newbie question, what encounter frequency means?

Discussion in 'Hydrodynamics and Aerodynamics' started by cristianon, Apr 9, 2017.

  1. cristianon
    Joined: Aug 2015
    Posts: 20
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: indonesia

    cristianon Junior Member

    hello sir,
    [​IMG]
    image hosting gif
    is there anyone who can explain what's the meaning of encounter frequency and it's formula on RAO graphic?
    thank you
     
  2. jehardiman
    Joined: Aug 2004
    Posts: 2,609
    Likes: 259, Points: 83, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Encounter frequency is the derived apparent seaway frequency corrected for vessel speed and direction. So a vessel heading into the seaway has an encounter frequency greater than the seaway. Likewise, when running with a seaway, the encounter frequency is lowered. Beam to seas gets kind on tricky as the encounter frequency could be zero, but not really due to the difference between a nominal single frequency calculated RAO response and an irregular seaway response.
     
  3. Ad Hoc
    Joined: Oct 2008
    Posts: 6,393
    Likes: 510, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    In addition to JEH's post

    Looking at the graph, the peaks are around 1.5 rad/s...so this means a period of roughly 4.2 seconds. So if the vessel encounters a wave every 4.2 seconds, from whatever speed direction (as noted by JEH), the response is increased. So for a unit wave of say 1.0m the response is almost 4 in heave. In other words, when encountering a wave every 4.2 s with a wave height of 1.0m the vessel shall heave 1 x 4 = 4.0m.

    Similarly if the encounter is say 6 rad/s or every 1.0 s the response is almost negligible; since 1.0m x 0 = 0 !!

    Conclusion, avoid the peaks, or resonance.
     
  4. cristianon
    Joined: Aug 2015
    Posts: 20
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: indonesia

    cristianon Junior Member

    thank you for explanation sir,
    Sorry but i wanna know, how do you get the period 4.2 seconds sir?
    thank you
     
  5. Ad Hoc
    Joined: Oct 2008
    Posts: 6,393
    Likes: 510, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Hmmm...sorry, if you are unable to work this simple one out...then the rest is way way beyond you.
     

  6. hoytedow
    Joined: Sep 2009
    Posts: 5,462
    Likes: 214, Points: 73, Legacy Rep: 2489
    Location: North of Cuba

    hoytedow Wood Butcher

    Isn't the period the amount of seconds it takes 2 consecutive wave crests to cross the same geographical coordinate? If the coordinate is constantly changing as in the bow of a ship the period between crests would be lengthened or shortened by the rate of travel of the vessel.

    I think I just wrote the same thing jehardiman wrote but in different words.

    http://serc.carleton.edu/introgeo/interactive/qotd/waves2.html
     
Loading...
Similar Threads
  1. six four
    Replies:
    3
    Views:
    1,498
  2. alesserfate
    Replies:
    12
    Views:
    1,549
  3. Brian Needham
    Replies:
    17
    Views:
    1,459
  4. massandspace
    Replies:
    11
    Views:
    1,036
  5. Southern Cross
    Replies:
    32
    Views:
    2,578
  6. laukejas
    Replies:
    40
    Views:
    4,230
  7. mydauphin
    Replies:
    1
    Views:
    1,130
  8. mcarling
    Replies:
    5
    Views:
    1,831
  9. Anatol
    Replies:
    13
    Views:
    3,310
  10. DTurnham
    Replies:
    10
    Views:
    1,809
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.