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Research Papers: CFD and VIV

Steady-State Hydrodynamic Power Attenuation by Finned-Buoys

[+] Author and Article Information
A. Spicer Bak

Murtech, Inc.,
820 Cromwell Park Drive,
Glen Burnie, MD 21061
e-mail: Spicer.Bak@usace.army.mil

Michael E. McCormick

Corbin A. McNeill Professor Emeritus,
Department of Naval Architecture and
Ocean Engineering,
U.S. Naval Academy,
Annapolis, MD 21402
e-mail: mmccormi@usna.edu

1Present address: U.S. Army Engineer Research and Development Center, Coastal & Hydraulics Laboratory, 1261 Duck Road, Kitty Hawk, NC 27949.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received February 25, 2015; final manuscript received September 1, 2017; published online October 11, 2017. Assoc. Editor: Wei Qiu.

J. Offshore Mech. Arct. Eng 140(1), 011801 (Oct 11, 2017) (8 pages) Paper No: OMAE-15-1017; doi: 10.1115/1.4037951 History: Received February 25, 2015; Revised September 01, 2017

Results of experimental and computational fluid dynamics (CFD) studies conducted to compare the flow-energy-attenuating performances of two different buoy configurations are presented. A finned-body was experimentally studied in two orientations—one with a splitter and one in a 22.5 deg yaw orientation (with no splitter). The finned-buoy is designed for both wave and current attenuation; however, only the current application is discussed here. Scaled models were subjected to wind tunnel testing and CFD analyses. For this study, the steady-state drag coefficient (CD) is considered to be the performance measure. The CFD model is used to match the physical testing by utilizing the k–ω turbulence model. Reynolds numbers (based on the tip-to-tip fin diameter) approaching the drag crisis are used to evaluate the bodies of interest, both of which have an aspect ratio (draft-to-diameter) of 1.85. The finned-bodies do encounter a drag crisis (as commonly seen with a cylinder), since the fins cause the buoys to act as a bluff body. The flow structures around the bodies are examined and compared to those predicted by established theories. For the finned-body, the 22.5 deg yaw orientation is found to have a consistently higher drag than the splitter orientation. The drag enhancement is explained by two phenomena. The first is a low-pressure area located in pockets adjacent to the upstream fins. The second is the absence of the drag-crisis, due to fixed separation points at the fin tips for all Reynolds numbers.

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References

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Figures

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Fig. 1

Sketches of the Finned-Spar Buoy and Antenna Buoy concepts for current energy and wave energy attenuation (Courtesy of Murtech, Inc.)

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Fig. 2

Notation and orientations for the FB and CB model cross sections. The sketches of the FB geometry are exaggerated for illustration.

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Fig. 3

Sketches of the CB model profiles and wind tunnel setup

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Fig. 4

Flow patterns for a cantilevered truncated circular cylinder. The sketch is patterned after that of Refs. [9], [10], and [13]. The existence and relative sizes depend on the Reynolds number.

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Fig. 5

Steady-state drag coefficient versus Reynolds number for all models and orientations

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Fig. 6

Yaw orientations of the FB and nominal wakes. Nonsplitter orientation shown in a/b and splitter configuration shown in c/d (Courtesy of Murtech, Inc.)

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Fig. 7

Pressure contours and velocity vectors at instant tV/D = 314 at Re = 4.65 × 105 for the 22.5 deg yaw orientation

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Fig. 8

Pressure contours and velocity vectors at instant tV/D = 314 at Re = 4.65 × 105 for the 0 deg yaw orientation

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Fig. 9

Comparison instantaneous velocity vectors (tV/D = 314) of the horshoe vortex at Re = 4.65 × 105 for both FB orientations and the CB: (a) 22.5 deg yaw orientation, (b) 0 deg yaw orientation, and (c) CB. Note: The figures are captured from the same position in relation to that of the center axis of the body, allowing the reader to compare the voritcal structures accurately.

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Fig. 10

Wake structures (λ2 = −20,000) for the bodies of interest Re = 4.65 × 105 at tV/D = 314: (a) CB, (b) 0 deg yaw, and (c) 22.5 deg yaw

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