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Bernoulli principle - negative pressure

  1. Mar 17, 2013 #1
    I will write a program about principle bernoulli, but i have a problem.

    My input data:

    Pressure(p1) = 1000
    Cross-section(A1) = 1
    Velocity(V1) = 1

    Cross-section(A2) = 0.5

    Velocity(v2)= (V1A1)/A2

    to simplify: h1=h2

    i'm counting the pressure p2 : p2 = p1 + 0.5*v1*v1 - 0.5*v2*v2 , so i getting negative pressure..


    This is a problem (negative pressure)

    I'm trying to add atmospheric pressure, but it doesn't solve my problem.

    Is a mathematical formula is good? Is there any other relationship?
  2. jcsd
  3. Mar 17, 2013 #2


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    Some units would be helpful in deciphering your problem.
  4. Mar 17, 2013 #3


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    Your equations look OK. You correctly calculated V2 = 2, so you must have made a mistake in the code.

    My calculator says 1000 + 0.5*1*1 - 0.5*2*2 = 998.5 whcih isn't negative.
  5. Mar 17, 2013 #4
    My mistake. I forgot write the density of liquid.

    Should : p2 = p1 + 0.5*v1*v1*ro - 0.5*v2*v2*ro


    p2 = 1000 + 0.5*1*1*1000 - 0.5*2*2*1000 = -500 .
  6. Mar 17, 2013 #5
    I replace Paskal to kPa, but it didn't help
  7. Mar 17, 2013 #6


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    Your "negative outlet pressure" just means the inlet pressure is too low to force the given mass flow through the nozzle.
  8. Mar 18, 2013 #7


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    Yep, although if the given pressures are gauge rather than absolute, it would work just fine (so long as you weren't trying to exhaust the flow with a negative gauge pressure to the atmosphere).
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