A key challenge in the Airborne Wind Energy industry is the minimisation of the time and energy required for the transfer and return phases inherent to the power cycles of ground-generator-based kite power systems. This thesis contributes to solving this problem by exploring the use of bleed air spoilers during the return phase in terms of their potential to improve the average cycle performance. To this end, a mathematical model is first introduced to evaluate the individual phases of the characteristic power pumping cycles in a general sense. This model is further used to demonstrate that a given kite configuration with the capacity to selectively lower its glide slope and/or resultant aerodynamic loading could improve the net power output of the system by around 30%. A CFD-based simulation environment for investigations of the effects induced by bleed air spoilers is developed using Simcenter STAR-CCM+. To establish the reliability of the setup, a first study revolves around its validation with experimental data from independent wind tunnel tests on bleed air spoilers effects on ram-air parachutes. The second CFD study is based on a standard kite geometry employed by SkySails Power GmbH. Two parameter studies testing the influence of spoiler size and chordwise position are carried out to investigate how bleed air spoilers are best used to reach the desired aerodynamic effects. The results show that bleed-air spoilers should ideally be applied in the vicinity of the suction peak and that relatively small spoiler geometries can already entail a major effect on the resultant flow field. It is predicted that the evaluated configurations would improve the system's overall performance by 9 to 18.3%, mostly due to substantial reductions in the glide ratio.

Wind turbines are operating in harsh environmental conditions, especially offshore. An implication of these conditions, caused by the impact of precipitation, is the development of leading edge erosion (LEE). This leads to degraded blade surfaces that result in lower aerodynamic performance. Leading edge erosion is researched in many ways but remote detection is still underdeveloped. Therefore, this thesis investigates the possibility to develop a LEE detection method by analysing real-life data from operational turbines in the field.

This research has been performed with the purpose of understanding the cause of the unstable pitch break experienced by the Flying V at a lift coefficient of CL = 0.95. Without understanding and possibly elimination of the pitch break, the flying V’s usable maximum lift coefficient is lower and the pitching moment behavior and gust response unpredictable. Three possible solutions are investigated empirically, with the ultimate goal of eliminating the unstable pitch break. First, the application of a trip strip to the suction and pressure side of the wing is investigated. Second and third, the implementation of vortilons and fences is investigated. The experiments are performed in the TU Delft’s Open Jet Facility using a 4.6% scaled half-span model of the Flying V. The wind tunnel experiments are evaluated using force and moment data and flow visualization using tufts. The application of trip strips results in an increase in pitching moment coefficient up to a lift coefficient of CL = 0.65 and introduces the unstable pitch break at CL = 0.80 compared to CL = 0.95 for the clean wing. Flow visualizations showed an improvement in the flow over the outboard wing when the trip strips were applied. Combining both observations, it is recommended to apply the trip strips on the outboard wing of the Flying V scaled flight testing model only. Vortilons have shown to have no effect on the unstable pitch break. The combination of a vortilon and the trip strips could decrease the pitching moment coefficient of the flying V up to a lift coefficient of CL = 0.70, depending on the spanwise location of the vortilon. Placing a fence could result in a favorable pitching moment coefficient characteristic, depending on its spanwise location and size. A fence placed at the spanwise location of the leading edge kink results in a decrease of pitching moment coefficient between a lift coefficient of CL = 0.30 and 1.1. Furthermore, a fence located at the leading edge kink and spanning the entire suction surface chord postponed the unstable pitch break experienced by the Flying V. Therefore, it is recommended to install that fence on the Flying V scaled flight testing model.

An experimental investigation was done regarding the propulsion system and airframe integration issues associated with the novel Flying V configuration. The experimental investigations involved a series of wind tunnel tests with a 4.6% scale half model, conducted in the TU Delft OJF low speed wind tunnel. Balance measurements allowed significant interference effects between the wing and engine to be identified. A nacelle mounted total pressure rake enabled the measurement of engine inlet total pressure, both in through-flow nacelle and powered conditions. At landing and take-off velocity (20m/s), an interference drag penalty is observed over the full range of positive incidence angles above 5 degrees, with a maximum of 60 counts (16.5% of isolated wing drag) at an incidence angle of 10 degrees. At incidence angles lower than 5∘ engine operation is somewhat beneficial, with a maximum contribution to thrust due to interference of approximately 20 counts. Engine inflow is shown to be distorted by the presence of the wing, which could contribute to the observed interference drag. Distortion DC(60) values higher than 0.2 are measured in off design conditions, while measurements at cruise condition show a DC(60) value of 0.08. However, distinguishing between loss of engine thrust and increase in airframe drag is not possible with the used setup. Increased suction around the engine intake contributes to added lift and nose down pitching moment at high thrust setpoints and incidence angles between 5 degrees and 12.5 degrees. An increase in nose up pitching moment is observed from 12.5 degrees to 22.5 degrees. To put the measured interference in perspective, the direct effects of the quantified interference on trimmed flight conditions are demonstrated. An increased power requirement of up to +8% is observed in trimmed flight conditions between 22m/s and 31m/s, while a maximum reduction of -11% in power requirement is predicted for velocities higher than 32m/s. The change in required control surface deflection for trim due to engine interference is evaluated to be less than +/ − 2.5 degrees for trimmed level flight. A trimmable flight envelope is demonstrated for maximum climb, level flight and optimum glide with control surface deflections in the range of −10 to 10 degrees.

The Flying V is a concept proposed and patented by Benad and Airbus Operations GmbH, which is a flying wing aircraft in the shape of the letter V. Multiple researches already showed promising results, with a 25% increased lift over drag ratio compared to conventional aircraft like the Airbus A350-900. Before research of sub-scale flight testing can be conducted, research into the flight characteristics is necessary. Based on the identification of the problems, the research question for the thesis research is defined as: "What are the flight characteristics of the Flying V sub-scale model at approach speed and high angles of attack?" In total, three different wind tunnel campaigns were conducted at the Open Jet Facility of the Delft University of Technology. During the wind tunnel campaigns, experiments were conducted to acquire data through balance measurements and flow visualisation techniques by means of oil, tufts ans smoke. The untrimmed maximum lift coefficient was estimated to be 1.09 at 41 degrees angle of attack and positive stability was found up to 20 degrees angle of attack. Flow visualisation results showed that large leading edge vortices acted on the surface of the wing, originating from the root towards the crank of the wing. A leading edge separated vortex spread over the wingtip, which originated at the kink of the leading edge. Investigations into the trimmed flight of the aircraft concluded that due to trim limitations the centre of gravity is bound between 1.345 and 1.425 meter behind the nose. The centre of gravity has an optimal location at 1.365 meter behind the nose, providing a static margin of about 9%. With the trimmed lift curve and the optimal centre of gravity location, the flight speed of 35 meter per second can be flown at 3.6 degrees angle of attack. Due to limitation in maximum deflection of the control surfaces, the maximum lift coefficient is estimated at 0.95, at 28.5 degrees angle of attack at a stall speed of 14.8 meter per second. At 20 degrees angle of attack, where the static stability switches from positive to negative, the lift coefficient is estimated at 0.73 at 17.2 meter per second and is defined as the 'safe' stall speed. Taking the FAA and ICAO regulations into account, the approach speed is estimated at 19.2 meter per second, at an angle of attack of 15.9 degrees at a lift coefficient of 0.58.

Recent decades, an ongoing trend has emerged in the upscaling of wind turbines in order to compete with traditional energy resources. For conventional wind turbines, aside from improving safety, the driving factors for new designs have always been increased efficiency and performance. This has led to novel wind turbine designs comprising thin-walled structures and light-weight composite materials that produce more power per unit. The current cost of wind energy is strongly dominated by operational and maintenance costs throughout the full lifetime of a wind turbine. Most wind turbines do not reach their design lifetime due to various reasons, from which fatigue failure is the most prominent one. The events responsible for gearbox or blade failure are caused by complex interactions between aerodynamics and structural responses that are inherently of unsteady nature. Aeroelasticity has become increasingly important for a safe and cost-effective design. Additionally, longer and lighter wind turbine blades undergo extremely large, low-strain deformations. A more accurate understanding of the aeroelastic behavior demands nonlinear analysis methods.

Most aeroelastic solvers in aerospace industry rely on linear structural models. This thesis work has modified the existing semi-nonlinear aeroelastic analysis method of the in-house developed CFD code at NLR by using Nastran's nonlinear structural module. Both analysis methods were utilized to obtain a converged static aeroelastic solution for two different operational conditions of the design curve. Subsequently, mode shapes of the deformed state of the structure were determined, to be used in the flutter analysis.

A 108-meters theoretically designed blade, provided by the Dutch blade design company We4Ce, was analysed using both methods. The results of static aeroelastic analysis and flutter analysis were compared to assess the effect of structural nonlinearities on the aeroelastic behavior. It is concluded that linear analysis overestimates the structural deformations and that pre-stressing due to nonlinear deformations and follower-forces alter the dynamic properties of the wind turbine blade. For the test case considered, the inclusion of geometrical nonlinearities resulted in a change of behavior of various modes. Naturally-low damped modes were affected negatively, eventually leading to dynamically diverging behavior. This thesis has successfully provided the first steps in the implementation of a nonlinear aeroelastic analysis for extreme scale wind turbines, including the capability of performing stability analysis.

Stall is traditionally divided in three types of stall behaviour, these have been regarded as two dimensional and dominated by the airfoil shape. However one of these types of stall behaviour, trailing edge stall, has been shown in literature to be three dimensional by forming stall cells along the wing span. Stall cells are coherent flow structures consisting of localised patches of separated flow that are surrounded by attached flow. Most of the recent research is focused on determining the requirements for the formation of stall cells in terms of the angle of attack and the Reynolds number. Published results of these investigations use different airfoils and are thus not comparable. In this master thesis a NACA 0012 wing with plain flap has been used to assess the effect of the airfoil thickness and the camber on the stall cell formation criteria. The experimental results of the NACA 0012 wing have been compared to published results of a NACA 0015 wing to assess the effect of the airfoil thickness. The effect of the camber is simulated by deflecting the plain flap. Experimental results suggested that stall cell behaviour needs to be split into a low and a high Reynolds number regime, where the change in regime is airfoil dependent. Thick airfoils have been found to promote the formation of stall cells in the high Reynolds number regime but delay the formation of stall cells in the low Reynolds number regime. Adding camber to an airfoil was found to promote stall cell formation in both Reynolds number regimes. Further investigation of the formation criteria for stall cells indicated the importance of aerodynamic parameters such as the lift coefficient in determining stall cell formation criteria. Additionally the size and steadiness of the stall cells has been investigated by tracking characteristic vortical structures of stall cells. Although the angle of attack and Reynolds number have an effect on the stall cell size in spanwise direction, it was found that the minimum and maximum size of a stall cell are determined by the airfoil. Larger flap deflections were found to result in larger stall cells in spanwise direction. The unsteadiness of the stall cell spanwise position was observed to decrease with increasing angle of attack and decreasing Reynolds number.

Laser diagnostic techniques such as Particle Image Velocimetry and Particle Tracking Velocimetry to measure the flow of a fluid around an object have been in prevalence for a few decades. Typically, the fluid, whose motion is of interest, is seeded with micron scale particles and illuminated with a laser. Due to scattering inefficiencies of these micron scale particles the maximum measurement volumes were about a few litres. Development of the neutrally buoyant sub-millimetre scale Helium-Filled Soap Bubbles allows for much larger scattering cross-sections thus enabling large-scale measurements in air (Caridi et.al. 2015).

Large scale Particle Tracking Velocimetry has been used to study the flow in the wake of a cyclist at typical time-trial speeds. A novel PTV algorithm known as Shake-The-Box (STB) (Schanz et.al. 2013) has been used to obtain the particle tracks from the images acquired in a thin volume in the cyclist’s wake using the wake-scanning method. The Lagrangian particle tracking technique was found to be more accurate than the Time Resolved Tomographic PIV through another experiment. Particularly for large field of views involving scanning at different positions, it produces whole-field results free from the boundary effects that arise between different positions of the scans. This yields accurate velocity components and their gradients that are crucial for pressure reconstruction.

Flow-field results from 4D-PTV show that the wake structure is similar to those found in the literature. Two large regions of momentum deficit are present, one behind the thighs and one near the lower legs and the wheel axis. Furthermore, they reveal the presence of some vortices which were not reported previously. Based on the observed signs of the vortices the points of origination of these vortices are identified based on separation mechanisms. There is a good agreement in the flow-field results with the literature. Pressure fields reconstructed from the velocity flow-field show low pressure pockets in the regions of the vortex cores and separated flow over the lower back.

Wake flow-field is utilised to compute the drag using the control volume approach (van Oudheusden 2007) and compared with the drag forces obtained from an external balance measurement to obtain the accuracy of large scale 4D-PTV. In total, five measurements at different velocities in the range of 13m/s to 15m/s are analysed. The accuracy of the drag estimates from 4D-PTV is obtained within 5%. Out of the three terms of the drag force, the momentum term contributes approximately 95% and the contribution from the pressure term is less than 0.3%. Rest is contributed by the Reynolds stresses in streamwise direction. Almost all the variation in the drag force comes from the momentum term.

Drag measurements on bluff-bodies such as bobsleighs using force balance systems have been in prevalence for a few decades. However these studies do not reveal anything on the flow behaviour around the body. In the recent past, various flow visualization techniques have been applied to investigate the flow behaviour around bobsleighs. Even though these studies have highlighted the qualitative flow features, there is a lack of a deep insight into the wake flow topology and the behaviour of the flow structures that are formed in the highly three-dimensional flow. Additionally, when the bobsleigh moves in a curvilinear path, the front cowling rotates with respect to the rear cowling, causing an increase in the frontal area exposed to the wind and in turn a higher drag. However, this perception is not quite straightforward and requires a thorough understanding of the mechanisms and sources of drag generation, in particular at different front cowling angles.

Stereoscopic Particle Image Velocimetry has been used to study the near-wake flow topology of a scaled bobsleigh model. From the velocity field information obtained from PIV, the pressure field in the wake of the bobsleigh model is determined from the Pressure Poisson Equation (PPE) using the methodology outlined by Oudheusden [41]. With the knowledge of the velocity and the pressure fields, the aerodynamic drag is computed using the control volume approach on a wake plane which is at approximately two diameters from the model. The drag obtained from PIV is compared to that obtained from balance measurements. The effects of different bobsleigh configurations on the aerodynamic drag is observed. Secondly, uncertainty quantification of the aerodynamic drag is performed. Finally, a Proper Orthogonal Decomposition (POD) analysis is performed for the velocity fluctuations in order to gain an insight on the motion of the flow structures formed in the wake.

The investigation of the near wake flow topology of the bobsleigh reveals the presence of two counter-rotating vortices with a significant downwash between them in case of the reference configuration. The motion and behaviour of these vortices is obtained by performing the PIV analysis on multiple vertical wake planes. The investigation of the effect of the front cowling misalignment on the drag reveals that there is a reduction of drag for smaller misalignment angles (upto 5°) and a marginal increase in drag for further increase of misalignment (from 5° to 20°). From the analysis of aerodynamic drag conducted on a vertical plane that is approximately two diameters behind the model, it can be seen that the momentum deficit is the highest contributor to the aerodynamic drag (88%) followed by the Reynolds stresses (9%) and pressure (3%). It is also observed that the aerodynamic drag obtained is in good agreement with the results obtained from the balance measurements with a variation of only 2 to 3%. The uncertainty analysis of drag shows that among the different flow variables which affect the drag, the average pressure has the maximum uncertainty. Finally, the POD analysis of the velocity fluctuations shows that the first two fluctuating modes which are the most energetic are not perfectly correlated to each other due to the highly three-dimensional nature of the flow.

In recent years, the research on Vertical Axis Wind Turbines (VAWTs) have been paving the way for the very large-scale (10-20 MW) floating offshore applications. For a cost effective utilization, VAWTs need to deliver superior aerodynamic and structural performance. Nowadays, the complex ow phenomenon of VAWTs have been better understood and the computational tools have reached to a mature level. It is time to explore the capabilities of this concept and improve its aerodynamic features by innovative design ideas. The main goal of this thesis is to enhance the performance of the lift-driven VAWTs by customized airfoil design and the active ow control with trailing edge aps. To achieve the research goals, four main lines of work are carried out: 1) performance exploration of an actuator cylinder, 2) design of the azimuthal flap control sequences, 3) comparison between the Actuator Cylinder Model and Panel Model in the presence of the active flap control and 4) airfoil design that is specified for effective flap operation on a VAWT blade. The flap control sequences are optimized with the use of the Actuator Cylinder Model for three aerodynamic objectives. These objectives are aiming to improve the power efficiency by maximizing the CP , provide a rated power control by minimizing the CP and decrease the cyclic load ranges by minimizing the CT . Whereas, RFOIL and NSGA-II genetic algorithm are coupled for the airfoil design, which aims for objectives such as superior single airfoil performance, extended flap sensitivity and high flap-wise bending stiffness. For a rotor solidity of 0.1 operating at tip speed ratio of 4, a 10% active flap is able to increase the CP by 7% , decrease the CP by 10% and alleviate 12% of the CT by sacrificing 3% of the CP . It is shown that depending on the solidity, tip speed ratio and the flap authority these figures could be increased. It is possible to brake the rotor with a relatively larger flap authority. The airfoils designed in this work are slightly cambered and span from 27% to 35% thickness, deliver higher maximum CP than a NACA 0018, show good performance in the dirty conditions and have high flap effectivity. Overall, the performance gains acquired by the new airfoils and flap control promises extensive improvements in multiple features of VAWTs such as power effciency, power control, load control that lead to reduced weight and decreased Cost of Energy.

During the development of an aerodynamic analysis tool to predict non-linear lift and drag coefficients, research was performed on arbitrary wing configurations focusing on airborne wind energy (AWE) applications. The purpose of this research is to develop and examine the accuracy of computationally fast aerodynamic methods. The non-linear part and maximum value of the lift curve, with the corresponding drag, are of great interest for AWE applied wings, since these y at high angles of attack. By means of a trail and error procedure, a most promising method is found. This research was performed via ASSET at the faculty of Aerospace Engineering and Ampyx Power, following the MSc Sustainable Energy Technology at the faculty of Applied Sciences at the Delft University of Technology. Kite technology has been discovered some 3,000 years ago. Since then, kites have been used to for example send messages, pull carts and perform weather measurements. Nowadays, the most advanced kites are used for sports such as kite surfing. Although higher altitudes can be reached with kites and (AWE) have been developed to extract energy from the wind, the wind energy market is dominated by wind turbines. The efficiency of these turbines is reaching the Betz limit, leaving one option to increase power output, which is increasing the operating altitude. However, structural issues will then appear for a wind turbine tower, where a tethered kite can easily y up to several kilometers altitude. The opportunities of kite power systems have been discovered by scientists, which has resulted in a number of 40 active AWE companies around the world. Ampyx Power is one of these companies, where a tethered rigid sailplane, called the PowerPlane, is used to extract energy from the wind…