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The field of remote sensing and information gathering is being revolutionized by the recent developments in Micro Air Vehicles, MAVs. The need for maneuverability and flight in confined spaces has directed the focus of research towards flapping flight. Biological flapping flyers exhibit all the characteristics that are desired by MAVs. Biological flyers are able to hover, make rapid changes in their attitude, and navigate through very narrow spaces. For the purpose of this Master thesis the hawkmoth was used as a starting point and source of inspiration. First and foremost the hawkmoth is a fairly large insect, with a wingspan of roughly 10 cm. The larger scale of the insect will translate to an easier design in terms of the scalability of structural and electronic components. Furthermore, the hawkmoth displays consistent and simple kinematics. The complex aerodynamics are not fully understood, it is still unclear how we can design flapping kinematics that will lead to an optimum performance with respect to maneuverability, speed, and energy efficiency and are feasible to manufacture. The problem is tackled through an experimental campaign in a water channel. The kinematics investigated are two pitch-plunge motions based on the hawkmoth and a third purely sinusoidal motion. The Reynolds number is 4, 800 and the reduced frequency is 0.38, similar to the hawkmoth. The kinematics are subjected to force data acquisition, flow visualization, and particle image velocimetry. The results are compared to flapping experiments and calculations using computational fluid dynamics and an unsteady vortex model. The results show that the effect of the elevation angle is very important when looking at the details of the force development. A strong correlation between the strength of the leading and trailing edge vortex and the forces was found. The force data acquired during the experiments compares well to the to the CFD calculations. The calculated force coefficients are between 82 and 87% of the magnitude measured during the experiments. The bias towards a lower value can be explained partially by the assumptions of the CFD model and the presence of blockage effects during the experiments. Comparison with the unsteady aerodynamic vortex model suggests that the dominant force generating mechanisms are the leading and trailing edge vortices. The impact of spanwise flow and tip vortices on the overall magnitude of the forces is not as significant. The comparison of pitch-plunge experiments with flapping experiments showed a good agreement as well. The force coefficients measured during flapping are about 50% smaller compared to the pitch-plunging case. The difference is attributed due to the fundamental difference in kinematics and the definition of the reference velocity. The agreement of the shape of the time history of the forces suggests that the underlying flow topology is analogues in both cases. It remains to be evaluated if the spanwise flow component is also present and whether the evolution of the tip vortices is comparable. The application of advanced and delayed rotation proved to have dramatic effects on the force generation. Delayed rotation is detrimental to the force production, thrust produced was 60% less compared to the baseline cases. Advanced rotation yields an increase in thrust of 34 to 47% with a reduction of up to 20% in efficiency. The flow topology was remarkably similar to the baseline kinematics with a slight shift in phase. Both the application of advanced and delayed rotation provides a large potential in maneuverability. An increase in reduced frequency to a value of 0.7 lead to an increase in the thrust produced. A qualitative study of the flow topology showed that the vortical structures were similar to the baseline cases. A small decrease in efficiency was measured for the bio-inspired cases and a small increase for the harmonic motion. It might thus be favorable for roboflyers to flap at higher reduced frequencies.

The installation of floating wind farms in deeper water is encouraged by the stronger and steadier wind, the lower visibility and noise impact, the absence of road restrictions, but also the absence or shortage of shallow water. In the summer of 2009, the first large-scale floating wind turbine ”Hywind” was installed. Hywind is a spar-buoy concept with three catenary mooring lines. The experience with modeling floating turbines is still limited. Furthermore, existing models for the design of offshore wind turbines are highly complex as they focus - by definition - mostly on the forces of the wind on the turbine. The correctness and applicability of existing simulation models for the design of floating wind turbines can therefore not be assumed a-priori and need to be researched. This requires that the driving physical processes governing the behaviour of floating wind turbines are investigated first. For this purpose, a new basic model A.T.FLOW has been developed. The requirement of A.T.FLOW is that it incorporates the most significant physical processes so as to be able to provide insight into the dominant physical behaviour of spar-type floating wind turbines. Assumptions have been made that illustrate the limitations of A.T.FLOW. Various verification methods show that the model simulates load cases as expected and is a useful tool for assessing the physical behaviour of spar-type floating wind turbines. The coming two years the body forces and behaviour of the operating full-scale Hywind demo project is monitored. This data should be used to further test and validate A.T.FLOW and to guide further development of the model.

This paper presents the database of the measurements that have been acquired over the years in the Offshore Wind farm Egmond aan Zee (OWEZ). In addition, two topics are discussed: the analyses on this database that were performed in the past, and the potential for new analyses in the framework of the EU/FP7 project MaRINET. The OWEZ database and the analyses performed on it offer a unique opportunity to study the external conditions of an offshore wind energy site.

The scope of this work was to investigate radial flow component for a Horizontal Axis Wind Turbine in axial flow conditions and to assess its impact on the turbine operation. This was done by means of Particle Image Velocimetry and numerical simulation with a 3D unsteady potential-flow panel model. A direct comparison between the numerical and experimental radial velocity results show differences in the tip and root regions. These differences have important implications on the wake development just at the moment of release of the tip vortex. Moreover, the impact of the radial velocities on the blade loading has been studied using the numerical results. The contribution of the radial velocity to the normal load on the blade is only slightly appreciable in the tip and root regions of the blade. However, as the numerical model does not account for viscous effects, further analysis of impact on boundary layer development is necessary.

The three-dimensional behavior of jet core breakdown is investigated with experiments conducted on a free water jet at Re = 5000 by time-resolved tomographic particle image velocimetry (TR-TOMO PIV). The investigated domain encompasses the range between 0 and 10 jet diameters. The characteristic pulsatile motion of vortex ring shedding and pairing culminates with the growth of four primary in-plane and out-of-plane azimuthal waves and leads to the formation of streamwise vortices. Vortex ring humps are tilted and ejected along the axial direction as they are subjected to higher axial velocities. By the end of the potential core, this process causes the breakdown of the vortex ring regime and the onset of streamwise filaments oriented at 30°-45° to the jet axis and “C” shaped peripheral structures. The latter re-organize further downstream in filaments oriented along the azimuthal direction at the jet periphery. Instead, in the vicinity of the jet axis the filaments do not exhibit any preferential direction resembling the isotropic turbulent regime. Following Powell's aeroacoustic analogy, the instantaneous spatial distribution of the acoustic source term is mapped by the second time derivative of the Lamb vector, revealing the highest activity during vortex ring breakdown. A three-dimensional modal analysis of velocity, vorticity, Lamb vector, and Lamb vector second time derivative fields is conducted by proper orthogonal decomposition (POD) within the first 10 modes. The decomposed velocity fluctuations describe a helical organization in the region of the jet core-breakdown and, further downstream, jet axis flapping and precession motions. By the end of the potential core, vorticity modes show that vortex rings are dominated by travelling waves of radial and axial vorticity with a characteristic 40°-45° inclination to the jet axis. The Lamb vector and the Lamb vector second time derivative modes exhibit similar patterns for the azimuthal component, whereas the vortex ring coherence is described by the radial and the axial components. While velocity, vorticity, and Lamb vector modes are typically associated with Strouhal numbers (St) smaller than 0.9, the modes of the Lamb vector second time derivative are also related to higher frequencies (1.05 ≤ St ≤ 1.9) ascribed to the three-dimensional travelling waves. Far-field acoustic predictions are obtained on the basis of direct evaluation of Powell's analogy with TR-TOMO PIV data. The spectral analysis returns peaks at pairing (St = 0.36) and shedding (St = 0.72) frequency. A broader distribution with a hump between St = 1 and 2.25 is observed, which corresponds to the breakdown of ring vortices.

This paper sets out to describe the measurements and computations to construct three components of velocity field around the blade. The primary aim of the measurements was to gain insight into the physics of the flow field produced by a horizontal axis wind turbine-HAWT blade. Stereo Particle Image Velocimetry experiments were performed on a two-bladed HAWT rotor in the open jet facility. Three components of velocity on 2D planar measurement planes were obtained from the defined field of views. The three components of velocity at the different radial positions are analysed in the present paper by comparing the experimental results with the panel code results. Besides having an insight about the flow field around the blade section, this comparison enables to improve and validate the panel code. The measurements show very well agreement with the computations except at the tip trailing edge region which is expected. The key observation of this work is inboard motion of the tip vortex. Also, clear outward motion of the radial flow on the suction side of the inboard sections of the blade is observed in the measurements and computations.

The linear scaling laws for upscaling wind turbine blades show a linear increase of stresses due to the weight. However, the stresses should remain the same for a suitable design. Application of linear scaling laws may lead to an upscaled blade that may not be any more a feasible design. In this paper a non-linear upscaling approach is presented with the aim of keeping the stresses in the upscaled blade the same as the reference blade. The stresses due to the weight, aerodynamics and centrifugal forces are taken into account and the blade is modeled as a beam with equivalent structural properties. This new methodology is used to upscale the 5 MW NREL wind turbine blade to a 20 MW wind turbine blade. As a result, a 20 MW wind turbine blade is obtained in which the stresses are the same as the 5 MW blade. This provides initial blade design solution for optimization studies that is feasible and enables the designer to explore other interesting aspects of larger scale wind turbines.

If at least one reference wind turbine is available, which provides sufficient information about the wind turbine loads, the loads acting on the neighbouring wind turbines can be predicted via an artificial neural network (ANN). This research explores the possibilities to apply such a network not only within a wind park but on turbines located at different sites. Following the idea to develop a tool to forecast the particular loads of any wind turbine in the field without the need to install additional measuring systems, a model has been developed needing only signals contained in the Supervisory Control and Data Acquisition (SCADA) data as input signals.

A detailed quantitative description of the aerodynamics of a horizontal axis wind turbine (HAWT) is difficult due to complexity of the flow field. Several methods from experimental to analytical are used to investigate the aerodynamics of a HAWT. In the present study, a wind tunnel experiment and computational fluid dynamics (CFD) simulations are used to explore the expansion of the wake. 2D actuator disc (AD) simulations are compared with the wind tunnel experiments. To understand the aerodynamic behavior of a model wind turbine blade, a detail flow field measurements in chordwise-spanwise directions and in the wake have been done. The measurements are performed on a 2 bladed rotor by means of Stereo Particle Image Velocimetry (Stereo PIV) in an open jet wind tunnel. In this paper, the velocity measurements performed in the wake region of the blade is presented. Actuator disc simulations are performed by applying a constant pressure jump on a permeable disc of zero thickness. Actuator disc simulations are carried out by using FLUENT 6.3.26 with the incompressible version of the Reynolds Averaged Navier-Stokes (RANS) equations. By validating the simulations with the experimental results, one may conclude that the unsteady CFD modeling works correctly and the wake expansion of the prescribed model is affected by the geometry of the Open Jet Facility (OJF).

The purpose of this paper is to integrate the controller design of wind turbines with structure and aerodynamic analysis and use the final product in the design optimization process (DOP) of wind turbines. To do that, the controller design is automated and integrated with an aeroelastic simulation tool. This integrated tool is linked with an optimization engine. The automated controller has two built-in control algorithms; a generator-torque controller and an above rated pitch-controller. This new tool is used in the DOP of the 5MW NREL research wind turbine. To show how this method works some parameters of both the generatortorque controller and the pitch-controller are introduced as design variables in the DOP. As the result of changing controller related design variables within each optimization iteration, the values of the objective function and the design constraint also change. This shows that by introducing the controller’s parameters as design variables in the DOP a more realistic assessment of the objective function and constraints is possible that helps the optimizer to search for better solutions.

A wind turbine blade has a complex shape and consists of different elements with dissimilar material properties. To do any aeroelastic simulation, the structural properties of the blade such as stiffnesses and mass per unit length should be known in advance, and extracting these properties is a difficult task. This paper presents an analytical model to extract these structural properties in a simple way. It starts with calculating an equivalent material property of the cross section using weighting method. Then the centroid of each section is obtained. Next the second moment of inertia of each element relevant to its local coordinates system is calculated and transferred to the centroid of the section using parallel axis theorem. A coordinate transformation is employed to rotate these second moment of inertias around any arbitrary axis. Finally, flapwise and edgewise stiffnesses are found by multiplying the equivalent modulus of elasticity to the second area moment of inertia in each section. Mass per unit length is calculated by multiplying the equivalent density to the real area of each section. The method is verified with the structural properties of a commercial 660 kW wind turbine blade. Despite the simplicity of the method the results show a good agreement.

The probabilistic method commonly applied to arrive at the ultimate loading is as follows: for several different mean wind speeds load simulations are performed. For each mean wind speed a conditional distribution can be fitted to the load maxima for that particular wind speed. The overall distribution of the load response is obtained by a weighted average of these conditional distributions taking into account the probability of occurrence of the wind speed bins (Weibull). Two practical issues are addressed: • The plotting positions • The averaging over the mean wind speeds The plotting positions (for the m-th ranked value out of N) are unique and given by: m/(N +1) . This means that the plotting positions do not depend on the particular application and/or the anticipated distribution function. The maximum of the 50 year estimates based on the exceedance probability Q short (LlUi) is an upper bound of the long term 50 year load value L50. A lower bound for L50 is given by the maximum of the estimates based on the relative exceedance probability Rshort (LlUi)=Qshort (LlUi) ni; with ni the fraction of time for wind speed bin Ui. In the situation that load data of just 1 wind speed bin is available it is in general not possible to determine L50. In case it is assumed that the considered wind speed bin governs the load, a good estimate (lower bound) of L50 is obtained by considering Rshort. If it is assumed that the load distributions of the other wind speed bins are about the same, a good estimate is obtained by considering Qshort (upper bound).

Wind energy in the urban environment faces complex and often unfavorable wind conditions. High turbulence, lower average wind velocities and rapid changes in the wind direction are common phenomena in the complex built environments. A possible way to improve the cost-efficiency of urban wind turbines is the application of flowenhancing structures on or near the turbines. For horizontal axis wind turbines (HAWTs), applying a diffuser has shown to have a beneficial impact on the power production, but it is still under development. For a vertical axis wind turbine (VAWT) it is expected that flow augmentation will also strongly increase the performance of the turbine, but very little research has been done in this field. The purpose of this research is to investigate the effects of a diffuser on the airflow through a VAWT. In order to investigate these effects, the turbine (with and without diffuser) is simulated using a 2-D unsteady free-wake potential-flow panel model. The local flow field, local angles of attack, shed vorticity, the shape and strength of the wake, and the rotor torque are investigated for both the case with and without the diffuser. The diffuser used in this research consists of two mirrored airfoil cross-sections. The size of the duct-opening in which the turbine operates is varied. This work shows that unlike for a 1-D actuator disc analysis, the area ratio B of the diffuser exit with respect to the diffuser nozzle area is not the only driving factor in the augmentation of the rotor torque of the VAWT. More important are the effect of the directional change of the rotor inflow and the faster downstream transport of the shed vorticity.

The unsteady flow conditions experienced by wind turbine blades lead to fatigue loads due to gusts, that increase the cost of energy. The decrease of the impact of these unsteady loads will most certainly lead to a decrease of the cost of energy. In order to alleviate unsteady loads the Smart Rotor Blade approach [2] applies spanwise-distributed smart load control devices, which sense the flow and consequently react on the flow. The smart load control devices are applied to avoid the fluctuating unsteady aerodynamic loads. In the context of alleviating these loads, the unsteady behaviour of the flow over a 2D airfoil due to the actuation of a 0.2 c flap is investigated. By building a database of unsteady flow experiments, reference material is created for the validation of computational fluid dynamics models simulating unsteady conditions. Eventually, the knowledge of unsteadiness of the flow acquired can be applied in projects like the Smart Rotor Blade with the purpose to reduce fluctuating blade loads. An airfoil model of the type DU96W180, with a span of 1.8 meter and a chord of 0.5 meter is tested. Using Particle Image Velocimetry (PIV), the flow is visualized as a function of flap position under unsteady conditions. The unsteadiness addressed is expressed in reduced frequency k, simulating a steady case at k = 0 and unsteady flows at k = 0.1 and k = 0.2. With the integration technique derived by F. Noca [1], the unsteady forces are calculated on the blade, using velocity fields obtained from PIV measurements around the model. Multiple PIV images are stitched together and interpolated on a general grid in order to obtain a velocity vector field of the flow around the model. Having the velocity vector fields at different time steps within one cycle of the flap motion, allows for the determination of a time dependent set of unsteady forces.

The main motivation behind this work is to create a purely analytical engineering model for wind turbine wake upward deflection due to shear flow, by developing a closed form solution of the velocity field due to an oblique vortex ring. The effectiveness of the model is evaluated by comparing the results with those of a free-wake model. The solution of the velocity field due to an oblique vortex ring is obtained by using the result of an upright ring along with an equivalent point method. The wake model is derived using oblique ring elements with a number of suitable assumptions. Results of wake vertical deflection are compared with a free-wake solution. A linear trend between wake deflection and shear flow exponent is found with both models. The oblique ring model shows some discrepancies from the free-wake result in terms of the dependence of the deflection on the reference tip speed ratio. The oblique ring model needs further refinements and validation with experimental work and is only currently suited for the determination of general wake kinematics. It however provides immediate results for a given input and can be useful in generating databases with wake geometry information.

The statistics of atmospheric stability and non-dimensional wind profiles are studied using the standard surface-layer theory at Egmond aan Zee in the North Sea. Measurements at 21, 70 and 116 m are used to validate the theoretical profiles. Charnock’s relation is used to estimate the sea surface roughness. Bulk Richardson number is used to estimate the Obukhov length. The measured sea water temperature has a positive bias of 0.82∘C resulting in the dominance of unstable conditions and a poor agreement of the theoretical wind profiles with the measurements. The conditions at Egmond aan Zee are dominated by unstable and neutral stabilities. The theoretical wind profiles agree very well with the measurements in the unstable and neutral conditions. In stable conditions, the wind profiles are over-predicted significantly as the height increases. The scaling of the wind profile with respect to the boundary layer height is necessary under stable conditions and the addition of another length scale parameter is preferred.

Airfoil characteristics at deep stall angles were investigated. It appeared that the maximum drag coefficient as a function of the airfoil upwind y/c ordinate at x/c=0.0125 can be approximated by a straight line. The lift-drag ratios in deep stall of a number of airfoils with moderate lower surface thickness coincide. It was found that the lift-drag ratio of airfoils with leading edge separation is independent of aspect ratio. The lift-drag ratios of the various sections of a non-rotating and a rotating blade in deep stall coincide with the two-dimensional curve.

The invention relates to a method for bonding a thermoplastic polymer to a thermosetting polymer component, the thermoplastic polymer having a melting temperature that exceeds the curing temperature of the thermosetting polymer. The method comprises the steps of providing a cured thermosetting polymer component comprising an implant of a thermoplastic polymer at least at the part of the thermosetting polymer component to be bonded, locating a thermoplastic polymer in contact with at least the part to be bonded, heating the assembly to the melting temperature of the thermoplastic polymer, whereby the thermoplastic polymer of the implant melts and fuses with the thermoplastic polymer, and cooling the assembly. The thermoplastic polymer has a melting temperature that exceeds the curing temperature of the thermosetting polymer, and the implant is designed such that heating above the maximum operating temperature of the thermosetting polymer at the interface of the implant with the thermosetting polymer component is avoided during the bonding step.

An experimental study of flow separation control with a nanosecond pulse plasma actuator was performed in wind-tunnel experiments. The discharge used had a pulse width of 12 ns and rising time of 3 ns with voltage up to 12 kV. Repetition frequency was adjustable up to 10 kHz. The first series of experiments was to measure integral effects of the actuator on lift and drag. Three different airfoil models were used, NACA-0015 with the chord of 20 cm, NLF-MOD22A with the chord of 60 cm and NACA 63-618 with the chord of 20 cm. Different geometries of the actuator were tested at flow speeds up to 80 m/s. In stall conditions the significant lift increase up to 20% accompanied by drag reduction (up to 3 times) was observed. The critical angle of attack shifted up to 5–7 degrees. The relation of the optimal discharge frequency to the chord length and flow velocity was proven. The dependence of the effect on the position of the actuator on the wing was studied, showing that the most effective position of the actuator is on the leading edge in case of leading edge separation. In order to study the mechanism of the nanosecond plasma actuation experiments using schlieren imaging were carried out. It shown the shock wave propagation and formation of large-scale vortex structure in the separation zone, which led to separation elimination. PIV diagnostics technique was used to investigate velocity field and quantitative properties of vortex formation. In flat-plate still air experiments small scale actuator effects were investigated. Measured speed of flow generated by actuator was found to be of order of 0.1 m/s and a span-wise nonuniformity was observed. The experimental work is supported by numerical simulations of the phenomena. The formation of vortex similar to that observed in experiments was simulated in the case of laminar leading edge separation. Model simulations of free shear layer shown intensification of shear layer instabilities due to shock wave to shear layer interaction.