Unsteady effects on a pitching airfoil at conditions relevant for large vertical axis wind turbines

C E Brunner,MOL Hansen,J Kiefer,M Hultmark

doi:10.1088/1742-6596/1618/5/052065

Abstract

Interest in offshore vertical axis wind turbines (VAWTs) has created a need to study VAWTs at much higher Reynolds numbers than they have previously been studied at. VAWTs are characterised by unsteady aerodynamics, and high Reynolds numbers have the potential to alter the blade aerodynamics significantly. Here, results are reported on an airfoil that is pitched sinusoidally around zero angle of attack at Rec = 1 × 106, at a reduced frequency k = 0.15 and amplitudes of 5° < α < 20°. Since the static stall angle is not exceeded, no stall effects occur. Nevertheless, lift, drag and moment coefficients show noticeable hysteresis. Despite the high amplitudes of oscillation, lift and moment coefficients show reasonable agreement with unsteady aerodynamic theories by Theodorsen and Motta et al. with regard to the width of the hysteresis loops, but have noticeably steeper slopes.

Highlights

Vertical axis wind turbines (VAWTs) have several advantages compared to conventional horizontal axis wind turbines (HAWTs) [1]
Due to the high Reynolds number and the relatively low aspect ratio of the wing model, the static stall angle αss = 21◦ is higher than in most vertical axis wind turbines (VAWTs) studies reported in the literature
Unsteady effects lead to hysteresis in the lift, drag and moment coefficients

Summary

Introduction

Vertical axis wind turbines (VAWTs) have several advantages compared to conventional horizontal axis wind turbines (HAWTs) [1]. They are agnostic to wind direction, have a simpler mechanical design, and have the potential to interact beneficially with each other when arranged in farms [2]. The non-dimensional parameters that characterise the aerodynamics of a VAWT are the Reynolds number, the tip speed ratio λ, and the solidity σ: ReD = U∞D ν ωD λ= 2U∞ nc σ= D. Most VAWTs deployed to date are significantly smaller than commercial HAWTs. Modern offshore HAWTs have diameters up to 220 m and produce up to 12 MW [3]. Offshore VAWTs would be considerably larger and experience higher wind speeds than previously studied VAWTs, leading to significantly

Methods

Results

Conclusion