1. Not finding help here? Sign up for a free 30min 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!

Change in pressure when merging tubes

  1. Dec 1, 2015 #1
    1. The problem statement, all variables and given/known data
    hello, I have a problem that I couldn't find a clear answer to. Attached is a diagram of a related problem that explains the problem

    Air traveling through two tubes with the same direction have the same PSI (say 75). If you merge the two tubes into a tube with the same diameter as the previous two, what happens to the pressure of the moving air? Does it double?

    20z7n6s.jpg

    2. Relevant equations


    3. The attempt at a solution
    I think the pressure would double, but im unsure
     
  2. jcsd
  3. Dec 1, 2015 #2

    gneill

    User Avatar

    Staff: Mentor

    Have you investigated the Bernoulli equation for this setup? If the tube diameters are all the same, how are the velocities of the air in each section related?
     
  4. Dec 1, 2015 #3
    I don't know if the original problem is that complicated. Lets say the source of air is two identical air compressors set to the same settings, so the PSI, the velocities, and the volume is the exact same for both tubes.
     
  5. Dec 1, 2015 #4

    gneill

    User Avatar

    Staff: Mentor

    Apply Bernoulli. Really :smile:

    You'll have to put a small bit of thought into deducing the velocity in the merged tube. Assume that the tubes are lying on the ground (you're looking down on them) so there's no change in height involved.
     
  6. Dec 2, 2015 #5
    I am still having troubles getting my head around this. The problem is that any time I've used the Bernoulli equation, its only looking at one tube (like a Laval nozzle where one tube gets smaller), but this question has two tubes feeding into one. So this equation:
    a71608f0f41473679c7dce52f525d232.png

    would need a p3 and v3 somewhere in there, but I can't figure out how to integrate it
     
  7. Dec 2, 2015 #6

    gneill

    User Avatar

    Staff: Mentor

    You said that the pressure and flow rate in the feeds were identical. Thus each feed moves the same volume of air into the exit tube per unit time. Knowing the dimensions of the tubes, that should allow you to determine what the flow rate (hence velocity) must be in the exit tube. A flow streamline can be chosen in one feed tube and followed to the exit tube. We assume no turbulence, hence laminar flow. Apply Bernoulli to that streamline. Two velocities, two pressures. You know the two velocities and one of the pressures...
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted