1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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 Equation

  1. Oct 16, 2015 #1
    Hi

    Can anyone advise how the following equation was derived.

    http://uploadpie.com/PYLrD [Broken]
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Oct 16, 2015 #2

    Mister T

    User Avatar
    Science Advisor

    Before we try to answer your question, you'll have to convince us that this is not a homework assignment.
     
  4. Oct 16, 2015 #3
    It is part of a document of examples not part of an assignment that's why I have all the answers
     
  5. Oct 16, 2015 #4

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Have you attempted this yourself yet?
     
  6. Oct 18, 2015 #5
    Yes but cannot fathom the 1/D wher eit came from
     
  7. Oct 18, 2015 #6
    Hey Boneh3ad,

    Have you noticed that the given answer is not dimensionally correct. They left out the width of the channel (if the really mean that Q is the volumetric flow rate).

    Chet
     
  8. Oct 18, 2015 #7
    I took the expression as meaning the velocity - I know it states for Q but it does not multiply by area anywhere.
     
  9. Oct 18, 2015 #8
    ##\frac{Q}{wD}=## velocity at the left of the figure, where w is the width of the channel. So the area is wD.

    chet
     
  10. Oct 18, 2015 #9

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Yes. I was able to reproduce the formula from the problem with the added ##w## term included, but I am not 100% convinced that the assumptions used to get there make a whole lot of sense to me at the moment.
     
  11. Oct 18, 2015 #10
    Me neither, if you are referring to the dip in the upper surface.

    Chet
     
  12. Oct 18, 2015 #11

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    I was referring to the fact that you have to assume that the pressure differenc causing acceleration is wholly explained by the slight change in hydrostatic pressure due to ##\Delta h##. The more I think about it, though, the more that makes sense. That assumption gives the same answer as the original post, except the expression describes ##Q/w## instead of just ##Q##.
     
  13. Oct 18, 2015 #12
    I would think a unit width is implied, which makes the expression easier to work with.

    Multiply by the whole width to obtain the total flow in the channel.
     
  14. Oct 18, 2015 #13

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    It may be but it specifically says volumetric flow rate and leaves it out. You could certainly assume unit width but you'd have to say that in order for the units to make sense. It's probably just one of those things that the author overlooked as obvious but would have confused me as an undergraduate.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Bernoulli's Equation
Loading...