Large-scale blockages like buildings affect the performance of urban cellular networks, especially in the millimeter-wave frequency band. Unfortunately, such blockage effects are either neglected or characterized by oversimplified models in the analysis of cellular networks. Leveraging concepts from random shape theory, this paper proposes a mathematical framework to model random blockages, and quantifies their effects on the performance of cellular networks. Specifically, random buildings are modeled as a process of rectangles with random sizes and orientations whose centers form a Poisson point process on the plane, which is called a Boolean scheme. The distribution of the number of blockages in a link is proven to be Poisson with parameter dependent on the length of the link, which leads to the distribution of penetration losses of a single link. A path loss model that incorporates the blockage effects is proposed, which matches experimental trends observed in prior work. The blockage model is applied to analyze blockage effects on cellular networks assuming blockages are impenetrable, in terms of connectivity, coverage probability, and average rate. Analytic results show while buildings may block the desired signal, they may still have a positive impact on network performance since they also block more interference. / text

Doctor of Philosophy / Department of Electrical and Computer Engineering / Balasubramaniam Natarajan / Triggered by the popularity of smart devices, wireless traffic volume and device connectivity have been growing exponentially during recent years. The next generation of wireless networks, i.e., 5G, is a promising solution to satisfy the increasing data demand through combination of key enabling technologies such as deployment of a high density of access points (APs), referred to as ultra-densification, and utilization of a large amount of bandwidth in millimeter wave (mmWave) bands. However, due to unfavorable propagation characteristics, this portion of spectrum has been under-utilized. As a solution, large antenna arrays that coherently direct the beams will help overcome the hostile characteristics of mmWave signals. Building networks of directional antennas has given rise to many challenges in wireless communication design. One of the main challenges is how to incorporate 5G technology into current networks and design uniform structures that bring about higher network performance and quality of service. In addition, the other factor that can be severely impacted is interference behavior. This is basically due to the fact that, narrow beams are highly vulnerable to obstacles in the environment. Motivated by these factors, the present dissertation addresses some key challenges associated with the utilization of mmWave signals. As a first step towards this objective, we first propose a framework of how 5G mmWave access points can be integrated into the current wireless structures and offer higher data rates. The related resource sharing problem has been also proposed and solved, within such a framework. Secondly, to better understand and quantify the interference behavior, we propose interference models for mmWave networks with directional beams for both large scale and finite-sized network dimension. The interference model is based on our proposed blockage model which captures the average number of obstacles that cause a complete link blockage, given a specific signal beamwidth. The main insight from our analysis shows that considering the effect of blockages leads to a different interference profile. Furthermore, we investigate how to model interference considering not only physical layer specifications but also upper layers constraints. In fact, upper network layers, such as medium access control (MAC) protocol controls the number of terminals transmitting simultaneously and how resources are shared among them, which in turn impacts the interference power level. An interesting result from this analysis is that, from the receiving terminal standpoint, even in mmWave networks with directional signals and high attenuation effects, we still need to maintain some sort of sensing where all terminals are not allowed to transmit their packets, simultaneously. The level of such sensing depends on the terminal density. Lastly, we provide a framework to detect the network regime and its relation to various key deployment parameters, leveraging the proposed interference and blockage models. Such regime detection is important from a network management and design perspective. Based on our finding, mmWave networks can exhibit either an interference-limited regime or a noise-limited regime, depending on various factors such as access point density, blockage density, signal beamwidth, etc.

CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / A constÃncia dos ventos brasileiros, e a necessidade de amenizar a demanda das grandes cidades encontraram na energia eÃlica uma forte parceira. Como uma alternativa na produÃÃo de energia elÃtrica, condomÃnios e prÃdios modernos, alÃm de algumas aplicaÃÃes rurais, tÃm recorrido Ãs turbinas eÃlicas de pequeno porte como uma alternativa para sanar suas necessidades. Contudo, a maioria da tecnologia encontrada no mercado à importada e nÃo foi desenvolvida exclusivamente para aplicaÃÃes no Brasil. A ferramenta mais importante na aerodinÃmica experimental à o tÃnel de vento. Experimentos controlados em escala fornecem um grande nÃmero de dados confiÃveis, alÃm de fornecer seguranÃa a quem o manuseia. Dada a sua importÃncia, este trabalho tem como objetivo o desenvolvimento, a prototipagem, e conhecer o comportamento de um rotor de turbina eÃlica em escala sujeito a testes em tÃnel de vento e comparar os resultados obtidos com os presentes na literatura para um protÃtipo em escala real. Para tanto, faz necessÃrio o uso de tÃcnicas de correÃÃo de efeitos de bloqueio de tÃnel de vento. Foi projetado e fabricado 4 conjuntos de rotores com valores de velocidade especÃfica de ponta de 6 atà 9 (λp=6, 7, 8 e 9). Os testes foram conduzidos em um tÃnel de vento onde foram coletados dados de velocidade de escoamento livre, velocidade de escoamento com a turbina em operaÃÃo, alÃm de medidas de velocidade angular e torque gerado pelas pÃs, com a finalidade de conhecer a curva de potÃncia de cada rotor. Foi observado que em situaÃÃes de escoamento em que as rotaÃÃes nÃo sejam representativas (o suficiente para atingir valores superiores ao intervalo de λ entre 3 e 5,6), indica-se um projeto com λp=6. Jà em situaÃÃes nas quais os valores de λ oscilam entre 4,7 e 7,3, λp=7 mostrou-se mais eficiente. Jà λp=9 mostrou-se nÃo vantajoso em comparaÃÃo aos demais projetos. Ao comparar os dados obtidos neste trabalho com os da literatura e do BEM, pode-se afirmar que o estudo de turbinas eÃlicas em tÃnel de vento à bastante confiÃvel. / The constancy of Brazilian winds and the need to mitigate the demand of large cities have found in wind energy a strong partner. As an alternative for the production of electrical power, modern buildings and condominiums, plus some rural applications, have resorted to small wind turbines as an alternative to solve your needs. However, most of the technology found in the market is imported and has not been developed exclusively for applications in Brazil. The most important tool in experimental aerodynamics is the wind tunnel. Scale controlled experiments provide a large number of reliable data, besides providing security to those who handle. Given its importance, this paper aims at the development, prototyping, and understands the behavior of a wind turbine rotor scale subjected to wind tunnel tests and compares the results with those in the literature for a prototype scale real. Therefore, it required the use of correction techniques blockage effects of the wind tunnel. It was designed and manufactured 4 sets of rotors with values specific tip speed of 6 to 9 (λp=6, 7, 8 e 9). The tests were conducted in a wind tunnel where velocity data free stream, stream velocity with the turbine in operation, and angular speed and torque generated by the blades, was collected in order to know the curve of each rotor. It was observed that in situations where the flow speeds are not representative enough to reach the higher values of λ range between 3 and 5,6, indicates a design with λp=6. Already in situations where the values of λ ranging between 4,7 and 7,3, λp=7 proved to be more efficient. Have λp=9 proved to be no advantage in comparison to other projects. By comparing the data obtained in this work with the literature and the BEM, it can be stated that the study of wind turbines in a wind tunnel is quite reliable.

No description available.

Yes / Stability and safety of offshore wind turbines with mono-pile foundations, affected by nonlinear wave effect and dynamic seabed response, are the primary concerns in offshore foundation design. In order to address these problems, the nonlinear wave effect on dynamic seabed response in the vicinity of mono-pile foundation is investigated using an integrated model, developed using OpenFOAM, which incorporates both wave model (waves2Foam) and Biot’s poro-elastic model. The present model was validated against several laboratory experiments and promising agreements were obtained. Special attention was paid to the systematic analysis of pore water pressure as well as the momentary liquefaction in the proximity of mono-pile induced by nonlinear wave effects. Various embedment depths of mono-pile relevant for practical engineering design were studied in order to attain the insights into nonlinear wave effect around and underneath the mono-pile foundation. By comparing time-series of water surface elevation, inline force, and wave-induced pore water pressure at the front, lateral, and lee side of mono-pile, the distinct nonlinear wave effect on pore water pressure was shown. Simulated results confirmed that the presence of mono-pile foundation in a porous seabed had evident blocking effect on the vertical and horizontal development of pore water pressure. Increasing embedment depth enhances the blockage of vertical pore pressure development and hence results in somewhat reduced momentary liquefaction depth of the soil around the mono-pile foundation. / Energy Technology Partnership (ETP), Wood Group Kenny, and University of Aberdeen; the National Science Fund for Distinguished Young Scholars (51425901) and the 111 project (B12032).

The research in this thesis focuses on the investigation of tidal turbines using a small scale horizontal axis tidal turbine and a 2D hydrofoil testing rig, combining experiments with simulations to provide comprehensive results and to better understand some of the variables that affect their performance. The experimental campaigns were carried out at the University of Victoria fluids research lab and the Sustainable Systems Design Lab (SSDL). The experimental testing rigs were re-designed by the author and are now fully automated, including a friendly graphical user interface for easy implementation. Particle image velocimetry (PIV) technique was used as the quantitative flow visualization method to obtain the time-averaged flow fields. This thesis presents three investigations. The first study aims to quantify the impacts of channel blockage, free surface effects and foundations on hydrokinetic turbine performance, using porous discs and an axial flow rotor. The results were used to cross-validate computational fluid dynamics (CFD) simulations. It was found that as wall blockage increases, thrust and power are incremented with and without the inclusion of free surface deformation. Discrepancies between simulations and experimental results on free surface effects compared to a slip wall were obtained and hence further research is recommended and the author gives some advice on how to proceed in this investigation. The second study determines the performance of four hydrofoil candidates over a range of low Reynolds number (Re), delivering useful information that can be applied to low Re energy conversion systems and, specifically in this case, to improve the performance of the small scale tidal turbine at the SSDL lab. The study combines the 2D hydrofoil test rig along with PIV measurements in order to experimentally obtain lift and drag coefficients. The experiments were carried out in the recirculating flume tank over the range of low Re expected for the small scale rotor rig, in order to provide more accurate results to improve rotor blade design. In addition, numerical simulations using XFOIL, a viscid-inviscid coupled method, were introduced to the study. These results were analysed against experiments to find the most suitable parameters for reliable performance prediction. The final results suggested that adding a numerical trip at a certain chordwise distance produced more reliable results. Finally, an experimental study on turbine rotor performance and tip vortex behavior was performed using again the rotor rig and PIV. Blade design and rotor performance were assessed, showing good agreement with Blade Element Momentum (BEM) simulations, particularly at predicting the tip speed ratio corresponding to the maximum power coefficient point. Regarding the wake structure, tip vortex locations (shed from the blade tips) were captured using PIV in the near wake region, showing evidence of wake expansion. The velocity and vorticity fields are also provided to contribute to the development and validation of CFD and potential flow codes. / Graduate / 0548 / 0547 / 0538 / iafranch@uvic.ca

Abstact: In order to address the wind-industry's need for a new generation of more advanced wake models, which accurately quantify the mean flow characteristics within a reasonably CPU-time, the two-dimensional analytical approach by Belcher et al. (2003) has been extended to a three-dimensional wake model. Hereby, the boundary-layer approximation of the Navier-Stokes equations has been linearized around an undisturbed baseflow, assuming that the wind turbines provoke a small perturbation of the velocity field. The conducted linearization of the well established actuator-disc theory brought valuable additional insights that could be used to understand the behavior (as well as the limitations) of a model based on linear methods. Hereby, one of the results was that an adjustment of the thrust coecient is necessary in order to get the same wake-velocity field within the used linear framework. In this thesis, two different datasets from experiments conducted in two different wind-tunnel facilities were used in order to validate the proposed model against wind-farm and single-turbine cases. The developed model is, in contrary to current engineering wake models, able to account for effects occurring in the upstream flow region. The measurement, as well as the simulations, show that the presence of a wind farm affects the approaching flow even far upstream of the first turbine row, which is not considered in current industrial guidelines. Despite the model assumptions, several velocity statistics above wind farms have been properly estimated, providing insight about the transfer of momentum inside the turbine rows. Overall, a promising preliminary version of a wake model is introduced, which can be extended arbitrarily depending on the regarded purpose.

Anthropogenic climate change is approaching predicted tipping points and there is an urgent need to de-carbonize energy systems on a global scale. Generation technologies that do not emit greenhouse gas need to be rapidly deployed, and energy grids need to be updated to accommodate an intermittent fluctuating supply. Rapidly advancing battery technology, cost reduction of solar and wind power and other emerging generation technologies are making the needed changes technically and economically feasible. Extracting energy from fast-flowing tidal currents using turbines akin to those used in wind farms, offers a reliable and predictable source of GHG free energy. The tidal power industry has established the technical feasibility of tidal turbines, and is presently up-scaling deployments from single isolated units to large tidal farms containing many turbines. However there remains significant economic uncertainty in financing such projects, partially due to uncertainty in predicting the long-term energy yield. Since energy yield is used in calculating the project revenue, it is of critical importance. Predicting yield for a prospective farm has not received sufficient attention in the tidal power literature. this task has been the primary motivation for this thesis work, which focuses on establishing and validating simulation-based procedures to predict flows through large tidal farms with many turbines, including the back effects of the turbines. This is a challenging problem because large tidal farms may alter tidal flows on large scales, and the slow-moving wake downstream of each rotor influences the inflow to other rotors, influencing their performance and loading. Additionally, tidal flow variation on diurnal and monthly timescales requires long-duration analysis to obtain meaningful statistics that can be used for forecasting. This thesis presents a hybrid simulation method that uses 2D coastal flow simulations to predict tidal flows over long durations, including the influence of turbines, combined with higher-resolution 3D simulations to predict how wakes and local bathymetry influence the power of each turbine in a tidal farm. The two simulation types are coupled using a method of bins to reduce the computational cost within reasonable limits. The method can be used to compute detailed 3D flow fields, power and loading on each turbine in the farm, energy yield and the impact of the farm on tidal amplitude and phase. The method is demonstrated to be computationally tractable with modest high-performance computing resources and therefore are of immediate value for informing turbine placement, comparing turbine farm-layout cases and forecasting yield, and may be implemented in future automated layout optimization algorithms. / Graduate