displacement wave

Discussion in 'Hydrodynamics and Aerodynamics' started by moTthediesel, Mar 28, 2013.

  1. moTthediesel
    Joined: Apr 2006
    Posts: 86
    Likes: 3, Points: 8, Legacy Rep: 21
    Location: 1k Islands

    moTthediesel Junior Member

    I posted this over on the WoodenBoat forum, but if you don't mind, I thought I'd like to try it here too, I'm just curious, and I thought some here might be interested...

    I noticed an interesting phenomenon yesterday while working on our summer house. The St. Lawrence Seaway opened last week, but shipping is only now starting to pick up. Our place is on the NY shore, and the river channel here is a little more than a mile wide over to Grenadier Islander in Ontario. The River is free of ice now in the main channels, but the bays, like where our dock is, are still ice bound.

    I heard a ship coming, so I walked around to the river side deck to see it, and recognized it as an up-bound Algoma Central laker. The River was perfectly calm, and as it got nearly abreast of our little bay, but still more than a half mile away, the ice around our dock began cracking and popping loudly, as though it was being suddenly lifted. Now I have noticed before (especially in narrower parts of the river) that the passage of ships causes a brief surge in water levels, and that it is most noticeable if you are in one of the narrow creeks leading to the channel. Still, I was surprised by the speed at which this reached our shore, as it typically takes several minutes after a ship has passed for the wake to reach us. So this clearly was not wake related, and I assume it is caused by the displacement of the water by the ship.

    I wonder then, if this is really a sort of shock wave, and that maybe it moves through the water in a way not unlike a tsunami, (long wave length, high speed, low amplitude) and at similar speed? In that case, the surging I have noticed while bullheading (springtime fishing for our local small catfish -- it's a Northern NY thing) up narrow creeks off the River is actually like watching a real tsunami, just in miniature? Also -- what are the implications of this to design performance of the ship? Surely, if some kind of shock wave is being created by the movement of the ship, it is causing drag. Is it simply a hidden component of normal wave formation drag?

    Tom
     
  2. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 145, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    It depends on the depth and width of the channel, and the speed of
    the ship, formally the depth-based Froude number, Fh = U/sqrt(g*h),
    where U is speed, g is gravitational acceleration, and h is water
    depth. Here are some wave patterns for different Fh...
    http://www.cyberiad.net/wakerowfd.htm

    Note the shock-wave like patterns when Fh is about 1.0.

    In a narrowish channel, the ship can also produce solitons which
    travel ahead of the ship and quite a bit faster.

    Tsunamis travel at a speed of U = sqrt(g*h). It is an interesting
    exercise to plug in values of g = 9.81 and h = 4000.0 metres to
    estimate how fast the Aceh tsunami travelled across the Indian Ocean
    before it hit the east coast of Africa.
     
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