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Ocean Engineering

Predictive Capability of a 2D FNPF Fluid-Structure Interaction Model

[+] Author and Article Information
Solomon C. Yim1

Department of Civil Engineering, Oregon State University, Corvallis, OR 97331-2302

Huan Lin

 The Boeing Company, Tukwila, WA 98108

David C. Robinson

 US Naval Academy, Annapolis, MD 21402

Katsuji Tanizawa

 National Maritime Research Institute, 6-38-1 Shinkawa, Mitaka, Tokyo, Japan

1

Corresponding author.

J. Offshore Mech. Arct. Eng 131(1), 011101 (Nov 10, 2008) (9 pages) doi:10.1115/1.2948945 History: Received December 13, 2006; Revised April 11, 2008; Published November 10, 2008

The predictive capability of two-dimensional (2D) fully-nonlinear-potential-flow (FNPF) models of an experimental submerged moored sphere system subjected to waves is examined in this study. The experimental system considered includes both single-degree-of-freedom (SDOF) surge-only and two-degree-of-freedom (2DOF) surge-heave coupled motions, with main sources of nonlinearity from free surface boundary, large geometry, and coupled fluid-structure interaction. The FNPF models that track the nonlinear free-surface boundary exactly hence can accurately model highly nonlinear (nonbreaking) waves. To examine the predictive capability of the approximate 2D models and keep the computational effort manageable, the structural sphere is converted to an equivalent 2D cylinder. Fluid-structure interaction is coupled through an implicit boundary condition enforcing the instantaneous dynamic equilibrium between the fluid and the structure. The numerical models are first calibrated using free-vibration test results and then employed to investigate the wave-excited experimental responses via comparisons of time history and frequency response diagrams. Under monochromatic wave excitations, both SDOF and 2DOF models exhibit complex nonlinear experimental responses including coexistence, harmonics, subharmonics, and superharmonics. It is found that the numerical models can predict the general qualitative nonlinear behavior, harmonic and subharmonic responses as well as bifurcation structure. However, the predictive capability of the models deteriorates for superharmonic resonance possibly due to three-dimensional (3D) effects including diffraction and reflection. To accurately predict the nonlinear behavior of moored sphere motions in the highly sensitive response region, it is recommended that the more computationally intensive 3D numerical models be employed.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Comparison of wave profiles (Test D2) of experimental result (solid), FNPF simulation (dotted), and sinusoidal approximation (dashed)

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Figure 2

Comparison of experimental and simulated responses of a sample SDOF free-vibration test (Test S2C)

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Figure 3

Comparison of 2DOF experimental and simulated responses of sample free-vibration test in (a) surge (Test T1F) and (b) heave (Test T2D)

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Figure 7

Comparison in frequency response diagram: experimental results (“○”) and FNPF simulations (“+”)

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Figure 8

Comparison of wave profile and harmonic responses (Test E2): (a) wave profile, (b) surge, and (c) heave displacement; solid line—experimental and dashed line—simulated

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Figure 9

Comparison of wave profile and subharmonic responses (Test E6): (a) wave profile, (b) surge, and (c) heave displacement; solid line—experimental and dashed line—simulated

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Figure 10

Comparison of wave profile and superharmonic responses (Test E4): (a) wave profile, (b) surge, and (c) heave displacement; solid line—experimental and dashed line—simulated

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Figure 11

Comparisons of experimental results (“○”) and FNPF predictions (“*”) in frequency response diagrams

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Figure 4

Wave profile and harmonic response near primary resonance (Test D14): experimental (solid) and numerical simulation (dotted)

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Figure 5

Wave profile and subharmonic response near secondary resonance (Test D2): experimental (solid) and numerical simulation (dotted)

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Figure 6

Wave profile and superharmonic response near secondary resonance (Test D3): experimental (solid) and numerical simulation (dotted)

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