On the circulation and the position of the forward stagnation point on airfoils

Lift and velocity circulation around airfoils are two aspects of the same phenomenon when airfoils are not stalled and the Kutta—Joukowski theorem applies. This theorem establishes a linear dependence between lift and circulation, which breaks when stalling occurs. As the angle of attack increases beyond this point, the circulation vanishes. Since the circulation determines to a great extent the position of the forward stagnation point on an airfoil, the measurement of this position is an easy and simple way to determine the circulation, which is of help in understanding the role of the latter in the generation of aerodynamic forces on airfoils.

Futuros urbanos: la reversibilidad del proceso de deterioro

Las dinámicas de reorganización de la producción y las transformaciones económicas determinaron la crisis de gran parte de las regiones industriales de Europa y Estados Unidos en las últimas décadas del siglo XX. Ante la persistencia e intensidad del declive de muchas de ellas, cabe preguntarse, ¿existen futuros para la ciudad industrial? O, en otras palabras, ¿es el declive un proceso irreversible? La incidencia dispar del declive así como la gradual regeneración de algunas regiones industriales en los últimos años parece apuntar hacia cierta capacidad de respuesta local ante unas dinámicas de escala global. Las múltiples trayectorias de las regiones de base industrial tan sólo parecen poder explicarse desde su singularidad. La presente tesis doctoral tiene por objeto analizar las posibilidades de orientar los procesos de deterioro urbano de las regiones industriales y examinar la capacidad de la planificación de intervenir sobre ellos a través de dos estudios de caso, El modo de abordar el análisis, estudiar la ciudad en evolución, debería proporcionar una mejor comprensión de las repercusiones del modelo de crecimiento de cada ciudad sobre su posterior declive y resaltar la influencia de las decisiones políticas y espaciales de cada etapa en el desarrollo futuro del territorio. Analizar la evolución de dos regiones industriales extremadamente diferentes dará la oportunidad de investigar, en primer lugar, uno de los casos paradigmáticos de deterioro urbano, Detroit y, tras haberse interrogado sobre las causas específicas de la persistencia y duración de su declive, estudiar la evolución urbana de la región de Nantes – Saint-Nazaire para comprender la singularidad de su regeneración tras años de estancamiento. El análisis de estos territorios debería permitir progresar en el conocimiento de los procesos de declive, comparar las diferentes estrategias y modelos urbanos, comprender las diferencias entre ellos e interrogarnos sobre la posibilidad de orientar los procesos de deterioro urbano. Las respuestas de este análisis quizás nos ayuden a afirmar la existencia de futuros múltiples para la ciudad industrial. Global dynamics such as economic transformations and reorganizations of production led to the crisis of most industrial cities in Europe and the U.S in the last decades of the 20th century. Most of them have suffered or are still suffering the consequences of urban decay and shrinkage. Given the severity and persistence of some of these processes, a significant question may be raised: are there alternative futures for former industrial cities? Or is urban decay an irreversible process? Nevertheless, the diverse evolution of these cities, as well as the gradual restructuring of some of them in recent years, seem to point toward the relevance of local response to these global dynamics. The different paths of development of industrial cities since 1970s may only be explained by their singularity and specific local conditions. This research aims to examine possibilities to guide urban decline and shrinkage in former industrial regions and to analyze the ability of urban planning to intervene in these processes through two case studies. The method of research, exploring cities in evolution, should provide a greater understanding of the effects of different modes of development during the city’s heyday on its subsequent shrinkage. Likewise, it should highlight the influence of each period’s local decisions on the future trajectory of the city. The evolutive analysis of two extremely different industrial regions will give us, first, the opportunity to study a paradigm of urban decay, Detroit, by exploring over time the specific causes of its decline’s prevalence. In the second place, we will be able to study Nantes - Saint-Nazaire region, examining the distinctiveness of its recent restructuring after years of shrinkage and stagnation. Through these examples, we would be able to analyze the consequences of decision-making on the evolution of each city. It should also let us compare diverse strategies and enable us to question the ability of planning to tackle decay. The conclusions of the analysis may help us to assert the existence of alternative futures for industrial cities.

Unearthing the roots of degradation of Quercus pyrenaica coppices: an integrative perspective from clonal structure to carbon budgets

Quercus pyrenaica es una especie rebrotadora de raíz intensa e históricamente aprovechada en monte bajo para la obtención de leñas, carbón y pastos. Debido al éxodo rural y a la aparición de nuevas fuentes energéticas, este aprovechamiento fue abandonado en la década de 1970. Desde entonces, las bajas producciones de madera y bellota y el puntisecado de los pies evidencian el generalizado estancamiento de estas masas. Uno de los mayores retos actuales de la selvicultura en el ámbito mediterráneo es encontrar usos alternativos para estos montes abandonados, siendo la conversión a monte alto una de las alternativas preferidas. Se han realizado resalveos de conversión, sin embrago, éstos se aplican sin un conocimiento integral de las causas de la degradación. En esta tesis doctoral, estudiamos un hipotético desequilibrio entre la parte radical y la parte aérea (R:S) de las cepas de rebollo como causa subyacente de su decaimiento. En una parcela experimental, aprovechada al menos desde el siglo XII, se realizaron análisis genéticos a priori para elucidar la estructura genética del rodal, y así estudiar la influencia del tamaño clonal en el funcionamiento de las cepas. Las cepas de mayor tamaño presentaron un menor crecimiento diametral de sus pies, así como mayores tasas de respiración radical, estimadas a partir de flujos internos de CO2 a través del xilema (FT) y de los flujos de CO2 del suelo. Estos resultados sugieren que el desequilibrio R:S aumenta con el tamaño clonal, dado que la eliminación periódica de órganos aéreos, al mismo tiempo que las raíces permanecen intactas, da lugar a un gran desarrollo del sistema radical que consume gran parte de los carbohidratos no estructurales (NSC) en respiración de mantenimiento, comprometiendo así el desarrollo de órganos aéreos. Se excavaron y pesaron dos cepas compuestas por cuatro y ocho pies, las cuales mostraron ratios R:S (0.5 y 1, respectivamente) superiores a los registrados en pies de origen sexual. Al igual que en otras especies rebrotadoras de raíz, se observaron altas concentraciones de NSC en las raíces (> 20% en primavera) y una gran proporción de albura en el sistema radical (52%) que alberga una notable reserva de NSC (87 kg en la cepa de mayor tamaño). En el sistema radical de dicha cepa, estimada mediante dataciones radiocarbónicas en 550 años de edad, se contaron 248 uniones radicales. La persistencia de sistemas radicales grandes, viejos, y altamente interconectados sugiere que la gran cantidad de recursos almacenados y consumidos en las raíces compensan un pobre desarrollo aéreo con una alta resiliencia vegetativa. Para un mejor entendimiento de los balances de carbono y del agotamiento de NSC en las cepas de rebollo, se midieron los flujos internos y externos de CO2 en troncos y los flujos de CO2 del suelo, y se estimó la respiración de órganos aéreos (RS) y subterráneos (RR). Estacionalmente, RS y RR reflejaron las dinámicas de flujo de savia y de crecimiento del tronco, y estuvieron determinadas principalmente por los flujos externos de CO2, dada la escasa contribución de FT a RS y RR (< 10% y < 2%, respectivamente). En una escala circadiana, la contribución de FT a RS aumentó hasta un 25% en momentos de alta transpiración. Las bajas concentraciones de CO2 en el xilema ([CO2] hasta un 0.11%) determinaron comparativamente unos bajos FT, probablemente causados por una limitada respiración del xilema y una baja resistencia a la difusión radial del CO2 impuestos por la sequía estival. Los pulsos de [CO2] observados tras las primeras lluvias de otoño apoyan esta idea. A lo largo del periodo vegetativo, el flujo medio de CO2 procedente del suelo (39 mol CO2 day-1) fue el mayor flujo respiratorio, tres y cuatro veces superior a RS (12 mol CO2 day-1) y RR (8-9 mol CO2 day-1), respectivamente. Ratios RR/RS menores que la unidad evidencian un importante peso de la respiración aérea como sumidero de carbono adicional. Finalmente, se ensayó el zanjado de raíces y el anillamiento de troncos como tratamientos selvícolas alternativos con el objetivo de aumentar las reservas de NSC en los troncos de las cepas. Los resultados preliminares desaconsejan el zanjado de raíces por el alto coste derivado posiblemente de la cicatrización de las heridas. El anillado de troncos imposibilitó el transporte de NSC a las raíces y aumentó la concentración de almidón por encima de la zona anillada, mientras que sistema radical se mantiene por los pies no anillados de la cepa. Son necesarias más mediciones y datos adicionales para comprobar el mantenimiento de esta respuesta positiva a largo plazo. Para concluir, destacamos la necesidad de estudios multidisciplinares que permitan una comprensión integral de la degradación de los rebollares ibéricos para poder aplicar a posteriori una gestión adecuada en estos montes bajos abandonados. ABSTRACT Quercus pyrenaica is a vigorous root-resprouting species intensively and historically coppiced for firewood, charcoal and woody pastures. Due to the rural exodus and the appearance of new energy sources, coppicing was abandoned towards 1970. Since then, tree overaging has resulted in stand stagnation displayed by slow stem growth, branch dieback, and scarce acorn production. The urgent need to find new alternative uses for abandoned coppices is recognized as one of the biggest challenges which currently faces Mediterranean silviculture; conversion into high forest by thinning is one of the preferred alternatives. For this aim, thinning has been broadly applied and seldom tested, although without a comprehensive understanding of the causes of stand stagnation. In this PhD study, we test the hypothesis of an imbalance between above- and below-ground organs, result of long term coppicing, as the underlying cause of Q. pyrenaica decay. In an experimental plot coppiced since at least the 12th century, genetic analyses were performed a priori to elucidate inconspicuous clonal structure of Q. pyrenaica to evaluate how clonal size affects the functioning of these multi-stemmed trees. Clonal size negatively affected diametric stem growth, whereas root respiration rates, measured by internal fluxes of CO2 through xylem (FT) and soil CO2 efflux, increased with clonal size. These results suggest root-to-shoot (R:S) imbalance intensifying with clonal size: periodic removal of aboveground organs whilst belowground organs remain undisturbed may have led to massive root systems which consume a great proportion of non-structural carbohydrates (NSC) for maintenance respiration, thus constraining aboveground performance. Furthermore, excavation of two multi-stemmed trees, composed by four and eight stems, revealed R:S ratios (0.5 and 1, respectively) greater than those reported for sexually regenerated trees. Moreover, as similarly observed in several root-resprouting species, NSC allocation to roots was favored ([NSC] > 20% in spring): a large proportion of sapwood maintained throughout the root system (52%) stored a remarkable NSC pool of 87 kg in the case of the largest clone. In this root system of the eight-stemmed tree, 248 root connections were counted and, by radiocarbon dating, its age was estimated to be 550-years-old. Persistence of massive, old and highly interconnected root systems suggests that enhanced belowground NSC storage and consumption reflects a trade-off between vegetative resilience and aboveground development. For a better understanding of tree carbon budget and the potential role of carbon starvation in Q. pyrenaica decay, internal and external stem CO2 fluxes and soil CO2 effluxes were monitored to evaluate respiratory costs above- and below-ground. On a seasonal scale, stem and root respiration (RS and RR) mirrored sap flow and stem growth dynamics. Respiration was determined to the greatest extent by external fluxes of CO2 to the atmosphere or soil, since FT accounted for a low proportion of RS and RR (< 10% and < 2%, respectively). On a diel scale, the contribution of FT to RS increased up to 25% at high transpiration rates. Comparatively low FT was determined by the low concentration of xylem CO2 registered ([CO2] as low as 0.11%), likely as a consequence of constrained xylem respiration and reduced resistance to CO2 radial diffusion imposed by summer drought. Xylem [CO2] pulses following first autumn rains support this idea. Averaged over the growing season, soil CO2 efflux was the greatest respiratory flux (39 mol CO2 day-1), three and four times greater than RS (12 mol CO2 day-1) and RR (8-9 mol CO2 day-1), respectively. Ratios of RR/RS below one evidence an additional and important weight of aboveground respiration as a tree carbon sink. Finally, root trenching and stem girdling were tested as complimentary treatments to thinning as a means to improve carbon reserves in stems of clonal trees. Preliminary results discouraged root trenching due to the high cost likely incurred for wound closure. Stem girdling successfully blocked NSC translocation downward, increasing starch concentrations above the girdled zone whilst the root system is fed by non-girdled stems within the clone. Further measurements and ancillary data are necessary to verify that this positive effect hold over time. To conclude, the need of multidisciplinary approaches for an integrative understanding on the functioning of abandoned Q pyrenaica coppices is highlighted for an appropriate management of these stands.

Crisis and the city: neoliberalism, austerity planning and the production of space

The current crisis, with its particularly severe configuration in Southern European countries, provides an opportunity to probe the interrelation of economic crunches and the production of space, and also to imagine potential paths of sociospatial emancipation from the dictates of global markets. This introductory chapter offers a preliminary interpretive framework exploring the fundamental role of urban and territorial restructuring in the formation, management and resolution of capitalist crises and, conversely, periods of crisis as key stages in the history of urbanization. I will begin by contextualizing the 2007-8 economic slump, the subsequent global recession and its uneven impact on states and cities in the longue durée of capitalist productions of space, studying the transformation of spatial configurations in previous episodes of economic stagnation. This broader perspective will then be used to analyze currently emerging formations of austerity urbanism, showing how the practices of crisis management incorporate a strategy for economic and institutional restructuring that eventually impacts on urban policy, and indeed in the production of urban space itself.

Regimes of spray vaporization and combustion in counterflow configurations

This article addresses the problem of spray vaporization and combustion in axisymmetric opposed-jet configurations involving a stream of hot air counterflowing against a stream of nitrogen carrying a spray of fuel droplets. The Reynolds numbers of the jets are assumed to be large, so that mixing of the two streams is restricted to a thin mixing layer that separates the counterflowing streams. The evolution of the droplets in their feed stream from the injection location is seen to depend fundamentally on the value of the droplet Stokes number, St, defined as the ratio of the droplet acceleration time to the mixing layer strain time close to the stagnation point. Two different regimes of spray vaporization and combustion can be identified depending on the value of St. For values of St below a critical value, equal to 1/4 for dilute sprays with small values of the spray liquid mass loading ratio, the droplets decelerate to approach the gas stagnation plane with a vanishing axial velocity. In this case, the droplets located initially near the axis reach the mixing layer, where they can vaporize due to the heat received from the hot air, producing fuel vapor that can burn with the oxygen in a diffusion flame located on the air side of the mixing layer. The character of the spray combustion is different for values of St of order unity, because the droplets cross the stagnation plane and move into the opposing air stream, reaching distances that are much larger than the mixing layer thickness before they turn around. The vaporization of these crossing droplets, and also the combustion of the fuel vapor generated by them, occur in the hot air stream, without significant effects of molecular diffusion, generating a vaporization-assisted nonpremixed flame that stands on the air side outside the mixing layer. Separate formulations will be given below for these two regimes of combustion, with attention restricted to the near-stagnation-point region, where the solution is self-similar and all variables are only dependent on the distance to the stagnation plane. The resulting formulations display a reduced number of controlling parameters that effectively embody dependences of the structure of the spray flame on spray dilution, droplet inertia, and fuel preferential diffusion. Sample solutions are given for the limiting cases of pure vaporization and of infinitely fast chemistry, with the latter limit formulated in terms of chemistry-free coupling functions that allow for general nonunity Lewis numbers of the fuel vapor.