866 resultados para Extreme Loads
Resumo:
This study characterises the abatement effect of large dams with fixed-crest spillways under extreme design flood conditions. In contrast to previous studies using specific hydrographs for flow into the reservoir and simplifications to obtain analytical solutions, an automated tool was designed for calculations based on a Monte Carlo simulation environment, which integrates models that represent the different physical processes in watersheds with areas of 150?2000 km2. The tool was applied to 21 sites that were uniformly distributed throughout continental Spain, with 105 fixed-crest dam configurations. This tool allowed a set of hydrographs to be obtained as an approximation for the hydrological forcing of a dam and the characterisation of the response of the dam to this forcing. For all cases studied, we obtained a strong linear correlation between the peak flow entering the reservoir and the peak flow discharged by the dam, and a simple general procedure was proposed to characterise the peak-flow attenuation behaviour of the reservoir. Additionally, two dimensionless coefficients were defined to relate the variables governing both the generation of the flood and its abatement in the reservoir. Using these coefficients, a model was defined to allow for the estimation of the flood abatement effect of a reservoir based on the available information. This model should be useful in the hydrological design of spillways and the evaluation of the hydrological safety of dams. Finally, the proposed procedure and model were evaluated and representative applications were presented
Resumo:
The study of pointer years of numerous tree-ring chronologies of the central Iberian Peninsula (Sierra de Guadarrama) could provide complementary information about climate variability over the last 405 yr. In total, 64 pointer years have been identified: 30 negative (representing minimum growths) and 34 positive (representing maximum growths), the most significant of these being 1601, 1963 and 1996 for the negative ones, and 1734 and 1737 for the positive ones. Given that summer precipitation was found to be the most limiting factor for the growth of Pinus in the Sierra de Guadarrama in the second half of the 20th century, it is also an explanatory factor in almost 50% of the extreme growths. Furthermore, these pointer years and intervals are not evenly distributed throughout time. Both in the first half of the 17th and in the second half of 20th, they were more frequent and more extreme and these periods are the most notable for the frequency of negative pointer years in Central Spain. The interval 1600–1602 is of special significance, being one of the most unfavourable for tree growth in the centre of Spain, with 1601 representing the minimum index in the regional chronology. We infer that this special minimum annual increase was the effect of the eruption of Huaynaputina, which occurred in Peru at the beginning of 1600 AD. This is the first time that the effects of this eruption in the tree-ring records of Southern Europe have been demonstrated.
Resumo:
In this paper, we propose a system for authenticating local bee pollen against fraudulent samples using image processing and classification techniques. Our system is based on the colour properties of bee pollen loads and the use of one-class classifiers to reject unknown pollen samples. The latter classification techniques allow us to tackle the major difficulty of the problem, the existence of many possible fraudulent pollen types. Also presented is a multi-classifier model with an ambiguity discovery process to fuse the output of the one-class classifiers. The method is validated by authenticating Spanish bee pollen types, the overall accuracy of the final system of being 94%. Therefore, the system is able to rapidly reject the non-local pollen samples with inexpensive hardware and without the need to send the product to the laboratory.
Resumo:
The flexural vibration of a homogeneous isotropic linearly elastic cylinder of any aspect ratio is analysed in this paper. Natural frequencies of a cylinder under uniformly distributed axial loads acting on its bases are calculated numerically by the Ritz method with terms of power series in the coordinate directions as approximating functions. The effect of axial loads on the flexural vibration cannot be described by applying infinitesimal strain theory, therefore, geometrically nonlinear strain–displacement relations with second-order terms are considered here. The natural frequencies of free–free, clamped–clamped, and sliding–sliding cylinders subjected to axial loads are calculated using the proposed three-dimensional Ritz approach and are compared with those obtained with the finite element method and the Bernoulli–Euler theory. Different experiments with cylinders axially compressed by a hydraulic press are carried out and the experimental results for the lowest flexural frequency are compared with the numerical results. An approach based on the Ritz formulation is proposed for the flexural vibration of a cylinder between the platens of the press with constraints varying with the intensity of the compression. The results show that for low compressions the cylinder behaves similarly to a sliding–sliding cylinder, whereas for high compressions the cylinder vibrates as a clamped–clamped one.
Resumo:
Tests used to simulate the separation of the lower stage of the Ariane Vehicle Equipment Bay (VEB) were carried out on a flat full scale model. Theoretical studies carried out prior to testing are described. Three different mathematical methods, finite element, component element, and wave propagation, were used. Comparison of the predicted theoretical results with the actual test results is planned.
Resumo:
Leonhardt demonstrated (2009) that the 2D Maxwell Fish Eye lens (MFE) can focus perfectly 2D Helmholtz waves of arbitrary frequency, i.e., it can transport perfectly an outward (monopole) 2D Helmholtz wave field, generated by a point source, towards a "perfect point drain" located at the corresponding image point. Moreover, a prototype with λ/5 superresolution (SR) property for one microwave frequency has been manufactured and tested (Ma et al, 2010). Although this prototype has been loaded with an impedance different from the "perfect point drain", it has shown super-resolution property. However, neither software simulations nor experimental measurements for a broad band of frequencies have yet been reported. Here we present steady state simulations for two cases, using perfect drain as suggested by Leonhardt and without perfect drain as in the prototype. All the simulations have been done using a device equivalent to the MFE, called the Spherical Geodesic Waveguide (SGW). The results show the super-resolution up to λ/3000, for the system loaded with the perfect drain, and up to λ/500 for a not perfect load. In both cases super-resolution only happens for discrete number of frequencies. Out of these frequencies, the SGW does not show super-resolution in the analysis carried out.
Resumo:
This paper presents the results of part of the research carried out by a committee in charge of the elaboration of the new Spanish Code of Actions in Railway Bridges. Following the work developed by the European Rail Research Institute (ERRI), the dynamic effects caused by the Spanish high-speed train TALGO have been studied and compared with other European trains. A simplified envelope of the impact coefficient is also presented. Finally, the train-bridge interactions has been analysed and the results compared with those obtained from simple models based on moving loads.
Resumo:
La fatiga superficial es uno de los principales problemas en las transmisiones mecánicas y es uno de los focos de atención de las investigaciones de los últimos anos en Tribología. La disminución de viscosidad de los lubricantes para la mejora de la eficiencia, el aumento de las potencias a transmitir, el aumento de la vida de los componentes o la mejora de su fiabilidad han supuesto que los fenómenos de fatiga superficial hayan cobrado especial relevancia, especialmente los fenómenos de pitting y micropitting en cajas multiplicadoras/reductoras de grandes potencias de aplicación, por ejemplo, en el sector eólico. Como todo fenómeno de fatiga, el pitting y micropitting son debidos a la aplicación de cargas ciclicas. Su aparición depende de las presiones y tensiones cortantes en el contacto entre dos superficies que al encontrarse en rodadura y deslizamiento varian con el tiempo. La principal consecuencia de la fatiga superficial es la aparición de hoyuelos de diferente magnitud segun la escala del fenómeno (pitting o micropitting) en la superficie del material. La aparición de estos hoyuelos provoca la perdida de material, induce vibraciones y sobrecargas en el elemento que finalmente acaba fallando. Debido a la influencia de la presión y tensión cortante en el contacto, la aparición de fatiga depende fuertemente del lubricante que se encuentre entre las dos superficies y de las condiciones de funcionamiento en las cuales este trabajando. Cuando el contacto trabaja en condiciones de lubricacion mixta-elastohidrodinamica tiende a aparecer micropitting debido a las altas tensiones localizadas en las proximidades de las asperezas, mientras que si el régimen es de lubricación completa el tipo de fatiga superficial suele ser pitting debido a las tensiones mas suavizadas y menos concentradas. En esta Tesis Doctoral se han analizado todos estos factores de influencia que controlan el pitting y el micropitting prestando especial atención al efecto del lubricante. Para ello, se ha dado un enfoque conjunto a ambos fenómenos resolviendo las ecuaciones involucradas en el contacto elastohidrodinamico no-Newtoniano (la ecuación de Reynolds, la deformación elástica de los sólidos y la reologia del lubricante) para conocer la presión y la tensión cortante en el contacto. Conocidas estas, se resuelve el campo de tensiones en el interior del material y, finalmente, se aplican criterios de fatiga multiaxial (Crossland, Dang Van y Liu-Mahadevan) para conocer si el material falla o no falla. Con la metodología desarrollada se ha analizado el efecto sobre las tensiones y la aparición de la fatiga superficial del coeficiente viscosidad-presion, de la compresibilidad, del espesor especifico de película y de la fricción así como de la influencia de las propiedades a fatiga del material y de las condiciones de funcionamiento (radios de contacto, velocidad, deslizamiento, carga y temperatura). Para la validación de los resultados se han utilizado resultados teóricos y experimentales de otros autores junto con normas internacionales de amplia utilización en el mundo industrial, entre otras, para el diseño y calculo de engranajes. A parte del trabajo realizado por simulación y cálculo de los diferentes modelos desarrollados, se ha realizado un importante trabajo experimental que ha servido no solo para validar la herramienta desarrollada sino que además ha permitido incorporar al estudio factores no considerados en los modelos, como los aditivos del lubricante. Se han realizado ensayos de medida del coeficiente de fricción en una maquina de ensayo puntual con la que se ha validado el cálculo del coeficiente de fricción y se ha desarrollado un proceso de mejora del coeficiente de fricción mediante texturizado superficial en contactos puntuales elastohidrodinamicos mediante fotolitografia y ataque quimico. Junto con los ensayos de medida de fricción en contacto puntual se han realizado ensayos de fricción y fatiga superficial en contacto lineal mediante una maquina de discos que ha permitido evaluar la influencia de diferentes aditivos (modificadores de fricción, antidesgaste y extrema-presion) en la aparición de fatiga superficial (pitting y micropitting) y la fricción en el contacto. Abstract Surface fatigue is one of the most important problems of mechanical transmissions and therefore has been one of the main research topics on Tribology during the last years. On the one hand, industrial demand on fuel economy has led to reduce lubricant viscosity in order to improve efficiency. On the other hand, the requirements of power and life of machine elements are continuously increasing, together with the improvements in reliability. As a consequence, surface fatigue phenomena have become critical in machinery, in particular pitting and micropitting in high power gearboxes of every kind of machines, e.g., wind turbines or cranes. In line with every fatigue phenomena, pitting and micropitting are caused by cyclic loads. Their appearance depends on the evolution of pressures and shear stresses with time, throughout the contact between surfaces under rolling and sliding conditions. The main consequence of surface fatigue is the appearance of pits on the surface. The size of the pits is related to the scale of the fatigue: pitting or micropitting. These pits cause material loss, vibrations and overloads until the final failure is reached. Due to the great influence of the pressures and shear stresses in surface fatigue, the appearance of pits depends directly on the lubricant and the operating conditions. When the contact works under mixed regime (or under elastohydrodynamic but close to mixed regime) the main fatigue failure is micropitting because of the high pressures located near the asperities. In contrast, when the contact works under elastohydrodynamic fully flooded conditions the typical fatigue failure is pitting. In this Ph.D. Thesis, the main factors with influence on pitting and micropitting phenomena are analyzed, with special attention to the effect of the lubricant. For this purpose, pitting and micropitting are studied together by solving the equations involved in the non-Newtonian elastohydrodynamic contact. Thus, pressure and shear stress distributions are found by taking into account Reynolds equation, elastic deflection of the solids and lubricant rheology. Subsequently, the stress field inside the material can be calculated and different multiaxial fatigue criteria (Crossland, Dang Van and Liu- Mahadevan) can be applied to predict whether fatigue failure is reached. The influences of the main parameters on pressure and surface fatigue have been studied, taking into account the lubricant compressibility and its viscosity-pressure coefficient, the specific film thickness, the friction coefficient and the fatigue properties of the contacting materials, together with the operating conditions (contact radius, mean velocity, sliding velocity, load and temperature). Several theoretical and experimental studies of different authors have been used to validate all the results obtained, together with international standards used worldwide in gear design industry. Moreover, an experimental stage has been carried out in order to validate the calculation methods and introduce additional influences not included previously, e.g., lubricant additives. The experimentation includes different friction tests in point contacts performed with a tribological equipment in order to validate the results given by the calculations. Furthermore, the reduction and optimization of the friction coefficient is analyzed by means of textured surfaces, obtained combining photolithography and chemical etching techniques. Besides the friction tests with point contact, friction and surface fatigue tests have also been performed with line contact in a tribological test rig. This equipment is also used to study the influence of different types of additives (friction modifiers, anti-wear and extreme-pressure additives) on surface fatigue (pitting and micropitting).
Resumo:
Solar thermal power plants are usually installed in locations with high yearly average solar radiation, often deserts. In such conditions, cooling water required for thermodynamic cycles is rarely available. Moreover, when solar radiation is high, ambient temperature is very high as well; this leads to excessive condensation temperature, especially when air-condensers are used, and decreases the plant efficiency. However, temperature variation in deserts is often very high, which drives to relatively low temperatures during the night. This fact can be exploited with the use of a closed cooling system, so that the coolant (water) is chilled during the night and store. Chilled water is then used during peak temperature hours to cool the condenser (dry cooling), thus enhancing power output and efficiency. The present work analyzes the performance improvement achieved by night thermal cool storage, compared to its equivalent air cooled power plant. Dry cooling is proved to be energy-effective for moderately high day–night temperature differences (20 °C), often found in desert locations. The storage volume requirement for different power plant efficiencies has also been studied, resulting on an asymptotic tendency.
Resumo:
Green façades constitute constructive technologies with a positive influence on sustainability in cities and several urban climate parameters such as thermal comfort, air quality and water management. According to the current research, the implementation of urban greenery contributes to increase the cooling effect and mitigate the urban heat island (UHI) phenomenon. This paper focuses on the role of vegetation in improving the urban environment of Madrid (Spain). The simulation results show that green walls could be more effective in urban morphologies with narrow streets. During overheated periods, the streets with green walls have a higher relative humidity in the surrounding areas than those with trees. The air temperature is found to be a little lower. The reduction of wind speed means a positive effect on urban hygrothermal comfort. Therefore, green walls could be taken into account as suitable tools to modify the outdoor thermal environment in cities with an extreme Continental Mediterranean climate.
Resumo:
The Santa Irene flood event, at the end of October 1982, is one of the most dramatically widely reported flood events in Spain. Its renown is mainly due to the collapse of the Tous dam, but its main message is to be the paradigm of the incidence of the maritime/littoral weather and its temporal sea level rise by storm surge accompanying rain process on the coastal plains inland floods. Looking at damages the presentation analyzes the adapted measures from the point of view of the aims of the FP7 SMARTeST Project related to the Flood Resilience improvement in urban areas through looking for Technologies, Systems and Tools an appropriate "road to de market".
Resumo:
The objectives of this work are to revisit the experimental measurements on dam break flow over a dry horizontal bed and to provide a detailed insight into the dynamics of the dam break wave impacting a vertical wall downstream the dam, with emphasis on the pressure loads. The measured data are statistically analyzed and critically discussed. As a result, an extensive set of data for validation of computational tools is provided.
Resumo:
Scientific missions constitute fundamental cornerstones of space agencies such as ESA and NASA. Modern astronomy could not be understood without the data provided by these missions. Scientists need to design very carefully onboard instruments. Payloads have to survive the crucial launch moment and later perform well in the really harsh space environ-ment. It is very important that the instrument conceptual idea can be engineered to sustain all those loads
Resumo:
During the last two decades the topic of human induced vibration has attracted a lot of attention among civil engineering practitioners and academics alike. Usually this type of problem may be encountered in pedestrian footbridges or floors of paperless offices. Slender designs are becoming increasingly popular, and as a consequence, the importance of paying attention to vibration serviceability also increases. This paper resumes the results obtained from measurements taken at different points of an aluminium catwalk which is 6 m in length by 0.6 m in width. Measurements were carried out when subjecting the structure to different actions:1)Static test: a steel cylinder of 35 kg was placed in the middle of the catwalk; 2)Dynamic test: this test consists of exciting the structure with singles impulses; 3)Dynamic test: people walking on the catwalk. Identification of the mechanical properties of the structure is an achievement of the paper. Indirect methods were used to estimate properties including the support stiffness, the beam bending stiffness, the mass of the structure (using Rayleigh method and iterative matrix method), the natural frequency (using the time domain and frequency domain analysis) and the damping ratio (by calculating the logarithmic decrement). Experimental results and numerical predictions for the response of an aluminium catwalk subjected to walking loads have been compared. The damping of this light weight structure depends on the amplitude of vibration which complicates the tuning of a structural model. In the light of the results obtained it seems that the used walking load model is not appropriate as the predicted transient vibration values (TTVs) are much higher than the measured ones.
Resumo:
If reinforced concrete structures are to be safe under extreme impulsive loadings such as explosions, a broad understanding of the fracture mechanics of concrete under such events is needed. Most buildings and infrastructures which are likely to be subjected to terrorist attacks are borne by a reinforced concrete (RC) structure. Up to some years ago, the traditional method used to study the ability of RC structures to withstand explosions consisted on a choice between handmade calculations, affordable but inaccurate and unreliable, and full scale experimental tests involving explosions, expensive and not available for many civil institutions. In this context, during the last years numerical simulations have arisen as the most effective method to analyze structures under such events. However, for accurate numerical simulations, reliable constitutive models are needed. Assuming that failure of concrete elements subjected to blast is primarily governed by the tensile behavior, a constitutive model has been built that accounts only for failure under tension while it behaves as elastic without failure under compression. Failure under tension is based on the Cohesive Crack Model. Moreover, the constitutive model has been used to simulate the experimental structural response of reinforced concrete slabs subjected to blast. The results of the numerical simulations with the aforementioned constitutive model show its ability of representing accurately the structural response of the RC elements under study. The simplicity of the model, which does not account for failure under compression, as already mentioned, confirms that the ability of reinforced concrete structures to withstand blast loads is primarily governed by tensile strength.