Body and wake influences on the 3D flow close to a HAWT rotor

Abstract

This work focuses on the hypothesis that flow three-dimensionality in the rotor proximity

is strongly affected by blade blockage and bound circulation.

The extent of flow three-dimensionality in the blade proximity needs to be assessed

particularly in sensitive regions such as the root and tip of the blade. Moreover the induction

needs to be decomposed into two components; that resulting due to the blade presence and

that due to the wake. In this regard, the estimation of bound vorticity is essential such that

its effects and those due to trailing vorticity can be understood.

To investigate the hypothesis, an experimental and a numerical approach is used. A

two bladed, 2m diameter rotor was tested in the Open Jet Facility at TU Delft measuring

6m×6.5m×13.5m using Stereo Particle Image Velocimetry (SPIV).

The flow field around the tested HAWT blade was successfully measured and the velocity

components around the entire blade were studied. The bound vorticity was measured using

the experimental data and compared well with the numerical measurements. The radial

flow, responsible for 3D effects was found to have high values not at the 100% span position

but rather at around 90% span. The flow components were also decomposed using the

potential flow model. Whilst the wake causes an increase in the flow three-dimensionality,

the blade effect was found to be indeed the opposite and hence to decrease the inductions

(and hence flow three-dimensionality).

While momentum approaches utilize correction functions to account for the finite nature

of the HAWT blade, we provide new understanding of the three-dimensional flow field which

may prove useful for future work in refining momentum formulations.