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Aerospace Total Pressure problem in a duct

  1. Jul 21, 2012 #1
    Hello
    Please help me, a physical relation which my friend is facing, i'm unable to get my head around..
    I created a 2D S-duct, the contours are as follows
    1.Static pressure
    2.Velocity contours
    3. Total pressure gradient graph

    intake Velocity 20m/s
    Guage pressure is taken as '0' pascals

    there is flow separation and reversal at + cruvature
    created a rake through the duct to find out the pressure gradient

    but the total pressure should be constant in duct, only static pressure changes but i was unable to find out what physical problem is causing such as change as shown in graph

    Thanks in Advance
     

    Attached Files:

  2. jcsd
  3. Jul 21, 2012 #2
    sorry for got to attach geometry, rake view
     

    Attached Files:

  4. Jul 21, 2012 #3

    boneh3ad

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    Inviscid or viscous?
     
  5. Jul 22, 2012 #4
    viscous, Laminar.

    Thanks
     
  6. Jul 22, 2012 #5

    boneh3ad

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    In a viscous flow, total pressure won't be conserved, so it shouldn't have constant total pressure.

    I also notice you have a negative static pressure, which makes no sense. That isn't physically possible.
     
  7. Jul 22, 2012 #6
    there is probably an offset used in the CFD code for static pressure, like 101325 Pa. There are good numerical reasons for doing this. 0 Pa would then actually be 101325 Pa.

    Also, the Casimir effect can cause negative pressures, but that's quantum physics, so naturally nothing works as it's supposed to.
     
  8. Jul 22, 2012 #7

    boneh3ad

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    The Casimir effect still doesn't cause negative fluid pressures. For starters, it is a quantum electrodynamic effect that occurs canonically in a vacuum. That isn't relevant in the least.
     
  9. Jul 22, 2012 #8
    Yeah as boneh3ad said total pressure isn't conserved in viscus flows. Did you do any hand calculations as a preliminary check of your results? I don't have much CFD experience personally, but it seems strange that the flow separates only at your first curve and not the second.
     
  10. Nov 26, 2012 #9
    ever heard "Pressure Recovery"....

    a duct with 98% Pressure recovery is nice....

    an "S" duct with a 96% recovery (a length-diameter ratio of say 4'ish) is good too....

    if all is perfect... you will loose total pressure from boundary layer effects...

    incorporate diffusion and see how messy it gets!! :O
     
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