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Research Papers

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

[+] Author and Article Information
D. Holst, B. Church, E. Tüzüner, C. N. Nayeri

Chair of Fluid Dynamics,
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany

G. Pechlivanoglou, J. Saverin

Chair of Fluid Dynamics,
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany

C. O. Paschereit

Chair of Fluid Dynamics,
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany
e-mail: David.Holst@TU-Berlin.de

1Corresponding author.

Manuscript received August 20, 2018; final manuscript received September 8, 2018; published online November 19, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(4), 041012 (Nov 19, 2018) (12 pages) Paper No: GTP-18-1576; doi: 10.1115/1.4041651 History: Received August 20, 2018; Revised September 08, 2018

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.

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References

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Worasinchai, S. , Ingram, G. L. , and Dominy, R. G. , 2011, “ A Low-Reynolds-Number, High-Angle-of-Attack Investigation of Wind Turbine Aerofoils,” Proc. Inst. Mech. Eng., Part A, 225(6), pp. 748−763. [CrossRef]
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Du, L. , Berson, A. , and Dominy, R. G. , 2014, “ NACA0018 Behaviour at High Angles of Attack and at Reynolds Numbers Appropriate for Small Wind Turbines,” Durham University, Durham, UK, Report No. ECS-TR 2014/08.
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Holst, D. , Balduzzi, F. , Bianchini, A. , Church, B. , Wegner, F. , Pechlivanoglou, G. , Ferrari, L. , Ferrara, G. , Nayeri, C. N. , and Paschereit, C. O. , 2018, “ Static and Dynamic Analysis of a NACA 0021 Airfoil Section at Low Reynolds Numbers Based on Experiments and CFD,” ASME J. Eng. Gas Turbines Power (accepted).
Holst, D. , Church, B. , Wegner, F. , Pechlivanoglou, G. , Nayeri, C. N. , and Paschereit, C. O. , 2018, “ Experimental Analysis of a NACA 0021 Airfoil Under Dynamic Angle of Attack Variation and Low Reynolds Numbers,” ASME J. Eng. Gas Turbines Power (accepted).
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Figures

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Fig. 1

Measurement setup for 180 deg polars: (a) motor including 1:10 gearbox, (b) splitter plates, (c) NACA 0021 airfoil with pressure taps, and (d) room for time-resolved pressure sensors

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Fig. 2

Tripping and pressure tap location

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Fig. 3

Signal processing and data reduction of a single polar measurement

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Fig. 4

NACA 0021: cp distribution: (a) pitch up and (b) pitch down

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Fig. 5

NACA 0021: Repeatability study at Re = 180 k. Single runs α⇑ lighter gray; α⇓ darker gray; mean in blue with triangles.

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Fig. 6

NACA 0021: Statistical analysis at Re = 180 k. The gray area marks the ±2σ level. Statistics based on 5000 data points per angle: (a) baseline and (b) tripped.

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Fig. 7

NACA 0021: cp distribution at Re = 180 k for angles within the second hysteresis loop for both pitching directions: (a) pitch up and (b) pitch down

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Fig. 8

NACA 0021: Reynolds study for baseline and tripped configuration: (a) baseline and (b) tripped

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Fig. 9

Validation against literature data. Literature drag data are the total drag cd while experimental data is pressure drag cdp: (a) lift polars up to stall angle and (b) full lift polars.

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Fig. 10

Trailing edge stall during α⇑ at Re = 180 k

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Fig. 11

NACA 0021: Comparison between baseline and tripped configuration for Re = 100 k and 180 k: (a) Re = 100 k and (b) Re = 180 k

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Fig. 12

NACA 0021: Effect of tripping on cp distribution for Re = 100 k and 180 k: (a) Re = 100 k and (b) Re = 180 k

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Fig. 13

NACA 0021: Effect of movement history on polars: (a) baseline and (b) tripped

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Fig. 14

NACA 0021: Effect of movement history on cp distribution: (a) pitch up and (b) pitch down

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