The interaction transfers wave energy from short waves generated by Miles mechanism to waves with frequencies slightly lower than the frequency of waves at the peak of the spectrum. Finally, the waves begin to interact among themselves to produce longer waves ( Hasselmann et al.The instability causes the wave to grow exponentially ( Miles 1957). The process is unstable because, as the wave gets bigger, the pressure differences get bigger, and the wave grows faster. Wind blowing over the wave produces pressure differences along the wave profile causing the wave to grow. Next, the wind acts on the small waves, causing them to become larger.The turbulence in the wind produces random pressure fluctuations at the sea surface, which produces small waves with wavelengths of a few centimeters ( Phillips 1957).What happens if the wind suddenly begins to blow steadily at say 8m/s? Three different physical processes begin: Suppose we begin with a mirror-smooth sea (Beaufort Number 0). We have, however, put off until now just how they are generated by the wind. We have seen in the last few paragraphs that waves are related to the wind. The latter turns out to be particularly important because it leads to enhanced non-linear interactions and a spectrum that changes in time according to the theory of Hasselmann 1966. The JONSWAP spectrum is similar to the Pierson-Moskowitz spectrum except that waves continues to grow with distance (or time) as specified by the a term, and the peak in the spectrum is more pronounced, as specified by the g term. Then they calculated the wave spectra for various wind speeds, and they found that the spectra were of the form (Figure 1): First, they selected wave data for times when the wind had blown steadily for long times over large areas of the North Atlantic. To obtain a spectrum of a fully developed sea, they used measurements of waves made by accelerometers on British weather ships in the North Atlantic. This is the concept of a fully developed sea (a sea produced by winds blowing steadily over hundreds of miles for several days).Here, a long time is roughly ten-thousand wave periods, and a "large area" is roughly five-thousand wave-lengths on a side.įigure 1 Wave spectra of a fully developed sea for different wind speeds according to Moskowitz 1964. They assumed that if the wind blew steadily for a long time over a large area, the waves would come into equilibrium with the wind. Perhaps the simplest is that proposed by Pierson and Moskowitz 1964. Various idealized spectra are used to answer the question in oceanography and ocean engineering. The concept of a wave spectrum can be quite abstract and is described in Waves and the Concept of a Wave Spectrum Pierson-Moskowitz Spectrum Peaks, one from distance swell and the other generated by the local wind. A typical ocean wave spectrum wil be much more complicated and variable. In very special conditions, namely the conditions after a wind with constant velocity has been blowing for a long It is important to realise that the spectra presented in the section are attempts to describe the ocean wave spectra What will be the spectrum of ocean waves at the downwind side of the area? Suppose the wind blows at 20m/s for many days over a large area of the North Atlantic. In designing ships or offshore structures we wish to know the biggest waves produced by a given wind speed.
The faster the wind, the longer the wind blows, and the bigger the area over which the wind blows, the bigger the waves.
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