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Research Papers

Static and Dynamic Analysis of a NACA 0021 Airfoil Section at Low Reynolds Numbers Based on Experiments and Computational Fluid Dynamics

[+] Author and Article Information
David Holst

Mem. ASME
Chair of Fluid Dynamics
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany
e-mail: david.holst@tu-berlin.de

Francesco Balduzzi

Department of Industrial Engineering,
Università degli Studi di Firenze,
Via di Santa Marta 3,
Italy 50139, Firenze
e-mail: francesco.balduzzi@unifi.it

Alessandro Bianchini

Mem. ASME
Department of Industrial Engineering,
Università degli Studi di Firenze,
Via di Santa Marta 3,
Italy 50139, Firenze

Benjamin Church, Felix Wegner, Georgios Pechlivanoglou, Christian Navid Nayeri

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

Lorenzo Ferrari

Mem. ASME
DESTEC,
Università di Pisa,
Largo Lucio Lazzarino,
Pisa 56122, Italy
e-mail: lorenzo.ferrari@unipi.it

Giovanni Ferrara

Department of Industrial Engineering,
Università degli Studi di Firenze,
Via di Santa Marta 3,
Italy 50139, Firenze

Christian Oliver Paschereit

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

1Corresponding author.

Manuscript received July 3, 2018; final manuscript received July 20, 2018; published online January 8, 2019. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(5), 051015 (Jan 08, 2019) (10 pages) Paper No: GTP-18-1435; doi: 10.1115/1.4041150 History: Received July 03, 2018; Revised July 20, 2018

The wind industry needs airfoil data for ranges of angle of attack (AoA) much wider than those of aviation applications, since large portions of the blades may operate in stalled conditions for a significant part of their lives. Vertical axis wind turbines (VAWTs) are even more affected by this need, since data sets across the full incidence range of 180 deg are necessary for a correct performance prediction at different tip-speed ratios. However, the relevant technical literature lacks data in deep and poststall regions for nearly every airfoil. Within this context, the present study shows experimental and numerical results for the well-known NACA 0021 airfoil, which is often used for Darrieus VAWT design. Experimental data were obtained through dedicated wind tunnel measurements of a NACA 0021 airfoil with surface pressure taps, which provided further insight into the pressure coefficient distribution across a wide range of AoAs. The measurements were conducted at two different Reynolds numbers (Re = 140 k and Re = 180 k): each experiment was performed multiple times to ensure repeatability. Dynamic AoA changes were also investigated at multiple reduced frequencies. Moreover, dedicated unsteady numerical simulations were carried out on the same airfoil shape to reproduce both the static polars of the airfoil and some relevant dynamic AoA variation cycles tested in the experiments. The solved flow field was then exploited both to get further insight into the flow mechanisms highlighted by the wind tunnel tests and to provide correction factors to discard the influence of the experimental apparatus, making experiments representative of open-field behavior. The present study is then thought to provide the scientific community with high quality, low-Reynolds airfoil data, which may enable in the near future a more effective design of Darrieus VAWTs.

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Figures

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Fig. 1

Experimental wind tunnel setup

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Fig. 2

Inflow velocity: Reynolds number, turbulence level, and PSD analysis starting at the center of the nozzle toward the outer edge for Re = 140 k at the LE position of the airfoil

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Fig. 3

Pressure tap locations

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Fig. 4

Experimental cp distribution during pitch up. Filled markers define the suction side, while empty markers define the pressure side. The darker, the higher the AoA.

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Fig. 5

Experimental cp distribution during pitch down. Filled markers define the suction side, while empty markers define the pressure side. The darker, the higher the AoA.

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Fig. 6

Experimental cl, cdp, and cm at Re = 140 k and 180 k. Markers are displayed every ten samples only for better readability.

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Fig. 7

Computational fluid dynamics domain: (a) wind tunnel domain and (b) open field domain

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Fig. 8

Streamlines and axial velocity contours for the NACA 0021 airfoil at α = 10 deg and Re = 140 k: (a) wind tunnel domain and (b) open field domain

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Fig. 9

Computational grid: (a) airfoil, (b) rotating domain, and (c) boundary layer at the leading edge

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Fig. 10

Experimental and CFD lift polars for different domains at Re = 140 k

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Fig. 11

Comparison of experimental and CFD pressure distributions at Re = 140 k

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Fig. 12

Experimental and CFD lift polars including the correction method at Re = 180 k

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Fig. 13

Sinusoidal movement between 0 deg and 20 deg by CFD and experiment (thin lines represent 40 experimental repetitions) at Re = 140 k and k = 0.05

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Fig. 14

Sinusoidal movement between 0 deg and 20 deg within CFD using the proposed correction and experiment (thin lines represent 40 experimental repetitions) at Re = 140 k and k = 0.05

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Fig. 15

Computational fluid dynamics vorticity contours and streamlines at different angular positions at Re = 140 k in open field domain: static polar (a), dynamic polar between 0 deg and 15.4 deg at k = 0.05 during pitch up (b) and pitch down (c)

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Fig. 16

Sinusoidal movement between 5 deg and 25 deg within CFD and experiment (thin lines represent 40 experimental repetitions) at Re = 180 k and k = 0.025

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Fig. 17

Sinusoidal movement between 10 deg and 30 deg within CFD using the proposed correction and experiment (thin lines represent 40 experimental repetitions) at Re = 180 k and k = 0.05

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Fig. 18

Corrected experimental lift polars for the NACA 0021 airfoil at Re = 140 k and Re = 180 k

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