Aerodynamic behavior of an airfoil under extreme wind conditions

Ingrid Neunaber,Caroline Braud

doi:10.1088/1742-6596/1618/3/032035

Abstract

Wind turbines operate in the naturally turbulent atmospheric boundary layer. Due to strong flow variations, the aerodynamics at the rotor blades are complex. Therefore, to gain a better understanding of the effect of strong velocity and angle fluctuations on the aerodynamic behavior of an airfoil, we present a new system capable of generating rapid, strong gusts in a wind tunnel, the chopper. It consists of a rotating bar cutting through the inlet of the wind tunnel, thus generating turbulent, strong flow perturbations. Using this system and exposing an airfoil to its flow, we investigate the lift variations caused by the simultaneous, rapid velocity and angle variations. The results show that the lift response of the airfoil is directly correlated with the velocity. The lift response to changes of the angle of attack is determined not only by the change of the angle, but also by the rapidity with which it changes.

Highlights

Wind energy converters operate in the turbulent atmospheric boundary layer
It can be seen that the recurring inverse gust events can be separated from the periods where the chopper blade does not block any part of the wind tunnel
When the chopper blade enters the wind tunnel, the velocity first briefly increases before the velocity drops in the wake of the chopper blade

Summary

Introduction

Wind energy converters operate in the turbulent atmospheric boundary layer. The atmospheric turbulence is highly complex with three-dimensional, multi-scale vortices of varying frequencies. In this study, we present a new setup that is capable of generating reproducible, rapid, strong velocity and flow angle variations in a wind tunnel while simultaneously including turbulent fluctuations. The background turbulence intensity of the empty wind tunnel is T I = 0.3%, and a regular grid with 16.4% blockage and a mesh width of 7 cm is installed inside the nozzle which increases the background turbulence intensity to T I ≈ 3.0% This was done to improve the aerodynamic response of the airfoil at low Reynolds numbers To generate complex three-dimensional flow structures, a new perturbation system was installed It is called the chopper, and it consists of a rotating bar, the chopper blade, that is passing through the inlet of the test section as illustrated in figure 1. Already a comparatively small number of gusts suffices to reach the statistical convergence

Results

Airfoil response

Discussion and Conclusions