Geomorphology and spectrophotometry of Philae's landing site on comet 67P/Churyumov-Gerasimenko

Context. On 12 November 2014 the European mission Rosetta succeeded in delivering a lander, named Philae, on the surface of one of the smallest, low-gravity and most primitive bodies of the solar system, the comet 67P/Churyumov-Gerasimenko (67P). Aims. The aim of this paper is to provide a comprehensive geomorphological and spectrophotometric analysis of Philae's landing site (Agilkia) to give an essential framework for the interpretation of its in situ measurements. Methods. OSIRIS images, coupled with gravitational slopes derived from the 3D shape model based on stereo-photogrammetry were used to interpret the geomorphology of the site. We adopted the Hapke model, using previously derived parameters, to photometrically correct the images in orange filter (649.2 nm). The best approximation to the Hapke model, given by the Akimov parameter-less function, was used to correct the reflectance for the effects of viewing and illumination conditions in the other filters. Spectral analyses on coregistered color cubes were used to retrieve spectrophotometric properties. Results. The landing site shows an average normal albedo of 6.7% in the orange filter with variations of similar to 15% and a global featureless spectrum with an average red spectral slope of 15.2%/100 nm between 480.7 nm (blue filter) and 882.1 nm (near-IR filter). The spatial analysis shows a well-established correlation between the geomorphological units and the photometric characteristics of the surface. In particular, smooth deposits have the highest reflectance a bluer spectrum than the outcropping material across the area. Conclusions. The featureless spectrum and the redness of the material are compatible with the results by other instruments that have suggested an organic composition. The observed small spectral variegation could be due to grain size effects. However, the combination of photometric and spectral variegation suggests that a compositional differentiation is more likely. This might be tentatively interpreted as the effect of the efficient dust-transport processes acting on 67P. High-activity regions might be the original sources for smooth fine-grained materials that then covered Agilkia as a consequence of airfall of residual material. More observations performed by OSIRIS as the comet approaches the Sun would help interpreting the processes that work at shaping the landing site and the overall nucleus.

A model for use by local public health departments to evaluate pandemic influenza plans

The events of the 1990's and early 2000's demonstrated the need for effective planning and response to natural and man-made disasters. One of those potential natural disasters is pandemic flu. Once defined, the CDC stated that program, or plan, effectiveness is improved through the process of program evaluation. (Centers for Disease Control and Prevention, 1999) Program evaluation should be accomplished not only periodically, but in the course of routine administration of the program. (Centers for Disease Control and Prevention, 1999) Accomplishing this task for a "rare, but significant event" is challenging. (Herbold, John R., PhD., 2008) To address this challenge, the RAND Corporation (under contract to the CDC) developed the "Facilitated Look-Backs" approach that was tested and validated at the state level. (Aledort et al., 2006).^ Nevertheless, no comprehensive and generally applicable pandemic influenza program evaluation tool or model is readily found for use at the local public health department level. This project developed such a model based on the "Facilitated Look-Backs" approach developed by RAND Corporation. (Aledort et al., 2006) Modifications to the RAND model included stakeholder additions, inclusion of all six CDC program evaluation steps, and suggestions for incorporating pandemic flu response plans in seasonal flu management implementation. Feedback on the model was then obtained from three LPHD's—one rural, one suburban, and one urban. These recommendations were incorporated into the final model. Feedback from the sites also supported the assumption that this model promotes the effective and efficient evaluation of both pandemic flu and seasonal flu response by reducing redundant evaluations of pandemic flu plans, seasonal flu plans, and funding requirement accountability. Site feedback also demonstrated that the model is comprehensive and flexible, so it can be adapted and applied to different LPHD needs and settings. It also stimulates evaluation of the major issues associated with pandemic flu planning. ^ The next phase in evaluating this model should be to apply it in a program evaluation of one or more LPHD's seasonal flu response that incorporates pandemic flu response plans.^

DISCHARGE PLANNING: A PROCESS MODEL

As the requirements for health care hospitalization have become more demanding, so has the discharge planning process become a more important part of the health services system. A thorough understanding of hospital discharge planning can, then, contribute to our understanding of the health services system. This study involved the development of a process model of discharge planning from hospitals. Model building involved the identification of factors used by discharge planners to develop aftercare plans, and the specification of the roles of these factors in the development of the discharge plan. The factors in the model were concatenated in 16 discrete decision sequences, each of which produced an aftercare plan.^ The sample for this study comprised 407 inpatients admitted to the M. D. Anderson Hospital and Tumor Institution at Houston, Texas, who were discharged to any site within Texas during a 15 day period. Allogeneic bone marrow donors were excluded from the sample. The factors considered in the development of discharge plans were recorded by discharge planners and were used to develop the model. Data analysis consisted of sorting the discharge plans using the plan development factors until for some combination and sequence of factors all patients were discharged to a single site. The arrangement of factors that led to that aftercare plan became a decision sequence in the model.^ The model constructs the same discharge plans as those developed by hospital staff for every patient in the study. Tests of the validity of the model should be extended to other patients at the MDAH, to other cancer hospitals, and to other inpatient services. Revisions of the model based on these tests should be of value in the management of discharge planning services and in the design and development of comprehensive community health services.^

Network-based analysis of genome-wide scans of natural selection

The genomic era brought by recent advances in the next-generation sequencing technology makes the genome-wide scans of natural selection a reality. Currently, almost all the statistical tests and analytical methods for identifying genes under selection was performed on the individual gene basis. Although these methods have the power of identifying gene subject to strong selection, they have limited power in discovering genes targeted by moderate or weak selection forces, which are crucial for understanding the molecular mechanisms of complex phenotypes and diseases. Recent availability and rapid completeness of many gene network and protein-protein interaction databases accompanying the genomic era open the avenues of exploring the possibility of enhancing the power of discovering genes under natural selection. The aim of the thesis is to explore and develop normal mixture model based methods for leveraging gene network information to enhance the power of natural selection target gene discovery. The results show that the developed statistical method, which combines the posterior log odds of the standard normal mixture model and the Guilt-By-Association score of the gene network in a naïve Bayes framework, has the power to discover moderate/weak selection gene which bridges the genes under strong selection and it helps our understanding the biology under complex diseases and related natural selection phenotypes.^

Robust effect sizes and their confidence intervals for group difference between trajectories in hierarchical linear growth model

Hierarchical linear growth model (HLGM), as a flexible and powerful analytic method, has played an increased important role in psychology, public health and medical sciences in recent decades. Mostly, researchers who conduct HLGM are interested in the treatment effect on individual trajectories, which can be indicated by the cross-level interaction effects. However, the statistical hypothesis test for the effect of cross-level interaction in HLGM only show us whether there is a significant group difference in the average rate of change, rate of acceleration or higher polynomial effect; it fails to convey information about the magnitude of the difference between the group trajectories at specific time point. Thus, reporting and interpreting effect sizes have been increased emphases in HLGM in recent years, due to the limitations and increased criticisms for statistical hypothesis testing. However, most researchers fail to report these model-implied effect sizes for group trajectories comparison and their corresponding confidence intervals in HLGM analysis, since lack of appropriate and standard functions to estimate effect sizes associated with the model-implied difference between grouping trajectories in HLGM, and also lack of computing packages in the popular statistical software to automatically calculate them. ^ The present project is the first to establish the appropriate computing functions to assess the standard difference between grouping trajectories in HLGM. We proposed the two functions to estimate effect sizes on model-based grouping trajectories difference at specific time, we also suggested the robust effect sizes to reduce the bias of estimated effect sizes. Then, we applied the proposed functions to estimate the population effect sizes (d ) and robust effect sizes (du) on the cross-level interaction in HLGM by using the three simulated datasets, and also we compared the three methods of constructing confidence intervals around d and du recommended the best one for application. At the end, we constructed 95% confidence intervals with the suitable method for the effect sizes what we obtained with the three simulated datasets. ^ The effect sizes between grouping trajectories for the three simulated longitudinal datasets indicated that even though the statistical hypothesis test shows no significant difference between grouping trajectories, effect sizes between these grouping trajectories can still be large at some time points. Therefore, effect sizes between grouping trajectories in HLGM analysis provide us additional and meaningful information to assess group effect on individual trajectories. In addition, we also compared the three methods to construct 95% confident intervals around corresponding effect sizes in this project, which handled with the uncertainty of effect sizes to population parameter. We suggested the noncentral t-distribution based method when the assumptions held, and the bootstrap bias-corrected and accelerated method when the assumptions are not met.^

Phosphate d18O pore-water profiles of sediment core GeoB11807-2 and GeoB11804-4

Phosphorus cycling in the ocean is influenced by biological and geochemical processes that are reflected in the oxygen isotope signature of dissolved inorganic phosphate (Pi). Extending the Pi oxygen isotope record from the water column into the seabed is difficult due to low Pi concentrations and small amounts of marine porewaters available for analysis. We obtained porewater profiles of Pi oxygen isotopes using a refined protocol based on the original micro-extraction designed by Colman (2002). This refined and customized method allows the conversion of ultra-low quantities (0.5 - 1 µmol) of porewater Pi to silver phosphate (Ag3PO4) for routine analysis by mass spectrometry. A combination of magnesium hydroxide co-precipitation with ion exchange resin treatment steps is used to remove dissolved organic matter, anions, and cations from the sample before precipitating Ag3PO4. Samples as low as 200 µg were analyzed in a continuous flow isotope ratio mass spectrometer setup. Tests with external and laboratory internal standards validated the preservation of the original phosphate oxygen isotope signature (d18OP) during micro extraction. Porewater data on d18OP has been obtained from two sediment cores of the Moroccan margin. The d18OP values are in a range of +19.49 to +27.30 per mill. We apply a simple isotope mass balance model to disentangle processes contributing to benthic P cycling and find evidence for Pi regeneration outbalancing microbial demand in the upper sediment layers. This highlights the great potential of using d18OP to study microbial processes in the subseafloor and at the sediment water interface.

Adaptive fuzzy knowledge-based systems for control metabots' mobility on virtual environments

The confluence of three-dimensional (3D) virtual worlds with social networks imposes on software agents, in addition to conversational functions, the same behaviours as those common to human-driven avatars. In this paper, we explore the possibilities of the use of metabots (metaverse robots) with motion capabilities in complex virtual 3D worlds and we put forward a learning model based on the techniques used in evolutionary computation for optimizing the fuzzy controllers which will subsequently be used by metabots for moving around a virtual environment.

Extended Triple-Junction Solar Cell 3D Distributed Model: Application to Chromatic Aberration-Related Losses

An extended 3D distributed model based on distributed circuit units for the simulation of triple‐junction solar cells under realistic conditions for the light distribution has been developed. A special emphasis has been put in the capability of the model to accurately account for current mismatch and chromatic aberration effects. This model has been validated, as shown by the good agreement between experimental and simulation results, for different light spot characteristics including spectral mismatch and irradiance non‐uniformities. This model is then used for the prediction of the performance of a triple‐junction solar cell for a light spot corresponding to a real optical architecture in order to illustrate its suitability in assisting concentrator system analysis and design process.

Simulation of a mechanical thrombectomy device based in the use of self-expandable stents for the blood clots extraction

Recently, we have presented some studies concerning the analysis, design and optimization of one experimental device developed in the UK - GPTAD - which has been designed to remove blood clots without the need to make contact with the clot itself, thereby potentially reducing the risk of problems such as downstream embolisation. Based on the idea of a modification of the previous device, in this work, we present a model based in the use of stents like the SolitaireTM FR, which is in contact with the clot itself. In the case of such devices, the stent is self-expandable and the extraction of the blood clot is faciliatated by the stent, which must be inside the clot. Such stents are generally inserted in position by using the guidewire inserted into the catheter. This type of modeling could potentially be useful in showing how the blood clot is moved by the various different forces involved. The modelling has been undertaken by analyzing the resistances, compliances and inertances effects. We model an artery and blood clot for range of forces for the guidewire. In each case we determine the interaction between blood clot, stent and artery.

Evolución y crisis de la región metropolitana de Madrid 1985-2007. Análisis de las diversas perspectivas para la transformación del gobierno urbano: una revisión crítica del planeamiento

La Región Metropolitana de Madrid (RMM) ha sufrido una gran transformación urbana en el periodo 1985-2007, en el cual ha crecido la población, ha crecido fuertemente el cuerpo físico, pero sobre todo han crecido su coste y su consumo, lo que supone que se ha vuelto más insostenible. Para tratar de comprender esta evolución asimétrica se ensayan sucesivos modelos que tratan de explicar la transformación de la realidad a través de la articulación de las formas de poder y sus políticas asociadas dentro del contexto local-metropolitano. Si se compara la transformación urbana en el periodo 1985-2007 respecto a la registrada durante el desarrollismo previo al presente periodo democrático, se encuentran similitudes, como el amplio consumo de suelo, pero el modelo desarrollista se inscribe en otras lógicas y tiene otros parámetros de contexto y es congruente ya que las últimas décadas del Régimen Franquista se caracterizan por un importantísimo aumento poblacional que se correspondía con el fuerte crecimiento industrial de la RMM. Esa congruencia relativa se pierde en el periodo estudiado, a pesar de que en 1985, se aprueba el Plan General de Ordenación Urbana de Madrid centrado en la ciudad existente y con un crecimiento contenido, y que puede considerarse un modelo abortado. Tras numerosas transformaciones políticas, económicas, sociales y urbanísticas se llega a una situación opuesta a la prevista en el citado Plan. Más de veinte años después, en 2007, se presentan no solo síntomas de agotamiento del modelo finalmente adoptado, sino su quiebra dramática tanto en su dimensión inmobiliario-financiera como del espacio del bienestar. Es precisamente la supresión de los mecanismos de regulación lo que ha caracterizado la evolución de los modelos urbanos, en correspondencia con la desregulación de las actividades económicas y de los flujos de capital propios del modelo "neoliberal". La actual crisis financiera internacional, en especial en algunos países periféricos europeos como España, ha demostrado cómo las políticas económicas que se han llevado a cabo, fuera de toda regulación, han resultado insostenibles. Pero no se trata solo de una crisis económica. En el caso español, de todas las dimensiones de la crisis, destaca la dimensión urbana, o el auge y caída del ciclo inmobiliario, debido a la urbanización intensiva del territorio en relación con el circuito secundario de la acumulación capitalista, habiendo tenido especial incidencia en algunos territorios como la RMM. En la Región Metropolitana de Madrid la situación actual es de crisis urbana, causada principalmente por el divorcio entre las necesidades y la producción de ciudad, pues no se ha basado el crecimiento en la creación de nuevos hogares, u otras cuestiones demográficas, sino en la acumulación de capital a través del crecimiento de la ciudad. Además, dicho crecimiento está conformado por una expansión urbana descontrolada, con mayores requerimientos energéticos que el modelo compacto y complejo tradicional, lo que unido a la escala de los procesos, supone un sistema urbano progresivamente ineficiente. El caso de la RMM resulta paradigmático, ya que la región ha desempeñado un papel como laboratorio de nuevas formas de gobierno y planificación que han dado un mayor protagonismo al espacio, que ha entrado en las dinámicas centrales principalmente por el apoyo al crecimiento físico, a la vez que han confluido circunstancias específicas, como un nuevo impulso al centralismo, lo que ha potenciado ciertas políticas, como considerar la ciudad como motor de crecimiento económico y de competitividad en el concierto europeo y mundial de ciudades. El estudio del papel de la planificación y sus crisis en la sucesión de los modelos, muestra su función nuclear en la propia constitución de estos —es parte fundamental de su aparato de regulación— y su valor no solo para poder entender el periodo, sino para poder proyectar otro futuro urbano. Este enfoque conduce a establecer la relación del planeamiento con las diferentes crisis económicas en el periodo de estudio lo que permite diferenciar tres momentos de dicha relación: la planificación urbanística austera bajo la influencia de la crisis fordista, la salida de la crisis a través de la imposición de un modelo urbano basado en el sobreproducción de espacio urbano, y la entrada en una crisis inmobiliaria y de financiarización en relación a la adopción de un modelo multidimensionalmente insostenible. El análisis de este periodo es la base para apuntar perspectivas que permitan transformar el gobierno urbano hacia un modelo urbano más deseable, o mejor aún, otros futuros posibles, que se enmarcan dentro de la alternativa principal que supone la sostenibilidad. Madrid's Metropolitan Region (MMR) has undergone a major urban transformation in the period 1985-2007, where the population has grown up, the built environment has grown strongly, but mostly its cost and consumption have grown, which means that it has become unsustainable. To try to understand this evolution successive asymmetric models are tested in order to explain the transformation of reality through the articulation of forms of power and its associated policies in that localmetropolitan context. Comparing the urban transformation in the period 1985-2007 to the existing during developmentalism in the current predemocratic period, both have similarities in terms of land consumption, but the previous developmentalism model is part of another logics and has got other context parameters. It is consistent since the last decades of the Franco Regime was characterized by an important population increase that corresponded to strong industrial growth of the MMR. This relative consistency is lost during the study period, although in 1985, with the approval of the Master Plan of Madrid that was focused on the existing city, with a limited growth, and it may be considered an interrupted model. After numerous political, economic, social and urban changes, there is the opposite situation to that foresight under that Plan. Over twenty years later, in 2007, there are not only signs of exhaustion of the model which was finally adopted, but also its dramatic collapse in both real estate and financial dimension of space as well. The urban transformation under analysis has relaunched the hegemony of the sectors that rule the growth of the Madrid's Metropolitan Region and it is supported by decision making and financing of the different administrations with the passivity of the social stakeholders and citizens. This has meant the removal of regulatory mechanisms that have characterized the evolution of urban models, corresponding to the deregulation of economic activities and capital flows according to "neoliberal" model. The current international financial crisis, especially in some European peripheral countries like Spain, has shown how economic policies that have been carried out, without any regulation, have proven unsustainable. But it is not only an economic crisis. In the Spanish case, of all the dimensions of the crisis, it is the urban dimension that is highlighted, or the rise and fall of real estate cycle, due to intensive urbanization of the territory in relation to the secondary circuit of capital accumulation, having had a particular impact in some territories such as the Madrid's Metropolitan Region. In Madrid's Metropolitan Region there is the current situation of urban crisis, mainly caused by the divorce between needs and the city (space) production, because no growth has been based on creating new homes, or other demographic issues, but in the capital accumulation through growth of the city. Furthermore, this growth is made up of urban sprawl, with higher energy requirements than the traditional compact and complex one, which together with the scale of processes, is increasingly an inefficient urban system. The case of Madrid's Metropolitan Region is paradigmatic, since the region has played a role as a laboratory for new forms of governance and planning have given a greater role to space, which has entered the core dynamics supported mainly by physical growth, while specific circumstances have come together as a new impulse to centralization. This has promoted policies such as considering the city as an engine of economic growth and competitiveness in the international and the European hierarchy of cities. The study of the role of planning and crisis in the succession of models, shows its nuclear role in the constitution of these models is a fundamental part of its regulatory apparatus- and also its value not only to understand the period, but to anticipate to other urban future. This approach leads to establish the relationship of planning with the various crises in the study period, allowing three different moments of that relationship: the austere urban planning under the influence of Fordist crisis, the output of the crisis through imposition of an urban model based on the overproduction of urban space, and entry into a housing crisis and financialisation in relation to the adoption of a multi-dimensionally unsustainable model. The analysis of this period is the basis for targeting prospects that translate urban governance towards a more desirable urban model, or better yet, other possible futures, which are part of the main alternative that is sustainability.

Experimental studies on transitional separated boundary layers

Separated transitional boundary layers appear on key aeronautical processes such as the flow around wings or turbomachinery blades. The aim of this thesis is the study of these flows in representative scenarios of technological applications, gaining knowledge about phenomenology and physical processes that occur there and, developing a simple model for scaling them. To achieve this goal, experimental measurements have been carried out in a low speed facility, ensuring the flow homogeneity and a low disturbances level such that unwanted transitional mechanisms are avoided. The studied boundary layers have been developed on a flat plate, by imposing a pressure gradient by means of contoured walls. They generate an initial acceleration region followed by a deceleration zone. The initial region is designed to obtain at the beginning of the deceleration the Blasius profile, characterized by its momentum thickness, and an edge boundary layer velocity, defining the problem characteristic velocity. The deceleration region is designed to obtain a linear evolution of the edge velocity, thereby defining the characteristic length of the problem. Several experimental techniques, both intrusive (hot wire anemometry, total pressure probes) as nonintrusive (PIV and LDV anemometry, high-speed filming), have been used in order to take advantage of each of them and allow cross-validation of the results. Once the boundary layer at the deceleration beginning has been characterized, ensuring the desired integral parameters and level of disturbance, the evolution of the laminar boundary layer up to the point of separation is studied. It has been compared with integral methods, and numerical simulations. In view of the results a new model for this evolution is proposed. Downstream from the separation, the flow near to the wall is configured as a shear layer that encloses low momentum recirculating fluid. The region where the shear layer remains laminar tends to be positioned to compensate the adverse pressure gradient associated with the imposed deceleration. Under these conditions, the momentum thickness remains almost constant. This laminar shear layer region extends up to where transitional phenomena appear, extension that scales with the momentum thickness at separation. These transitional phenomena are of inviscid type, similar to those found in free shear layers. The transitional region analysis begins with a study of the disturbances evolution in the linear growth region and the comparison of experimental results with a numerical model based on Linear Stability Theory for parallel flows and with data from other authors. The results’ coalescence for both the disturbances growth and the excited frequencies is stated. For the transition final stages the vorticity concentration into vortex blobs is found, analogously to what happens in free shear layers. Unlike these, the presence of the wall and the pressure gradient make the large scale structures to move towards the wall and quickly disappear under certain circumstances. In these cases, the recirculating flow is confined into a closed region saying the bubble is closed or the boundary layer reattaches. From the reattachment point, the fluid shows a configuration in the vicinity of the wall traditionally considered as turbulent. It has been observed that existing integral methods for turbulent boundary layers do not fit well to the experimental results, due to these methods being valid only for fully developed turbulent flow. Nevertheless, it has been found that downstream from the reattachment point the velocity profiles are self-similar, and a model has been proposed for the evolution of the integral parameters of the boundary layer in this region. Finally, the phenomenon known as bubble burst is analyzed. It has been checked the validity of existing models in literature and a new one is proposed. This phenomenon is blamed to the inability of the large scale structures formed after the transition to overcome with the adverse pressure gradient, move towards the wall and close the bubble. El estudio de capas límites transicionales con separación es de gran relevancia en distintas aplicaciones tecnológicas. Particularmente, en tecnología aeronáutica, aparecen en procesos claves, tales como el flujo alrededor de alas o álabes de turbomaquinaria. El objetivo de esta tesis es el estudio de estos flujos en situaciones representativas de las aplicaciones tecnológicas, ganando por un lado conocimiento sobre la fenomenología y los procesos físicos que aparecen y, por otra parte, desarrollando un modelo sencillo para el escalado de los mismos. Para conseguir este objetivo se han realizado ensayos en una instalación experimental de baja velocidad específicamente diseñada para asegurar un flujo homogéneo y con bajo nivel de perturbaciones, de modo que se evita el disparo de mecanismos transicionales no deseados. La capa límite bajo estudio se ha desarrollado sobre una placa plana, imponiendo un gradiente de presión a la misma por medio de paredes de geometría especificada. éstas generan una región inicial de aceleración seguida de una zona de deceleración. La región inicial se diseña para tener en al inicio de la deceleración un perfil de capa límite de Blasius, caracterizado por su espesor de cantidad de movimiento, y una cierta velocidad externa a la capa límite que se considera la velocidad característica del problema. La región de deceleración está concebida para que la variación de la velocidad externa a la capa límite sea lineal, definiendo de esta forma una longitud característica del problema. Los ensayos se han realizado explotando varias técnicas experimentales, tanto intrusivas (anemometría de hilo caliente, sondas de presión total) como no intrusivas (anemometrías láser y PIV, filmación de alta velocidad), de cara a aprovechar las ventajas de cada una de ellas y permitir validación cruzada de resultados entre las mismas. Caracterizada la capa límite al comienzo de la deceleración, y garantizados los parámetros integrales y niveles de perturbación deseados se procede al estudio de la zona de deceleración. Se presenta en la tesis un análisis de la evolución de la capa límite laminar desde el inicio de la misma hasta el punto de separación, comparando con métodos integrales, simulaciones numéricas, y proponiendo un nuevo modelo para esta evolución. Aguas abajo de la separación, el flujo en las proximidades de la pared se configura como una capa de cortadura que encierra una región de fluido recirculatorio de baja cantidad de movimiento. Se ha caracterizado la región en que dicha capa de cortadura permanece laminar, encontrando que se posiciona de modo que compensa el gradiente adverso de presión asociado a la deceleración de la corriente. En estas condiciones, el espesor de cantidad de movimiento permanece prácticamente constante y esta capa de cortadura laminar se extiende hasta que los fenómenos transicionales aparecen. Estos fenómenos son de tipo no viscoso, similares a los que aparecen en una capa de cortadura libre. El análisis de la región transicional comienza con un estudio de la evolución de las vii viii RESUMEN perturbaciones en la zona de crecimiento lineal de las mismas y la comparación de los resultados experimentales con un modelo numérico y con datos de otros autores. La coalescencia de los resultados tanto para el crecimiento de las perturbaciones como para las frecuencias excitadas queda demostrada. Para los estadios finales de la transición se observa la concentración de la vorticidad en torbellinos, de modo análogo a lo que ocurre en capas de cortadura libres. A diferencia de estas, la presencia de la pared y del gradiente de presión hace que, bajo ciertas condiciones, la gran escala se desplace hacia la pared y desaparezca rápidamente. En este caso el flujo recirculatorio queda confinado en una región cerrada y se habla de cierre de la burbuja o readherencia de la capa límite. A partir del punto de readherencia se tiene una configuración fluida en las proximidades de la pared que tradicionalmente se ha considerado turbulenta. Se ha observado que los métodos integrales existentes para capas límites turbulentas no ajustan bien a las medidas experimentales realizadas, hecho imputable a que no se obtiene en dicha región un flujo turbulento plenamente desarrollado. Se ha encontrado, sin embargo, que pasado el punto de readherencia los perfiles de velocidad próximos a la pared son autosemejantes entre sí y se ha propuesto un modelo para la evolución de los parámetros integrales de la capa límite en esta región. Finalmente, el fenómeno conocido como “estallido” de la burbuja se ha analizado. Se ha comprobado la validez de los modelos existentes en la literatura y se propone uno nuevo. Este fenómeno se achaca a la incapacidad de la gran estructura formada tras la transición para vencer el gradiente adverso de presión, desplazarse hacia la pared y cerrar la burbuja.