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!

Homework Help: Fluids Problem - Dimensional analysis, pipe flow, propeller

  1. Jul 17, 2013 #1
    1. The problem statement, all variables and given/known data

    I have a tutorial question I have been struggling with. The problem is:

    The flow through a closed, circular sectioned pipe may be metered by measuring the speed of
    rotation of a propeller having its axis along the pipe central line. Derive a relation between
    the volume flow rate and the rotational speed of the propeller in terms of the diameter of the
    pipe and the propeller and the density and viscosity of the fluid. A propeller of 75mm diameter
    installed in a 150mm pipe carrying water at 42.5 l/s is found to rotate at 20.7rps. If a geometrically similar propeller of 375mm diameter rotates at 10.9rps in air flow through a pipe of
    750mm diameter, estimate the volume flow rate of the air. (The air density is 1:28kg/m3 with
    viscosity of 1.93E-05 Ns/m2 as and the viscosity of water is 1:145E-03 Ns/m2).

    3. The attempt at a solution

    I think my main problem is just finding all the pertinent variables to start with. After that I should be able to use the Buckingham Pi theorem, form dimensionless groups the use similarity to find the value of volume flow rate for air.

    The variables I can think of are:

    Volume flow rate, Q
    pipe diameter, D
    propeller diameter, d
    density, ρ
    viscosity, μ
    rotational speed, N

    I have a feeling I should also include angular velocity or just theta for the angle but I'm cant figure it out.
  2. jcsd
  3. Jul 17, 2013 #2
    I think you have the right idea. Regarding angular velocity, that is proportional to rotational speed N. In my judgement, you have identified all the parameters you need.


    PS, welcome to Physics Forums.
  4. Jul 18, 2013 #3
    Ok so using the six original variables I listed I formed three Pi terms:

    Pi 1: Q/(N)(d3)

    Pi 2: D/d

    Pi 3: ρN(d2)/μ

    From geometric similarity I found the scale is 1:5.

    Then just using similarity of Pi 1 with the data given I found Q for air to be 2797.40 l/s.

    I thought this seemed high but using similarity of Pi 3 with the given data I worked out the density of water to be 999.34 kg/m3 which makes sense. Is the flow rate right, I thought it should be more complicated.
  5. Jul 18, 2013 #4
    I got it now, I was right, it just seems high to me when flow rate is in l/s instead of m^3/s. I was really expecting the problem to be more complicated. Thanks for the help.
  6. Apr 5, 2014 #5
    Confirmation of Answer

    I just finished the exact same problem and I'm having a problem with it.

    The value for rotational velocity isn't squared for the answer. The value used is just N and not N^2, can anyone explain this?

    It's in this book. The exercise is 5.4 and the answer is at the back.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted