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Research Papers: Fundamental Issues and Canonical Flows

Vortex-Induced Vibration and Frequency Lock-In of an Airfoil at High Angles of Attack

[+] Author and Article Information
Fanny M. Besem

Department of Mechanical Engineering and
Materials Science,
Duke University,
Durham, NC 27708
e-mail: fb46@duke.edu

Joshua D. Kamrass

Department of Mechanical Engineering and
Materials Science,
Duke University,
Durham, NC 27708
e-mail: jdk34@duke.edu

Jeffrey P. Thomas

Department of Mechanical Engineering and
Materials Science,
Duke University,
Durham, NC 27708
e-mail: jthomas@duke.edu

Deman Tang

Department of Mechanical Engineering and
Materials Science,
Duke University,
Durham, NC 27708
e-mail: demant@duke.edu

Robert E. Kielb

Fellow ASME
Department of Mechanical Engineering and Materials Science,
Duke University,
Durham, NC 27708
e-mail: rkielb@duke.edu

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received January 16, 2015; final manuscript received July 18, 2015; published online August 21, 2015. Assoc. Editor: Sharath S. Girimaji.

J. Fluids Eng 138(1), 011204 (Aug 21, 2015) (9 pages) Paper No: FE-15-1036; doi: 10.1115/1.4031134 History: Received January 16, 2015; Revised July 18, 2015

Vortex-induced vibration is a fluid instability where vortices due to secondary flows exert a periodic unsteady force on the elastic structure. Under certain circumstances, the shedding frequency can lock into the structure natural frequency and lead to limit cycle oscillations. These vibrations may cause material fatigue and are a common source of structural failure. This work uses a frequency domain, harmonic balance (HB) computational fluid dynamics (CFD) code to predict the natural shedding frequency and lock-in region of an airfoil at very high angles of attack. The numerical results are then successfully compared to experimental data from wind tunnel testings.

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References

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Figures

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Fig. 1

NACA0012 airfoil in Duke University low-speed wind tunnel. The flow speed is determined by hot wire, located upstream of the test section.

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Fig. 2

Position of the pressure transducers located at midspan of the NACA0012 airfoil

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Fig. 3

Magnitude of the Fourier transformed surface pressure on the pressure side of the airfoil for α = 50 deg and Re = 250,000

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Fig. 4

Strouhal number versus flow speed. The Strouhal number is based on the characteristic length perpendicular to the flow direction, or St = 2π·f·c· sin(α)/U∞. The different curves represent the different angles of attack, from 25 deg to 90 deg.

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Fig. 5

Corrected Strouhal number versus flow speed. The different curves represent the different angles of attack, from 25 deg to 90 deg. The filled symbols represent the CFD results.

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Fig. 6

Close-up of the 2D grid used in the CFD simulations

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Fig. 7

Change in unsteady lift phase per iteration for different reduced frequency input to the HB code. Re = 200,000 and α=40 deg.

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Fig. 8

Unsteady pressure contours in the flow field at four snapshots in time. The arrow indicates the direction of the flow (40 deg in this case).

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Fig. 9

Representation of the airfoil motion during one period of oscillation

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Fig. 10

Surface pressure contours at four snapshots in time. The pressure is normalized by the dynamic pressure in the far-field. The solution is taken at Re = 200,000, α = 40 deg, fenforced/fshed = 0.998, and Amp = 0.5 deg.

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Fig. 11

Amplitude of pitching oscillations versus nondimensionalized frequency (fenforced/fshed) for α = 40 deg and Re = 200,000. The circles represent locked-in solutions with positive (filled circle) or negative (empty circles) aerodynamic damping. The squares represent unlocked solutions. The dashed lines delimitate the lock-in region.

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Fig. 12

Experimental lock-in region of an NACA0012 airfoil forced to oscillate about quarter-chord. The nondimensionalized frequency is defined as fenforced/fshed. The angle of attack is 40 deg and the Reynolds number is 100,000. The flow is assumed incompressible. The circles are locked-in, the diamonds are unlocked, and the squares have a chaotic behavior.

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Fig. 13

Time-averaged surface pressure for five nondimensional frequencies at Amp = 1.5 deg for Re = 200,000 and α = 40 deg

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Fig. 14

Real part of the surface pressure first harmonic for five nondimensional frequencies at Amp = 1.5 deg for Re = 200,000 and α = 40 deg

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Fig. 15

Imaginary part of the surface pressure first harmonic for five nondimensional frequencies at Amp = 1.5 deg for Re = 200,000 and α = 40 deg

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Fig. 16

Eigenvalues of the first 14 POD modes

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Fig. 17

Unsteady pressure contours for the first three POD modes at one time snapshot. The top three images are extracted from snapshots with positive aerodynamic damping. The bottom three images are extracted from snapshots with negative aerodynamic damping. The time snapshot is taken when the trailing edge is at the lowest.

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Fig. 18

Representation of the airfoil in free-play

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Fig. 19

Lock-in of the NACA0012 when the flow speed is decreased. The airfoil is free to vibrate with a free-play region.

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