An aerodynamic study on flexed blades for VAWT applications

Abstract

There is renewed interest in aerodynamics research of VAWT rotors. Lift type,

Darrieus designs sometimes use flexed blades to have an ’egg-beater shape’ with an optimum

Troposkien geometry to minimize the structural stress on the blades. While straight bladed

VAWTs have been investigated in depth through both measurements and numerical modelling,

the aerodynamics of flexed blades has not been researched with the same level of detail.

Two major effects may have a substantial impact on blade performance. First, flexing at the

equator causes relatively strong trailing vorticity to be released. Secondly, the blade performance

at each station along the blade is influenced by self-induced velocities due to bound vorticity.

The latter is not present in a straight bladed configuration. The aim of this research is to

investigate these effects in relation to an innovative 4kW wind turbine concept being developed

in collaboration with industry known as a self-adjusting VAWT (or SATVAWT). The approach

used in this study is based on experimental and numerical work. A lifting line free-wake vortex

model was developed. Wind tunnel power and hot-wire velocity measurements were performed

on a scaled down, 60cm high, three bladed model in a closed wind tunnel.

Results show a substantial axial wake induction at the equator resulting in a lower power

generation at this position. This induction increases with increasing degree of flexure. The

self-induced velocities caused by blade bound vorticity at a particular station was found to be

relatively small.