4,811 research outputs found

    High Voltage DC-biased Oil Type Medium Frequency Transformer; A Green Solution for Series DC Wind Park Concept

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    The electric energy generated by remote offshore wind parks is transported to the consumers using high voltage submarine cables. On the generation site, such transmissions are realized today by collecting the energy produced by several wind turbines in a bulky and expensive transformer placed on a dedicated platform. An alternative solution has been proposed recently, which allows to reduce the installation and maintenance costs by eliminating such a platform. It is suggested to equip each wind turbine in the wind park by an individual DC/DC converter and connect them in series to reach the DC voltage level required for an efficient HVDC energy transportation to the shore. The DC/DC converter is supposed to be a Dual Active Bridge (DAB) converter, which can be made reasonably small to be placed on the wind turbine tower or even in its nacelle. The key element of the converter defining its size and mass is a special transformer, which operates at voltages comprising a high (switching) frequency component superimposed on a high DC offset voltage. DC insulation design of such a transformer and investigation of the effects of a high DC insulation level on the other electromagnetic properties of the transformer is the subject of the present research.In order to verify the concept a prototype of the transformer was built, and its evaluation presented. The unit has been manufactured for the rated power of 50 kW and rated voltages 0.4/5 kV including DC offset of 125 kV and square-shaped oscillations with the frequency of 5 kHz. The magnetic system was made of ferrite material and consisted of 10 shell-type core segments. The magnetic properties have been verified by measuring magnetization and losses at various frequencies in the range 1-10 kHz to cover the operational range of the DAB. The types and dimensions of the windings and their conductors were chosen to minimize the proximity and eddy current effects at higher frequencies. To reduce the size of the transformer and to allow for its efficient cooling, the active part was immersed in oil and cellulose-based materials (paper and pressboard) were used to build the high voltage insulation system. The principles for dimensioning the insulation of the transformer are discussed. The criteria used for selecting insulating distances were based on the consideration of the electric field strength obtained from FEM simulations and using the non-linear Maxwell-Wagner model accounting for local variations of the electric field caused by accumulation of interfacial charges induced by DC stresses. The properties of the materials needed for the calculations were obtained by measuring their dielectric constants and electric conductivities. The methodology used for the measurements conducted for conventional mineral oil and eco-friendly biodegradable transformer oils and, respectively, for oil-impregnated paper/pressboard, is presented. The methodologies used for obtaining parameters of the built transformer prototype needed for its integration in the power electric circuit of the DAB are introduced. A method developed for accurate calculations of the leakage inductance for the shell-type multi core transformers with circular windings is described. Two innovative methods for evaluations of parasitic capacitances based on high frequency equivalent circuits of the transformer are presented. The results of their verifications against performed Frequency Response Analysis measurements and FEM calculations as well as their accuracy are discussed.Thermal performance of the developed transformer prototype is analysed based on the results of computer simulations of heat transfer in its active part under rated load. Identified hot spots and solutions for their elimination are presented.Finally, the expected dimensions, weight, and efficiency of an actual DC/DC converter with the rated parameters corresponding to a 6 MW, 1.8 kV real wind turbine having a 250 kV offset DC voltage are estimated assuming that the developed transformer prototype is scalable. It is shown that the proposed solution allows for installing the full-scale converter having 2.2 Tons in weight and 1.8 m3 in volume on the bottom of the wind turbine’s tower

    Level-Set Mass-Conservative Front-Tracking Technique for Multistep Simulations of In-Flight Ice Accretion

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    This paper presents a novel level-set-based approach to model evolving boundary problems for in-flight ice accretion. No partial differential equations are solved as in the standard level-set formulation, but simple geometrical quantities are employed to provide an implicit discretization of the updated boundary. This method avoids mesh entanglements and grid intersections typical of algebraic and mesh deforming techniques, making it suitable for generating a body-fitted discretization of arbitrarily complex geometries as in-flight ice shapes, including the collision of separate ice fronts. Moreover, this paper presents a local ice thickness correction, which accounts for the body's curvature, to conserve the prescribed iced mass locally. The verification includes ice accretion over an ellipse and a manufactured example to show the proposed strategy's advantages and robustness compared to standard algebraic methods. Finally, the method is applied to ice accretion problems. A temporal and grid convergence study is presented for automatic multistep in-flight simulations over a NACA0012 airfoil in rime, glaze, and mixed ice conditions

    Designs of Blackness

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    Across more than two centuries Afro-America has created a huge and dazzling variety of literary self-expression. Designs of Blackness provides less a narrative literary history than, precisely, a series of mappings—each literary-critical and comparative while at the same time offering cultural and historical context. This carefully re-edited version of the 1998 publication opens with an estimation of earliest African American voice in the names of Phillis Wheatley and her contemporaries. It then takes up the huge span of autobiography from Frederick Douglass through to Maya Angelou. "Harlem on My Mind," which follows, sets out the literary contours of America’s premier black city. Womanism, Alice Walker’s presiding term, is given full due in an analysis of fiction from Harriet E. Wilson to Toni Morrison. Richard Wright is approached not as some regulation "realist" but as a more inward, at times near-surreal, author. Decadology has its risks but the 1940s has rarely been approached as a unique era of war and peace and especially in African American texts. Beat Generation work usually adheres to Ginsberg and Kerouac, but black Beat writing invites its own chapter in the names of Amiri Baraka, Ted Joans and Bob Kaufman. The 1960s has long become a mythic change-decade, and in few greater respects than as a black theatre both of the stage and politics. In Leon Forrest African America had a figure of the postmodern turn: his work is explored in its own right and for how it takes its place in the context of other reflexive black fiction. "African American Fictions of Passing" unpacks the whole deceptive trope of "race" in writing from Williams Wells Brown through to Charles Johnson. The two newly added chapters pursue African American literary achievement into the Obama-Trump century, fiction from Octavia Butler to Darryl Pinkney, poetry from Rita Dove to Kevin Young

    (b2023 to 2014) The UNBELIEVABLE similarities between the ideas of some people (2006-2016) and my ideas (2002-2008) in physics (quantum mechanics, cosmology), cognitive neuroscience, philosophy of mind, and philosophy (this manuscript would require a REVOLUTION in international academy environment!)

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    (b2023 to 2014) The UNBELIEVABLE similarities between the ideas of some people (2006-2016) and my ideas (2002-2008) in physics (quantum mechanics, cosmology), cognitive neuroscience, philosophy of mind, and philosophy (this manuscript would require a REVOLUTION in international academy environment!

    Estudio y desarrollo de un cuadro de descenso

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    L'objectiu del present projecte és estudiar el procés de desenvolupament d'un quadre de bicicleta de descens. Es presenten i s'analitzen al llarg del projecte les diferents etapes. S'avalua el material a utilitzar, una decisió fonamental per al disseny, per la seva influència en la resistència, durabilitat, rigidesa i pes del quadre, i se sospesen diferents alternatives. Es discuteixen aspectes tan fonamentals com la geometria i la cinemàtica de la suspensió, així com el procediment i les eines utilitzades per a la seva definició. També s'aborda el disseny i desenvolupament d'un model virtual i la realització d'assajos segons la norma ISO 4210, relativa als requisits de seguretat per a bicicletes, utilitzant un mètode de simulació per elements finits. S'han emès informes d'assaig sobre el compliment d'aquesta norma, així com una fulla d'assaig en la qual es resumeixen els resultats de la simulació. També s'ha realitzat una anàlisi econòmica dels costos per a dur a terme el projecte, concloent que el cost per a la fabricació d'un prototip real, tal com estava previst inicialment, és prohibitiu. No obstant això, en aquest projecte s'han tingut en compte i recollit tots els passos i consideracions per a la creació d'un prototip. A més, s'analitza la sostenibilitat de les tendències actuals en la indústria de la bicicleta, i es presenten algunes alternatives per a reduir el seu impacte mediambiental.El objetivo del presente proyecto es estudiar el proceso de desarrollo de un cuadro de bicicleta de descenso. Se presentan y se analizan a lo largo del proyecto las diferentes etapas. Se evalúa el material a utilizar, una decisión fundamental para el diseño, por su influencia en la resistencia, durabilidad, rigidez y peso del cuadro, y se sopesan diferentes alternativas. Se discuten aspectos tan fundamentales como la geometría y la cinemática de la suspensión, así como el procedimiento y las herramientas utilizadas para su definición. También se aborda el diseño y desarrollo de un modelo virtual y la realización de ensayos según la norma ISO 4210, relativa a los requisitos de seguridad para bicicletas, utilizando un método de simulación por elementos finitos. Se han emitido informes de ensayo sobre el cumplimiento de esta norma, así como una hoja de ensayo en la que se resumen los resultados de la simulación. También se ha realizado un análisis económico de los costes para llevar a cabo el proyecto, concluyendo que el coste para la fabricación de un prototipo real, tal y como estaba previsto inicialmente, es prohibitivo. No obstante, en este proyecto se han tenido en cuenta y recogido todos los pasos y consideraciones para la creación de un prototipo. Además, se analiza la sostenibilidad de las tendencias actuales en la industria de la bicicleta, y se presentan algunas alternativas para reducir su impacto medioambiental.The objective of the present project is to study the development process of a downhill bicycle frame. The different stages involved are presented and then analysed in the course of the project. Material to use, a fundamental decision for the design because of its influence in resistance, durability, rigidity, and weight of the frame, is evaluated and different alternatives are balanced. Such fundamental aspect as geometry and suspension kinematics are discussed, as well as the procedure and tools used to define them. Design and development of a virtual model and testing as per ISO 4210, regarding safety requirement for bicycles, using a simulation method by finite elements is also covered in the following. Test reports on compliance with this standard have been issued, as well as a test sheet summarising the simulation results. An economic analysis of the costs involved to complete the project has also been conducted, concluding the cost for the manufacturing of a real prototype, as initially planned, is prohibitive. Nevertheless, all steps and considerations for prototyping have been considered and collected in this project. Additionally, sustainability of current tendencies in the bicycle industry is analysed, and some alternatives are presented to reduce its environmental impact

    3D RMHD simulations of jet-wind interactions in high-mass X-ray binaries

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    Context. Relativistic jets are ubiquitous in the Universe. In microquasars, especially in high-mass X-ray binaries, the interaction of jets with the strong winds driven by the massive and hot companion star in the vicinity of the compact object is fundamental for understanding the jet dynamics, nonthermal emission, and long-term stability. However, the role of the jet magnetic field in this process is unclear. In particular, it is still debated whether the magnetic field favors jet collimation or triggers more instabilities that can jeopardize the jet evolution outside the binary. Aims. We study the dynamical role of weak and moderate to strong toroidal magnetic fields during the first several hundred seconds of jet propagation through the stellar wind, focusing on the magnetized flow dynamics and the mechanisms of energy conversion. Methods. We developed the code Lóstrego v1.0, a new 3D relativistic magnetohydrodynamics code to simulate astrophysical plasmas in Cartesian coordinates. Using this tool, we performed the first 3D relativistic magnetohydrodynamics numerical simulations of relativistic magnetized jets propagating through the clumpy stellar wind in a high-mass X-ray binary. To highlight the effect of the magnetic field in the jet dynamics, we compared the results of our analysis with those of previous hydrodynamical simulations. Results. The overall morphology and dynamics of weakly magnetized jet models is similar to previous hydrodynamical simulations, where the jet head generates a strong shock in the ambient medium and the initial overpressure with respect to the stellar wind drives one or more recollimation shocks. On the timescales of our simulations (i.e., t < 200 s), these jets are ballistic and seem to be more stable against internal instabilities than jets with the same power in the absence of fields. However, moderate to strong toroidal magnetic fields favor the development of current-driven instabilities and the disruption of the jet within the binary. A detailed analysis of the energy distribution in the relativistic outflow and the ambient medium reveals that magnetic and internal energies can both contribute to the effective acceleration of the jet. Moreover, we verified that the jet feedback into the ambient medium is highly dependent on the jet energy distribution at injection, where hotter, more diluted and/or more magnetized jets are more efficient. This was anticipated by feedback studies in the case of jets in active galaxies

    'A way of life': practising place in the small press

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    This thesis is a study of place in the practice and publications of three small presses: Moschatel Press, Coracle Press and Corbel Stone Press. Practice is central to my approach, both in situating place as something practised, unfinished and ongoing, and in the repetitive everyday acts that make running a press ‘a way of life’. I examine the ways in which small press practice shapes and responds to a variety of places. Beginning with the home, the thesis moves gradually outwards to larger-scale spaces: the local area, public spaces, the wider landscape. The thesis is founded upon the press model as one of collaboration, both between artists, and with the places they inhabit. Chapter One establishes the domestic space as central to the activities and publications of the press. The home is a site of production enmeshed with the everyday, and is the intended habitat of many small press pieces. I trace the influence of domestic intimacy and tactility across small press poetics, and the importance of ‘the domestic scale’ is foregrounded throughout the thesis. Chapter Two is an exploration of small press localness. I build upon the domestic chapter to examine how the local is shaped by its relationship to the home. I frame small press localness as distinctly embodied, examining the charting of local places on foot and the gathering of texts and objects by hand. Chapter Three examines site-specific work, exploring the presence of small press pieces in public, communal spaces. I focus particularly upon the hospital-based works of Thomas A Clark, and how they provoke questions around attention, contemplation and care. The chapter closes by reflecting upon how these pieces facilitate thinking about the more- than-human. Chapter Four sustains a focus upon the more-than-human to explore the small press relationship with the wider landscape. The chapter scrutinises an ambivalent attitude towards books as a means of relating to and recording landscapes. I consider work across deep timescales and study the embodied landscape-based practices of Corbel Stone Press, such as burial and the leaving of offerings
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