B. Akay

The origins of a wind turbine tip vortex

The tip vortex of a wind turbine rotor blade originates as a result of a complex distribution of vorticity along the blade tip thickness. While the tip vortex evolution was extensively studied previously in other work, the mechanism of the initiation of the tip vorticity in a 3D rotating environment is still somewhat obscured due to lack of detailed experimental evidence. This paper therefore aims at providing an understanding of how tip vorticity is formed at the wind turbine blade tip and what happens just behind the tip trailing edge. Stereo Particle Image Velocimetry (SPIV) is used to measure the flow field at the tip of a 2m diameter, two- bladed rotor at the TU Delft Open Jet Facility (OJF). The rotor has a rectangular blade tip. Spanwise measurements were performed for both axial and yawed flow conditions with a very small azimuthal increment. A 3D, unsteady, potential flow panel method is also used for the purpose of better understanding the tip bound vorticity. A validation study is carried out with positive results. This paper is focused on axial flow results. A complex distribution of vorticity is found along the blade tip thickness. Just after release, the tip vortex becomes almost immediately round and well defined. Observations from the MEXICO rotor are confirmed again by a slight inboard convection of the tip vortex. This is explained by means of the effect of chordwise vorticity at the tip from the numerical solutions. The results presented in this work suggest that a more physical interpretation of the tip loss effect is required. Currently, inclusion of tip effects are based primarily on either wake induced effects or on an empirical 3D correction for airfoil data. This research should stimulate a more rigorous approach, where the effects of the blade tip chordwise vorticity are implemented in tip correction models.

Effects of geometry and tip speed ratio on the HAWT blade's root flow

In this study, the effect of the parameters playing a role in the root flow behavior of HAWT are only partly understood. To better reveal the root flow properties, this study presents the progression of HAWT blade root flow at two different blade geometries and at two different tip speed ratios. The effects of the geometry and the tip speed ratio on the root flow behavior and on the evolution of the root flow features are investigated. This study aims to answer the following questions: (i) What are the effects of the blade geometry and tip speed ratio on the root flow behavior? (ii) How are the blade wake and the root vortex evolution affected by the change of these parameters? The analysis of the velocity fields shows that the radial flow behavior changes with different blade geometries but a remarkable difference in the radial flow behavior is not observed with the change of tip speed ratio. The formation of the wake is different at three test cases because of different loading that the blades are encountered. From the circulation distribution along the blades, while a strong root vortex can be observed in Blade 1, the bound vorticity along Blade 2 builds up gradually when moving outboard, and do not show a trace of a strong root vortex.

Experimental investigation of the wind turbine blade root flow

Several methods from experimental to analytical are used to investigate the aerodynamics of a horizontal axis wind turbine. To understand 3D and rotational effects at the root region of a wind turbine blade, correct modeling of the flow field is essential. Aerodynamic models need to be validated by accurate experimental data. In this paper, the experimental results of the aerodynamic behavior of a model wind turbine blade, by focusing on the blade root flow, are presented. The measurements are performed on a 2 bladed rotor having 1 m radius by means of Stereo Particle Image Velocimetry in a wind tunnel. The spanwise velocity distribution on the suction side of the blade is determined in detail. It shows a complex flow pattern in the root region and positive spanwise flow component apparent at radial stations beyond r/R=0.4 at the leading edge (z/c=0.25).

Experimental and Numerical Quantification of Radial Flow in the Root Region of a HAWT

This paper explores the evolution of radial flow in a Horizontal Axis Wind Turbine (HAWT) blade root region. The radial flow is analyzed in the potential flow and viscous flow regions. An experiment carried out by means of stereo Particle Image Velocimetry to measure the velocity field produced by a HAWT blade. While the radial flow in the potential flow region was obtained from the measurements, the radial flow in the boundary layer was derived from CFD. By the direct observations obtained from the experiment, an insight is gained about the nature of the radial flow in the suction side of the blade as well as in the near wake. An outboard radial flow motion is observed in the root region. This tendency of the flow changes dramatically when it reaches the maximum chord position of the blade, where the radial flow moves inboard. The trace of the viscous region due to merging of the boundary layers and trailing vorticity are observed clearly in the radial velocity and vorticity distributions at 135o azimuth angle of the blade. In the viscous flow region the radial flow is more pronounced than in the potential flow region. The performed CFD simulations are able to predict the vortex formation in the maximum chord region and its interaction with the nacelle.