Comparing linear and non linear wind flow models

Assessing wind conditions on complex terrain has become a hard task as terrain complexity increases. That is why there is a need to extrapolate in a reliable manner some wind parameters that determine wind farms viability such as annual average wind speed at all hub heights as well as turbulence intensities. The development of these tasks began in the early 90��s with the widely used linear model WAsP and WAsP Engineering especially designed for simple terrain with remarkable results on them but not so good on complex orographies. Simultaneously non-linearized Navier Stokes solvers have been rapidly developed in the last decade through CFD (Computational Fluid Dynamics) codes allowing simulating atmospheric boundary layer flows over steep complex terrain more accurately reducing uncertainties. This paper describes the features of these models by validating them through meteorological masts installed in a highly complex terrain. The study compares the results of the mentioned models in terms of wind speed and turbulence intensity.

RANS simulations of wind flow at the Bolund experiment

As part of their development, the predictions of numerical wind flow models must be compared with measurements in order to estimate the uncertainty related to their use. Of course, the most rigorous such comparison is under blind conditions. The following paper includes a detailed description of three different wind flow models, all based on a Reynolds-averaged Navier-Stokes approach and two-equation k-�� closure, that were tested as part of the Bolund blind comparison (itself based on the Bolund experiment which measured the wind around a small coastal island). The models are evaluated in terms of predicted normalized wind speed and turbulent kinetic energy at 2 m and 5 m above ground level for a westerly wind direction. Results show that all models predict the mean velocity reasonably well; however accurate prediction of the turbulent kinetic energy remains achallenge.

Experimental and numerical study of wind flow behind windbreaks

The shelter effect of a windbreak protects aggregate piles and provides a reduction of particle emissions in harbours. RANS (Reynolds-averaged Navier���Stokes equations) simulations using three variants of k����� (standard k�����, RNG k����� and realizable k�����) turbulence closure models have been performed to analyse wind flow characteristics behind an isolated fence located on a flat surface without roughness elements. The performance of the three turbulence models has been assessed by wind tunnel experiments. Cases of fences with different porosities (��) have been evaluated using wind tunnel experiments as well as numerical simulations. The aim is to determine an optimum porosity for sheltering effect of an isolated windbreak. A value of 0.35 was found as the optimum value among the studied porosities (��=0, 0.1, 0.24, 0.35, 0.4, 0.5).

A method to correct the flow distortion of offshore wind data using CFD simulation and experimental wind tunnel tests

The assessment of wind energy resource for the development of deep offshore wind plants requires the use of every possible source of data and, in many cases, includes data gathered at meteorological stations installed at islands, islets or even oil platforms���all structures that interfere with, and change, the flow characteristics. This work aims to contribute to the evaluation of such changes in the flow by developing a correction methodology and applying it to the case of Berlenga island, Portugal. The study is performed using computational fluid dynamic simulations (CFD) validated by wind tunnel tests. In order to simulate the incoming offshore flow with CFD models a wind profile, unknown a priori, was established using observations from two coastal wind stations and a power law wind profile was fitted to the existing data (a=0.165). The results show that the resulting horizontal wind speed at 80 m above sea level is 16% lower than the wind speed at 80 m above the island for the dominant wind direction sector.

Analysis of Forest Effects in Wind Farm Performance

The objective of the thesis is to analyze the behaviour of the wind flow when it is passing beside the forest. To complete this analysis, a parametric study was done based upon generalized situations. Some abacus have been made, which are related to forest and wind characteristics. The abacus were compared with a particular real case, namely Alexandrovo (Bulgaria), where it was concluded that the applicability of the abacus in projects with complex terrain is low and they must be used, from a quantitative point of view, for flat terrain, being hc the most important parameter.

Large-eddy simulation of wind flows over complex terrains for wind energy applications

Wind energy has obtained outstanding expectations due to risks of global warming and nuclear energy production plant accidents. Nowadays, wind farms are often constructed in areas of complex terrain. A potential wind farm location must have the site thoroughly surveyed and the wind climatology analyzed before installing any hardware. Therefore, modeling of Atmospheric Boundary Layer (ABL) flows over complex terrains containing, e.g. hills, forest, and lakes is of great interest in wind energy applications, as it can help in locating and optimizing the wind farms. Numerical modeling of wind flows using Computational Fluid Dynamics (CFD) has become a popular technique during the last few decades. Due to the inherent flow variability and large-scale unsteadiness typical in ABL flows in general and especially over complex terrains, the flow can be difficult to be predicted accurately enough by using the Reynolds-Averaged Navier-Stokes equations (RANS). Large- Eddy Simulation (LES) resolves the largest and thus most important turbulent eddies and models only the small-scale motions which are more universal than the large eddies and thus easier to model. Therefore, LES is expected to be more suitable for this kind of simulations although it is computationally more expensive than the RANS approach. With the fast development of computers and open-source CFD software during the recent years, the application of LES toward atmospheric flow is becoming increasingly common nowadays. The aim of the work is to simulate atmospheric flows over realistic and complex terrains by means of LES. Evaluation of potential in-land wind park locations will be the main application for these simulations. Development of the LES methodology to simulate the atmospheric flows over realistic terrains is reported in the thesis. The work also aims at validating the LES methodology at a real scale. In the thesis, LES are carried out for flow problems ranging from basic channel flows to real atmospheric flows over one of the most recent real-life complex terrain problems, the Bolund hill. All the simulations reported in the thesis are carried out using a new OpenFOAM�� -based LES solver. The solver uses the 4th order time-accurate Runge-Kutta scheme and a fractional step method. Moreover, development of the LES methodology includes special attention to two boundary conditions: the upstream (inflow) and wall boundary conditions. The upstream boundary condition is generated by using the so-called recycling technique, in which the instantaneous flow properties are sampled on aplane downstream of the inlet and mapped back to the inlet at each time step. This technique develops the upstream boundary-layer flow together with the inflow turbulence without using any precursor simulation and thus within a single computational domain. The roughness of the terrain surface is modeled by implementing a new wall function into OpenFOAM�� during the thesis work. Both, the recycling method and the newly implemented wall function, are validated for the channel flows at relatively high Reynolds number before applying them to the atmospheric flow applications. After validating the LES model over simple flows, the simulations are carried out for atmospheric boundary-layer flows over two types of hills: first, two-dimensional wind-tunnel hill profiles and second, the Bolund hill located in Roskilde Fjord, Denmark. For the twodimensional wind-tunnel hills, the study focuses on the overall flow behavior as a function of the hill slope. Moreover, the simulations are repeated using another wall function suitable for smooth surfaces, which already existed in OpenFOAM�� , in order to study the sensitivity of the flow to the surface roughness in ABL flows. The simulated results obtained using the two wall functions are compared against the wind-tunnel measurements. It is shown that LES using the implemented wall function produces overall satisfactory results on the turbulent flow over the two-dimensional hills. The prediction of the flow separation and reattachment-length for the steeper hill is closer to the measurements than the other numerical studies reported in the past for the same hill geometry. The field measurement campaign performed over the Bolund hill provides the most recent field-experiment dataset for the mean flow and the turbulence properties. A number of research groups have simulated the wind flows over the Bolund hill. Due to the challenging features of the hill such as the almost vertical hill slope, it is considered as an ideal experimental test case for validating micro-scale CFD models for wind energy applications. In this work, the simulated results obtained for two wind directions are compared against the field measurements. It is shown that the present LES can reproduce the complex turbulent wind flow structures over a complicated terrain such as the Bolund hill. Especially, the present LES results show the best prediction of the turbulent kinetic energy with an average error of 24.1%, which is a 43% smaller than any other model results reported in the past for the Bolund case. Finally, the validated LES methodology is demonstrated to simulate the wind flow over the existing Muukko wind farm located in South-Eastern Finland. The simulation is carried out only for one wind direction and the results on the instantaneous and time-averaged wind speeds are briefly reported. The demonstration case is followed by discussions on the practical aspects of LES for the wind resource assessment over a realistic inland wind farm.

The impact of urban wind environments on natural ventilation

The aim of this paper is to illustrate the impact of urban wind environments when assessing the availability of natural ventilation. A numerical study of urban airflow for a complex of five building blocks located at the University of Reading, UK is presented. The computational fluid dynamics software package ANSYS was used to simulate six typical cases of urban wind environments and the potential for natural ventilation assessed. The study highlights the impact of three typical architectural forms (street canyons, semi-enclosures and courtyards) on the local wind environment. Simulation results have also been compared with experimental data collected from six locations on the building complex. The study demonstrates that ventilation strategies formed using regional weather data, may have a propensity to over-estimate the potential for natural ventilation and cooling, due to the impact of urban form which creates a unique microclimate. Characteristics of urban wind flow patterns are presented as a guideline and can be used to assess the design and performance of natural or hybrid ventilation and the opportunity for passive cooling.

Implications of the recent low solar minimum for the solar wind during the Maunder minimum

The behavior of the Sun and near-Earth space during grand solar minima is not understood; however, the recent long and low minimum of the decadal-scale solar cycle gives some important clues, with implications for understanding the solar dynamo and predicting space weather conditions. The speed of the near-Earth solar wind and the strength of the interplanetary magnetic field (IMF) embedded within it can be reliably reconstructed for before the advent of spacecraft monitoring using observations of geomagnetic activity that extend back to the mid-19th century. We show that during the solar cycle minima around 1879 and 1901 the average solar wind speed was exceptionally low, implying the Earth remained within the streamer belt of slow solar wind flow for extended periods. This is consistent with a broader streamer belt, which was also a feature of the recent low minimum (2009), and yields a prediction that the low near-Earth IMF during the Maunder minimum (1640-1700), as derived from models and deduced from cosmogenic isotopes, was accompanied by a persistent and relatively constant solar wind of speed roughly half the average for the modern era.

Reconstruction of geomagnetic activity and near-Earth interplanetary conditions over the past 167 yr - Part 4: Near-Earth solar wind speed, IMF, and open solar flux

In the concluding paper of this tetralogy, we here use the different geomagnetic activity indices to reconstruct the near-Earth interplanetary magnetic field (IMF) and solar wind flow speed, as well as the open solar flux (OSF) from 1845 to the present day. The differences in how the various indices vary with near-Earth interplanetary parameters, which are here exploited to separate the effects of the IMF and solar wind speed, are shown to be statistically significant at the 93% level or above. Reconstructions are made using four combinations of different indices, compiled using different data and different algorithms, and the results are almost identical for all parameters. The correction to the aa index required is discussed by comparison with the Ap index from a more extensive network of mid-latitude stations. Data from the Helsinki magnetometer station is used to extend the aa index back to 1845 and the results confirmed by comparison with the nearby St Petersburg observatory. The optimum variations, using all available long-term geomagnetic indices, of the near-Earth IMF and solar wind speed, and of the open solar flux, are presented; all with ��2sigma��� uncertainties computed using the Monte Carlo technique outlined in the earlier papers. The open solar flux variation derived is shown to be very similar indeed to that obtained using the method of Lockwood et al. (1999).

Centennial changes in the solar wind speed and in the open solar flux

We use combinations of geomagnetic indices, based on both variation range and hourly means, to derive the solar wind flow speed, the interplanetary magnetic field strength at 1 AU and the total open solar flux between 1895 and the present. We analyze the effects of the regression procedure and geomagnetic indices used by adopting four analysis methods. These give a mean interplanetary magnetic field strength increase of 45.1 �� 4.5% between 1903 and 1956, associated with a 14.4 �� 0.7% rise in the solar wind speed. We use averaging timescales of 1 and 2 days to allow for the difference between the magnetic fluxes threading the coronal source surface and the heliocentric sphere at 1 AU. The largest uncertainties originate from the choice of regression procedure: the average of all eight estimates of the rise in open solar flux is 73.0 �� 5.0%, but the best procedure, giving the narrowest and most symmetric distribution of fit residuals, yields 87.3 �� 3.9%.

Recent Ionospheric Observations Relating to Solar-Wind--Magnetosphere Coupling [and Discussion]

Simultaneous observations in the high-latitude ionosphere and in the near-Earth interplanetary medium have revealed the control exerted by the interplanetary magnetic field and the solar wind flow on field-perpendicular convection of plasma in both the ionosphere and the magnetosphere. Previous studies, using statistical surveys of data from both low-altitude polar-orbiting satellites and ground-based radars and magnetometers, have established that magnetic reconnection at the dayside magnetopause is the dominant driving mechanism for convection. More recently, ground-based data and global auroral images of higher temporal resolution have been obtained and used to study the response of the ionospheric flows to changes in the interplanetary medium. These observations show that ionospheric convection responds rapidly (within a few minutes) to both increases and decreases in the reconnection rate over a range of spatial scales, as well as revealing transient enhancements which are also thought to be related to magnetopause phenomena. Such results emphasize the potential of ground-based radars and other remote-sensing instruments for studies of the Earth's interaction with the interplanetary medium.

Wind tunnel analysis of the aerodynamic loads on rolling stock over railway embankments: the effect of shelter windbreaks

Wind-flow pattern over embankments involves an overexposure of the rolling stock travelling on them to wind loads. Windbreaks are a common solution for changing the flow characteristic in order to decrease unwanted effects induced by the presence of crosswind. The shelter effectiveness of a set of windbreaks placed over a railway twin-track embankment is experimentally analysed. A set of two-dimensional wind tunnel tests are undertaken and results corresponding to pressure tap measurements over a section of a typical high-speed train are herein presented.The results indicate that even small-height windbreaks provide sheltering effects to the vehicles. Also, eaves located at the windbreak tips seem to improve their sheltering effect.

Aerodynamic loads on bluff bodies under wind gusts

Esta tesis doctoral se ha centrado en el estudio de las cargas aerodin��micas no estacionario en romos cuerpos o no aerodin��micos (bluff bodies). Con este objetivo se han identificado y analizado los siguientes puntos: -Caracterizaci��n del flujo medido con diferentes tipos de tubos de Pitot y anem��metro de hilo caliente en condiciones de flujo no estacionario inestable generado por un t��nel aerodinamico de r��fagas. -Dise��o e integraci��n de los montajes experimentales requeridos para medir las cargas de viento internas y externas que act��an sobre los cuerpos romos en condiciones de flujo de viento con r��fagas. -Implementaci��n de modelos matem��ticos semi-emp��ricos basados en flujo potencial y las teor��as fenomenol��gicas pertinentes para simular los resultados experimentales. -En diversan condiciones de flujo con r��fagas, la identificaci��n y el an��lisis de la influencia de los par��metros obtenida a partir de los modelos te��ricos desarrollados. -Se proponen estimaciones emp��ricas para averiguar los valores adecuados de los par��metros que influyente, mediante el ajuste de los resultados experimentales y los predichos te��ricamente. Los montajes experimentales se has reakizado en un tunel aerodinamico de circuito abierto, provisto de baja velocidad, c��mara de ensayes cerrada, un nuevo concepto de mecanismo generador de r��faga sinusoidal, dise��ado y construido en el Instituto de Microgravedad "Ignacio Da Riva" de la Universidad Polit��cnica de Madrid, (IDR / UPM). La principal caracter��stica de este t��nel aerodynamico es la capacidad de generar un flujo con un perfil de velocidad uniforme y una fluctuaci��n sinusoidal en el tiempo. Se han realizado pruebas experimentales para estudiar el efecto de los flujos no estacionarios en cuerpos romos situados en el suelo. Se han propuesto dos modelos te��ricos para diterminar las cargas de presi��n externas e internas respectivamente. Con el fin de satisfacer la necesidad de la crea r��fagas de viento sinusoidales para comprobar las predicciones de los modelos te��ricos, se han obtenido velocidades de hasta 30 m/s y frecuencias r��faga de hasta 10 Hz. La secci��n de la c��mara de ensayos es de 0,39 m x 0,54 m, dimensi��nes adecuadas para llevar a cabo experimentos con modelos de ensayos. Se muestra que en la gama de par��metros explorados los resultados experimentales est��n en buen acuerdo con las predicciones de los modelos te��ricos. Se han realizado pruebas experimentales para estudiar los efectos del flujo no estacionario, las cuales pueden ayudar a aclarar el fen��meno de las cargas de presi��n externa sobre los cuerpos romos sometidos a r��fagas de viento: y tambien para determinan las cargas de presi��n interna, que dependen del tama��o de los orificios de ventilaci��n de la construcci��n. Por ��ltimo, se ha analizado la contribuci��n de los t��rminos provenientes del flujo no estacionario, y se han caracterizado o los saltos de presi��n debido a la p��rdida no estacionario de presi��n a trav��s de los orificios de ventilaci��n. ABSTRACT This Doctoral dissertation has been focused to study the unsteady aerodynamic loads on bluff bodies. To this aim the following points have been identified and analyzed: -Characterization of the flow measured with different types of Pitot tubes and hot wire anemometer at unsteady flow conditions generated by a gust wind tunnel. -Design and integrating of the experimental setups required to measure the internal and external wind loads acting on bluff bodies at gusty wind flow conditions. -Implementation of semi-empirical mathematical models based on potential flow and relevant phenomenological theories to simulate the experimental results.-At various gusty flow conditions, extracting and analyzing the influence of parameters obtained from the developed theoretical models. -Empirical estimations are proposed to find out suitable values of the influencing parameters, by fitting the experimental and theoretically predicted results. The experimental setups are performed in an open circuit, closed test section, low speed wind tunnel, with a new sinusoidal gust generator mechanism concept, designed and built at the Instituto de Microgravedad ���Ignacio Da Riva��� of the Universidad Polit��cnica de Madrid, (IDR/UPM). The main characteristic of this wind tunnel is the ability to generate a flow with a uniform velocity profile and a sinusoidal time fluctuation of the speed. Experimental tests have been devoted to study the effect of unsteady flows on bluff bodies lying on the ground. Two theoretical models have been proposed to measure the external and internal pressure loads respectively. In order to meet the need of creating sinusoidal wind gusts to check the theoretical model predictions, the gust wind tunnel maximum flow speed and, gust frequency in the test section have been limited to 30 m/s and 10 Hz, respectively have been obtained. The test section is 0.39 m �� 0.54 m, which is suitable to perform experiments with testing models. It is shown that, in the range of parameters explored, the experimental results are in good agreement with the theoretical model predictions. Experimental tests have been performed to study the unsteady flow effects, which can help in clarifying the phenomenon of the external pressure loads on bluff bodies under gusty winds: and also to study internal pressure loads, which depend on the size of the venting holes of the building. Finally, the contribution of the unsteady flow terms in the theoretical model has been analyzed, and the pressure jumps due to the unsteady pressure losses through the venting holes have been characterized.

Urban wind energy: empirical optimization of high-rise building roof shape for the wind energy exploitation

El programa Europeo HORIZON2020 en Futuras Ciudades Inteligentes establece como objetivo que el 20% de la energ��a el��ctrica sea generada a partir de fuentes renovables. Este objetivo implica la necesidad de potenciar la generaci��n de energ��a e��lica en todos los ��mbitos. La energ��a e��lica reduce dr��sticamente las emisiones de gases de efecto invernadero y evita los riesgos geo-pol��ticos asociados al suministro e infraestructuras energ��ticas, as�� como la dependencia energ��tica de otras regiones. Adem��s, la generaci��n de energ��a distribuida (generaci��n en el punto de consumo) presenta significativas ventajas en t��rminos de elevada eficiencia energ��tica y estimulaci��n de la econom��a. El sector de la edificaci��n representa el 40% del consumo energ��tico total de la Uni��n Europea. La reducci��n del consumo energ��tico en este ��rea es, por tanto, una prioridad de acuerdo con los objetivos "20-20-20" en eficiencia energ��tica. La Directiva 2010/31/EU del Parlamento Europeo y del Consejo de 19 de mayo de 2010 sobre el comportamiento energ��tico de edificaciones contempla la instalaci��n de sistemas de suministro energ��tico a partir de fuentes renovables en las edificaciones de nuevo dise��o. Actualmente existe una escasez de conocimiento cient��fico y tecnol��gico acerca de la geometr��a ��ptima de las edificaciones para la explotaci��n de la energ��a e��lica en entornos urbanos. El campo tecnol��gico de estudio de la presente Tesis Doctoral es la generaci��n de energ��a e��lica en entornos urbanos. Espec��ficamente, la optimization de la geometr��a de las cubiertas de edificaciones desde el punto de vista de la explotaci��n del recurso energ��tico e��lico. Debido a que el flujo del viento alrededor de las edificaciones es exhaustivamente investigado en esta Tesis empleando herramientas de simulaci��n num��rica, la mec��nica de fluidos computacional (CFD en ingl��s) y la aerodin��mica de edificaciones son los campos cient��ficos de estudio. El objetivo central de esta Tesis Doctoral es obtener una geometr��a de altas prestaciones (u ��ptima) para la explotaci��n de la energ��a e��lica en cubiertas de edificaciones de gran altura. Este objetivo es alcanzado mediante un an��lisis exhaustivo de la influencia de la forma de la cubierta del edificio en el flujo del viento desde el punto de vista de la explotaci��n energ��tica del recurso e��lico empleando herramientas de simulaci��n num��rica (CFD). Adicionalmente, la geometr��a de la edificaci��n convencional (edificio prism��tico) es estudiada, y el posicionamiento adecuado para los diferentes tipos de aerogeneradores es propuesto. La compatibilidad entre el aprovechamiento de las energ��as solar fotovoltaica y e��lica tambi��n es analizado en este tipo de edificaciones. La investigaci��n prosigue con la optimizaci��n de la geometr��a de la cubierta. La metodolog��a con la que se obtiene la geometr��a ��ptima consta de las siguientes etapas: - Verificaci��n de los resultados de las geometr��as previamente estudiadas en la literatura. Las geometr��as b��sicas que se someten a examen son: cubierta plana, a dos aguas, inclinada, abovedada y esf��rica. - An��lisis de la influencia de la forma de las aristas de la cubierta sobre el flujo del viento. Esta tarea se lleva a cabo mediante la comparaci��n de los resultados obtenidos para la arista convencional (esquina sencilla) con un parapeto, un voladizo y una esquina curva. - An��lisis del acoplamiento entre la cubierta y los cerramientos verticales (paredes) mediante la comparaci��n entre diferentes variaciones de una cubierta esf��rica en una edificaci��n de gran altura: cubierta esf��rica estudiada en la literatura, cubierta esf��rica integrada geom��tricamente con las paredes (planta cuadrada en el suelo) y una cubierta esf��rica acoplada a una pared cilindrica. El comportamiento del flujo sobre la cubierta es estudiado tambi��n considerando la posibilidad de la variaci��n en la direcci��n del viento incidente. - An��lisis del efecto de las proporciones geom��tricas del edificio sobre el flujo en la cubierta. - An��lisis del efecto de la presencia de edificaciones circundantes sobre el flujo del viento en la cubierta del edificio objetivo. Las contribuciones de la presente Tesis Doctoral pueden resumirse en: - Se demuestra que los modelos de turbulencia RANS obtienen mejores resultados para la simulaci��n del viento alrededor de edificaciones empleando los coeficientes propuestos por Crespo y los propuestos por Bechmann y S��rensen que empleando los coeficientes est��ndar. - Se demuestra que la estimaci��n de la energ��a cin��tica turbulenta del flujo empleando modelos de turbulencia RANS puede ser validada manteniendo el enfoque en la cubierta de la edificaci��n. - Se presenta una nueva modificaci��n del modelo de turbulencia Durbin k ��� e que reproduce mejor la distancia de recirculaci��n del flujo de acuerdo con los resultados experimentales. - Se demuestra una relaci��n lineal entre la distancia de recirculaci��n en una cubierta plana y el factor constante involucrado en el c��lculo de la escala de tiempo de la velocidad turbulenta. Este resultado puede ser empleado por la comunidad cient��fica para la mejora del modelado de la turbulencia en diversas herramientas computacionales (OpenFOAM, Fluent, CFX, etc.). - La compatibilidad entre las energ��as solar fotovoltaica y e��lica en cubiertas de edificaciones es analizada. Se demuestra que la presencia de los m��dulos solares provoca un descenso en la intensidad de turbulencia. - Se demuestran conflictos en el cambio de escala entre simulaciones de edificaciones a escala real y simulaciones de modelos a escala reducida (t��nel de viento). Se demuestra que para respetar las limitaciones de similitud (n��mero de Reynolds) son necesarias mediciones en edificaciones a escala real o experimentos en t��neles de viento empleando agua como fluido, especialmente cuando se trata con geometr��as complejas, como es el caso de los m��dulos solares. - Se determina el posicionamiento m��s adecuado para los diferentes tipos de aerogeneradores tomando en consideraci��n la velocidad e intensidad de turbulencia del flujo. El posicionamiento de aerogeneradores es investigado en las geometr��as de cubierta m��s habituales (plana, a dos aguas, inclinada, abovedada y esf��rica). - Las formas de aristas m��s habituales (esquina, parapeto, voladizo y curva) son analizadas, as�� como su efecto sobre el flujo del viento en la cubierta de un edificio de gran altura desde el punto de vista del aprovechamiento e��lico. - Se propone una geometr��a ��ptima (o de altas prestaciones) para el aprovechamiento de la energ��a e��lica urbana. Esta optimizaci��n incluye: verificaci��n de las geometr��as estudiadas en el estado del arte, an��lisis de la influencia de las aristas de la cubierta en el flujo del viento, estudio del acoplamiento entre la cubierta y las paredes, an��lisis de sensibilidad del grosor de la cubierta, exploraci��n de la influencia de las proporciones geom��tricas de la cubierta y el edificio, e investigaci��n del efecto de las edificaciones circundantes (considerando diferentes alturas de los alrededores) sobre el flujo del viento en la cubierta del edificio objetivo. Las investigaciones comprenden el an��lisis de la velocidad, la energ��a cin��tica turbulenta y la intensidad de turbulencia en todos los casos. ABSTRACT The HORIZON2020 European program in Future Smart Cities aims to have 20% of electricity produced by renewable sources. This goal implies the necessity to enhance the wind energy generation, both with large and small wind turbines. Wind energy drastically reduces carbon emissions and avoids geo-political risks associated with supply and infrastructure constraints, as well as energy dependence from other regions. Additionally, distributed energy generation (generation at the consumption site) offers significant benefits in terms of high energy efficiency and stimulation of the economy. The buildings sector represents 40% of the European Union total energy consumption. Reducing energy consumption in this area is therefore a priority under the "20-20-20" objectives on energy efficiency. The Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings aims to consider the installation of renewable energy supply systems in new designed buildings. Nowadays, there is a lack of knowledge about the optimum building shape for urban wind energy exploitation. The technological field of study of the present Thesis is the wind energy generation in urban environments. Specifically, the improvement of the building-roof shape with a focus on the wind energy resource exploitation. Since the wind flow around buildings is exhaustively investigated in this Thesis using numerical simulation tools, both computational fluid dynamics (CFD) and building aerodynamics are the scientific fields of study. The main objective of this Thesis is to obtain an improved (or optimum) shape of a high-rise building for the wind energy exploitation on the roof. To achieve this objective, an analysis of the influence of the building shape on the behaviour of the wind flow on the roof from the point of view of the wind energy exploitation is carried out using numerical simulation tools (CFD). Additionally, the conventional building shape (prismatic) is analysed, and the adequate positions for different kinds of wind turbines are proposed. The compatibility of both photovoltaic-solar and wind energies is also analysed for this kind of buildings. The investigation continues with the buildingroof optimization. The methodology for obtaining the optimum high-rise building roof shape involves the following stages: - Verification of the results of previous building-roof shapes studied in the literature. The basic shapes that are compared are: flat, pitched, shed, vaulted and spheric. - Analysis of the influence of the roof-edge shape on the wind flow. This task is carried out by comparing the results obtained for the conventional edge shape (simple corner) with a railing, a cantilever and a curved edge. - Analysis of the roof-wall coupling by testing different variations of a spherical roof on a high-rise building: spherical roof studied in the litera ture, spherical roof geometrically integrated with the walls (squared-plant) and spherical roof with a cylindrical wall. The flow behaviour on the roof according to the variation of the incident wind direction is commented. - Analysis of the effect of the building aspect ratio on the flow. - Analysis of the surrounding buildings effect on the wind flow on the target building roof. The contributions of the present Thesis can be summarized as follows: - It is demonstrated that RANS turbulence models obtain better results for the wind flow around buildings using the coefficients proposed by Crespo and those proposed by Bechmann and S0rensen than by using the standard ones. - It is demonstrated that RANS turbulence models can be validated for turbulent kinetic energy focusing on building roofs. - A new modification of the Durbin k ��� e turbulence model is proposed in order to obtain a better agreement of the recirculation distance between CFD simulations and experimental results. - A linear relationship between the recirculation distance on a flat roof and the constant factor involved in the calculation of the turbulence velocity time scale is demonstrated. This discovery can be used by the research community in order to improve the turbulence modeling in different solvers (OpenFOAM, Fluent, CFX, etc.). - The compatibility of both photovoltaic-solar and wind energies on building roofs is demonstrated. A decrease of turbulence intensity due to the presence of the solar panels is demonstrated. - Scaling issues are demonstrated between full-scale buildings and windtunnel reduced-scale models. The necessity of respecting the similitude constraints is demonstrated. Either full-scale measurements or wind-tunnel experiments using water as a medium are needed in order to accurately reproduce the wind flow around buildings, specially when dealing with complex shapes (as solar panels, etc.). - The most adequate position (most adequate roof region) for the different kinds of wind turbines is highlighted attending to both velocity and turbulence intensity. The wind turbine positioning was investigated for the most habitual kind of building-roof shapes (flat, pitched, shed, vaulted and spherical). - The most habitual roof-edge shapes (simple edge, railing, cantilever and curved) were investigated, and their effect on the wind flow on a highrise building roof were analysed from the point of view of the wind energy exploitation. - An optimum building-roof shape is proposed for the urban wind energy exploitation. Such optimization includes: state-of-the-art roof shapes test, analysis of the influence of the roof-edge shape on the wind flow, study of the roof-wall coupling, sensitivity analysis of the roof width, exploration of the aspect ratio of the building-roof shape and investigation of the effect of the neighbouring buildings (considering different surrounding heights) on the wind now on the target building roof. The investigations comprise analysis of velocity, turbulent kinetic energy and turbulence intensity for all the cases.