Experimental Analysis of a NACA 0021 Airfoil Section Through 180-Degree Angle of Attack at Low Reynolds Numbers for Use in Wind Turbine Analysis

Abstract

Wind turbine industry has a special need for accurate post stall airfoil data. While literature often covers incidence ranges [−10deg,+25deg] smaller machines experience a range of up to 90 deg for horizontal axis and up to 360 deg for vertical axis wind turbines (VAWTs). The post stall data of airfoils is crucial to improve the prediction of the start-up behavior as well as the performance at low tip speed ratios. The present paper analyzes and discusses the performance of the symmetrical NACA 0021 airfoil at three Reynolds numbers (Re = 100k, 140k, and 180k) through 180 deg incidence. The typical problem of blockage within a wind tunnel was avoided using an open test section. The experiments were conducted in terms of surface pressure distribution over the airfoil for a tripped and a baseline configuration. The pressure was used to gain lift, pressure drag, moment data. Further investigations with positive and negative pitching revealed a second hysteresis loop in the deep post stall region resulting in a difference of 0.2 in moment coefficient and 0.5 in lift.

Time‐accurate blade surface static pressure behaviour on a rotating H‐Darrieus wind turbine

“…Within the limit of the resolution provided by the finite number of pressure tappings, the laminar separation bubble can be identified at around 0.1 c ~0.2 c for θ = 35 ° and θ = 45 ° (see Figure ). It must be noted that at θ = 45 ° , the theoretical incidence angle is about α = 30 ° , which is already far beyond the static stall angle of ~13 ° (NACA0021) at similar Reynolds numbers . Even taking the curvature effect into consideration, the dynamic stall effect that significantly delays the stall angle is successfully captured in this study.…”

“…It must be noted that at θ = 45°, the theoretical incidence angle is about α = 30°, which is already far beyond the static stall angle of~13°(NACA0021) at similar Reynolds numbers. 43,44 Even taking the curvature effect into consideration, the dynamic stall effect that significantly delays the stall angle is successfully captured in this study. The laminar separation bubble bursts when the blade reaches about θ = 50°F (note: Since at this low tip speed ratio, the blade will experience all possible incidence angle from α = −180°to α = 180°; the radius axis for incidence angle data is significantly larger than that for torque coefficient data.…”

Experimental Analysis of a NACA 0021 Airfoil Under Dynamic Angle of Attack Variation and Low Reynolds Numbers

“…The first and second stall of the static polars, as well as their according hysteresis, are known to literature. Holst et al [1] showed a hysteresis loop at the very high angles in a full range investigation between 0 and 180 deg. This loop is also visible in the full range static polars shown in figs.…”

“…There are studies of symmetric airfoils that investigate thinner airfoils like the NACA 0012 [12,13] and NACA 0015 [13,14], but data of thicker airfoils, especially at lower Reynolds numbers, are rare. There are experimental studies of a NACA 0018 airfoil done by Rainbird et al [15,16] and Du et al [17,18] but the present study investigates the NACA 0021, providing additional data referring to the existing work of Holst et al [1,19]. This paper concentrates on experiments with dynamic AoA changes at two different Reynolds numbers of Re = 140 k and 180 k.…”

“…However, at the same time detailed knowledge of the airfoil performance is crucial for a good prediction [5][6][7][8]. Darrieus vertical axis wind turbines (VAWTs) are even more sensitive to airfoil polars, as their blades operate in stalled conditions for a significant part of their life [1,8]. They experience many dynamic effects, i.e.…”