The origins of a wind turbine tip vortex

Abstract

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.