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!

Mechanical Energy Balance

  1. Sep 14, 2009 #1
    1. The problem statement, all variables and given/known data
    In this era of high energy prices, there is a focus on renewable sources of energy such as wind energy. Consider a large windmill 30m in diameter. On a windy day, suppose that the windmill entrains a stream of air 25 m in diameter at a speed of 14 m/s. Downstream of the windmill, the entrained stream exits over a large diameter at a speed of 5 m/s. The pressure equals atmospheric pressure at both inlet and outlet. Density of air: ρ = 1.2 kg/m3

    Find the power (in megawatts) generated by the windmill.


    2. Relevant equations

    1/2 Δ(v^2) + gΔz + (ΔPressure)/density + lost work + shaft work/mass flow rate = 0

    3. The attempt at a solution

    So I know that that is no change in Pressure and no lost work so those go to 0. I don't think that you would need the gΔz part either?? So basically I'm thinking plug in the velocities given and that would only leave shaft work which I'm solving for and mass flow rate. Now I don't know what velocity and area I would use for that either so that's where I'm stuck?? Am I headed in the right direction?
  2. jcsd
  3. Sep 15, 2009 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to Physics Forums. You're correct that gΔz is not needed (it is zero).

    You can use the initial velocity and area, and density, to get the mass flow rate.

    Also, note that "shaft work" would actually be power here.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook