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Research Papers: Fundamental Issues and Canonical Flows

Characterization of Vortex Dynamics in the Near Wake of an Oscillating Flexible Foil

[+] Author and Article Information
Firas F. Siala, Alexander D. Totpal, James A. Liburdy

School of Mechanical, Industrial and
Manufacturing Engineering,
Oregon State University,
Corvallis, OR 97331

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received November 13, 2015; final manuscript received May 23, 2016; published online July 18, 2016. Assoc. Editor: D. Keith Walters.

J. Fluids Eng 138(10), 101202 (Jul 18, 2016) (12 pages) Paper No: FE-15-1837; doi: 10.1115/1.4033959 History: Received November 13, 2015; Revised May 23, 2016

An experimental study was conducted to explore the effect of surface flexibility at the leading and trailing edges on the near-wake flow dynamics of a sinusoidal heaving foil. Midspan particle image velocimetry (PIV) measurements were taken in a closed-loop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies (k = fc/U) from 0.09 to 0.20. Time-resolved and phase-locked measurements are used to describe the mean flow characteristics and phase-averaged vortex structures and their evolution. Large-eddy scale (LES) decomposition and swirling strength analysis are used to quantify the vortical structures. The results demonstrate that trailing edge flexibility has minimal influence on the mean flow characteristics. The mean velocity deficit for the flexible trailing edge and rigid foils remains constant for all reduced frequencies tested. However, the trailing edge flexibility increases the swirling strength of the small-scale structures, resulting in enhanced cross-stream dispersion. Flexibility at the leading edge is shown to generate a large-scale leading edge vortex (LEV) for k ≥ 0.18. This results in a reduction in the swirling strength due to vortex interactions when compared to the flexible trailing edge and rigid foils. Furthermore, it is shown that the large-scale LEV is responsible for extracting a significant portion of energy from the mean flow, reducing the mean flow momentum in the wake. The kinetic energy loss in the wake is shown to scale with the energy content of the LEV.

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Figures

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Fig. 1

Cartoons of (a) the PIV setup in the wind tunnel, (b) side view of the foil indicating the flexible leading and trailing edges, and (c) motion control device

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Fig. 2

Measured instantaneous heaving position as well as the leading and trailing edge deflection angles for k = 0.12, 0.18, and 0.2

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Fig. 3

Mean velocity profiles in the near wake: top row—velocity profile at x/c = 0.2 for (a) k = 0.12, (b) k = 0.18, and (c) k = 0.2; bottom row—velocity profile at x/c = 0.5 for (d) k = 0.12, (e) k = 0.18, and (f) k = 0.2

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Fig. 4

Mean vorticity contour for the flexible LE, TE, and rigid foil cases at k = 0.12, 0.18, and 0.2

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Fig. 5

Phase-averaged vorticity distribution at k = 0.2 for flexible LE, TE, and rigid foil cases

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Fig. 6

Phase variation of vorticity distribution in the near wake for k = 0.2 for the flexible LE foil; the cross shown for Φ = 45 deg and Φ = 225 deg indicates the predicted location of LEV 1 and LEV 2 which matches the observed region of high vorticity at these phases

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Fig. 7

Instantaneous small-scale LES filtered flow field at the top heaving position (Φ = 0 deg) for k = 0.2 overlaid with swirling strength contour for (a) flexible LE, (b) flexible TE, and (c) rigid foils

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Fig. 8

Instantaneous large-scale LES filtered velocity for the flexible LE case overlaid with swirling strength based on low-pass velocity field at Φ = 0 deg showing the LEV for (a) k = 0.18, (b) k = 0.19, and (c) k = 0.20

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Fig. 9

Autocorrelation of the longitudinal turbulent velocity for k = 0.09, 0.12, 0.18, and 0.2 for (a) flexible LE, (b) flexible TE, and (c) rigid foils

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Fig. 10

Normalized length scale based on the longitudinal velocity fluctuations as a function of k for the flexible LE, TE, and rigid foils

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Fig. 11

The variation of (a) mean swirling strength and (b) mean convective velocity as a function of k

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Fig. 12

Mean swirling strength of the LEV as a function of k

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Fig. 13

Variation of energy content of the LEV structure and kinetic energy loss in the wake at x/c = 0.2 as a function of k

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