The placement of wind turbines in wind farms is economically beneficial to the industry. Consequently, the wake generated by the upstream turbines introduces interactions with downstream turbines that significantly affect power generation. However, it is common practice to control the individual turbines to operate at their optimal settings and neglect the wake effect. Turbines in a wake experience lower wind speeds and an increase in turbulence. This results in lower power generation and increased fatigue loads on the downstream turbine. Wind farm control aims to increase overall power generation and reduce fatigue loads by considering all turbines in the wind farm. Recently, numerous studies have been conducted to control the wake itself. The helix approach is one such strategy and seeks to initiate better mixing of the wake. For this, it uses IPC to create sinusoidal blade bending moments acting on the rotor disc. This in turn creates a helical structure in the velocity of the wind, allowing it to better mix with the surrounding energy-rich wind. As a result, the waked downstream turbine experiences an increase in power due to the reduced velocity deficit in the wake. An extensive load analysis of the helix method has shown that the rotational frequency of this helical wake can be measured in the bending moments of the waked downstream turbine. To further develop the field of dynamic wake mixing, this work investigated the initialization of the helix method in a downstream turbine by amplifying these signals through synchronization with the incoming helical wake. This method was tested in simulation environments with increasing levels of fidelity and number of turbines simulated. The results with low fidelity showed that synchronization could be achieved without loss of performance. In addition, less pitch actuation was required to generate large bending moments due to amplification. The final experiment was conducted for a three-turbine setup and was performed in a high-fidelity model using CFD methods. Here it was found that -90 degrees out-of-phase synchronization with the incoming helical wake resulted in a 9.8% increase in power generation in a turbine further downstream. These results demonstrate the clear potential of wake synchronization in a multi-turbine setting. This study, therefore, contributes to the field of dynamic wake mixing by introducing a novel field of wake synchronization.

Wind turbines experience a wide variety of load cases, both when in operation and when parked. While a turbine in operating conditions often experiences larger loads than in parked conditions, uniquely large inflow angles may occur in parked conditions resulting in less well-understood aeroelastic conditions. Vibrations under these conditions are referred to as stall and vortex-induced vibrations (SVIV). While there has been some success in simulating these conditions, there has been a lack of experimental studies in the literature. To address this gap, this study analyses the experimental data obtained from a 3.8 MW research wind turbine. This turbine was equipped with strain gauges at the root of 2 blades and in the tower, as well as pressure sensors on one blade and accelerometers in the nacelle and tower. Several experiments were performed with a focus on simulating various conditions where SVIV may occur. This included pitch traverses of a single blade, yaw traverses, and a traverse of the azimuth angle at a yaw angle of 90 degrees to determine the effect of the inclination angle. Severe stall-induced vibrations were identified at the yaw angle of 110 degrees, when a single blade pointing up in the sky was pitched 180 degrees, and the other two blades were in vane positions at 85 degrees pitch. The severity of these vibrations strongly varied with the wind speed, at an average wind speed of 19.5 m/s the test needed to be stopped early for the safety of the wind turbine, while at 16.6 m/s, the vibrations appeared to reach a limit that was still considered to be safe. The first tower mode experienced the most severe increase in magnitude during these conditions of SIV. In other conditions, no severe case of SIV was identified. However, under several conditions, smaller increases in vibration magnitude were identified. These are likely the result of slightly reduced aerodynamic damping. No vortex shedding was identified in the data obtained from these experiments. This means that the tested turbine/blade design either did not experience vortex shedding or that it simply could not be measured by the installed instruments. Having pressure sensors installed in multiple locations along the blade would likely help in identifying vortex shedding for future experiments. Simulations from the experimental conditions were performed using the aeroelastic software PHATAS together with the aero module from ECN (now part of TNO). This simulation used beam-based structural modeling with either a BEM or free vortex wake aerodynamic model. These simulations confirmed the need for realistic turbulent inflow conditions and the need for a good dynamic stall model. However, the structural model failed to simulate all the natural frequencies with the expected accuracy. This should be improved upon for future use of this structural model in parked conditions.

In recent years, there has been renewed interest in propeller research due to environmental concerns. Propellers are known for their low-speed efficiency and compatibility with electric motors. The utilization of regenerative energy during deceleration has been demonstrated to be beneficial for automobiles and has the potential to be applied to propeller-driven aircraft. This technology allows for the recharging of batteries and could increase the overall efficiency of the propulsion system by utilizing the regenerated energy in other flight maneuvers. The negative thrust mode of propellers has several additional advantages, including weight reduction, improved safety, and increased maneuverability. However, when operated in reverse thrust conditions, the performance of conventional propellers is suboptimal because they are designed for forward thrust. This suboptimal operation can lead to boundary layer separation on the blades, increasing broadband noise and potentially making it more dominant over tonal noise. To assess the relative importance between an isolated propeller's tonal and broadband noise sources under positive and negative thrust conditions, an experimental study was conducted using a scaled propeller setup in a low-turbulence wind tunnel (LTT). Aeroacoustic measurements were taken using a microphone array with 63 microphones placed in the floor and wall of the test section. The advance ratio was varied across different wind speeds at Reynolds numbers ranging from Rec0.7R = 0.8·105 to Rec0.7R = 1.9·105. Signal processing methods were used to analyze the data and quantify tonal and broadband noise sources, including fast Fourier transformation (FFT), phase averaging, beamforming, and blade element momentum (BEM) analysis. The results showed that the broadband noise component of the propeller significantly increased in negative thrust mode compared to positive thrust mode due to fully separated boundary layers. The tonal propeller components were often masked by interference from wind tunnel and motor noise, suggesting the need for further research to validate propellers in regenerative mode.

Wind turbines operate in the earth's atmospheric boundary layer - an environment where the climate is turbulent and unsteady. In addition, wind turbines usually operate in clusters (wind farms) and are thus subjected to the unsteady wakes generated by upstream turbines. Because of the unsteady inflow, wind turbines suffer from structural fatigue damages. There is therefore motivation to better analyse and model the unsteady loading in order to design better turbines and reduce the cost of energy. To study the unsteady aerodynamics phenomena, an actuator disc model under unsteady loading was simulated using experimental and numerical methods. Experimentally, a porous disc rotor was constructed and tested in the Open Jet Facility, TU Delft. The design involved a novel method to dynamically control the rotor's thrust coefficient through its porosity. Numerically, a dynamically loaded actuator disc was implemented in the RANS equations and solved by the commercial CFD solver Star-CCM+. Results from the experiment, CFD simulations, as well as a free wake vortex ring model were used to benchmark the performance of dynamic inflow engineering models. Results show that the wake of a wind turbine under steady loading is also turbulent and meandering. The Strouhal number of the wake fluctuation closely agrees to that from a previous experiment of a 2-bladed wind turbine (Medici & Alfredsson, 2006). This suggests that the wakes of blade rotors are comparable to that from disc rotors. The tower shadow effect also bears a major influence on the inner wake profile which causes further velocity deficit and higher turbulent intensity. For rotors with higher CT , the velocity recovery process occurs relatively earlier. This is because of the enhanced momentum entrainment arising from a steeper velocity gradient at the shear layer. For the disc rotor under unsteady loading, the wake is affected by unsteady aerodynamics or the dynamic inflow effect. The transient velocity profile experiences an overshoot due to the passage of the 'old' and 'new' shed vortices, which are generated at the wake edge as a consequence of the uniform load profile on the rotor. The subsequent decay of the velocity in the wake or inflow at the rotor plane to the 'new' equilibrium state is a result of the rate of convection of the 'new' shed vortices to the far field. From this research, numerical results showed lower inflow velocity decay rates (velocity response due to the change in loading on the rotor) than that predicted by engineering models. While this is so, it was found that the decay rate is highly influenced by the ambient flow's turbulence. At high ambient turbulence, the 'new' equilibrium state is achieved in a shorter time due to the enhanced momentum mixing process. It is hypothesised that this is the primary reason behind the higher decay rates predicted by empirical-based engineering models.

Wind farm control is an area of research that looks to maximise the performance of individual but most importantly the collective performance of wind farms. The performance of turbines located in a wind farm is heavily dependent on the effects of turbine wakes, and improving the efficiency of wind farms by mitigating the wake effects is an ongoing and promising field of research. Simulating the wake effects of a turbine is generally done with computationally expensive physics based modelling techniques like CFD simulations. The computational complexity of these models limits the scope of optimisation that can be performed on wake mitigating controls strategies such as dynamic induction control. A substitute engineering model like the free vortex wake model could provide better insight into dynamic induction control by virtue of its computational efficiency allowing a wider optimisation parameter space. In this thesis an optimisation framework based on a 2D implementation of the free vortex wake model and genetic algorithm optimisation is used to investigate dynamic induction control for a simple two turbine wind farm beyond the research that has been done up to this point. While yet to be confirmed with higher fidelity simulations, initial results corroborate earlier findings for simple dynamic induction signals and additionally indicate that dynamic induction control could benefit from multiple harmonics. The optimisation framework achieves its goal in allowing a wider parameter space to be searched for optimisation, even on consumer grade desktop hardware, and shows potential as a tool for further investigation of dynamic induction control.

As the world grapples with the challenges of global warming, there has been a significant push for the development of new methods of harnessing renewable energy. Energy harvesters have been a new avenue to generate electrical power locally for remote applications. An energy harvester is a device that taps into aeroelastic phenomena to convert vibrational energy into electrical energy. Additional benefits of harvesters include being lower in complexity, ease of maintenance, and the resulting cost-effectiveness. The splitter plate represents a wake control device that alters the wake to make the energy capture more efficient and practical when attached to a cylinder. However, the flow mechanisms under different conditions have not been fully understood. The motivation of the present work is to bridge this gap by characterizing the mechanisms and wake dynamics of a cylinder with a flexible splitter plate. This will help advance the understanding of the wake behavior of these devices to make safe and efficient devices. To characterize the behavior of the wake, measurements are made at different flow speeds. A preliminary analysis of the splitter plate motion showed that increasing the Reynolds number produced four distinct regimes to be studied further: crossover, baseline, reduced amplitude, and chaotic regimes. The initial analysis also revealed a condition named burst, defined as a period of resonant, two-dimensional, high amplitude oscillations of the splitter plate. All regimes are compared to a bare cylinder with no splitter plate to ascertain differences in wake behavior. The use Helium Filled Soap Bubbles (HFSB) in flow diagnostic tools such as Particle Image Velocity (PIV) experiments has permitted large-scale measurements. A Lagrangian Particle Tracking (LPT) experiment was performed in which the HFSB is tracked individually with the help of an algorithm. The present study employs Shake-The-Box (STB), a novel LPT tracking algorithm that uses time-resolved experimental data to predict future tracer particle locations. The benefit of such an algorithm includes reduced ghost particle detection and computational efficiency. The details of this experimental method have been covered extensively in this thesis. The raw experimental data was processed to perform statistical analysis, proper orthogonal decomposition, and spectral analysis to provide insights to meet the thesis objective.

Due to its immense potential to harvest larger amounts of energy from the wind at a higher capacity factor, offshore wind energy is the key to reach the goal of a more sustainable future. The wind energy research community has been focusing all its efforts to bring the new concept of floating offshore wind turbines (FOWT) into the market. By installing the turbine on top of a floating platform the restrictions of offshore turbines regarding the sea depth are lifted and numerous new sites now become available for the wind energy market. Due to the motions that FOWT experience during their operation several questions arise regarding their aerodynamics, structural integrity and control efficiency. Since aerodynamic models are utilized in almost every field involved in the design of a wind farm, its particularly important to make sure that the current industrial aerodynamic codes, like BEM, are able to capture the unsteady aerodynamics of this complex system. High-fidelity CFD codes and experiments is the only way to truly assess the performance of BEM. Surge motion is recognized by all authors as one of the most important motions imposed to FOWT. Thus, in the present thesis the impact of surge motion in the aerodynamics of FOWT was investigated by employing a high-fidelity blade-resolved CFD model in OpenFoam. After the extensive validation of the CFD model with the high-fidelity experiment of the UNAFLOW project, two test cases were examined. One with low surge frequency/amplitude which was employed to test whether momentum theory can accurately predict the induction of FOWT and one with an extreme surge frequency/amplitude which was used to test whether propeller and vortex ring states occur during the operation of FOWT - rendering BEM invalid. In the first test case, Ferreira-Micallef induction computation model was utilized to extract the induction field from the CFD field data. After extensively comparing the induction computed by Ferreira-Micallef model and momentum theory it became apparent that due to the 2D nature of both models it was not possible to draw any significant conclusions about the accuracy of momentum theory. Specifically, due to the existence of 3D radial flow during the operation of FOWT Ferreira-Micallef model is not reliable and can not be used as a basis to compute the error of momentum theory. Having said that, in the mid-span region of the blade and when the FOWT is operating in the mild surge conditions, 3D flow was not present and a comparison of induction fields between momentum theory and Ferreira-Micallef model yielded very similar results. In the second extreme surge test case, two observations were made. First, even though thrust was acquiring negative values during the surge motion of the rotor, the stream-tube did not seem to experience any propeller state since it was expanding at all times. Second, in all the 3D iso-surface q-criterion scenes that were produced there was no large vortex ring visible in the wake, thus there was no indication that vortex ring state occurs for a FOWT surging under these harsh surge conditions. The present thesis contributes in the better understanding of FOWT aerodynamics and provides clarity on the ambiguous research topic of whether propeller/vortex ring states occur during their operation. Furthermore, the high-fidelity blade-resolved CFD model which was developed can be used as a starting point for several future research efforts.

Understanding the fatigue load history of wind turbines is critical for taking decisions regarding the lifetime of a project. However, direct measurement of fatigue loads at each turbine in a wind farm is unfeasible. For this reason, surrogate models offer a useful alternative. In this thesis, a methodology for creating surrogate models for emulating fatigue loads of offshore wind turbines is presented. The methodology is unique in that it accounts for the variability of site-specific conditions that may be present between wind turbines of the same class. First, a method for creating simplified structural models which depends only on a few degrees of freedom is derived. After this, a database of simulation data is assembled by varying the geometric, dynamic, and environmental degrees of freedom within ranges which capture a high degree of variability of possible site-specific conditions. This database is then used to train surrogate models using neural networks.

The hunt for new methods to harness wind energy is at an all-time high as wind energy quickly overtakes other renewable energy sources. Deep waters are promising alternatives with higher average wind speed (thus, higher yields) and also due to lesser availability of land on shore. Even though the design of the blades and rotor of a FOWT is largely similar to that of onshore counterparts, they are mounted on floating platforms, unlike the fixed platforms on onshore wind turbines. Due to the excitation from wind and waves, these foundations are made to oscillate in all six degrees of freedom. Pitch and surge motions have the most significant effects out of all the possible types of motions. Since surging motion comes closest to the pitching motion, this study attempts to compare the aerodynamics of these two motions by simulating a pitching wind turbine using an actuator disk CFD model. This study can also be considered a continuation of the actuator disk study of the surging motion. The primary objective of this thesis is to understand the differences between the aerodynamics of a surging and a pitching wind turbine using an actuator disk model. The actuator disk is made to pitch sinusoidally with the prescribed pitching frequency and amplitude. In order to identify the effects of motion and thrust prescription separately, three different types of actuator disk models were created. Using the first model, which simulates a pitching actuator disk with constant thrust, the effects of pitch motion are captured. The second model which simulates a still actuator disk with dynamic loading gives the effects of change in thrust on the rotor. The last model, which is a combination of the first two with a pitching actuator disk under dynamic loading, attempts to simulate a more realistic pitching actuator. On comparing the surge and pitch motions of the actuator disk, the stark difference observed is the meandering in wake due to flow shear and asymmetric tip vortex shedding. At high thrust and low-frequency cases, turbulent wake states are also visible in the wake. At higher frequencies, they are nearly as strong as the tip vortices and introduce non-linear effects on the induction field. They also tend to introduce varied dynamic responses based on the radial location. It was also observed that the motion introduces a phase delay that varies with the radial position on the rotor. Thus, this project provides a comprehensive view of the effects of platform pitching motion on a FOWT and it differences with respect to the surging dynamics. This project also confirms that an actuator disc model is capable of reproducing the effects of pitching in a FOWT. Using the insights provided in this thesis, a better dynamic inflow can be developed for use in industries

Large scale wind farms consisting with floating offshore wind turbines (FOWTs) will be a solution for offshore wind energy industry to access more wind resources. However, wake structures and wake interactions of FOWTs subject to motions are still not yet been fully understand, especially when they are under turbulent inflow conditions with realistic turbulent intensities. These will be critical for designing floating offshore wind farms. Note that the majority of previous research are conducted with single rotor using models having relatively low fidelities and/or focusing on laminar inflow conditions. To advance the knowledge about wake and wake interactions of FOWTs, numerical studies about rotors of FOWT with prescribed harmonic surging motions are conducted in this thesis project with high fidelity CFD models, namely large eddy simulation (LES) coupled with actuator line model (ALM). Cases with single rotor without controller, dual rotors in tandem without controller, and dual rotors in tandem with controller are simulated with various of settings, including different surging settings and different inflow turbulence intensities. For the cases with single rotor without controller, it is found that the differences of wake structures between fixed and surging rotors are pronounced when under laminar inflow conditions, where the periodic structures related to the harmonic surging motions can be detected straightforwardly; while the differences are much less significant when under inflow conditions with realistic turbulence intensities, and the periodic structures are clearly revealed only after phase-locked averaging. Moreover, surging cases with laminar inflow conditions have wake recovery rates which are significantly higher than the fixed case with laminar inflow; however, with turbulent inflow, wake recovery rates for surging cases are only slightly higher than the fixed case. For the cases with dual rotors without controller, it is found that the wake interaction modes between the two rotors are significantly affected by the surging settings for the laminar cases, while the turbulent cases are insensitive. However, the power performances of the downstream rotors will be increased slightly with surging upstream rotors for the turbulent cases. For the cases with dual rotors with controller, it is found that the implemented simple controller cannot improve the performances of the rotors as designed due to the large rotational inertia, and thus the modes of wake interactions are not altered a lot. However, the downstream rotors' operational parameters were successfully changed to more desirable values by the controller, demonstrating the controller's potential for numerical analysis of wake interactions between wind turbine rotors. The findings of this effort demonstrates that the wake structures due to the surging motions of FOWT rotors will be smeared out by the ambient turbulence; and to achieve better power performances, more advanced controlling strategies may have to be implemented for FOWTs subject to surging motions.

In order to reduce the levelised cost of energy, the rotors of wind turbines are increasing in size. To increase the energy yield, wind turbine rotors need to have an innovative tip design; such as winglets. Winglets are used widely in aircraft design; however, they remain mostly absent in state-of-the-art wind turbine design. The low-fidelity wind turbine design models used by industry, such as BEM, are insufficient to capture the full 3D flow physics in such an innovative design. Therefore, high-fidelity methods, such as vortex methods, are becoming more and more important in such a design phase in wind energy research. Single-objective optimisation has been applied in earlier works to maximise power production. Winglets have shown the potential of increasing power production while simultaneously increasing the design-driving loads (DDLs), for example, the thrust or flapwise bending moment. This work focuses on optimisation using machine learning of a winglet that increases power production without increasing DDLs. A parameterised design for a winglet on a wind turbine is created. Different design constraints, such as DDLs or a diameter constraint, are explored to determine under which constraints and conditions a winglet can have an added value to the wind turbine blade design. Multi-objective Bayesian optimisation is used to maximise the rotor's power production and minimise DDLs. Surrogate models, created using machine learning techniques such as Gaussian Processes and Bayesian Neural Networks, are used in combination with an acquisition function, to determine what designs should be evaluated by the lifting line model AWSM. This has the goal to obtain designs that lie on the Pareto front of two or more objectives. The recent Bayesian Neural Networks were able to find the Pareto front most effectively in this work. Furthermore, the results show that different DDL constraints led to different winglet designs, with noticeable differences between upwind and downwind winglet designs obtained by the optimiser. Both a downwind and upwind winglet were found to be able to increase power without increasing the thrust, root flapwise bending moment and flapwise bending moment at 80% of the rotor radius.

The thesis presents an analysis of unsteady vortex shedding and vortex induced vibrations (VIV) on wind turbine blades using a combination of Stereoscopic Particle Image Velocimetry (SPIV) and Computational Fluid Dynamics (CFD). A spanwise uniform blade with a symmetric NACA0021 airfoil section, 0.075 m chord, and 0.4 m span was tested experimentally for a static, plunging, and surging blade oscillating at 90 deg angle of attack, 5 Hz, and 2.5 Hz frequency, 1 chord oscillation amplitude. Equivalent CFD simulation cases were run to compare the results and validate the CFD setup, which was used to analyse additional cases of interest. The extent of the three-dimensional flow behaviour due to the tip vortex was described, along with determining the lock-in region for the two motions using additional simulations. The results indicated that the spanwise convection of the tip vortex and the transition to a ‘2D’ flow were only dependent on the blade aspect ratio and independent of the freestream wind speed, the type of motion, and the oscillation frequency. The lock-in region for the plunging and surging blades was presented, noting that all the surging cases were locked in, most likely due to the large oscillation amplitude. However, the surging cases had a positive aerodynamic damping and hence, VIV would not be excited for a freely vibrating blade. It is suggested as future work to test a flexible blade and study the onset and development of VIV.

The main objective of this master’s thesis is to experimentally investigate the effects of dynamic inflow conditions on the wake of an actuator disc. Dynamic inflow is the unsteady wake in momentum theory. Dynamic inflow can experimentally be generated by a gust generator. During the experiments, in which Particle Image Velocimetry (PIV) was used as a flow measurement technique, the gust generator produced different gust frequencies and gust amplitudes. The oscillating gust results in the oscillating wake of the actuator disc. The oscillations of the wake cause wake expansion and compression. The expansion and compression of the wake is mainly dependent on the gust frequency. The higher the gust frequency is, the more oscillations the wake will see. So, the actuator disc’s wake was influenced by the gust.

As the need for abundant and reliable renewable energy increases, there is a growing interest in floating wind turbines, which would allow to harness the wind resource in areas where bottom-founded wind turbines cannot be used. However, the movements of the platform are expected to cause unsteady aerodynamics effects, including different wake dynamics, a variation of the induction field at the rotor and blade-vortex interactions. Consequently, there is no general consensus on whether Blade-Element Momentum (BEM) codes could be employed to model the aerodynamics of floating wind turbines. This poses a serious issue as BEM models are widely used in industry practice. This project proposes to analyze the impact of surge motion on the induction field of a horizontal-axis wind turbine. A suitable actuator disc model is developed, followed by an actuator line model that allows to study the effect of the finite number of blades. Both models are implemented in OpenFOAM, an open-source CFD software. The simulations are run for a range of case studies with imposed baseline thrust, amplitude of the thrust variations, surge frequency and surge amplitude and the resulting induction factors are compared to those obtained with a dynamic inflow model, to assess whether a momentum method could lead to accurate predictions. Particular attention is given to the identification of the wake states of the streamtube, since both turbulent wake state and vortex ring state imply a breakdown of momentum theory. The results of the actuator disc simulations show that in all cases there is a good agreement with the induction factors obtained with the examined dynamic inflow model. Turbulent wake state is only entered when a high thrust coefficient is reached at low frequencies, while propeller state is only entered when a negative thrust coefficient is reached at low frequencies. Furthermore, no signs of vortex ring state were detected. Some of the cases considered during the first part of the project were also run with the actuator line model, to examine the finite blade effect. A rotor with three blades was modelled. The resulting disc average induction factors are in excellent agreement with those obtained with the actuator disc model, while the induction at the disc center is lower. The contour plots show that the conclusions on the wake states entered by the streamtube remain valid. It is advised to test this model at different tip speed ratios and for rotors with different numbers of blades. Overall, this project contributes to a better understanding of the aerodynamics of floating wind turbines and gives confidence in the possibility of using momentum methods during their design phase. Furthermore, the CFD models developed are a flexible tool that may be used for future research on related topics.

A 3D panel method code, named AWSM3D, is developed as an extension to the AeroModule aerodynamic software suite of TNO which includes a Blade Element Momentum theory (BEM) and a Lifting Line (LL) code. The underlying panel code method is presented along with a gradual process of simulation validation culminating in validating the code against the New MEXICO experimental dataset and an established panel code. In view of exploring the avenue of sweeping wind turbine blades as a way of increasing wind turbine efficiency, the capabilities of the three proposed aerodynamic models in solving flows about swept geometries are studied. The unmodified BEM model is incapable of adequately modeling flows over swept geometries. The limits of the cross-flow theory which the LL simulation hinges on for the simulation of swept flows are put to the test. Both a fixed wing and a conventional Horizontal Axis Wind Turbine case show that the LL solution in proximity to body extremities where the cross-flow component is prominent fails to capture smaller-scale local effects caused by said cross-flow. Despite this, the larger-scale effect is reproduced well. Although AWSM3D requires no airfoil polars and produces an accurate inviscid flow solution for more complex body geometries, it currently excludes viscosity and has a much higher computation time in comparison to an LL. A number of further AWSM3D development and research into the effects of sweep suggestions are formulated.

The wind turbine wake is a downstream region of kinetic energy and velocity deficit, higher turbulence and has a complex helical vortex structure. The wind turbine wake’s stability depends mainly on the ambient turbulence and the wind turbine tip- speed ratio. Researchers have developed several wake control techniques to mitigate this issue, such as static axial induction control by torque or pitch control, dynamic axial induction control by pitch control and actuating flaps or other actively controlled add- on devices. Some gaps are present in these methods, in terms of applicability of these techniques in terms of full scale validation and practicality to have the devices as add on to existing blades. To address these research gaps, this study focuses on designing and evaluating segmented Gurney flaps (in line with the ECN (now TNO Wind Energy) patent by Edwin Bot and Arne Van Garrel) for wind turbine blades to enhance the wake recovery by inducing turbulence in the wake to excite the tip vortices. 4 Gurney flaps were attached in the tip region of each blade of a GE 3.8MW research wind turbine. Field tests were conducted in this study for the wind turbine wake (using a scanning LiDAR to scan a sector up to 5.5D downstream at different altitudes; with a scan time of roughly 3 minutes) and performance analysis (using 10 minute averaged measurement data). Free vortex wake simulations were conducted to validate the faster wake breakdown by the change in lift distribution upon addition of segmented Gurney flaps. Simulations using dynamic blade element momentum theory with IEC NTM inflow conditions from cut in to cut out wind speed were conducted to assess the structural impacts on the retrofitted wind turbine. The field tests’ wake analysis was quantified with different wind speed, turbulence intensity, wind shear, wind direction conditions. Post processing of LiDAR data involved filtering, creating bin averaged data set, using Gaussian process regression and retrieval of the required wind component. The results show a consistent increase in wake recovery, generally at all downstream distances. The retrofitted configuration results are associated with a higher standard error because of a shorter testing period (due to increased noise levels) than the baseline configuration. The wake simulations indicate an earlier wake break down position, by roughly 2D. The simulations indicate AEP increase for the retrofitted wind turbine to be roughly +0.2% (+50MWh), at the expense of increased blade and tower structural loads. The wind speed weighted damage equivalent blade flapwise bending moment was found to increase by roughly +2% to +4%; with peaks observed in partial load region of the wind turbine, associated with the operating angle of attack in this region.