Numerical simulations of the Navier-Stokes equations are conducted to achieve a better understanding of the behavior of wakes generated by the wind turbines. The simulations are performed by combining the in-house developed code EllipSys3D with the actuator line technique. In step one of the project, a numerical study is carried out focusing on the instability onset of the trailing tip vortices shed from a 3-bladed wind turbine. To determine the critical frequency, the wake is perturbed using low-amplitude excitations located near the tip spirals. Two basic flow cases are studied; symmetric and asymmetric setups. In the symmetric setup a 120 degree flow symmetry condition is dictated due to the confining the polar computational grid to 120 degree or introducing identical excitations. In the asymmetric setup, uncorrelated excitations are imposed near the tip of the blades. Both setups are analyzed using proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). By analysing the dominant modes, it was found that in the symmetric setup the amplification of specific waves (traveling structures) traveling along the tip vortex spirals is responsible for triggering the instability leading to wake breakdown, while by breaking the symmetry almost all the modes are involved in the tip vortex destabilization. The presence of unstable modes in the wake is related to the mutual inductance (vortex pairing) instability where there is an out-of-phase displacement of successive helix turns. Furthermore, using the non-dimensional growth rate, it is found that the mutual inductance instability has a universal growth rate equal to Π/2. Using this relationship, and the assumption that breakdown to turbulence occurs once a vortex has experienced sufficient growth, an analytical relationship is provided for determining the length of the stable wake. This expression shows that the stable wake length is inversely proportional to thrust, tip speed ratio and the logarithmic of the turbulence intensity. In second study, large eddy simulations of the Navier-Stokes equations are also performed to investigate the wake interaction. Previous actuator line simulations on the single model wind turbine show that the accuracy of the results is directly related to the quality of the input airfoil characteristics. Therefore, a series of experiments on a 2D wing are conducted to obtain high quality airfoil characteristics for the NREL S826 at low Reynolds numbers. The new measured data are used to compute the rotor performance. The results show that the power performance as well as the wake development behind the rotor are well-captured. There are, however, some difficulties in prediction of the thrust coefficients. The continuation of this work considers the wake interaction investigations of two turbines inline (full-wake interaction) and two turbines with spanwise offset (half wake interaction). It is demonstrated that the numerical computations are able to predict the rotor performances as well as the flow field around the model rotors, and it can be a suitable tool for investigation of the wind turbine wakes. In the last study, an evaluation of the performance and near-wake structure of an analytical vortex model is presented. The vortex model is based on the constant circulation along the blades (Joukowsky rotor) and it is able to determine the geometry of the tip vortex filament in the rotor wake, allowing the free wake expansion and changing the local tip vortex pitch. Two different wind turbines have been simulated: a wind turbine with constant circulation along the blade and the other setup with a realistic circulation distribution, to compare the outcomes of the vortex model with real operative wind turbine conditions. The vortex model is compared with the actuator line approach and the presented comparisons show that the vortex method is able to approximate the single rotor performance and qualitatively describe the flow field around the wind turbine but with a negligible computational effort. This suggests that the vortex model can be a substitute of more computationally-demanding methods like actuator line technique to study the near-wake behavior. / <p>QC 20141010</p>

This thesis focuses on modelling of wind turbine wakes when they are affected by real complex terrain features, such as hills and forests, and also examines the effect of the rotational momentum imparted to the downstream wake from the rotor blades. Modelling work is carried out using the commercial Computational Fluid Dynamics (CFD) solver FLUENT. Motivation for this project was the fact that there is currently limited knowledge on several issues that affect the operation of a wind farm in a complex terrain environment. Wind developers normally use commercial, easy-to-use software (such as WAsP) to predict the potential wind farm output , which are based on simple linear models to model wakes and wind flow orographic effects and have been calibrated for cases of simple terrain. In cases of complex terrain, they are expected to give errors due to arising non-linearities. After a review of the relevant literature, the chosen CFD procedure is explained. This involves the use of 3-D Reynolds Averaged Navier-Stokes equations using the Reynolds Stress Model for the turbulence closure, in order to account for the anisotropy in atmospheric turbulence. The Virtual Blade Model in FLUENT is demonstrated as a useful tool for modelling the rotor effects without the need of meshing the rotor geometry in detail and avoiding significant computational cost. The approach is initially validated with the widely documented Nibe measurements, which involved full-scale observations of a single wake over at terrain. The model is also tested in the case of a wind turbine operating at the summit of an ideal, Gaussian hill. The wake development is examined in detail and in comparison with another CFD approach. Most notably, a slight divergence is found in the wake path as it evolves downwind. Additionally, the proposed approaches of modelling the neutral atmospheric ow over a real hill and over a forest are validated with full-scale measurements. Ultimately, the work includes the modelling of real wind farms over complex terrain and validating the results with measurements. A coastal complex terrain wind farm is initially examined and results are validated with SCADA measurements and compared with results using the WAsP wind modelling software. Finally, a wind farm over hilly terrain and near forests is also considered and the effect of the forest in the wake is studied. Results are also validated with full-scale measurements.

Over the last few decades, wind power has shown a continuous and significant developmentin the energy market globally. Having reached a certain level in both technologyand in dimensions, the role of optimizing wind turbines as well as wind farms hasbecome an additional aspect to future development and research. Since turbine wakescan cause significant power deficits within a farm, research in this area has the potentialfor large improvements in wind farm design. A wake is described as the downstream flow behind the rotor of an operating windturbine. The two main characteristics of wakes are a velocity (momentum) deficit and anincreased turbulence level. The velocity deficit behind the upwind turbine results in apower loss of the downstream turbines, whereas the higher turbulence causes additionalloads on the downstream turbines’ structures resulting in fatigue problems. The study of wakes is a complex topic, they are influenced by an interconnection of anumber of parameters like ambient wind speed and turbulence, atmospheric stabilityconditions (stable, unstable, and neutral), the turbines’ operational characteristics, andthe terrain properties. In order to assess the power deficits affected by wake interaction between turbines,an analysis can be realized by processing SCADA data of turbines in a wind farm. The collected data is treated by a comprehensive filtration process, excluding events of icing, curtailment, faults, etc. and by grouping into different atmospheric conditions, windspeed intervals and wind speed sectors. Finally, power deficit values, as a function ofwind direction, are calculated and quantified, and thereafter analyzed to assess the wakebehavior at different conditions for different cases.In this thesis, the wake-induced power deficit has been investigated in a specificstudy case for three pairs of two neighboring turbines in a forested moderately complexterrain using SCADA data. The production losses amounted between the range of 32% to 67% for the specific site with turbine spacing around 4D. The obtained results werepartially unsatisfactory, caused by the reasons of inaccurate wind direction values due toyaw misalignment issues and challenging separation into different stability conditions. Moreover, the power deficits showed a clear reduction of losses with increasing windspeed. A conclusion regarding the differences between stable and near neutral conditionscould not be determined from the data.

Aerogeradores, ou turbinas eólicas, são máquinas instaladas em grandes parques eólicos que convertem a energia cinética do vento em energia elétrica. A definição da separação e da interação entre máquinas é um fator fundamental de análise durante a fase de projeto, pois os chamados efeitos de esteira podem inviabilizar o desenvolvimento de um parque eólico. Em geral, a esteira de um aerogerador está caracterizada por um significativo déficit de velocidade e uma intensificação dos níveis de turbulência, o que ocasiona a diminuição da eficiência aerodinâmica e a redução da vida útil das máquinas localizadas a sotavento. Embora existam diferentes pesquisas destinadas à compreensão e previsão dos efeitos de esteira, o problema permanece como uma questão desafiadora que exige a adoção de ferramentas de alta precisão para sua identificação. Este trabalho apresenta uma metodologia experimental em túnel de vento, para a caracterização e avaliação do campo de escoamento na esteira aerodinâmica de um modelo reduzido, sob diferentes condições de escoamento incidente. Especificamente, investiga-se a influência da turbulência atmosférica para quatro perfis de escoamento: i) uniforme-suave; ii) uniforme-turbulento; iii) lei potencial com expoente α = 0,11; iv) lei potencial com expoente α = 0,23. Todos os casos foram conduzidos sob condições de estratificação neutra, e foi utilizado anemômetro de fio-quente para efetivar as medições dos perfis de velocidade média e intensidade da turbulência, em diferentes posições da esteira. Os resultados mostraram diferenças substanciais no comportamento dos perfis de esteira, em função dos níveis de turbulência incidente. Particularmente, observou-se que o incremento da turbulência atmosférica reduz o déficit de velocidade e promove uma maior mistura turbulenta, o que acelera a dissipação dos efeitos de esteira. Assim, a metodologia experimental em túnel de vento evidencia-se como uma importante ferramenta de análise que possibilita amplo espectro para a investigação, precisão e confiabilidade de projetos eólicos. / Wind turbines are machines installed in large wind farms to convert the wind's kinetic energy into electrical power. For an optimal wind farm siting, it is necessary to take into account the interaction between wind turbine wakes. In general, wake effects are associated with velocity deficit and enhanced turbulence intensity. This may reduce the aerodynamic efficiency and lifetime of downwind turbines, making the project unfeasible. Several experimental and numerical studies have been conducted to unravel the behavior of wind turbine wakes under different inflow conditions. However, current wind farm siting tools are incapable of accurately predicting and assessing its effects. This document presents an experimental methodology in the wind tunnel to survey the influence of the atmospheric turbulence on the wake flow field of a wind turbine model. Specifically, four different flow conditions were investigated: i) uniform-laminar; ii) uniform-turbulent; iii) power law exponent α = 0.11; iv) power law exponent α = 0.23. All cases were developed under neutrally stratified conditions. Hot-wire anemometry was used to obtain high-resolution measurements of the mean velocity and turbulence intensity profiles at different downwind positions. Results show that different turbulence intensity levels of the incoming flow lead to substantial differences in the spatial distribution of the wakes. Particularly, higher ambient turbulence promotes a faster wake recovery and lower velocity deficit. In conclusion, the use of wind tunnel experiments is a trustworthy alternative that brings precision and reliability to wind projects.

Aerogeradores, ou turbinas eólicas, são máquinas instaladas em grandes parques eólicos que convertem a energia cinética do vento em energia elétrica. A definição da separação e da interação entre máquinas é um fator fundamental de análise durante a fase de projeto, pois os chamados efeitos de esteira podem inviabilizar o desenvolvimento de um parque eólico. Em geral, a esteira de um aerogerador está caracterizada por um significativo déficit de velocidade e uma intensificação dos níveis de turbulência, o que ocasiona a diminuição da eficiência aerodinâmica e a redução da vida útil das máquinas localizadas a sotavento. Embora existam diferentes pesquisas destinadas à compreensão e previsão dos efeitos de esteira, o problema permanece como uma questão desafiadora que exige a adoção de ferramentas de alta precisão para sua identificação. Este trabalho apresenta uma metodologia experimental em túnel de vento, para a caracterização e avaliação do campo de escoamento na esteira aerodinâmica de um modelo reduzido, sob diferentes condições de escoamento incidente. Especificamente, investiga-se a influência da turbulência atmosférica para quatro perfis de escoamento: i) uniforme-suave; ii) uniforme-turbulento; iii) lei potencial com expoente α = 0,11; iv) lei potencial com expoente α = 0,23. Todos os casos foram conduzidos sob condições de estratificação neutra, e foi utilizado anemômetro de fio-quente para efetivar as medições dos perfis de velocidade média e intensidade da turbulência, em diferentes posições da esteira. Os resultados mostraram diferenças substanciais no comportamento dos perfis de esteira, em função dos níveis de turbulência incidente. Particularmente, observou-se que o incremento da turbulência atmosférica reduz o déficit de velocidade e promove uma maior mistura turbulenta, o que acelera a dissipação dos efeitos de esteira. Assim, a metodologia experimental em túnel de vento evidencia-se como uma importante ferramenta de análise que possibilita amplo espectro para a investigação, precisão e confiabilidade de projetos eólicos. / Wind turbines are machines installed in large wind farms to convert the wind's kinetic energy into electrical power. For an optimal wind farm siting, it is necessary to take into account the interaction between wind turbine wakes. In general, wake effects are associated with velocity deficit and enhanced turbulence intensity. This may reduce the aerodynamic efficiency and lifetime of downwind turbines, making the project unfeasible. Several experimental and numerical studies have been conducted to unravel the behavior of wind turbine wakes under different inflow conditions. However, current wind farm siting tools are incapable of accurately predicting and assessing its effects. This document presents an experimental methodology in the wind tunnel to survey the influence of the atmospheric turbulence on the wake flow field of a wind turbine model. Specifically, four different flow conditions were investigated: i) uniform-laminar; ii) uniform-turbulent; iii) power law exponent α = 0.11; iv) power law exponent α = 0.23. All cases were developed under neutrally stratified conditions. Hot-wire anemometry was used to obtain high-resolution measurements of the mean velocity and turbulence intensity profiles at different downwind positions. Results show that different turbulence intensity levels of the incoming flow lead to substantial differences in the spatial distribution of the wakes. Particularly, higher ambient turbulence promotes a faster wake recovery and lower velocity deficit. In conclusion, the use of wind tunnel experiments is a trustworthy alternative that brings precision and reliability to wind projects.

Aerogeradores, ou turbinas eólicas, são máquinas instaladas em grandes parques eólicos que convertem a energia cinética do vento em energia elétrica. A definição da separação e da interação entre máquinas é um fator fundamental de análise durante a fase de projeto, pois os chamados efeitos de esteira podem inviabilizar o desenvolvimento de um parque eólico. Em geral, a esteira de um aerogerador está caracterizada por um significativo déficit de velocidade e uma intensificação dos níveis de turbulência, o que ocasiona a diminuição da eficiência aerodinâmica e a redução da vida útil das máquinas localizadas a sotavento. Embora existam diferentes pesquisas destinadas à compreensão e previsão dos efeitos de esteira, o problema permanece como uma questão desafiadora que exige a adoção de ferramentas de alta precisão para sua identificação. Este trabalho apresenta uma metodologia experimental em túnel de vento, para a caracterização e avaliação do campo de escoamento na esteira aerodinâmica de um modelo reduzido, sob diferentes condições de escoamento incidente. Especificamente, investiga-se a influência da turbulência atmosférica para quatro perfis de escoamento: i) uniforme-suave; ii) uniforme-turbulento; iii) lei potencial com expoente α = 0,11; iv) lei potencial com expoente α = 0,23. Todos os casos foram conduzidos sob condições de estratificação neutra, e foi utilizado anemômetro de fio-quente para efetivar as medições dos perfis de velocidade média e intensidade da turbulência, em diferentes posições da esteira. Os resultados mostraram diferenças substanciais no comportamento dos perfis de esteira, em função dos níveis de turbulência incidente. Particularmente, observou-se que o incremento da turbulência atmosférica reduz o déficit de velocidade e promove uma maior mistura turbulenta, o que acelera a dissipação dos efeitos de esteira. Assim, a metodologia experimental em túnel de vento evidencia-se como uma importante ferramenta de análise que possibilita amplo espectro para a investigação, precisão e confiabilidade de projetos eólicos. / Wind turbines are machines installed in large wind farms to convert the wind's kinetic energy into electrical power. For an optimal wind farm siting, it is necessary to take into account the interaction between wind turbine wakes. In general, wake effects are associated with velocity deficit and enhanced turbulence intensity. This may reduce the aerodynamic efficiency and lifetime of downwind turbines, making the project unfeasible. Several experimental and numerical studies have been conducted to unravel the behavior of wind turbine wakes under different inflow conditions. However, current wind farm siting tools are incapable of accurately predicting and assessing its effects. This document presents an experimental methodology in the wind tunnel to survey the influence of the atmospheric turbulence on the wake flow field of a wind turbine model. Specifically, four different flow conditions were investigated: i) uniform-laminar; ii) uniform-turbulent; iii) power law exponent α = 0.11; iv) power law exponent α = 0.23. All cases were developed under neutrally stratified conditions. Hot-wire anemometry was used to obtain high-resolution measurements of the mean velocity and turbulence intensity profiles at different downwind positions. Results show that different turbulence intensity levels of the incoming flow lead to substantial differences in the spatial distribution of the wakes. Particularly, higher ambient turbulence promotes a faster wake recovery and lower velocity deficit. In conclusion, the use of wind tunnel experiments is a trustworthy alternative that brings precision and reliability to wind projects.

<p>This work is a five-article-based collection of published and to-be-published research articles that explore a novel combination of inlet conditions, wind turbine design, and non-flat topography by performing scaled-down experiments in a wind tunnel.</p>

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