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Wind turbine structures and components suffer excessive loads and premature failures when key aerodynamic phenomena are not well characterized, fail to be understood, or are inaccurately predicted. Turbine blade rotational augmentation remains incompletely characterized and understood, thus limiting robust prediction for design. Pertinent rotational augmentation research including experimental, theoretical, and computational work has been pursued for some time, but large scale wind tunnel testing is a relatively recent development for investigating wind turbine blade aerodynamics. Because of their large scale and complementary nature, the MEXICO and UAE Phase VI wind tunnel experiments offer unprecedented synergies to better characterize and understand rotational augmentation of blade aerodynamics. Cn means, Cn standard deviations, two-dimensional cp distributions, and three-dimensional planform surface pressure topologies from these two experiments were analyzed and compared. Rotating blade data were evaluated against analogous stationary blade data. Rotational augmentation effects were found to be pervasive and were present over the blade radius and throughout blade operating envelopes at all radial locations investigated. Rotational effects manifested themselves in both mean and time varying statistics, in both two-dimensional sectional data as well as three-dimensional planform data. Comparative analyses of MEXICO and UAE data validated and generalized current knowledge regarding rotationally augmented blade flow fields. In addition to confirming prior research, results also provided new insights not attainable by considering either data set in isolation of the other.

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 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.

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.

Investigating the flow physics in the vicinity of the wind turbine blade is a challenging endeavour. In the past, focus was placed on the understanding of near wake flows arising from wake vorticity and the rotor loading. In this work, a different approach is taken by considering the flow field in the blade vicinity as a consequence of the separate effects of bound and wake vorticity. This enables new insight regarding the role of the blade as having a direct influence on the three-dimensional flow. The approach is applied for the reference axial flow condition and hence for the yawed flow condition where the issue of flow three-dimensionality takes a new level of complexity. Three research hypotheses are investigated in this work: 1. Radial flow components especially close to the wind turbine blade are not negligible. This contradicts the classical momentum approach which treats the flow as two-dimensional. The situation for yawed flow is even more important since wake vorticity not only exhibits an expansion but also a skewness. A fundamental understanding of the behaviour of the radial flow component is hence of paramount importance. 2. The three-dimensional flow field close to a Horizontal Axis Wind Turbine (HAWT) rotor is due to the effects of body and wake vorticity. The blade tip shape plays a fundamental role on the behaviour of the flow field near the blade. 3. The tip vorticity for axial and yawed flow results in a different tip flow behaviour. The hypotheses are linked by a common goal; to establish new insight into three-dimensional flows in the proximity of the rotor in yawed flow, using axial flow as a baseline investigation. Both numerical and experimental approaches have been used to investigate these hypotheses. A 3D unsteady potential flow panel model is used for the numerical computations. The model permits to decompose flow due to diff erent vorticity components. Stereo Particle Image Velocimetry (SPIV) is used for the experimental measurements. This enables measurement of all velocity components in a 2D plane and can then be used to construct a 3D volume of data. Flow data from three different rotors is used: SPIV measurements from the Model Experiments in Controlled Conditions (MEXICO) rotor in the German-Dutch DNW wind tunnel and experiments performed in the Open Jet Facility of TU Delft on two different 2m diameter rotors. The thesis is structured into six parts as follows: Part I - Literature review to support and contextualize the research Part II - Analysis of the hypotheses on ow three-dimensionality Part III - Decomposition of velocities in the rotor proximity Part IV - Origins and dynamics of vorticity Part V - Conclusions Part VI - Appendices The results presented in this thesis challenge the current understanding of flow three-dimensionality in the rotor plane particularly for the yawed flow case. The blade's role as a vorticity generator as well as its active role in disturbing the flow due to its vorticity distribution are both supported. The creation of a HAWT tip vortex over the blade thickness is studied leading to important implications about the induced flow field at the tip. The details of flow three-dimensionality due to the complex behaviour of the tip vortex upon release are presented and the implications of this discussed. The outcome of this research bridges the gap between existing knowledge of the flow on the rotor scale to future lines of research which will be directed to the study of boundary layer flows of rotating blades. By extensively analyzing the rotor blade scale outer flow (outside of the boundary layer) this research gives impetus to a necessary revision of tip corrections in the application to the industry standard BEM design codes which to this day rely on models which are not based on the detailed knowledge of rotor blade flow which this research provides.