Talk:Wave period
Merge this article?[edit]
Does this page really need to exist? All it says is that period is the inverse of frequency, which is already stated in the period article. The rest of the article is a list of formulas like "period = 1 / frequency" and "frequency = 1 / period" which are the same formula restated and can be found in the reciprocal article, talk about some SI units (Hertz and seconds), and another list of formulas like "wavelength / time = velocity" that are restated 6 times. Seems like a pointless article to me. Opinions? HN 22:10 24 Jul 2003 (UTC)
- It is fairly light, content-wise. Conceiveably it could be revised to change that, but I'm not up to it today. Michael Hardy 22:13 24 Jul 2003 (UTC)
- I agree with HN. It should be merged into Frequency. Also, as an electronics engineer, I've never actually heard the term 'wave period', it's always just 'period'. If this is a term specific to ocean surface waves, it should be so stated. --ChetvornoTALK 21:44, 26 January 2008 (UTC)
- I agree with the proposed merge into Frequency. I also notified the merge proposal on Wikipedia:Proposed_mergers. Kraaiennest (talk) 18:06, 29 January 2008 (UTC)
- I disagree with the merger. Studying wave forms and energy in a coastal geomporhology sense, listing wave period separately helps people looking for information in the context of coastal geomorphology. If it is joined into its physics roots, it becomes harder to find. Wikipeadia is so good because it aligns information with disciplines, and coastal geomorphology is its own discipline. Pushing things into master disciplines is going to water down the strength of Wikipedia. (I hope I have done this right) Peter Drescher (talk) 02:32, 13 November 2012 (UTC)
It's totally useless to show all possible permutations of a formula...must be changed IMHO. dave 04:26 25 Jul 2003 (UTC)
- I've made a lot of changes. I hope it doesn't tick anyone off, but there was a ton of redundancy in the article as it was. dave 04:40 25 Jul 2003 (UTC)
How about this lot for ocean waves if tidied up and equations sorted out?: Subsea (talk) 17:22, 17 December 2007 (UTC)
Period estimators
Mean zero up-crossing period, TZ or Tm0,2
When recordings were first taken this was onto charts and simple counts could be made. First the charts were zero-meaned (the average and trend calculated and drawn through the plot to provide a new axis for measuring) and then the number of times the wave record crossed the mean going up (or sometimes down) was counted and this gave the number of waves and, as a time measure, the zero-crossing period. The parameter is estimated by taking the mean of these periods for a given wave record. For wave records on paper the mean level is found by eye and Tvisual is estimated from the record length and the number of zero up-crossings counted on the record. This method can also be applied to digitised data using a computer but if the wave records are available in machine readable form it is preferable to estimate from the moments of the spectrum using,
Tz=
where the mi values are spectral moments. The alternative symbol of Tm0,2 is derived from the moment equation. It can be seen that TZ is very dependent on the higher frequency end of the spectrum and although TZ is the most commonly used period estimator it is not very stable.
Significant wave period, Ts
The significant wave period is the mean of the zero up-crossing periods associated with the highest one third of the waves. It is sometimes denoted by Ts. Note that this parameter cannot be obtained directly from the wave spectrum. It is not very useful, but sometimes is used!
Spectral peak period, Tp
The spectral peak period is the inverse of the frequency at which the value of the frequency spectrum is a maximum. It cannot be defined satisfactorily in multi-peaked spectra. fp is very important in characterising spectra.
Period associated with the most likely highest wave, Tmax
The most likely height of the highest wave in a record of duration 3 hours is Hmax and Tmax is its period. It is often obtained indirectly from Tz or Tp using empirical relationships, or from Hmax and steepness assumptions – usually to obtain a range of possible associated periods. These methods should be applied only in the water depth for which the empirical relationships have been found, usually deep water (i.e. depth >1/2 wavelength). It should be possible to use the steepness method in shallow water provided that refraction is minimal and that allowance can be made for shoaling effects. It cannot, however, be derived directly from the wave spectrum.
Energy period, Te
This period is important for power estimation and is used in wave power design as a preferred comparator. The most appropriate way to consider energy period is as the period of the regular wave that has the same significant height and the same power density as the sea-state under consideration. It is defined as,
Te=
Because of the relationship with power it is worth giving the expression for time averaged power associated with a spectrum,
power = rho g^2 m0 Te / 4Pi
Hence we can find an expression for Te as follows,
Te=(64 Pi Power)/ rho g^2 Hs^2
Average wave period, Tav
This is the inverse of the average frequency calculated from the mean of all component sine waves weighted by the spectral energy. It cannot be measured in the time domain unless the waves are simple sine curves.
Average crest period, Tc
Defined as, Tc=
This is equivalent to dividing the length of the wave record by the number of crests where a crest is any point either side of which the surface elevation decreases. Crests are not necessarily associated with zero up-crossings. Clearly this is very much influenced by the ‘tail’ of the spectrum through the fourth moment.