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B Is the water pressure below ocean waves constant?

  1. May 19, 2017 #1
    If you are stationary say 10m below the ocean surface does the water pressure at your location vary with the waves.

    For example:

    When a wave crest is above you than means perhaps 12m of water is above you.
    Then a few seconds later a trough is above you so only 8m of water is above you.

    So you might expect your pressure reading to be going up and down with the waves?
     
  2. jcsd
  3. May 19, 2017 #2

    Bystander

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    What wavelength?
     
  4. May 19, 2017 #3

    Merlin3189

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    I hadn't considered wavelength. I assumed a water depth very much greater than wavelength and amplitude.
     
  5. May 19, 2017 #4

    davenn

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    wouldn't matter
    the point is that the head of water above you is varying depending if it is a peak or trough
     
    Last edited: May 19, 2017
  6. May 19, 2017 #5

    anorlunda

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    I tried to find a graphic that illustrates @davenn 's correct answer. I found this. It's pretty extreme, but the principle applies to all waves, big and small.

    buoys.fw.png
     
  7. May 19, 2017 #6

    jbriggs444

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    And what is the wavelength of a tsunami?

    The point is that if the pressure at depth varies according to the depth below the peak or trough of surface waves then the water at depth must be moving due to the resulting pressure gradients. If the water at depth is not oscillating back and forth due to this effect, the pressure at depth must not be changing due to the surface waves.

    In a shallow water wave, water at depth is indeed moving under the effect of the wave. In a deep water wave this is not the case. It may be surprising, but a tsunami is a shallow water wave.
     
    Last edited: May 19, 2017
  8. May 19, 2017 #7

    anorlunda

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    I'm sorry but I don't understand your point. Water is incompressible for practical purposes. Why is movement at depth necessary to have pressure change?

    I also don't see why wavelength is relevant to the question.

    I could understand an argument including the speed of sound in water that pressure oscillations fan out and merge with adjacent crests at very deep depths (the tsunami sensor was 19800 feet deep). But the OP specified only 10m depth.
     
  9. May 19, 2017 #8

    Svein

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    That reminds me - in the start of the 1980s my research company were hired to do the instrumentation of the collapsed oil platform "Alexander Kielland" before attempting to turn it right side up. I was asked to create a backup system (in the case that the expensive measurement system crashed). The main function of my system was to show the depth of one of the flotation pods.

    When I asked what precision they wanted on the depth, they more or less automatically answered "1 cm". To which I answered "below what?" This was in a rather large inlet, with waves seldom below half a meter. The answer: "Below sea level". No use in asking any further. I got a very precise pressure sensor, measured the sensor output frequency, calculated the apparent depth, did some low-pass filtering and presented the answer - they got a number and they were happy.

    Back to the original question - is the pressure at, say, 10 m below the surface, independent on the amplitude of the waves on the surface? I still do not know, but my low-pass filter was meant to minimize any such effect. AFAIK, the frequency spectrum of sea waves is in the 0.1Hz - 1Hz band, so my filter could not be all that effective.
     
  10. May 19, 2017 #9

    anorlunda

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    @Svein , even sea level (neglecting tides, waves, seasons, and weather) is dynamic, depending on your time scale.:wink:

    sea+level.jpg
     
  11. May 19, 2017 #10

    Svein

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    Yes, but they were trying to turn it around in a couple of days... Seriously, since the platform was floating in the same water, I was only supposed to inform the turnaround guys of the position of the pod in relation to the current sea level. My take on it was purely as a measurement guy - with the huge size of the platform a precision of half a meter would be more than precise enough.

    I have a list of such anecdotes regarding silly requirements - I try to use them sparingly.
     
  12. May 19, 2017 #11
    I agree with this reply. A wave is held up by the dynamics of movement, its inertia, and the pressure effect must die out when we get a few wavelengths below the surface. We only see seaweed waving in fairly shallow water. It is rather like the situation when an airliner flies overhead - we are not all crushed by an increase in pressure.
     
  13. May 19, 2017 #12

    anorlunda

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    An airliner is a poor comparison because air is so compressible.

    Of course I can always be wrong. You and @jbriggs444 could be right. I tried looking it up. https://en.wikipedia.org/wiki/Wind_wave#Physics_of_waves talks about the motion but not the pressure.

    If I had a hollow cylinder, sealed at the base, and anchored to the sea bottom, with a hole in the side just barely deep enough to stay submerged. Its level would go up and down as waves pass (not as high as the waves, but some) but horizontal motion would be blocked. What about pressure in the bottom of that cylinder, constant or oscillatory?
     
  14. May 19, 2017 #13
    The water particles near the surface execute a circular motion as a wave passes, and the tube would alter their motion to an up-and-down one. It seems like a waveguide carrying energy down to the sea floor.
     
  15. May 19, 2017 #14

    jbriggs444

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    The reasoning runs the other way. If there is a pressure difference between the water under the crest and the water under the trough then water will flow under that pressure gradient. If we do not observe such a flow then we can infer the lack of a gradient. We do not, in fact, observe such a flow (for deep water waves).
     
  16. May 19, 2017 #15

    berkeman

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    But the tsunami wave is wide enough that an ocean floor monitor can pick it up? That might make sense.
     
  17. May 19, 2017 #16

    jbriggs444

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    Yes. Tsunami waves have huge wavelengths, huge enough that the ocean depth is smaller than the wavelength. Accordingly, they qualify as shallow water waves.

    [Mind you, my education in the details of tsunamis is limited to what I could Google up in a few minutes]
     
  18. May 19, 2017 #17

    berkeman

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    Interesting. Learn something new every dang day here at the PF! :smile:
     
  19. May 19, 2017 #18

    anorlunda

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    I feel like I'm being denser than water :nb) because I still don't see it. Yes water does flow under that pressure gradient. Isn't that what causes the circular motions of the molecules in the wave?

    I would like a link to a tutorial on the math of the pressure-motion interactions in those water waves. As I said, the Wikipedia article doesn't even talk about the pressure.
     
  20. May 19, 2017 #19

    jbriggs444

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    Yes, near the surface, water moves in a circular pattern under the wave. But water farther from the surface does not move that much. Otherwise, the energy in a wave of small amplitude and small wavelength in deep water would be enormous.

    If you drop a rock off the back end of an ocean liner, it makes waves. That proves that the deep water does not move appreciably.
     
  21. May 19, 2017 #20

    anorlunda

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    Ah, thanks @jbriggs444 , now the first light of understanding is beginning to penetrate. That's a clever argument.

    I would still like to see the math.
     
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