3D flows near a HAWT rotor

Abstract

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.