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Research Papers: CFD and VIV

Computational Fluid Dynamics Reproduction of Nonlinear Loads on a Vertical Column During Extreme Irregular Wave Events

[+] Author and Article Information
Erin E. Bachynski

Department of Marine Technology,
Norwegian University of Science and
Technology (NTNU),
Trondheim 7491, Norway
e-mail: erin.bachynski@ntnu.no

Csaba Pákozdi

SINTEF Ocean (former MARINTEK),
Trondheim 7052, Norway
e-mail: csaba.pakozdi@sintef.no

Anders Östman

SINTEF Ocean (former MARINTEK),
Trondheim 7052, Norway
e-mail: anders.ostman@sintef.no

Carl Trygve Stansberg

Norwegian Marine Technology Research Institute
(MARINTEK),
Trondheim 7052, Norway
e-mail: ctstansberg.marinteknikk@gmail.com

1Corresponding 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 October 19, 2016; final manuscript received May 23, 2018; published online July 12, 2018. Assoc. Editor: Robert Seah.

J. Offshore Mech. Arct. Eng 140(6), 061804 (Jul 12, 2018) (12 pages) Paper No: OMAE-16-1129; doi: 10.1115/1.4040442 History: Received October 19, 2016; Revised May 23, 2018

Recently, a method for numerical reproduction of measured irregular wave events has been developed. The measured motion of the wave maker flaps defines the wave kinematics at the boundary of the numerical simulation in order to generate the waves. When such data are not available, the control signal of the wave maker can, instead, be generated from a given free surface elevation following the same procedure as in model tests. This procedure is applied to a model test case with extreme irregular wave events and resulting nonlinear global wave loads on a vertical cylinder, focusing on higher-order ringing excitation. The purpose of the investigation is twofold: (1) to validate the wave reconstruction procedure and (2) to validate the resulting computational fluid dynamics (CFD) ringing loads with the given waves. In order to better understand the frequency content in the CFD-generated loads, wavelet analysis as well as the response of a single degree-of-freedom (SDOF) oscillator is examined and compared with the corresponding results for the third-order wave forcing based on the MacCamy–Fuchs (MF) and Faltinsen, Newman, Vinje (FNV) formulations. The results show generally good agreement between CFD and experiment both in the waves and in the loads; discrepancies found in the loads mainly originate from corresponding uncertainties in the wave reconstruction. Wave breaking may be one source of uncertainty. The MF + FNV formulation showed reasonable prediction of the maximum responses of an SDOF oscillator, but could not capture the loads well at all of the important frequencies.

Copyright © 2018 by ASME
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References

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Figures

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Fig. 4

Wave flaps in the simulation with explicit wave maker

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Fig. 2

SINTEF Ocean's BM1 wave maker [14]

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Fig. 1

Procedure for generating the wave maker control signal for CFD reproduction of an irregular wave event

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Fig. 5

Applied boundary conditions. All boundaries are visualized in the figure, except the side wall boundary at y = 0 m.

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Fig. 6

The main part of the computation domain and the start of the stretched mesh part

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Fig. 7

Mesh in a longitudinal cut located at the center of the geometry

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Fig. 16

Visualization of the space–time history of the CFD free surface elevation in event B

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Fig. 14

Time series of the force (top) and response of the SDOF oscillator with various natural periods (tn) for event A. 2% critical damping is applied in all time series shown here

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Fig. 8

Comparison of the free surface elevation at several locations against model test time series, CFD reproduction of event A versus experiment

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Fig. 9

Visualization of the space–time history of the CFD free surface elevation in event A

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Fig. 10

Wave elevation (measured calibrated wave, CFD calibration reproduction, and linearized measured wave) and corresponding force on the cylinder, event A. Thin vertical dotted lines refer to the time instants shown in Fig. 11.

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Fig. 11

Fluid visualization of event A. (Top: free surface elevation; bottom: dynamic pressure distribution). Time stamps differ from the experimental time stamps due to the reduced length of the CFD simulations. The time instants shown here are indicated in Fig. 10 by thin vertical dotted lines.

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Fig. 12

Wavelet transformation of the horizontal force (N) from experimental measurements (top), CFD (middle), and analytical methods (bottom) for event A. Dashed lines indicate the peak frequency fp = 0.42 Hz, and multiples 2fp, 3fp.

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Fig. 13

Components of the hydrodynamic load according to MacCamy–Fuchs (top) and FNV second-order (middle) and third-order (bottom) for event A

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Fig. 15

Comparison of maximum positive and negative responses, event A

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Fig. 17

Comparison of the free surface elevation at several locations against model test time series, CFD reproduction of event B versus experiment

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Fig. 18

Fluid visualization of event B. (Top: free surface elevation; bottom: dynamic pressure distribution). Time stamps differ from the experimental time stamps due to the reduced length of the CFD simulations. The time instants shown here are indicated in Fig. 19 by thin vertical dotted lines.

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Fig. 19

Wave elevation (measured calibrated wave, CFD calibration reproduction, linearized measured wave) and corresponding force on the cylinder, event B. Thin vertical dotted lines refer to the time instants shown in Fig. 18.

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Fig. 20

Wavelet transformation of the horizontal force (N) from experimental measurements (top), CFD (middle), and analytical methods (bottom) for event B. Dashed lines indicate the peak frequency fp = 0.42 Hz, and multiples 2fp, 3fp.

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Fig. 21

Time series of the force (top) and response of the SDOF oscillator with various natural periods (tn) for event B. 2% critical damping is applied in all time series shown here

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Fig. 22

Comparison of maximum positive and negative responses, event B

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