by the model in the thermodynamic limit and shown that the networks exhibit similar properties to those observed in other complex networks. With the aim to study in more detail topological properties of these networks, the presence of soil pore community structures is studied. The detection of communities of pores, as groups densely connected with only sparser connections between groups, could contribute to understand the mechanisms of the diffusion phenomena in soils.

Modelling of chloride transport in non-saturated concrete : from microscale to macroscale

Corrosion of steel bars embedded in concrete has a great influence on structural performance and durability of reinforced concrete. Chloride penetration is considered to be a primary cause of concrete deterioration in a vast majority of structures. Therefore, modelling of chloride penetration into concrete has become an area of great interest. The present work focuses on modelling of chloride transport in concrete. The differential macroscopic equations which govern the problem were derived from the equations at the microscopic scale by comparing the porous network with a single equivalent pore whose properties are the same as the average properties of the real porous network. The resulting transport model, which accounts for diffusion, migration, advection, chloride binding and chloride precipitation, consists of three coupled differential equations. The first equation models the transport of chloride ions, while the other two model the flow of the pore water and the heat transfer. In order to calibrate the model, the material parameters to determine experimentally were identified. The differential equations were solved by means of the finite element method. The classical Galerkin method was employed for the pore solution flow and the heat transfer equations, while the streamline upwind Petrov Galerkin method was adopted for the transport equation in order to avoid spatial instabilities for advection dominated problems. The finite element codes are implemented in Matlab® . To retrieve a good understanding of the influence of each variable and parameter, a detailed sensitivity analysis of the model was carried out. In order to determine the diffusive and hygroscopic properties of the studied concretes, as well as their chloride binding capacity, an experimental analysis was performed. The model was successfully compared with experimental data obtained from an offshore oil platform located in Brazil. Moreover, apart from the main objectives, numerous results were obtained throughout this work. For instance, several diffusion coefficients and the relation between them are discussed. It is shown how the electric field set up between the ionic species depends on the gradient of the species’ concentrations. Furthermore, the capillary hysteresis effects are illustrated by a proposed model, which leads to the determination of several microstructure properties, such as the pore size distribution and the tortuosity-connectivity of the porous network. El fenómeno de corrosión del acero de refuerzo embebido en el hormigón ha tenido gran influencia en estructuras de hormigón armado, tanto en su funcionalidad estructural como en aspectos de durabilidad. La penetración de cloruros en el interior del hormigón esta considerada como el factor principal en el deterioro de la gran mayoría de estructuras. Por lo tanto, la modelización numérica de dicho fenómeno ha generado gran interés. El presente trabajo de investigación se centra en la modelización del transporte de cloruros en el interior del hormigón. Las ecuaciones diferenciales que gobiernan los fenómenos a nivel macroscópico se deducen de ecuaciones planteadas a nivel microscópico. Esto se obtiene comparando la red porosa con un poro equivalente, el cual mantiene las mismas propiedades de la red porosa real. El modelo está constituido por tres ecuaciones diferenciales acopladas que consideran el transporte de cloruros, el flujo de la solución de poro y la transferencia de calor. Con estas ecuaciones se tienen en cuenta los fenómenos de difusión, migración, advección, combinación y precipitación de cloruros. El análisis llevado a cabo en este trabajo ha definido los parámetros necesarios para calibrar el modelo. De acuerdo con ellas, se seleccionaron los ensayos experimentales a realizar. Las ecuaciones diferenciales se resolvieron mediante el método de elementos finitos. El método clásico de Galerkin se empleó para solucionar las ecuaciones de flujo de la solución de poro y de la transferencia de calor, mientras que el método streamline upwind Petrov-Galerkin se utilizó para resolver la ecuación de transporte de cloruros con la finalidad de evitar inestabilidades espaciales en problemas con advección dominante. El código de elementos finitos está implementado en Matlab® . Con el objetivo de facilitar la comprensión del grado de influencia de cada variable y parámetro, se realizó un análisis de sensibilidad detallado del modelo. Se llevó a cabo una campaña experimental sobre los hormigones estudiados, con el objeto de obtener sus propiedades difusivas, químicas e higroscópicas. El modelo se contrastó con datos experimentales obtenidos en una plataforma petrolera localizada en Brasil. Las simulaciones numéricas corroboraron los datos experimentales. Además, durante el desarrollo de la investigación se obtuvieron resultados paralelos a los planteados inicialmente. Por ejemplo, el análisis de diferentes coeficientes de difusión y la relación entre ellos. Así como también se observó que el campo eléctrico establecido entre las especies iónicas disueltas en la solución de poro depende del gradiente de concentración de las mismas. Los efectos de histéresis capilar son expresados por el modelo propuesto, el cual conduce a la determinación de una serie de propiedades microscópicas, tales como la distribución del tamaño de poro, además de la tortuosidad y conectividad de la red porosa.

Time derivatives in air temperature and enthalpy as non-invasive welfare indicators during long distance animal transport in Spain and Portugal.

High temperatures and relative humidity can compromise animal welfare on the farm level, but less is known about those changes during long distance transport of domestic animals to slaughter. Although upper temperature limits have been established to transport pigs in Europe, few indices include relative or absolute humidity maxima or mention appropriate enthalpy ranges.

The Role of Multilateral Coordination in Transport Policy: The Case of the Maghreb Countries

El WCTR es un congreso de reconocido prestigio internacional en el ámbito de la investigación del transporte y aunque las actas publicadas están en formato digital y sin ISSN ni ISBN, lo consideramos lo suficientemente importante como para que se considere en los indicadores. This paper aims at describing how multilateral cooperation policies are influencing national transport policies in developing countries. It considers the evolution of national transport policies and institutional frameworks in Algeria, Morocco and Tunisia in the last 10 years, and analyses the influence that EU cooperation programmes (particularly those within the Euromed programme initiative) and international coordination activities have played in the evolution towards efficient, sustainable transport systems in those countries. Notwithstanding the significant socioeconomic, political and institutional differences among the three countries, three major traits are common to the transport policy framework in all cases: a focus on megaprojects; substitution of traditional ministerial services by ad hoc public agencies to develop those megaprojects, and progressive involvement of international private players for the operation (and eventually the design and construction) of new projects, focusing on know-how transfer rather than investment needs. The hypotheses is that these similarities are largely due to the influence of the international cooperation promoted by the European Union since the mid- 1990s. The new decision making situation is characterized by the involvement of two new relevant stakeholders, the EU and a limited number of global transport operators. The hierarchical governance model evolves towards more complex structures, which explain the three common traits mentioned above. International coordination has been crucial for developing national transport visions, which are coherent with a regional, transnational system.

Strategies for Minimum Distance in Simulated Evacuation of Transport Airplanes

The present paper describes the preliminary stages of the development of a new, comprehensive model conceived to simulate the evacuation of transport airplanes in certification studies. Two previous steps were devoted to implementing an efficient procedure to define the whole geometry of the cabin, and setting up an algorithm for assigning seats to available exits. Now, to clarify the role of the cabin arrangement in the evacuation process, the paper addresses the influence of several restrictions on the seat-to-exit assignment algorithm, maintaining a purely geometrical approach for consistency. Four situations are considered: first, an assignment method without limitations to search the minimum for the total distance run by all passengers along their escaping paths; second, a protocol that restricts the number of evacuees through each exit according to updated FAR 25 capacity; third, a procedure which tends to the best proportional sharing among exits but obliges to each passenger to egress through the nearest fore or rear exits; and fourth, a scenario which includes both restrictions. The four assignment strategies are applied to turboprops, and narrow body and wide body jets. Seat to exit distance and number of evacuees per exit are the main output variables. The results show the influence of airplane size and the impact of non-symmetries and inappropriate matching between size and longitudinal location of exits.

An integrated modeling approach to assess the impact on road freight transport demand of allowing longer and heavier vehicles (LHVs) in Spain

The assessment on introducing Longer and Heavier Vehicles (LHVs) on the road freight transport demand is performed in this paper by applying an integrated modeling approach composed of a Random Utility-Based Multiregional Input-Output model (RUBMRIO) and a road transport network model. The approach strongly supports the concept that changes in transport costs derived from the LHVs allowance as well as the economic structure of regions have both direct and indirect effects on the road freight transport system. In addition, we estimate the magnitude and extent of demand changes in the road freight transportation system by using the commodity-based structure of the approach to identify the effect on traffic flows and on pollutant emissions over the whole network of Spain by considering a sensitivity analysis of the main parameters which determine the share of Heavy-Goods Vehicles (HGVs) and LHVs. The results show that the introduction of LHVs will strengthen the competitiveness of the road haulage sector by reducing costs, emissions, and the total freight vehicles required.