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MIT2_019S11_OWE - 2.019 Desi 2 019 D ign of O f Ocean...

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2 019 D 2.019 Design of Ocean Systems f O t Lecture 9 Ocean Wave Environment March 7, 2011
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Ocean Surface Wave Generation Waves important to offshore structure design and operation: Wind waves or gravity waves with wave period T= 5 ~ 20 seconds, wavelength O(10)m to O(500)m. Source of forcing: wind air Source of restoring: gravity Source of damping: wave breaking and viscous effects water •When wind starts (0.5 ~ 2.0 knots), capillary waves form (e.g Vp =24 cm/s → λ =1.73 cm) •As wind becomes stronger, waves become longer Wind energy input into water: Frictional drag Separation drag Bernoulli effect
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Wave evolution as wind blows Nonlinear wave-wave interactions cause energy to be transferred into longer waves Certain distance and duration (for wind to blow) are necessary for effective energy transfer Equilibrium sea: when energy input from the wind is balanced by dissipation When wind input energy is larger than dissipation, waves grow When wind input energy is smaller than dissipation, waves decay. Short waves decay faster. Shorter waves are steeper, and easier to break
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Required fetch and storm duration: •Wind must blow over long periods of time and large distances to reach fully- developed state. •At fully-developed state, U w ~ V p (i.e. ω limit ~ g/U w ) •Swell: waves are not generated by local wind •Sea: waves generated by local wind
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Standard Wave Spectra S ( ω ) Based on measured spectra and theoretical results, standard spectrum forms have been developed: Bretschneider spectrum: S ( ω ) = 1 . 25 ω 4 H 2 ¡ ¢ 4 m ω m 4 ω 5 s exp { 1 . 25 ω } ω m is peak or modal frequency H s is signi fi cant wave
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