Modelización numérica de emisores puntuales de contaminación atmosférica. Aplicación a La Oroya (Perú)

Este trabajo se centra en el análisis comparado de diferentes métodos numéricos para la resolución de problemas de calidad del aire cerca de grandes emisores puntuales. Es un tema de elevado interés científico, por su complejidad y relación con la calidad ambiental. La simulación numérica es una herramienta potente para estimar si las concentraciones de contaminantes respeta o no los límites de inmisión previstos en las normativas. Los fenómenos que intervienen en el proceso de emisión de contaminantes en la atmosfera son convección, difusión, reacción entre las especies y el carácter transitorio del problema. Estos procesos dan lugar a unas ecuaciones diferenciales parabólicas que pueden ser modelizadas mediante el método de los elementos finitos. Se ha realizado un estudio detallado, en principio, de un modelo simplificado en dos dimensiones con dominios pequeños (de 400 a 2000 elementos) para estudiar, a continuación, el caso tridimensional de La Oroya (Perú) con un dominio de 17 x 23 Km2 y alrededor de 1500000 elementos. De este caso de referencia se disponen de datos meteorológicos y de inmisión en superficie. Para los ejemplos 2D ha sido utilizado un código MATLAB, mientras en el 3D un código FORTRAN. La modelización del problema incluye la interpolación del campo de viento, la sobreelevación del penacho, discretización del dominio y la resolución de las ecuaciones diferenciales. Se ha analizado la influencia de los valores de difusión vertical y horizontal, con distintas mallas de elementos finitos, realizadas por Albert Oliver Serra en el marco del proyecto “Modelos numéricos predictores para gestión medio-ambiental", Ministerio de Ciencia e Innovación, CGL2008-06003-C03-02. Finalmente se ha modificado el código FORTRAN en formulación estabilizada LS (Least-squares) por SUPG (Streamline-upwind Petrov-Galerkin stabilization) simetrizado. De los resultados numéricos presentados se concluye que los modelos pueden describir adecuadamente el problema de calidad del aire cerca de grandes emisores puntuales, en condiciones de baja difusión vertical. Las formulaciones estabilizadas de elementos finitos con discretizaciones espaciales de precisión adecuada, pueden simular la formación de capas de aire limpio bajo capas de aire contaminado en condiciones de alta estabilidad atmosférica

Overlapping two standing-waves in a microcavity for a multi-atom photon interface

We develop a light-matter interface enabling strong and uniform coupling between a chain of cold atoms and photons of an optical cavity. This interface is a fiber Fabry-Perot cavity, doubly resonant for both the wavelength of the atomic transition and for a geometrically commensurate red-detuned intracavity trapping lattice. Fulfilling the condition of a strong and uniform atom-photon coupling requires optimization of the spatial overlap between the two standing waves in the cavity. In a strong-coupling cavity, where the mode waists and Rayleigh range are small, we derive the expression of the optimal trapping wavelength taking into account the Gouy phase. The main parameter controlling the overlap of the standing waves is the relative phase shift at the reflection on the cavity mirrors between the two wavelengths, for which we derive the optimal value. We have built a microcavity optimized according to these results, employing custom-made mirrors with engineered reflection phase for both wavelengths. We present a method to measure with high precision the relative phase shift at reflection, which allows us to determine the spatial overlap of the two modes in this cavity.Comment: 14 pages, 7 figure

Influence of the excitation force estimator methodology within a predictive controller framework on the overall cost of energy minimisation of a wave energy converter

A large amount of energy is freely roaming around the world each day, without us being able to exploit it: wave energy is a largely untapped source of renewable energy, which can have a substantial influence in the future energy mix. The reason behind the inability of using this free resource is linked to the cost of the energy (CoE) produced from the different wave energy converters (WEC). The CoE from the different WECs is not yet comparable with other energy resources, due to a relative low efficiency coupled with the high structural costs. Within the sector a large effort has been addressed to optimize the WEC efficiency by means of different control strategies. In several articles [1, 2], it has been shown that with simple modifications of the control law, the absorbed energy can be doubled or quadrupled. Whilst the improvement of the efficiency will increase the revenue of the machine, the application of an advance control strategy will most probably increase the loads exerted on the structure, leading to an increment of the structural cost. Therefore, the problem of minimising the CoE produced by a WEC is at least a 2D problem. In a previous article [3], the minimisation problem has been investigated with a sequential approach, and the results have been reported for different control strategies. The Model Predictive Controller (MPC) seemed to have superior performance in terms of energy maximisation and loads on the structure, leading to a minimal CoE. But as presented in [3] the MPC was implemeted with perfect knowledge of the future load time series, which is physically not achivable. This article is an extension of the work presented in [3] with a closer focus on the influence of the excitation force prediction on the capability of the MPC architecture. Different estimator models of the excitation force time series are benchmarked, and validated with laboratory results