Hybrid foundations for offshore wind turbines

Nowadays offshore wind turbines represents a valid answer for energy production but with an increasing in costs mainly due to foundation technology required. Hybrid foundations composed by suction caissons over which is welded a tower supporting the nacelle and the blades allows a strong costs reduction. Here a monopod configuration is studied in a sandy soil in a 10 m water depth. Bearing capacity, sliding resistance and pull-out resistance are evaluated. In a second part the installation process occurring in four steps is analysed. considering also the effect of stress enhancement due to frictional forces opposing to penetration growing at skirt sides both inside and outside. In a three dimensional finite element model using Straus7 the soil non-linearity is considered in an approximate way through an iterative procedure using the Yokota empirical decay curves.

Stabilizzazione del Blu di Prussia con film politiofenici depositati per via elettrochimica

Hydrogen peroxide (H2O2) is a powerful oxidant which is commonly used in a wide range of applications in the industrial field. Several methods for the quantification of H2O2 have been developed. Among them, electrochemical methods exploit the ability of some hexacyanoferrates (such as Prussian Blue) to detect H2O2 at potentials close to 0.0 V (vs. SCE) avoiding the occurrence of secondary reactions, which are likely to run at large overpotentials. This electrocatalytic behaviour makes hexacyanoferrates excellent redox mediators. When deposited in the form of thin films on the electrode surfaces, they can be employed in the fabrication of sensors and biosensors, normally operated in solutions at pH values close to physiological ones. As hexacyanoferrates show limited stability in not strongly acidic solutions, it is necessary to improve the configuration of the modified electrodes to increase the stability of the films. In this thesis work, organic conducting polymers were used to fabricate composite films with Prussian Blue (PB) to be electro-deposited on Pt surfaces, in order to increase their pH stability. Different electrode configurations and different methods of synthesis of both components were tested, and for each one the achievement of a possible increase in the operational stability of Prussian Blue was verified. Good results were obtained for the polymer 3,3''-didodecyl-2,2':5',2''-terthiophene (poly(3,3''-DDTT)), whose presence created a favourable microenvironment for the electrodeposition of Prussian Blue. The electrochemical behaviour of the modified electrodes was studied in both aqueous and organic solutions. Poly(3,3''-DDTT) showed no response in aqueous solution in the potential range where PB is electroactive, thus in buffered aqueous solution is was possible to characterize the composite material, focusing only on the redox behaviour of PB. A combined effect of anion and cation of the supporting electrolyte was noticed. The response of Pt electrodes modified with films of the PB /poly(3,3''-DDTT) composite was evaluated for the determination of H2O2. The performance of such films was found better than that of the PB alone. It can be concluded that poly(3,3''-DDTT) plays a key role in the stabilization of Prussian Blue causing also a wider linearity range for the electrocatalytic response to H2O2.

Query rewriting in information retrieval: automatic context extraction from local user documents to improve query results

The central objective of research in Information Retrieval (IR) is to discover new techniques to retrieve relevant information in order to satisfy an Information Need. The Information Need is satisfied when relevant information can be provided to the user. In IR, relevance is a fundamental concept which has changed over time, from popular to personal, i.e., what was considered relevant before was information for the whole population, but what is considered relevant now is specific information for each user. Hence, there is a need to connect the behavior of the system to the condition of a particular person and his social context; thereby an interdisciplinary sector called Human-Centered Computing was born. For the modern search engine, the information extracted for the individual user is crucial. According to the Personalized Search (PS), two different techniques are necessary to personalize a search: contextualization (interconnected conditions that occur in an activity), and individualization (characteristics that distinguish an individual). This movement of focus to the individual's need undermines the rigid linearity of the classical model overtaken the ``berry picking'' model which explains that the terms change thanks to the informational feedback received from the search activity introducing the concept of evolution of search terms. The development of Information Foraging theory, which observed the correlations between animal foraging and human information foraging, also contributed to this transformation through attempts to optimize the cost-benefit ratio. This thesis arose from the need to satisfy human individuality when searching for information, and it develops a synergistic collaboration between the frontiers of technological innovation and the recent advances in IR. The search method developed exploits what is relevant for the user by changing radically the way in which an Information Need is expressed, because now it is expressed through the generation of the query and its own context. As a matter of fact the method was born under the pretense to improve the quality of search by rewriting the query based on the contexts automatically generated from a local knowledge base. Furthermore, the idea of optimizing each IR system has led to develop it as a middleware of interaction between the user and the IR system. Thereby the system has just two possible actions: rewriting the query, and reordering the result. Equivalent actions to the approach was described from the PS that generally exploits information derived from analysis of user behavior, while the proposed approach exploits knowledge provided by the user. The thesis went further to generate a novel method for an assessment procedure, according to the "Cranfield paradigm", in order to evaluate this type of IR systems. The results achieved are interesting considering both the effectiveness achieved and the innovative approach undertaken together with the several applications inspired using a local knowledge base.

Mac protocols for linear wireless (sensor) networks

Wireless sensor networks (WSNs) consist of a large number of sensor nodes, characterized by low power constraint, limited transmission range and limited computational capabilities [1][2].The cost of these devices is constantly decreasing, making it possible to use a large number of sensor devices in a wide array of commercial, environmental, military, and healthcare fields. Some of these applications involve placing the sensors evenly spaced on a straight line for example in roads, bridges, tunnels, water catchments and water pipelines, city drainages, oil and gas pipelines etc., making a special class of these networks which we define as a Linear Wireless Network (LWN). In LWNs, data transmission happens hop by hop from the source to the destination, through a route composed of multiple relays. The peculiarity of the topology of LWNs, motivates the design of specialized protocols, taking advantage of the linearity of such networks, in order to increase reliability, communication efficiency, energy savings, network lifetime and to minimize the end-to-end delay [3]. In this thesis a novel contention based Medium Access Control (MAC) protocol called L-CSMA, specifically devised for LWNs is presented. The basic idea of L-CSMA is to assign different priorities to nodes based on their position along the line. The priority is assigned in terms of sensing duration, whereby nodes closer to the destination are assigned shorter sensing time compared to the rest of the nodes and hence higher priority. This mechanism speeds up the transmission of packets which are already in the path, making transmission flow more efficient. Using NS-3 simulator, the performance of L-CSMA in terms of packets success rate, that is, the percentage of packets that reach destination, and throughput are compared with that of IEEE 802.15.4 MAC protocol, de-facto standard for wireless sensor networks. In general, L-CSMA outperforms the IEEE 802.15.4 MAC protocol.

Implementazione su fpga di un pre-distorsore digitale a banda larga per sistemi a microonde di backhaul

The development of next generation microwave technology for backhauling systems is driven by an increasing capacity demand. In order to provide higher data rates and throughputs over a point-to-point link, a cost-effective performance improvement is enabled by an enhanced energy-efficiency of the transmit power amplification stage, whereas a combination of spectrally efficient modulation formats and wider bandwidths is supported by amplifiers that fulfil strict constraints in terms of linearity. An optimal trade-off between these conflicting requirements can be achieved by resorting to flexible digital signal processing techniques at baseband. In such a scenario, the adaptive digital pre-distortion is a well-known linearization method, that comes up to be a potentially widely-used solution since it can be easily integrated into base stations. Its operation can effectively compensate for the inter-modulation distortion introduced by the power amplifier, keeping up with the frequency-dependent time-varying behaviour of the relative nonlinear characteristic. In particular, the impact of the memory effects become more relevant and their equalisation become more challenging as the input discrete signal feature a wider bandwidth and a faster envelope to pre-distort. This thesis project involves the research, design and simulation a pre-distorter implementation at RTL based on a novel polyphase architecture, which makes it capable of operating over very wideband signals at a sampling rate that complies with the actual available clock speed of current digital devices. The motivation behind this structure is to carry out a feasible pre-distortion for the multi-band spectrally efficient complex signals carrying multiple channels that are going to be transmitted in near future high capacity and reliability microwave backhaul links.