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Technical Brief

Wake Topology of a Cylinder Undergoing Vortex-Induced Vibrations With Elliptic Trajectories

[+] Author and Article Information
Sina Kheirkhah

Institute for Aerospace Studies,
University of Toronto,
Toronto, ON M3H 5T6, Canada
e-mail: kheirkah@utias.utoronto.ca

Serhiy Yarusevych

Department of Mechanical and Mechatronics Engineering,
University of Waterloo,
Waterloo, ON N2L 3G1, Canada
e-mail: syarus@uwaterloo.ca

Sriram Narasimhan

Department of Civil and Environmental Engineering,
University of Waterloo,
Waterloo, ON N2L 3G1, Canada
e-mail: snarasim@uwaterloo.ca

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received February 2, 2015; final manuscript received October 13, 2015; published online January 8, 2016. Assoc. Editor: Peter Vorobieff.

J. Fluids Eng 138(5), 054501 (Jan 08, 2016) (7 pages) Paper No: FE-15-1086; doi: 10.1115/1.4031971 History: Received February 02, 2015; Revised October 13, 2015

Wake vortex shedding topology of a cylinder undergoing vortex-induced vibrations (VIV) is investigated experimentally. Vibration measurements and flow visualization are utilized to study the connection between the cylinder response and the wake topology. The experiments were performed for two different orientations of the elliptic trajectories relative to the incoming flow at a fixed Reynolds number, moment of inertia ratio, mass ratio, and reduced velocity. Similar to the classical 2P regime, two counter-rotating vortex pairs are produced per oscillating cycle for both cases of elliptic trajectories examined here. However, significant changes in wake vortex dynamics are observed along the cylinder span. These changes include merging of vortices, which leads to shedding patterns similar to 2S and P + S modes downstream of the vortex formation region. The observed changes in vortex dynamics are accompanied by splitting of spanwise vortex filament and are attributed primarily to the changes in the local amplitude of vibrations along the span of the pivoted cylinder. It is shown that, being dependent on both the local amplitude of vibrations and vortex dynamics, the observed wake topology cannot be captured by the classical map of shedding regimes developed for VIV of one degree-of-freedom (DOF) cylinders.

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Figures

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Fig. 1

Schematics of the experimental setup. Flow visualization was conducted in horizontal planes at Z/L = 0.16 and Z/L = 0.35. Note that the dimensions in the figure are not to scale.

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Fig. 2

Cylinder tip trajectories. Flow direction is from top to bottom. For (a) ξ = 0.007 and Uc*=28.8 and for (b) ξ = 0.01 and Uc*=13.8.

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Fig. 3

(a) and (b) Spectra of vibrations for cases 1 and 2, respectively. In the figures, spectra of streamwise and transverse vibrations are denoted by fx and fy, respectively, and are stepped by 40 dB in magnitude for clarity.

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Fig. 4

Near-wake flow visualization for case 1. (a) and (b) pertain to Z/L = 0.16 and 0.35, respectively.

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Fig. 5

Representative vortex shedding patterns at Z/L = 0.35. (a) and (d) correspond to cases 1 and 2, respectively. (b) and (c) are insets of (a).

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Fig. 6

Vortex merging observed for case 1 at Z/L = 0.35. (a)–(d) are four consecutive PLIF images separated by 10% of the time period of oscillations. (e)–(h) are insets of (a)–(d), respectively. In the figures, V1 and V2 correspond to the corotating vortices that undergo merging. Approximate cylinder center positions associated with (a)–(d) are presented in (i)–(l).

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Fig. 7

Representative vortex shedding patterns at Z/L = 0.16. (a) and (b) correspond to cases 1 and 2, respectively.

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Fig. 8

Schematics of vortex splitting along the cylinder span. (a) and (b) correspond to cases 1 and 2, respectively.

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Fig. 9

Results overlaid on the vortex shedding map developed by Williamson and Roshko [18] and updated in a subsequent study [20]. Ay* and U* are the normalized amplitude of vibrations in the transverse direction and the reduced velocity, respectively. Hollow and solid circles are associated with cases 1 and 2, respectively.

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