Analysis of air flow past and through the 2415-3S airfoil for an unmanned aerial vehicle with internal propulsion system

This paper deals with the prediction of velocity fields on the 2415-3S airfoil which will be used for an unmanned aerial vehicle with internal propulsion system and in this way analyze the air flow through an internal duct of the airfoil using computational fluid dynamics. The main objective is to evaluate the effect of the internal air flow past the airfoil and how this affects the aerodynamic performance by means of lift and drag forces. For this purpose, three different designs of the internal duct were studied; starting from the base 2415-3S airfoil developed in previous investigation, basing on the hypothesis of decreasing the flow separation produced when the propulsive airflow merges the external flow, and in this way obtaining the best configuration. For that purpose, an exhaustive study of the mesh sensitivity was performed. It was used a non-structured mesh since the computational domain is three-dimensional and complex. The selected mesh contains approximately 12.5 million elements. Both the computational domain and the numerical solution were made with commercial CAD and CFD software, respectively. Air, incompressible and steady was analyzed. The boundary conditions are in concordance with experimental setup in the AF 6109 wind tunnel. The k-e model is utilized to describe the turbulent flow process as followed in references. Results allowed obtaining velocity contours as well as lift and drag coefficients and also the location of separation and reattachment regions in some cases for zero degrees of angle of attack on the internal and external surfaces of the airfoil. Finally, the selection of the configuration with the best aerodynamic performance was made, selecting the option without curved baffles.

Numerical analysis of air flow past the 2415-3S airfoil for an unmanned aerial vehicle with internal propulsion system

This paper deals with the prediction of pressure and velocity fields on the 2415-3S airfoil which will be used for and unmanned aerial vehicle with internal propulsion system and in this way analyze the air flow through an internal duct of the airfoil using computational fluid dynamics. The main objective is to evaluate the effect of the internal air flow past the airfoil and how this affects the aerodynamic performance by means of lift and drag forces. For this purpose, three different designs of the internal duct were studied; starting from the base 2415-3S airfoil developed in previous investigation, basing on the hypothesis of decreasing the flow separation produced when the propulsive airflow merges the external flow, and in this way obtaining the best configuration. For that purpose, an exhaustive study of the mesh sensitivity was performed. It was used a non-structured mesh since the computational domain is tridimensional and complex. The selected mesh contains approximately 12.5 million elements. Both the computational domain and the numerical solution were made with commercial CAD and CFD software respectively. Air, incompressible and steady was analyzed. The boundary conditions are in concordance with experimental setup in the AF 6109 wind tunnel. The k-ε model is utilized to describe the turbulent flow process as followed in references. Results allowed obtaining pressure and velocity contours as well as lift and drag coefficients and also the location of separation and reattachment regions in some cases for zero degrees of angle of attack on the internal and external surfaces of the airfoil. Finally, the selection of the configuration with the best aerodynamic performance was made, selecting the option without curved baffles.

Hysteresis in transverse galloping: the role of the inflection points

Transverse galloping is here considered as a one-degree-of-freedom oscillator subjected to aerodynamic forces, which are described by using the quasi-steady hypothesis. The hysteresis of transverse galloping is also analyzed. Approximate solutions of the model are obtained by assuming that the aerodynamic and damping forces are much smaller than the inertial and stiffness ones. The analysis of the approximate solution, which is obtained by means of the method of Krylov–Bogoliubov, reveals the existing link between the hysteresis phenomenon and the number of inflection points at the aerodynamic force coefficient curve, Cy(α)Cy(α); CyCy and αα being, respectively, the force coefficient normal to the incident flow and the angle of attack. The influence of the position of these inflection points on the range of flow velocities in which hysteresis takes place is also analyzed.

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.

An engineering modification of the blade element momentum combination applicable in vertical descent flight :fundamentals and validation

An engineering modification of blade element/momentum theory is applied to describe the vertical autorotation of helicopter rotors. A full non‐linear aerodynamic model is considered for the airfoils, taking into account the dependence of lift and drag coefficients on both the angle of attack and the Reynolds number. The proposed model, which has been validated in previous work, has allowed the identification of different autorotation modes, which depend on the descent velocity and the twist of the rotor blades. These modes present different radial distributions of driven and driving blade regions, as well as different radial upwash/downwash patterns. The number of blade sections with zero tangential force, the existence of a downwash region in the rotor disk, the stability of the autorotation state, and the overall rotor autorotation efficiency, are all analyzed in terms of the flight velocity and the characteristics of the rotor. It is shown that, in vertical autorotation, larger blade twist leads to smaller values of descent velocity for a given thrust generated by the rotor in the autorotational state.

An engineering modification of the blade element momentum equation for vertical descent : an autoration case study

An engineering modification of blade element/momentum theory is applied to describe the vertical autorotation of helicopter rotors. A full non-linear aerodynamic model is considered for the airfoils, taking into account the dependence of lift and drag coefficients on both the angle of attack and the Reynolds number. The proposed model, which has been validated in previous work, has allowed the identification of different autorotation modes, which depend on the descent velocity and the twist of the rotor blades. These modes present different radial distributions of driven and driving blade regions, as well as different radial upwash/downwash patterns. The number of blade sections with zero tangential force, the existence of a downwash region in the rotor disk, the stability of the autorotation state, and the overall rotor autorotation efficiency, are all analyzed in terms of the flight velocity and the characteristics of the rotor. It is shown that, in vertical autorotation, larger blade twist leads to smaller values of descent velocity for a given thrust generated by the rotor in the autorotational state.

Análisis de la influencia del borde de salida abierto en perfiles aerodinámicos a bajos números de Reynolds

El objetivo del presente trabajo es analizar la influencia que tiene sobre el comportamiento aerodinámico del perfil el hecho de que este presente un borde de salida más grueso que el perfil original del que se partía. Este estudio se ha centrado fundamentalmente en la influencia sobre su sustentación aerodinámica, resistencia aerodinámica y, especialmente, sobre la eficiencia aerodinámica del perfil, es decir sobre la relación entre la sustentación y la resistencia aerodinámica. También se ha analizado su influencia en otros aspectos aerodinámicos de los perfiles, como la entrada en pérdida, el ángulo de ataque de sustentación máxima, el ángulo de ataque de eficiencia máxima, el coeficiente de momento aerodinámico y la posición del centro aerodinámico. Estas imperfecciones en el borde de salida pueden aparecer en algunos procesos de fabricación de determinados elementos aerodinámicos, como alas de aviones no tripulados o palas de aeroturbina. Este fenómeno no ha sido analizado en profundidad en la literatura científica, aunque si que se ha analizado por varios autores la influencia sobre el perfil con el borde de salida truncado, o perfiles con la parte final regruesada, utilizados en otras aplicaciones. Para la realización de este estudio se han analizado perfiles de distinto tipo, laminares y no laminares, perfiles simétricos y con curvatura, así como perfiles con distinto espesor, a fin de comparar el grado de influencia del fenómeno estudiado sobre cada tipo de perfil para comparar su grado de sensibilidad a dicha anomalía geométrica. El estudio se ha realizado experimentalmente utilizando una cámara de ensayos diseñada específicamente a tal efecto, así como una balanza electrónica para medir las fuerzas y los momentos sobre el perfil, y un escáner de presiones para medir la distribución de presiones en determinados casos. También se ha abordado el estudio del comportamiento de perfiles con borde de salida más grueso que el nominal pero redondeado en vez de romo, con el objeto de analizar la eficacia de redondear el borde de salida, que es uno de los métodos que se puede utilizar para mitigar este efecto. Por otro lado, como el comportamiento de los perfiles aerodinámicos tiene una fuerte dependencia del número de Reynolds, el estudio se ha centrado en el análisis del comportamiento a bajos números de Reynolds debido a su uso reciente en una amplia gama de aplicaciones, desde vehículos aéreos no tripulados (UAV) hasta palas de aeroturbinas de baja potencia, e incluso debido a su uso potencial en aeronaves diseñadas para volar en atmósferas de baja densidad como la que existe en Marte. El interés de este estudio está orientado al establecimiento de criterios para cuantificar la influencia que tiene el hecho de que el borde de salida sea más grueso que el nominal en la degradación de su eficiencia aerodinámica máxima, con el objeto de poder establecer los límites de aceptación o rechazo de estas piezas una vez fabricadas, según el tipo de perfil aerodinámico utilizado. Del resultado del análisis de los casos estudiados se puede concluir que según aumenta el espesor del borde de salida, dentro del intervalo de estudio, la sustentación aerodinámica aumenta, así como la sustentación máxima, pero aumenta en mayor proporción la resistencia aerodinámica, por lo que se produce una reducción de la eficiencia aerodinámica, en particular de su valor máximo. Por otro lado, el hecho de redondear el borde de salida del perfil ayuda ligeramente a reducir este efecto. ABSTRACT The aim of this thesis is to analyze the effects of airfoil trailing edges thickness when this is thicker than the airfoil nominal. Several factors may lead to an airfoil trailing edge being thicker than the nominal airfoil, and this may affect various aerodynamic parameters. This study has focus on its influence on the airfoil’s aerodynamic lift, drag and, particularly on the aerodynamic efficiency of the airfoil, that is, the relationship between the aerodynamic lift and drag. It has also been studied how this fact may alter some other aerodynamic aspects of airfoils, such as stall, angle of attack of maximum lift, angle of maximum efficiency, aerodynamic moment coefficient and aerodynamic center position. These imperfections in the trailing edge may appear in some manufacturing processes of certain aerodynamic elements, such as unmanned aircraft wings or wind turbine blades. This phenomenon has not been deeply analyzed in the literature, although several authors have discussed its influence on airfoil with truncated trailing edge, or airfoils with thickened end, used in other applications. Various types of airfoils have been analyzed, laminar and non-laminar, symmetric and curved airfoils, and airfoils with different thickness, in order to compare the degree of influence of the phenomenon studied on each airfoil type and thus, to estimate the degree of sensitivity to the anomaly geometry. The study was carried out experimentally using a test chamber designed specifically for this purpose, as well as an electronic balance to measure the forces and moments on the airfoil, and a pressure scanner to measure distribution of pressures in certain cases. It has also been investigated the behavior of airfoils with trailing edge thicker than the nominal, but rounded instead of blunt, in order to analyze the effectiveness of the trailing edge rounding, which is one of the methods that can be used to mitigate this phenomenon. Moreover, as the behavior of the airfoil is highly dependent on the Reynolds number, the study has been focused on the analysis of the behavior at low Reynolds numbers due to recent use of low Reynolds numbers airfoils in a wide range of applications, from unmanned aerial vehicles (UAV) to low power wind turbine blades, or even due to their potential use in aircraft designed to fly in low density atmospheres as the one existing in Mars. The main purpose of this research is to establish a set of criteria for quantifying the influence that a thicker-than–nominal-trailing edge has in the degradation of maximum aerodynamic efficiency, aiming at establishing the acceptance limits for these pieces when they are manufactured, according to the type of airfoil used. Based on the results obtained from the analysis of the cases under study it can be concluded that increasing the thickness of the trailing edge, within the range of study, increases aerodynamic lift, as well as maximum lift, but the aerodynamic drag increases in a higher proportion, and consequently there is a reduction of aerodynamic efficiency, particularly, of its maximum value. On the other hand, rounding the trailing edge of the airfoil slightly helps to reduce this effect.

Fenómenos de galope en piezas prismáticas con sección en H

El objetivo de esta Tesis es el estudio sistemático del fenómeno aeroelástico de galope de una viga prismática con sección transversal en H. En particular, se pretende analizar la influencia de determinados parámetros geométricos que definen la geometría de la sección y el efecto del ángulo de ataque de la corriente sobre la estabilidad del fenómeno de oscilación. El interés en el estudio de esta forma de la sección transversal de una viga se basa en el hecho de que, con cierta frecuencia, se sigue utilizando, por su buen comportamiento estructural, en construcciones civiles como tableros y tirantes de puentes de gran longitud, señalizaciones, luminarias y, en general, en grandes estructuras metálicas. Los parámetros geométricos seleccionados para su estudio son tres: el espesor de las dos alas verticales, su porosidad y el espesor de sendas ranuras en la zona de unión entre el alma y las alas de la sección. Inicialmente se han realizado ensayos estáticos en un túnel aerodinámico con objeto de obtener las cargas aerodinámicas y poder aplicar el criterio casi-estático de Glauert - Den Hartog. En estos ensayos, se han medido tanto las fuerzas de sustentación y resistencia aerodinámicas como las distribuciones de presiones en la superficie de la zona central de la sección. Posteriormente, se han realizado ensayos de visualización de flujo, utilizando un túnel de humos, para poder comprender mejor el comportamiento físico del aire alrededor del cuerpo. El estudio estático se ha completado realizando ensayos con PIV, que permiten realizar una medida precisa de la velocidad del campo fluido. Por último, se han realizado ensayos dinámicos en otro túnel aerodinámico con objeto de contrastar la aplicabilidad del criterio casi-estático, la velocidad de inicio de galope y la amplitud de las oscilaciones producidas. Los resultados muestran que el espesor de las alas verticales, aunque modifica apreciablemente la magnitud de las cargas aerodinámicas, no afecta sustancialmente a la estabilidad a galope, mientras que su porosidad sí ejerce un control efectivo que permite reducir este fenómeno e incluso evitarlo, en determinados casos. En todas las situaciones el criterio de Glauert-Glauert - Den Hartog ha resultado ser aplicable y, en ocasiones, más restrictivo que los resultados obtenidos en ensayos dinámicos. La presencia de una ranura en la zona de unión entre el alma y las dos alas, o su combinación con la porosidad en las alas, reduce la intensidad de galope, incrementando la velocidad crítica de su inicio, pero no logra hacer que desaparezca, como se justificará en el desarrollo del trabajo. ABSTRACT Galloping is a type of aeroelastic instability characterized by a large amplitude oscillation at the natural frequency of the structure, producing normal motion to wind. It usually occurs in slender bodies lightly damped at sufficiently high speeds. In this thesis an experimental study has been developed on the galloping instability of a beam with H cross section, which is inscribed in a rectangle with a slender 1: 2. A systematic study has been carried out of the influence of three different geometric parameters on galloping, in the range of 0 to 90° angle of attack of the incoming stream. These parameters are the thickness of the flanges of the section, the porosity of the flanges, the thickness of two slots along the span, in the area between the flange and the central core of the section, and the combination effect of the last two parameters. First of all, static tests have been performed in a wind tunnel to determine the lift and drag forces by using a balance and then the quasi-static stability criterion due to Glauert-Den Hartog has been determined. Later, to better understand and verify the results previously obtained, it has also been tested the pressure distribution on the surface of the model, flow visualization in a second, smoke, wind tunnel, and Particle Image Velocimetry (PIV) study of the flow around the section, in a third tunnel. Finally, dynamic tests have been performed, on a fourth wind tunnel, for determining the amplitude and frequency of the oscillations in each case. The results have been collected in stability diagrams for each geometric parameter studied. These results show that the more critical angles of attack of the stream for galloping behavior are close to 0 and 90º. It has been found that the thickness of the flanges, although changes the galloping behavior on the section, does not reduce it substantially. However, the porosity in the flanges has been proved to be an efficient control mechanism on galloping, and even above 40% porosity, it disappears. The thickness of the slot studied and its combination with the porosity in the flanges in some cases reduces the aerodynamic forces appreciably but fail to prevent galloping at all angles of attack.

Investigación de fenómenos de galope en cuerpos de diversa sección transversal

Los fenómenos aeroelásticos son relativamente frecuentes en las construcciones civiles modernas como edificios de oficinas, terminales de aeropuertos o fábricas. En este tipo de arquitectura aparecen con frecuencia estructuras flexibles sometidas a la acción del viento, como por ejemplo persianas formadas por láminas con distintos perfiles. Uno de estos perfiles es el perfil en Z, formado por un elemento central y dos alas laterales. Las inestabilidades de tipo galope se determinan en la práctica utilizando el criterio Glauert-Den Hartog. Este criterio precisa de la predicción exacta de la dependencia de los coeficientes aerodinámicos del ángulo de ataque. En esta tesis se presenta un estudio sistemático, tanto por métodos experimentales como numéricos de una familia completa de perfiles en Z que permite determinar sus regiones de inestabilidad frente al galope. Los análisis numéricos han sido validados con ensayos estáticos realizados en túnel de viento. Para la parte numérica se ha utilizado el código DLR TAU, que es un código de amplia utilización en la industria aeronáutica europea. En esta tesis se enfoca sobre todo a la predicción del galope en este tipo de perfiles en Z. Los resultados se presentan en forma de mapas de estabilidad. A lo largo del trabajo se realizan también comparaciones entre resultados numéricos y experimentales para varios niveles de detalle de las mallas empleadas y diversos modelos de turbulencia. ABSTRACT Aeroelastic effects are relatively common in the design of modern civil constructions such as office blocks, airport terminal buildings, and factories. Typical flexible structures exposed to the action of wind are shading devices, normally slats or louvers. A typical cross-section for such elements is a Z-shaped profile,made out of a central web and two-sidewings. Galloping instabilities are often determined in practice using the Glauert-DenHartog criterion.This criterion relies on accurate predictions of the dependence of the aerodynamic force coefficients with the angle of attack. The results of a parametric analysis based on both experimental and numerical analysis and performed on different Z-shaped louvers to determine translational galloping instability regions are presented in this thesis. These numerical analysis results have been validated with a parametric analysis of Z-shaped profiles based on static wind tunnel tests. In order to perform this validation, the DLR TAU Code, which is a standard code within the European aeronautical industry, has been used. This study highlights the focus on the numerical prediction of the effect of galloping, which is shown in a visible way, through stability maps. Comparisons between numerical and experimental data are presented with respect to various meshes and turbulence models.

Aerodynamic database error filtering via high order singular value decomposition

Esta Tesis presenta un nuevo método para filtrar errores en bases de datos multidimensionales. Este método no precisa ninguna información a priori sobre la naturaleza de los errores. En concreto, los errrores no deben ser necesariamente pequeños, ni de distribución aleatoria ni tener media cero. El único requerimiento es que no estén correlados con la información limpia propia de la base de datos. Este nuevo método se basa en una extensión mejorada del método básico de reconstrucción de huecos (capaz de reconstruir la información que falta de una base de datos multidimensional en posiciones conocidas) inventado por Everson y Sirovich (1995). El método de reconstrucción de huecos mejorado ha evolucionado como un método de filtrado de errores de dos pasos: en primer lugar, (a) identifica las posiciones en la base de datos afectadas por los errores y después, (b) reconstruye la información en dichas posiciones tratando la información de éstas como información desconocida. El método resultante filtra errores O(1) de forma eficiente, tanto si son errores aleatorios como sistemáticos e incluso si su distribución en la base de datos está concentrada o esparcida por ella. Primero, se ilustra el funcionamiento delmétodo con una base de datosmodelo bidimensional, que resulta de la dicretización de una función transcendental. Posteriormente, se presentan algunos casos prácticos de aplicación del método a dos bases de datos tridimensionales aerodinámicas que contienen la distribución de presiones sobre un ala a varios ángulos de ataque. Estas bases de datos resultan de modelos numéricos calculados en CFD. ABSTRACT A method is presented to filter errors out in multidimensional databases. The method does not require any a priori information about the nature the errors. In particular, the errors need not to be small, neither random, nor exhibit zero mean. Instead, they are only required to be relatively uncorrelated to the clean information contained in the database. The method is based on an improved extension of a seminal iterative gappy reconstruction method (able to reconstruct lost information at known positions in the database) due to Everson and Sirovich (1995). The improved gappy reconstruction method is evolved as an error filtering method in two steps, since it is adapted to first (a) identify the error locations in the database and then (b) reconstruct the information in these locations by treating the associated data as gappy data. The resultingmethod filters out O(1) errors in an efficient fashion, both when these are random and when they are systematic, and also both when they concentrated and when they are spread along the database. The performance of the method is first illustrated using a two-dimensional toymodel database resulting fromdiscretizing a transcendental function and then tested on two CFD-calculated, three-dimensional aerodynamic databases containing the pressure coefficient on the surface of a wing for varying values of the angle of attack. A more general performance analysis of the method is presented with the intention of quantifying the randomness factor the method admits maintaining a correct performance and secondly, quantifying the size of error the method can detect. Lastly, some improvements of the method are proposed with their respective verification.

Characterization of a Right Angle Transfer Point in a Fruit Packing Line

Characteristics of the impacts su!ered by the fruit on a transfer point of an experimental fruit packing line were analysed. The transfer is made up by two transporting belts at di!erent heights forming an angle of 903. These transfer points are very common in fruit packing lines, in which fruits receive two impacts: the "rst onto the belt base and the second into the lateral plate. Diferent tests were carried out to study the e!ect of transfer height, velocity, belt structure and padding on the acceleration values recorded by an instrumental sphere (IS 100). Results showed that transfer height and belt structure a!ect mainly impact values on the belt base, and padding a!ects mainly impact values registered for lateral contact. The elect of belt velocity in both impacts is less important when compared to the rest of the variables. Additionally, two powered transfer decelerators were tested at the same point with the aim of decreasing impacts su!ered by the fruit. Comparing impacts registered using these decelerators to those analysed in the first part of the study without decelerators, a high reduction of the impact values was observed.

Evaluation of angle-dependent absorption: Computer simulation and listening test for an objective and subjective evaluation

The aim of this thesis is the subjective and objective evaluation of angledependent absorption coefficients. As the assumption of a constant absorption coefficient over the angle of incidence is not always held, a new model acknowledging an angle-dependent reflection must be considered, to get a more accurate prediction in the sound field. The study provides information about the behavior of different materials in several rooms, depending on the reflection modeling of incident sound waves. An objective evaluation was run for an implementation of angle-dependent reflection factors in the image source and ray tracing simulation models. Results obtained were analysed after comparison to diffuse-field averaged data. However, changes in acoustic characteristics of a room do not always mean a variation in the listener’s perception. Thus, additional subjective evaluation allowed a comparison between the different results obtained with the computer simulation and the response from the individuals who participated in the listening test. The listening test was designed following a three-alternative forced-choice (3AFC) paradigm. In each interaction asked to the subjects a sequence of either three pink noise bursts or three natural signals was alternated. These results were supposed to show the influence and perception of the two different ways to implement surface reflection –either with diffuse or angle-dependent absorption properties. Results show slightly audible effects when material properties were exaggerated. El objetivo de este trabajo es la evaluación objetiva y subjetiva del coeficiente de absorción en función del ángulo de incidencia de la onda de sonido. La suposición de un coeficiente de absorción constante con respecto al ángulo de incidencia no siempre se sostiene. Por ello, un nuevo modelo considerando la reflexión dependiente del ángulo se debe tener en cuenta para obtener predicciones más certeras en el campo del sonido. El estudio proporciona información sobre el comportamiento de diferentes materiales en distintos recintos, dependientes del modelo de reflexión de las ondas de sonido incidentes. Debido a las dificultades a la hora de realizar las medidas y, por lo tanto, a la falta de datos, los coeficientes de absorción dependientes del ángulo a menudo no se tienen en cuenta a la hora de realizar las simulaciones. Hoy en día, aún no hay una tendencia de aplicar el coeficiente de absorción dependiente del ángulo para mejorar los modelos de reflexión. Por otra parte, para una medición satisfactoria de la absorción dependiente del ángulo, sólo hay unos pocos métodos. Las técnicas de medición actuales llevan mucho tiempo y hay algunos materiales, condiciones y ángulos que no pueden ser reproducidos y, por lo tanto, no es posible su medición. Sin embargo, en el presente estudio, los ángulos de incidencia de las ondas de sonido son conocidos y almacenados en una de base de datos para cada uno de los materiales, de modo que los coeficientes de absorción para el ángulo dado pueden ser devueltos siempre que sean requeridos por el usuario. Para realizar el estudio se llevó a cabo una evaluación objetiva, por medio de la implementación del factor de reflexión dependiente del ángulo en los modelos de fuentes imagen y trazado de rayos. Los resultados fueron analizados después de ser comparados con el promedio de los datos obtenidos en medidas en el campo difuso. La simulación se hizo una vez se configuraron un número de materiales creados por el autor, a partir de los datos existentes en la literatura y los catálogos de fabricantes. Los modelos de Komatsu y Mechel sirvieron como referencia para los materiales porosos, configurando la resistividad al aire o el grosor, y para los paneles perforados, introduciendo el radio de los orificios y la distancia entre centros, respectivamente. Estos materiales se situaban en la pared opuesta a la que se consideraba que debía alojar a la fuente sonora. El resto de superficies se modelaban con el mismo material, variando su coeficiente de absorción y/o de dispersión. Al mismo tiempo, una serie de recintos fueron modelados para poder reproducir distintos escenarios de los que obtener los resultados. Sin embargo, los cambios en las características acústicas de un recinto no significan variaciones en la percepción por parte del oyente. Por ello, una evaluación subjetiva adicional permitió una comparación entre los diferentes resultados obtenidos mediante la simulación informática y la respuesta de los individuos que participaron en la prueba de escucha. Ésta fue diseñada bajo las pautas del modelo de test three-alternative forced-choice (3AFC), con treinta y dos preguntas diferentes. En cada iteración los sujetos fueron preguntados por una secuencia alterna entre tres señales, siendo dos de ellas iguales. Éstas podían ser tanto ráfagas de ruido rosa como señales naturales, en este test se utilizó un fragmento de una obra clásica interpretada por un piano. Antes de contestar al cuestionario, los bloques de preguntas eran ordenados al azar. Para cada ensayo, la mezcla era diferente, así los sujetos no repetían la misma prueba, evitando un sesgo por efectos de aprendizaje. Los bloques se barajaban recordando siempre el orden inicial, para después almacenar los resultados reordenados. La prueba de escucha fue realizada por veintitrés personas, toda ellas con conocimientos dentro del campo de la acústica. Antes de llevar a cabo la prueba de escucha en un entorno adecuado, una hoja con las instrucciones fue facilitada a cada persona. Los resultados muestran la influencia y percepción de las dos maneras distintas de implementar las reflexiones de una superficie –ya sea con respecto a la propiedad de difusión o de absorción dependiente del ángulo de los materiales. Los resultados objetivos, después de ejecutar las simulaciones, muestran los datos medios obtenidos para comprender el comportamiento de distintos materiales de acuerdo con el modelo de reflexión utilizado en el caso de estudio. En las tablas proporcionadas en la memoria se muestran los valores del tiempo de reverberación, la claridad y el tiempo de caída temprana. Los datos de las características del recinto obtenidos en este análisis tienen una fuerte dependencia respecto al coeficiente de absorción de los diferentes materiales que recubren las superficies del cuarto. En los resultados subjetivos, la media de percepción, a la hora de distinguir las distintas señales, por parte de los sujetos, se situó significativamente por debajo del umbral marcado por el punto de inflexión de la función psicométrica. Sin embargo, es posible concluir que la mayoría de los individuos tienden a ser capaces de detectar alguna diferencia entre los estímulos presentados en el 3AFC test. En conclusión, la hipótesis de que los valores del coeficiente de absorción dependiente del ángulo difieren es contrastada. Pero la respuesta subjetiva de los individuos muestra que únicamente hay ligeras variaciones en la percepción si el coeficiente varía en intervalos pequeños entre los valores manejados en la simulación. Además, si los parámetros de los materiales acústicos no son exagerados, los sujetos no perciben ninguna variación. Los primeros resultados obtenidos, proporcionando información respecto a la dependencia del ángulo, llevan a una nueva consideración en el campo de la acústica, y en la realización de nuevos proyectos en el futuro. Para futuras líneas de investigación, las simulaciones se deberían realizar con distintos tipos de recintos, buscando escenarios con geometrías irregulares. También, la implementación de distintos materiales para obtener resultados más certeros. Otra de las fases de los futuros proyectos puede realizarse teniendo en cuenta el coeficiente de dispersión dependiente del ángulo de incidencia de la onda de sonido. En la parte de la evaluación subjetiva, realizar una serie de pruebas de escucha con distintos individuos, incluyendo personas sin una formación relacionada con la ingeniería acústica.

Influence of an upstream building on the wind-induced mean suction on the flat roof of a low-rise building

The effect of an upstream building on the suction forces on the flat roof of a low-rise building placed in the wake of the former is analyzed. The analysis has been performed by wind tunnel testing of a flat roof, low-rise building model equipped with pressure taps on the roof and different block-type buildings (only configurations where the upstream building is as high or higher than the downstream one are considered in this paper). The influence of the distance between both buildings on the wind loads on the downstream building roof is analyzed, as well as the height of the upstream one and the wind angle of incidence. Experimental results reveal that the wind load increases as the relative height of the upstream building increases, the wind load being highest for intermediate distances between buildings, when a passage between them is formed.

Optimization of energy extraction in transverse galloping

A numerical method to analyse the stability of transverse galloping based on experimental measurements, as an alternative method to polynomial fitting of the transverse force coefficient Cz, is proposed in this paper. The Glauert–Den Hartog criterion is used to determine the region of angles of attack (pitch angles) prone to present galloping. An analytic solution (based on a polynomial curve of Cz) is used to validate the method and to evaluate the discretization errors. Several bodies (of biconvex, D-shape and rhomboidal cross sections) have been tested in a wind tunnel and the stability of the galloping region has been analysed with the new method. An algorithm to determine the pitch angle of the body that allows the maximum value of the kinetic energy of the flow to be extracted is presented.

Development of Efficient Techniques for the Analysis and Design of Antennas in Dual-Reflectarray Configuration

This thesis contributes to the analysis and design of printed reflectarray antennas. The main part of the work is focused on the analysis of dual offset antennas comprising two reflectarray surfaces, one of them acts as sub-reflector and the second one acts as mainreflector. These configurations introduce additional complexity in several aspects respect to conventional dual offset reflectors, however they present a lot of degrees of freedom that can be used to improve the electrical performance of the antenna. The thesis is organized in four parts: the development of an analysis technique for dualreflectarray antennas, a preliminary validation of such methodology using equivalent reflector systems as reference antennas, a more rigorous validation of the software tool by manufacturing and testing a dual-reflectarray antenna demonstrator and the practical design of dual-reflectarray systems for some applications that show the potential of these kind of configurations to scan the beam and to generate contoured beams. In the first part, a general tool has been implemented to analyze high gain antennas which are constructed of two flat reflectarray structures. The classic reflectarray analysis based on MoM under local periodicity assumption is used for both sub and main reflectarrays, taking into account the incident angle on each reflectarray element. The incident field on the main reflectarray is computed taking into account the field radiated by all the elements on the sub-reflectarray.. Two approaches have been developed, one which employs a simple approximation to reduce the computer run time, and the other which does not, but offers in many cases, improved accuracy. The approximation is based on computing the reflected field on each element on the main reflectarray only once for all the fields radiated by the sub-reflectarray elements, assuming that the response will be the same because the only difference is a small variation on the angle of incidence. This approximation is very accurate when the reflectarray elements on the main reflectarray show a relatively small sensitivity to the angle of incidence. An extension of the analysis technique has been implemented to study dual-reflectarray antennas comprising a main reflectarray printed on a parabolic surface, or in general in a curved surface. In many applications of dual-reflectarray configurations, the reflectarray elements are in the near field of the feed-horn. To consider the near field radiated by the horn, the incident field on each reflectarray element is computed using a spherical mode expansion. In this region, the angles of incidence are moderately wide, and they are considered in the analysis of the reflectarray to better calculate the actual incident field on the sub-reflectarray elements. This technique increases the accuracy for the prediction of co- and cross-polar patterns and antenna gain respect to the case of using ideal feed models. In the second part, as a preliminary validation, the proposed analysis method has been used to design a dual-reflectarray antenna that emulates previous dual-reflector antennas in Ku and W-bands including a reflectarray as subreflector. The results for the dualreflectarray antenna compare very well with those of the parabolic reflector and reflectarray subreflector; radiation patterns, antenna gain and efficiency are practically the same when the main parabolic reflector is substituted by a flat reflectarray. The results show that the gain is only reduced by a few tenths of a dB as a result of the ohmic losses in the reflectarray. The phase adjustment on two surfaces provided by the dual-reflectarray configuration can be used to improve the antenna performance in some applications requiring multiple beams, beam scanning or shaped beams. Third, a very challenging dual-reflectarray antenna demonstrator has been designed, manufactured and tested for a more rigorous validation of the analysis technique presented. The proposed antenna configuration has the feed, the sub-reflectarray and the main-reflectarray in the near field one to each other, so that the conventional far field approximations are not suitable for the analysis of such antenna. This geometry is used as benchmarking for the proposed analysis tool in very stringent conditions. Some aspects of the proposed analysis technique that allow improving the accuracy of the analysis are also discussed. These improvements include a novel method to reduce the inherent cross polarization which is introduced mainly from grounded patch arrays. It has been checked that cross polarization in offset reflectarrays can be significantly reduced by properly adjusting the patch dimensions in the reflectarray in order to produce an overall cancellation of the cross-polarization. The dimensions of the patches are adjusted in order not only to provide the required phase-distribution to shape the beam, but also to exploit the crosses by zero of the cross-polarization components. The last part of the thesis deals with direct applications of the technique described. The technique presented is directly applicable to the design of contoured beam antennas for DBS applications, where the requirements of cross-polarisation are very stringent. The beam shaping is achieved by synthesithing the phase distribution on the main reflectarray while the sub-reflectarray emulates an equivalent hyperbolic subreflector. Dual-reflectarray antennas present also the ability to scan the beam over small angles about boresight. Two possible architectures for a Ku-band antenna are also described based on a dual planar reflectarray configuration that provides electronic beam scanning in a limited angular range. In the first architecture, the beam scanning is achieved by introducing a phase-control in the elements of the sub-reflectarray and the mainreflectarray is passive. A second alternative is also studied, in which the beam scanning is produced using 1-bit control on the main reflectarray, while a passive subreflectarray is designed to provide a large focal distance within a compact configuration. The system aims to develop a solution for bi-directional satellite links for emergency communications. In both proposed architectures, the objective is to provide a compact optics and simplicity to be folded and deployed.