Wave Spectrums

Discussion in 'Hydrodynamics and Aerodynamics' started by Crowsnest, Aug 23, 2013.

  1. Crowsnest
    Joined: Jun 2012
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    Crowsnest Junior Member

    Hi all:
    Im stupidly stucked with an Excel Wave Spectrum spreadsheet.

    I've correctly got the P-M and the Bretschneider spectrums, but when trying with the Jonswap I got quite odd results.
    I get quite big or quite little numbers with this last spectrum that do not agree with the other two.

    DATA for S(w):
    Fetch 1200 Nm; U= 30 kts

    CONSTANTS:
    Gamma= 0.3; X= g Fetch/U²
    a= 0.076*X^(-0.22)
    B= -5/4
    wm= 2π*3.5*(g/U)*X^(-0.33)
    s --- 0.07 or 0.09 depending on relation "w/wm"

    EXPRESION
    r= -(w-wm)²/(2 s² wm²)

    S(w)= Gamma^r * (ag²/W^5) exp(B (wm/w)^4)

    For Fetch=1200 and U=30 kts I get....
    S(wm)= 0.014
    H(1/3)= 0.4m

    None of them looks reasonable.

    I've tried with different functions for S(w) and S(f), in all cases results are "stupid".

    Cannot find my mistake ¡¡¡¡

    Regards
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

  3. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Excellent links, AH. Thanks! :)
     
  4. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Gamma constant should be 3.3

    Check how you implemented the exponential function and make sure you convereted to constant units, i.e. fetch in meters with wind speed in meters/sec, etc.. You may need to implement the exponential in a different fashion because of precision rounding depending on which version of Excel you are using
     
  5. Crowsnest
    Joined: Jun 2012
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    Location: Madrid

    Crowsnest Junior Member

    Thanks all:
    Of course my results (on JONSWAP) make no sence.

    Jehardiman: Im for the units-precission affaire. But, before posting here, I've been checking that matter for a few days.
    Gamma constant may be 3.3 or 0.33 depending on the formula arrangement.
    About Excel: Im running Excel 2007 on 8x64 system.

    Ad Hoc: Going to check the links. Thanks a lot.

    Regards
     
  6. Crowsnest
    Joined: Jun 2012
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    Location: Madrid

    Crowsnest Junior Member

    Hi again:
    I've checked your links and modified my spreadsheet according to the first one.

    Results keep on making no sence.
    The curve shape looks correct, but values are out of any common sence.

    Time for stopping now. May be a rest will clear my mind up. Cannot understand how can I be stucked in such a simple calculation.

    Best regards.
     
  7. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Crowsnest

    When faced with such "issues", you can't beat the "old fashioned way". Perform several iterations using hand calculations and a calculator only. Then double check against your excel values, are they the same, if not, why not?

    When doing the calc's, work out the 'sum' for each part (product) of the formula, break it down into small lumps so you can see if, or where, you have made an error, or excel has made an error. It is so easy to mistype a value or have an incorrect sign or too many parentheses etc in excel.
     

  8. Crowsnest
    Joined: Jun 2012
    Posts: 44
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    Location: Madrid

    Crowsnest Junior Member

    Hi Ad Hoc:
    I've reworked the spreadsheet, breaking the expression into parts (as per your advice). It now works all right.
    I think there was, as you've pointed, a parenthesis or mistype problem.

    Thanks all for your advices and interest.

    Regards
     
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