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Bernoulli saving submarine

  1. Jul 1, 2014 #1
    Hi people,

    It may be a strange question, or impossible, but maybe you could help me out.

    It's about submarines, high pressure and Bernoulli's law.

    What if we build some kind of jet engine-like thing on the front of the submarine, not to 'pull' it forward, but to speed up the water direct next to the submarine. In this was, following Bernoulli's law, when the velocity of the water increases, the pressure must decrease?

    Will the submarine implode later than a similar submarine without this stuff? If we assume that the water coming out of the engine covers the whole side.

    More simplified: Can you use the decreasing pressure effect from Bernoulli's law to help increase the depth range of a submarine?

    Just curious...

    The attachment includes a simple picture to make the story more clear.

    Attached Files:

  2. jcsd
  3. Jul 1, 2014 #2


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    Speeding up the water doesn't decrease the pressure, depending on how the water's speed is increased. In the case of a turbine, the exhaust at the exit nozzle would have higher velocity and higher pressure, since the engine peforms work on the water.
  4. Jul 1, 2014 #3
    How about a rocket engine to speed up the submarine? In case of torpedos it even enables super cavitation.
  5. Jul 1, 2014 #4


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    In general, moving through a fluid makes the otherwise uniform pressure distribution non-uniform. If the structure is optimized for this, it might survive at greater depths when moving than when static.
  6. Jul 2, 2014 #5

    Yes, why not. Nice idea. (Water stream pump decreasing the pressure)

    We could also spin the water around the submarine. (A centrifugal pump decreasing the pressure)
    Last edited: Jul 2, 2014
  7. Jul 2, 2014 #6
    Thank another question to make something clear. Please see the attachment picture. Is it true what i'm describing? The black thing is an jet engine-like thing which sucks the water in the front.

    Attached Files:

  8. Jul 2, 2014 #7


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    No - the pump is adding energy to the stream, so the static pressure in the high velocity jet will be basically the same as in the surrounding water.
  9. Jul 2, 2014 #8


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    I know others have said it, but I'll say it a slightly different way

    The reason that the pressure is lower when velocity is higher is essentially because of conservation of energy (namely bernouilli's equation,) so an increase in kinetic energy comes from a decrease in pressure

    But, like with any form of conservation of energy, it only applies when no energy is being added to the system.

    In your situation, though, the engine is adding energy, so the water does indeed have more velocity, but that velocity comes from the energy of the engine, instead of the energy of the pressure. (So pressure is still the same.)
  10. Jul 2, 2014 #9


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    The exit pressure from a turbine has to be higher than ambient, otherwise there would be no flow out of the turbine. Once past the exit nozzle, then no more work is done, Bernoulli applies, and the pressure decreases as the water continues to accelerate (until the pressure gradient becomes zero or negative and the pressure returns to ambient). There are complicating factors such as interaction with the surrounding flow and viscosity.
    Last edited: Jul 2, 2014
  11. Jul 3, 2014 #10
    Ah well, that makes sense. Bernoulli is actually just some application of the first law of thermodynamics. Thanks!
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