1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Fluid mechanics - Water flowing through a pipe

  1. Mar 3, 2013 #1
    1. The problem statement, all variables and given/known data

    3. The attempt at a solution
    I am interested in why my method is flawed:

    Alright I know how to do this problem. Essentially you solve mass flow rate = rho * double integral of u dA. HOWEVER, I decided to do this differently. I know that I can solve for the average u from the equation mass flow rate = rho * u_avg * A. I also know that the average u is just the integral of u over the integral of 1. I tried this and I get 1.528 m/s which is clearly not one of the answers. Why does my method not work?
  2. jcsd
  3. Mar 3, 2013 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    In the expression: ##\dot{m} = \rho u_{avg}A##,
    ##u_{avg}## needs to be the average over the cross-sectional area of the pipe, not the average over the radius.
  4. Mar 3, 2013 #3
    Hmmm, according to the equation doesn't u only depend on r? In other words u is constant for some circular ring r = k?
  5. Mar 3, 2013 #4


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Consider two intervals of radius, one near the center of the pipe and one out at the edge. Suppose the interval dr is the same for both. There are more patches of area in the same interval dr out near the edge. So, the value of the velocity out at the edge contributes more to the average over the cross–section than the value near the center.

    Attached Files:

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

Have something to add?
Draft saved Draft deleted

Similar Threads - Fluid mechanics Water Date
Calculating water pressure in a horizontal pipe Dec 22, 2017
Angle of Inclination of a Water Surface Mar 8, 2015
Water in a pipe Dec 24, 2014
Fluid mechanics cork in water Dec 15, 2012
Fluid mechanics water tank level Nov 21, 2012