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Research Papers: Ocean Renewable Energy

Computational Modeling of Rolling Wave-Energy Converters in a Viscous Fluid1

[+] Author and Article Information
Yichen Jiang

Department of Mechanical Engineering,
University of California at Berkeley,
Berkeley, CA 94720
e-mail: yichen.e.jiang@gmail.com

Ronald W. Yeung

American Bureau of Shipping Inaugural
Chair in Ocean Engineering;
Director of Computational Marine Mechanics
Laboratory (CMML),
Department of Mechanical Engineering,
University of California at Berkeley,
Berkeley, CA 94720
e-mail: rwyeung@berkeley.edu

2Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received April 20, 2012; final manuscript received August 5, 2015; published online October 12, 2015. Assoc. Editor: Hideyuki Suzuki.

J. Offshore Mech. Arct. Eng 137(6), 061901 (Oct 12, 2015) (9 pages) Paper No: OMAE-12-1039; doi: 10.1115/1.4031277 History: Received April 20, 2012; Revised August 05, 2015

The performance of an asymmetrical rolling cam as an ocean-wave energy extractor was studied experimentally and theoretically in the 70s. Previous inviscid-fluid theory indicated that energy-absorbing efficiency could approach 100% in the absence of real-fluid effects. The way viscosity alters the performance is examined in this paper for two distinctive rolling-cam shapes: a smooth “Eyeball Cam (EC)” with a simple mathematical form and a “Keeled Cam (KC)” with a single sharp-edged keel. Frequency-domain solutions in an inviscid fluid were first sought for as baseline performance metrics. As expected, without viscosity, both shapes, despite their differences, perform exceedingly well in terms of extraction efficiency. The hydrodynamic properties of the two shapes were then examined in a real fluid, using the solution methodology called the free-surface random-vortex method (FSRVM). The added inertia and radiation damping were changed, especially for the KC. With the power-take-off (PTO) damping present, nonlinear time-domain solutions were developed to predict the rolling motion, the effects of PTO damping, and the effects of the cam shapes. For the EC, the coupled motion of sway, heave and roll in waves was investigated to understand how energy extraction was affected.

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Figures

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Fig. 1

EC and KC with radius R=25 cm, reference shape with origin at y = 0

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Fig. 2

EC, KC, and Salter Cams with their center of rotation submerged

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Fig. 3

Computational domain D for a rolling body in waves

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Fig. 4

The comparison of the cam's efficiency between experiments and FSRVM simulations for the case that the MOI of the Salter Cam, Î=I/(ρR4)=3.56

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Fig. 5

Optimal ideal efficiency η, calculated from inviscid-fluid models based on potential theory

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Fig. 6

RAO α0/kA, required to achieve the optimum ideal efficiency (inviscid-fluid solution)

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Fig. 7

Comparison between the viscous and inviscid hydrodynamic coefficients of the KC

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Fig. 8

Comparison between the viscous and inviscid hydrodynamic coefficients of the EC

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Fig. 9

Roll amplitude and efficiency of the two cams over a range of extractor damping, B̃g, using viscous-fluid modeling

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Fig. 10

Visualization of vortex blobs for both cams with the optimum Bg in one 30th period of oscillation (ω̃inc=0.6). a) Eyeball Cam in the first half of 30th period, b) Eyeball Cam in the second half of 30th period, c) Keeled Cam in the first half of 30th period, and d) Keeled Cam in the second half of 30th period

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Fig. 11

Mooring system of the 3DOF model

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Fig. 12

Roll extraction efficiency in 3DOF motion with the effects of mooring cables

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Fig. 13

Time history of sway, heave and roll displacement in five periods

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