Experimental Analysis of a NACA 0021 Airfoil Section Through 180-Deg 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 [−10 deg, +25 deg], 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 = 100 k, 140 k, and 180 k) 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.

Near Stall Unsteady Flow Responses to Morphing Flap Deflections

“…A few possible reasons could explain those discrepancies observed between the two numerical predictions and the test data. In the experimental work performed by Church et al [37] investigating an NACA 0021 airfoil at lower Reynolds numbers (less than 200,000), for the same Reynolds number it was suggested that Sheldahl et al's data [36] undermeasured both the stall angle and the CL,max value. It was concluded that Sheldahl et al's experiment is not representative of the NACA 0021 airfoil, mainly because the data prior to stall and early stall were calculated using an in-house PROFILE computer code [36].…”

Preliminary assessment of the NACA0021 trailing edge wedge for wind turbine application

“…The current study chose the NACA 0021 airfoil with a chord length (c) of 140 mm. This particular airfoil has undergone both experimental and computational examinations by several researchers in the past [ 37 , 43 , 44 ]. The effects of various WT and FWT dimensions were simulated and benchmarked against the clean airfoil case as follows:.…”

The effect of trailing-edge wedge-tails on the aerodynamic characteristics of wind turbine airfoil

“…The graph in Figure 4 presents computational and experimental data for the NACA 0021 at ranges of AOAs from 0 օ to 20 օ . The literature data was taken out of some previous works, the wind tunnel experiment by Holst [17] and CFD simulation work recently done by Balduzzi [23].…”