Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Bernoulli's Principle explaination

  1. Mar 29, 2004 #1
    Can someone explain this principle to me in as few words as possible (less than 500) whilst retaining quality?
     
  2. jcsd
  3. Mar 29, 2004 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Bernoulli's principle relates the pressure, velocity, and height between two points along a fluid under certain conditions (such as incompressible, steady flow, non-viscous). It is a statement of conservation of energy along the fluid.

    Bernoulli's equation looks like this:

    [tex]P_1 + \frac{1}{2} \rho v_1^2 + \rho g y_1 = P_2 + \frac{1}{2} \rho v_2^2 + \rho g y_2[/tex]

    Want more? Google. :smile:
     
    Last edited: Mar 29, 2004
  4. Mar 30, 2004 #3
    Is that the same for air flow? ie wind turbines?

    I dont like google, i get too much useless information, spam, porn, untruths, bad attempts and general waffle.
     
  5. Mar 30, 2004 #4

    enigma

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Bernoulli applies to all incompressible fluids, which low speed air can be approximated as.
     
  6. Mar 30, 2004 #5
    enigma said: Bernoulli applies to all incompressible fluids, which low speed air can be approximated as.

    I don't think approximating low speed air as an incompressible fluid is a very good approximation. Don't you mean high speed?
     
  7. Mar 30, 2004 #6

    enigma

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, at high speeds the air compresses.

    It comes from thermodynamics and the ideal gas law

    [tex]p=\rho*R*T[/tex]

    If you restrict the space which air can take up (by putting a wing in its path, for instance), the temperature rises, the density increases, and the pressure rises. According to thermodynamic properties, how much each changes depends entirely on the Mach number.

    For low Mach number flows (less than .3), the density changes less than 5%, so it can be safely modeled as incompressible. For high Mach numbers (modern aircraft or rocket nozzles), using Bernoulli will give you very wrong numbers. In those cases, the more complicated thermodynamic properties must be used. If you're interested, Introduction to Flight, by John D. Anderson is a very well written textbook which has a chapter or three on it.
     
  8. Mar 31, 2004 #7
    rattis,
    Enigma and Doc Al are absolutely steering you in the right direction. A good book on fluid mechanics would help, and google too(send us some porn links).
    -Mike
     
    Last edited by a moderator: Mar 31, 2004
  9. Mar 31, 2004 #8
    What is the word equation for this principle?
     
  10. Mar 31, 2004 #9

    russ_watters

    User Avatar

    Staff: Mentor

    To amplify what enigma said, treating airflow above 220mph as compressible is the rule of thumb I learned.

    And what is a "word equation"?
     
  11. Mar 31, 2004 #10
    Ummmmm a word equations is an equation in words, or is this priciple to complex to write in words???!!!! :confused:
     
  12. Mar 31, 2004 #11

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Read Doc Al's post, he gives the key information.

    Examine the equation the first thing to note is that the 2 sides only differ by the subscripts, this means it is relating the same properties in different regions.

    The first term is a P or pressure, since all the terms are added they must all have the units of pressure. The second term is the density times the square of the velocity, this looks suspiciously like a kinetic energy. Notice that Doc Al mention conservation of energy? So this expression corresponds to a pressure due to the motion of the fluid. The last expression is a similar to a potential energy, this is a pressure due to fluid depth.
     
  13. Mar 31, 2004 #12
    Absolute pressure plus kinetic energy per unit volume plus potential energy per unit volume has the same value at all points along a streamline.

    or if you like:

    Absolute pressure plus dynamic pressure plus potential energy per unit volume has the same value at all points along a streamline.

    or in a level system(no gravitational potential energy):
    The sum of absolute pressure plus dynamic pressure remains constant along a streamline.

    I hope that this is what you were looking for.
    -Mike
     
  14. Mar 31, 2004 #13
    thanks, although that maybe too advanced to tell to the 15/16 year olds that i am trying to find this out for.
     
  15. Mar 31, 2004 #14
    I found a better version in an encyclopedia.

    "Bernoulli’s principle states that as the velocity of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases."
     
  16. Mar 31, 2004 #15

    enigma

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Alright... how about:

    As the velocity of a flow increases, the pressure drops. The pressure will not go any higher than the pressure of a stagnant (zero velocity) flow.

    EDIT: crosspost
     
  17. Mar 31, 2004 #16

    Absolutely.
    -Mike
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Bernoulli's Principle explaination
  1. Bernoulli Principle (Replies: 1)

  2. Bernoulli's Principle (Replies: 1)

Loading...