Wave Spectrum The wave spectrum is the term that describes mathematically the distribution of wave energy with frequency and direction. The wave spectrum consists of a range of frequencies. Remember that ocean waves are composed of a multitude of sine waves, each having a different frequency. For purposes of explanation, these frequencies are arranged in ascending order from left to right, ranging from the low-valued frequencies on the left to the high-valued frequencies on the right, as illustrated in figure 6-3.  A particular range of frequencies, for instance, from 0.05 to 0.10 does not, however, represent only six different frequencies of sine waves, but an infinite number of sine waves whose frequencies range between 0.05 and 0.10. Each sine wave contains a certain amount of energy, and the energy of all the sine waves added together is equal to the total energy present in the ocean waves. The total energy present in the ocean waves is not distributed equally throughout the range of frequencies; instead, in every spectrum, the energy is concentrated around a particular frequency (fmax), that corresponds to a certain wind speed. For instance, for a wind speed of 10 knots (kt) fcnax is 0.248; for 20 kt, 0.124; for 30 kt, 0.0825; for 40 kt, 0.0619. For more information refer to the publication Practical Methods for Observing and Forecasting Ocean Waves (H.O. Publication 603), which gives the complete range of fu values and the corresponding periods for wind speeds, starting from 10 kt, at 2-kt intervals, Notice that the frequency decreases as the wind speed increases, This suggests that the higher wind speeds produce higher ocean waves. The table mentioned above can be graphed for each wind speed, An example of such a graph can also be found in H.O. publication 603. It is difficult to work with actual energy values of these sine waves; for this reason the square of the wave amplitude has been substituted for energy. This value is proportional to wave energy. The square of the wave amplitude plotted against frequency for a single value of wind speed constitutes the spectrum of waves. Thus, a graph of the spectrum is needed for each wind speed, and the energy associated with each sine wave can be determined from these graphs. Each wind speed produces a particular spectrum; and the higher the wind speed, the larger the spectrum.
|
