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Technical Briefs

Effect of Wave Age on Wind Gust Spectra Over Wind Waves

[+] Author and Article Information
Dag Myrhaug

Department of Marine Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norwaydag.myrhaug@ntnu.no

Muk Chen Ong

Department of Marine Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway

J. Offshore Mech. Arct. Eng 131(3), 034501 (Jun 03, 2009) (6 pages) doi:10.1115/1.3124130 History: Received April 04, 2008; Revised December 15, 2008; Published June 03, 2009

This paper considers the effect of wave age on wind gust spectra over wind waves. Here, two wind gust spectra based on maritime data are used to serve the purpose of demonstrating the effect of wave age; the spectra according to Wills (1986 “Offshore Mean Wind Profile,” Department of Energy, Offshore Technical Report No. OTH86226) and Ochi and Shin (1988, “Wind Turbulent Spectra for Design Considerations of Offshore Structures,” Proceedings of the 20th Offshore Technology Conference, Houston, TX, Paper No. 5736, pp. 461–467). The effect of wave age is taken into account by using the Volkov (2001, “The Dependence on Wave Age  ,” in Wind Stress Over the Ocean, Cambridge University Press, Cambridge, UK, pp. 206–217) sea surface roughness formula. The wave age independent Charnock (1955, “Wind Stress on a Water Surface,” Q. J. R. Meteorol. Soc., 81, pp. 639–640) sea surface roughness is used as a reference. The results are valid for wind waves in local equilibrium with steady wind in laterally homogeneous flow. An example of results demonstrates a clear effect of wave age on the wind gust spectra; also, distinct differences between the two spectra for a given wave age are demonstrated. Finally, an example of application is provided, demonstrating clear effects of the two spectra and the sea surface roughness on slowly varying surge motion of a marine structure due to wind gust.

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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Sea surface drag coefficient versus wave age

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Figure 2

Turbulence energy density versus frequency at the 5 m, 10 m, 20 m, 40 m, and 100 m elevations for the two spectra for Charnock (9) and Volkov (8) with wave age=0.4

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Figure 8

Turbulence intensity for f≤0.01 Hz versus elevation above the sea surface

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Figure 5

Turbulence energy density versus frequency for the spectrum of Wills (7) at given elevations for Volkov (8) with wave age=0.4

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Figure 4

Turbulence energy density versus frequency for the Ochi and Shin (5) spectrum at given elevations for Volkov (8) with wave age=0.4

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Figure 3

Turbulence energy density versus frequency for the Ochi and Shin (5) spectrum at the 10 m elevation for Charnock (9) and for given wave ages according to Volkov (8)

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Figure 7

Turbulence intensity for f>0.01 Hz versus elevation above the sea surface

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Figure 6

Turbulence intensity for all frequencies versus elevation above the sea surface

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