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Research Papers: Techniques and Procedures

A Nonlinear Computational Model of Tethered Underwater Kites for Power Generation

[+] Author and Article Information
Amirmahdi Ghasemi

Mechanical Engineering Department,
Worcester Polytechnic Institute,
Worcester, MA 01609
e-mail: aghasemi@wpi.edu

David J. Olinger

Mechanical Engineering Department,
Worcester Polytechnic Institute,
Worcester, MA 01609

Gretar Tryggvason

Department of Aerospace
and Mechanical Engineering,
University of Notre Dame,
Notre Dame, IN 46556

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received September 14, 2015; final manuscript received July 7, 2016; published online September 12, 2016. Assoc. Editor: Zhongquan Charlie Zheng.

J. Fluids Eng 138(12), 121401 (Sep 12, 2016) (10 pages) Paper No: FE-15-1665; doi: 10.1115/1.4034195 History: Received September 14, 2015; Revised July 07, 2016

The dynamic motion of tethered undersea kites (TUSK) is studied using numerical simulations. TUSK systems consist of a rigid winged-shaped kite moving in an ocean current. The kite is connected by tethers to a platform on the ocean surface or anchored to the seabed. Hydrodynamic forces generated by the kite are transmitted through the tethers to a generator on the platform to produce electricity. TUSK systems are being considered as an alternative to marine turbines since the kite can move at a high-speed, thereby increasing power production compared to conventional marine turbines. The two-dimensional Navier–Stokes equations are solved on a regular structured grid to resolve the ocean current flow, and a fictitious domain-immersed boundary method is used for the rigid kite. A projection method along with open multiprocessing (OpenMP) is employed to solve the flow equations. The reel-out and reel-in velocities of the two tethers are adjusted to control the kite angle of attack and the resultant hydrodynamic forces. A baseline simulation, where a high net power output was achieved during successive kite power and retraction phases, is examined in detail. The effects of different key design parameters in TUSK systems, such as the ratio of tether to current velocity, kite weight, current velocity, and the tether to kite chord length ratio, are then further studied. System power output, vorticity flow fields, tether tensions, and hydrodynamic coefficients for the kite are determined. The power output results are shown to be in good agreement with the established theoretical results for a kite moving in two dimensions.

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References

Figures

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Fig. 1

Flowchart of the algorithm for numerical modeling of a tethered undersea kite, based on the Navier–Stokes equations

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Fig. 2

(a) Schematic diagram of the kite–tether system and computational domain. Not to scale. (b) Numerical grid.

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Fig. 3

Unstretched and actual tether lengths versus time

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Fig. 4

Grid refinement study showing the effect of different grid resolutions on kite position and velocity: (a) vertical position of the kite versus time and (b) vertical velocity of the kite versus time

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Fig. 5

The effect of varying Reynolds numbers (based on kite chord length) on kite position and velocity: (a) vertical position of the kite versus time and (b) vertical velocity of the kite versus time

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Fig. 6

Snapshots of the kite position during a power-retraction cycle showing vorticity contours. The tethers are attached 5 m above the top off the computational domain. (a) Initial position of the kite, (b) power phase, (c) transition phase between power and retraction phases, and (d) retraction phase.

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Fig. 7

Angle of attack of the kite versus time

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Fig. 8

Trajectory of the kite versus time. The direction of the kite motion is shown by the arrows.

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Fig. 9

Hydrodynamic lift and drag coefficient of the kite versus time

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Fig. 10

Tether tension versus time

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Fig. 11

Reel-in and reel-out velocity of tethers versus time

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Fig. 12

Power versus time

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Fig. 13

Effect of the ratio of tether to current velocity on power output. Enlarged views show details of the power output during the power and retraction phases.

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Fig. 14

Comparison of power coefficients from the simulation with Loyd [1] prediction at L/D = 0.6

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Fig. 15

Effect of the kite density on power

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Fig. 16

Effect of the kite to water density ratio on power coefficient

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Fig. 17

Power generated in different ocean current velocities

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Fig. 18

Comparison of power generated in different ocean currents with V3 curve fit

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Fig. 19

Output power versus time for different ratios of tether to chord length

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Fig. 20

Kite position and vorticity contours with Lt/c=30 at various times: (a) initial position of the kite, (b) initial position of the kite—expand view, (c) position of the kite at 78 s during power phase, (d) position of the kite at 78 s—expand view, (e) position of the kite at 91 s during retraction phase, and (f) position of the kite at 91 s—expand view

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Fig. 21

Trajectory of the kite center of gravity with Lt/c=30

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