• Home
  • Download PDF
  • Order CD-ROM
  • Order in Print
Definitions of Other Terms
FORECASTING SEA WAVES

Aerographers Mate 1 & C
Page Navigation
  112    113    114    115    116  117  118    119    120    121    122  
present in the fetch or to account for waves generated by a rapidly changing wind. . E value. E is equal to the sum of the squares of the individual amplitudes of the individual sine waves that make up the actual waves. Since it is proportional to the total energy accumulated in these waves, it is used to describe the energy present in them and in several formulas involving wave energy. l  Co-cumulative  spectra.  The  co-cumulative spectra are graphs in which the total accumulated energy is plotted against frequency for a given wind speed. The co-cumulative  spectra  have  been  devised  for  two situations: a fetch limited wind and a duration time limited  wind. l Upper limit of frequencies (fu). The upper limit of  frequencies  represents  the  lowest  valued  frequencies produced by a fetch or that are present at a forecast point. This term gets its name from the fact that the period associated with this frequency is the period with the highest   value. The   waves   associated   with   this frequency are the largest waves. l Lower limit of frequencies (fL,). The lower limit of  frequencies  represents  the  highest  value  frequencies produced by a fetch or that are present at a forecast point. This term gets its name from the fact that the period associated with this frequency is the period with the lowest  value.  The  waves  associated  with  this  frequency are the smallest waves. . Filter area. That area between the fetch and the forecast point through which swell waves propagate. This area is so termed because it filters the frequencies and permits only certain ones to arrive at a forecast point at a forecast time. l  Significant  frequency  range.  The  significant frequency range is the range of frequencies between the  upper  limit  of  frequencies  and  the  lower  limit of  frequencies. The  term  significant  range  is used   because   those   low-valued   frequencies whose  E  values  are  less  than  5  percent  of  the total  E  value  and  those  high-valued  frequencies whose  E  values  are  less  than  3  percent  of  the total E value are eliminated because of insignificance. The  significant  range  of  frequencies  is  used  to determine the range of periods present at the forecast point. l Propagation.    Propagation as applied to ocean waves refers to the movement of the swell through the area between the fetch and the forecast point. . Dispersion. The spreading out effect caused by the different group speeds of the spectral frequencies in the  original  disturbance  at  the  source.  Dispersion  can be  understood  by  thinking  of  the  different  speeds  of  the different  frequencies.  The  faster  wave  groups  will  get ahead of the slower ones; the total area covered is thereby extended. The effect applies to swell only. l Angular spreading.    Angular  spreading  results from  waves  traveling  radially  outward  from  the generating area rather than in straight lines or banks because  of  different  wind  direction  in  the  fetch. Although all waves are subject to angular spreading, the effect of such spreading is compensated for only with swell  waves  because  the  spreading  effect  is  negligible for sea waves still in the generating area. Angular spreading  dissipates  energy. 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 Figure 6-3.-A typical frequency range of a wave spectrum. 6-4







Western Governors University

Privacy Statement
Press Release
Contact

© Copyright Integrated Publishing, Inc.. All Rights Reserved. Design by Strategico.