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B Water drag lines - what's it called?

  1. Sep 15, 2018 #1
    there's a stationary pebble on the flat shore of the beach. As the last of the wave heads back out to the ocean, it moves over the pebble and creates.. "lines" similar in shape to eg air stream around a jet, etc

    Wondering what these 'waves' are called???
     
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  3. Sep 15, 2018 #2

    Bandersnatch

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  4. Sep 15, 2018 #3

    Tom.G

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  5. Sep 16, 2018 at 4:34 AM #4

    sophiecentaur

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    . . . . or bow wave from a moving boat. It's a wave that's continually being formed against the water flow and it falls away on either side, travelling at its wave speed and that forms a vee. Once formed, the wave can travel a long way because there are very few losses (apart from all the other stuff going on).
     
  6. Sep 16, 2018 at 5:46 AM #5

    tech99

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    Yes. It is also a supersonic shock wave, not quite the same thing as, say, ordinary flow over a wing.
     
  7. Sep 16, 2018 at 8:48 AM #6

    boneh3ad

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    I don't know who Oxford hired to write the definition for a streamline, but whoever it was clearly wasn't familiar with fluid mechanics. That is not how a streamline is defined.

    A streamline is a line that is everywhere tangent to the velocity field.
     
  8. Sep 17, 2018 at 6:51 AM #7
    Hmm... is it wake if the object is stationary and the water is moving around it??

    Looking to name it in order to find an online model that shows the angles accurately.. in the 'wake' examples, there is usually a single 'V' with quite an acute angle, whereas with the wave and the pebble there are 2-3 'Vs' with increasingly obtuse angles...
     
  9. Sep 17, 2018 at 6:56 AM #8

    sophiecentaur

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    Would you expect a different answer? The motion is relative, just the same.
    Multiple wakes would perhaps not be surprising as the water velocity is different at different depths in shallow water. I can't think of an equivalent to the pebble situation for a boat. You could, perhaps observe the Vee formed by a pole in deep water.
     
  10. Sep 17, 2018 at 4:05 PM #9
    It's not that I would expect a different answer, it's that I'm looking to find online a model or illustration of the rippling effect that happens on the water surface in that exact situation - a shallow wave moving over an object on a flat surface...

    When I look for 'wake', 'streamline' etc online there's some great examples, but they don't look like what happens with the pebble...
     
  11. Sep 17, 2018 at 4:11 PM #10
    A picture would help here.
     
  12. Sep 17, 2018 at 4:16 PM #11

    sophiecentaur

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    I have to assume that's because there is a velocity gradient in the water around the pebble and not in a simplified model of a hull moving through deep water.
    Also, in shallow water, surface waves are noticeably modified and they can break.
    as @mfig says, a picture (with a comparison perhaps) would help. Is it the angle of the V that is different?
     
  13. Sep 19, 2018 at 2:43 AM #12
    Can I post a pic from my device? I can only see the URL option...
     
  14. Sep 19, 2018 at 4:09 AM #13

    A.T.

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  15. Sep 19, 2018 at 9:14 AM #14

    berkeman

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    Yes, use the Upload button in the lower right corner of the Edit window to Upload a JPEG or PDF copy of your picture. :smile:
     
  16. Sep 19, 2018 at 9:14 AM #15

    berkeman

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    LOL :biggrin:
     
  17. Sep 20, 2018 at 7:26 PM #16

    Mister T

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    I don't see why not. When I'm water skiing the boat that pulls me is at rest (relative to me) and the water is in motion. As long as the speed of the boat relative to the water exceeds the speed of the water waves, I'll get bow waves. Or shock waves, or a wake, or whatever you want to call it.

    My guess is that any asymmetry in the rock's shape causes bow waves to be created by more than one spot on the rock. I believe that if you happened to leave the ladder hanging off the stern as you sped away in your motor boat, you'd see more than one vee trailing away in the downstream wake.
     
  18. Sep 20, 2018 at 10:21 PM #17

    Was

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  19. Sep 20, 2018 at 10:28 PM #18

    olivermsun

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    I think what the OP is describing might be better described as a stationary wave, which is propagating upstream at the speed of the current, rather than a standing wave, which would be oscillating in place without propagating.
     
    Last edited: Sep 21, 2018 at 8:07 AM
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