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Research Papers: Research Papers

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

[+] Author and Article Information
D. Holst

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

B. Church, F. Wegner, G. Pechlivanoglou, C. N. Nayeri, C. O. Paschereit

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

1Corresponding author.

Manuscript received July 3, 2018; final manuscript received July 20, 2018; published online October 17, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(3), 031020 (Oct 17, 2018) (10 pages) Paper No: GTP-18-1434; doi: 10.1115/1.4041146 History: Received July 03, 2018; Revised July 20, 2018

The wind industry needs reliable and accurate airfoil polars to properly predict wind turbine performance, especially during the initial design phase. Medium- and low-fidelity simulations directly depend on the accuracy of the airfoil data and even more so if, e.g., dynamic effects are modeled. This becomes crucial if the blades of a turbine operate under stalled conditions for a significant part of the turbine's lifetime. In addition, the design process of vertical axis wind turbines needs data across the full range of angles of attack between 0 and 180 deg. Lift, drag, and surface pressure distributions of a NACA 0021 airfoil equipped with surface pressure taps were investigated based on time-resolved pressure measurements. The present study discusses full range static polars and several dynamic sinusoidal pitching configurations covering two Reynolds numbers Re = 140k and 180k, and different incidence ranges: near stall, poststall, and deep stall. Various bistable flow phenomena are discussed based on high frequency measurements revealing large lift-fluctuations in the post and deep stall regime that exceed the maximum lift of the static polars and are not captured by averaged measurements. Detailed surface pressure distributions are discussed to provide further insight into the flow conditions and pressure development during dynamic motion. The experimental data provided within the present paper are dedicated to the scientific community for calibration and reference purposes, which in the future may lead to higher accuracy in performance predictions during the design process of wind turbines.

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Figures

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Fig. 1

Setup for dynamic and static measurements including A—motor, B—splitter plates, C—NACA 0021 airfoil with pressure taps, D—hollow shaft with room to connect time resolved pressure sensors, E—nozzle, F—collector, G—bearings, H—flexible coupling, I—1:10 angular gearbox, J—shaft, T—staggered surface pressure taps: (a) side view, (b) wind tunnel setup [8], and (c) top view

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Fig. 2

Pressure sensors (circled) in the hollow shaft on the opposite side of the motor. Flexible tubing connection to the airfoil's pressure taps (left arrow) and electrical connection to the amplifier (right arrow).

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Fig. 3

Pressure tap location

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Fig. 4

Literature validation in the stall range: (a) Re = 140 k and (b) Re = 180 k

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Fig. 5

Literature validation in the full 180 deg range: (a) lift coefficient and (b) drag coefficient

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Fig. 6

Effect of correction methods in the stall range including corrections proposed by the NATO-AGARD [23,24] and a correction proposed by Holst et al. [19]: (a) Re = 140 k and (b) Re = 180 k

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Fig. 7

Effect of correction methods in the full 180 deg range proposed by the NATO-AGARD [23,24] and a correction proposed by Holst et al. [19]: (a) lift coefficient and (b) drag coefficient

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Fig. 8

Near stall investigation between 0 and 20 deg at Re = 140 k for different reduced frequencies k: (a) k = 0.0500 and (b) k = 0.0250

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Fig. 9

Binning method for bistable configurations: (a) using all repetitions, (b) binning the repetitions, and (c) calculating mean of binned repetitions

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Fig. 10

Poststall investigation between 10 and 30 deg for a reduced frequency of k = 0.0500 at various Reynolds numbers Re: (a) Re = 140 k and (b) Re = 180 k

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Fig. 11

Poststall investigation between 10 and 30 deg at Re = 140 k for various reduced frequencies k: (a) k = 0.0500, (b) k = 0.0250, and (c) k = 0.0100

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Fig. 12

Deep stall investigation between 0 and 90 deg (0 to 70 deg shown) at Re = 180 k for various reduced frequencies k: (a) k = 0.0100, (b) k = 0.0050, and (c) k = 0.0025

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Fig. 13

Full range dynamic investigation between 0 and 180 deg for a reduced frequency of k = 0.0025 at various Reynolds numbers Re: (a) Re = 140 k and (b) Re = 180 k

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Fig. 14

Near stall investigation of cp between 0 and 20 deg at Re = 140 k for various angles of attack during pitch down: (a) cl for k = 0.0500, (b) cp: mean of repetitions in bin 1, (c) cp: mean of all repetitions, and (d) cp: mean of repetitions in bin 2

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Fig. 15

Poststall investigation of cp between 10 and 30 deg at Re = 140 k for various angles of attack during pitch up: (a) cl for k = 0.0500, (b) cp: mean of repetitions in bin 1, (c) cp: mean of all repetitions, and (d) cp: mean of repetitions in bin 2

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Fig. 16

Deep stall investigation of cp between 0 and 90 deg at Re = 180 k for various angles of attack during pitch up: (a) cl for k = 0.0050, (b) cp: mean of repetitions in bin 1, (c) cp: mean of all repetitions, and (d) cp: mean of repetitions in bin 2

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