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Postprint (published version

Estimation of wall shear stress using 4D flow cardiovascular MRI and computational fluid dynamics

Electronic version of an article published as Journal of mechanics in medicine and biology, 0, 1750046 (2016), 16 pages. DOI:10.1142/S0219519417500464 © World Scientific Publishing CompanyIn the last few years, wall shear stress (WSS) has arisen as a new diagnostic indicator in patients with arterial disease. There is a substantial evidence that the WSS plays a significant role, together with hemodynamic indicators, in initiation and progression of the vascular diseases. Estimation of WSS values, therefore, may be of clinical significance and the methods employed for its measurement are crucial for clinical community. Recently, four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has been widely used in a number of applications for visualization and quantification of blood flow, and although the sensitivity to blood flow measurement has increased, it is not yet able to provide an accurate three-dimensional (3D) WSS distribution. The aim of this work is to evaluate the aortic blood flow features and the associated WSS by the combination of 4D flow cardiovascular magnetic resonance (4D CMR) and computational fluid dynamics technique. In particular, in this work, we used the 4D CMR to obtain the spatial domain and the boundary conditions needed to estimate the WSS within the entire thoracic aorta using computational fluid dynamics. Similar WSS distributions were found for cases simulated. A sensitivity analysis was done to check the accuracy of the method. 4D CMR begins to be a reliable tool to estimate the WSS within the entire thoracic aorta using computational fluid dynamics. The combination of both techniques may provide the ideal tool to help tackle these and other problems related to wall shear estimation.Peer ReviewedPostprint (author's final draft

Distributed time-critical coordination strategies for unmanned aerial systems in cluttered environments

This thesis addresses the problem of cooperative motion planning and control for a group of cooperating unmanned aerial systems through cluttered and uncertain environments, subject to a broad range of coordination and temporal constraints. The proposed solution expands the type of time-critical missions that can be automated using cooperative motion control frameworks. This work introduces the use of novel geometric queries to aid a sample-based motion-planning algorithm guide the growth of a rapidly-exploring random tree through the narrow passages in cluttered and uncertain scenarios. To this effect, specific silhouette and tolerance verification queries are designed for the geometric objects that represent vehicle motion and environmental obstacles. The combination of the silhouette-informed path planner with a CNC-inspired path-smoothing method, and a centralized cooperative speed-assignment algorithm yields a set of C2 continuous trajectories that maintain safe separation with all uncertain obstacles and cooperating peers, meet desired mission constraints, and satisfy a set of simplified dynamic constraints. The vehicles are then tasked to follow their assigned paths and coordinate online to meet mission objectives, desired inter-agent spacing constraints, and temporal constraints—such as a time of arrival or a window of arrival. The thesis introduces two types of inter-agent spacing constraints—tight and loose coordination—and three types of temporal constraints—unenforced, relaxed, and strict—that result in six general time-critical coordination strategies. This thesis presents six distributed coordination protocols to enforce this range of constraints. These coordination protocols rely on a lossy communication network that can be disconnected pointwise in time at all times, but is connected in an integral sense over a sliding temporal window. This work derives transient and steady-state performance bounds for the tight coordination protocols. Simulation results through a cluttered urban-like environment, where vehicles are subject to wind disturbances, corroborate the theoretical results

Desarrollo y análisis de un generador de inducción de doble devanado en el estátor aplicado en sistemas eólicos de velocidad variable

El present treball de tesi doctoral tracta sobre la utilització d'una màquina asíncrona, amb un rotor de gàbia d'esquirol estàndard i un estator amb dos debanaments independents amb diferent nombre de pols, per a l'obtenció d'energia elèctrica a partir d'un sistema motriu accionat per una turbina eòlica. S'ha desenvolupat un prototip i el model matemàtic, s'han realitzat simulacions i assajos experimentals per demostrar la validesa del generador. Així mateix, s'ha realitzat el disseny de l'etapa de control de la màquina. En el generador s'utilitza un dels debanats per a controlar la producció d'energia elèctrica en règim de baixa velocitat a través d'un convertidor bidireccional, el qual pot estar connectat a un bus de contínua o a la xarxa elèctrica. L'altre debanat proporciona directament l'energia elèctrica a la xarxa, de forma conjunta al primer, a partir d'un cert valor de velocitat. Aconseguint-se una òptima eficiència per a diferents condicions de velocitat del vent i una bona qualitat de l'energia elèctrica subministrada. És la principal novetat d'aquest treball, la utilització d'aquest tipus de màquina per a l'aprofitament de l'energia eòlica en un rang de velocitat variable, amb la utilització d'un control senzill i en llaç obert en el control del convertidor del costat del generador.El presente trabajo de tesis doctoral trata sobre la utilización de una máquina asíncrona, con un rotor de jaula de ardilla estándar y un estátor con dos devanados independientes con diferente número de polos, para la obtención de energía eléctrica a partir de un sistema motriz accionado por una turbina eólica. Se ha desarrollado un prototipo y el modelo matemático, se han realizado simulaciones y ensayos experimentales para demostrar la validez del generador. Así mismo, se ha realizado el diseño de la etapa de control de la máquina. En el generador se utiliza uno de los devanados para controlar la producción de energía eléctrica en régimen de baja velocidad a través de un convertidor bidireccional, el cual puede estar conectado a un bus de continua o a la red eléctrica. El otro devanado proporciona directamente la energía eléctrica a la red, de forma conjunta al primero, a partir de un cierto valor de velocidad. Lográndose una óptima eficiencia para diferentes condiciones de velocidad del viento y calidad en el suministro de energía eléctrica. Siendo la principal novedad del presente trabajo, la utilización de este tipo de máquina para el aprovechamiento de la energía eólica en un rango de velocidad variable, con la utilización de un control sencillo y en lazo abierto en el control del convertidor del lado del generador.The present doctoral dissertation deals with the use of an asynchronous machine, with a standard rotor squirrel cage and a stator with two independent windings with a different number of poles, for obtaining electrical energy from a motor system powered by a wind turbine. We have developed a prototype and the mathematical models; we have performed simulations and have undertaken experimental tests to demonstrate the effectiveness of the generator. We have also designed the machine control stage. In the generator, one of the windings is used to control the production of electricity in a low-speed regime through a bidirectional converter, which can be connected to a DC bus or to the mains. The other winding provides power directly to the mains, together with the previous winding, when reaching a given speed value, thereby achieving optimum efficiency for different wind speed conditions and quality in the power supply. The main new development is the use of this type of machine for the exploitation of wind energy in a variable speed range with the use of a simple open-loop control