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How to be sure your pitot-static tube is aligned correctly with the flow?

  1. Sep 19, 2012 #1
    Hello everyone, I just found this site and I'm really hoping to find an answer to my problem!

    As the title tells my problem is this: I'm working on a project with pitot-static tube and I'm wondering how to get the tube correctly placed in a wind tunnel. Every guidance I can find just says pitot tube must be properly aligned with the flow. Exactly how can one align it properly?

    I have this theory that it's correctly aligned when the static ports of the tube shows the lowest reading (meaning it's 90 degrees to flow) and the stagnation pressure (total pressure) is it's highest (meaning it's head-on to flow). I'm thinking that would indicate the tube is coaxial with the flows direction.

    Could someone wiser please confirm if my theory is right or wrong.

    I'm not yet sure what kind of a differential pressure transducer I'm using but in case it's digital that shows just the difference between these two pressures how can I tell when the static pressure is it's lowest and stagnation pressure it's highest?
     
    Last edited: Sep 19, 2012
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  3. Sep 19, 2012 #2

    boneh3ad

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    That would be a good way to align it.
     
  4. Sep 19, 2012 #3
    If the stagnation pressure is highest and the static pressure is at its lowest point then the difference between the two would be maximum.

    If you wanted to isolate them then only connect one of them to the transducer at a time and leave the other end of the transducer open to the ambient pressure.
     
  5. Sep 22, 2012 #4
    Ok, thank you guys!
     
  6. Oct 16, 2012 #5
    I've tried to measure the pressures but it seems I do something wrong because the static and stagnation pressures are too small.

    Dynamic pressure seems to be legit (I've checked it by calculating the speed of flow from it, and it is fairly accurate to the known flow speed) but measured stagnation pressure is way too small. Actually it's smaller than the measured dynamic pressure which is in my understanding impossible since stagnation pressure = dynamic pressure + static pressure.

    I've measured the dynamic pressure by connecting two hoses (static and stagnation) from pitot tube to the transducer and resetting the pressure to zero before starting the wind tunnel. Stagnation and static pressures I measured exactly same way but left the other hose unplugged.

    So why am I getting way too small stagnation (and maybe static) pressure? For instance, if my measured dynamic pressure is 233 Pa, then my measured static pressure is 38,5 Pa and measured stagnation pressure is 194,5 Pa. So where's the error? [233 + 38,5 >> 194,5 but then again: 233 - 38,5 = 194,5] So is it correct to subtract static pressure from dynamic pressure even though it is known that stagnation pressure = dynamic pressure + static pressure?
     
    Last edited: Oct 16, 2012
  7. Oct 16, 2012 #6

    boneh3ad

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    No, it looks more like you are misinterpreting your results. Neither static nor dynamic pressure should ever be higher than total pressure and the two should always add up to total pressure for an incompressible flow.

    What sort of transducer is it? Is it a differential transducer and you are hooking up one hose to each side so you are measuring the pressure difference?
     
  8. Oct 16, 2012 #7
    The hoses goes to a Testo 452 (I have tried also Testo 510) static pressure to - input and stagnation pressure to + input.
     
  9. Oct 16, 2012 #8

    boneh3ad

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    And how do you have your hoses mounted in the tunnel to measure the pressures?
     
  10. Oct 17, 2012 #9
    Not sure if I understood you correctly..

    But the hoses are connected to pitot static tube that is half inside the wind tunnel and half outside. Hoses are connected to pitot by the outside part of it.
     
  11. Mar 27, 2014 #10

    Bobbywhy

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    Your theory seems correct since we know that when a pitot-static tube is mounted on an aircraft, or in a wind tunnel , the center tube is always pointed in the direction of the flow and the outside holes are perpendicular to the center tube. On some airplanes the pitot-static tube is put on a longer boom sticking out of the nose of the plane or the wing.

    Here is a procedure to check this:

    "To insure accurate velocity pressure readings, the Pitot tube tip must be pointed directly into (parallel with) the air stream. As the Pitot tube tip is parallel with the static pressure outlet tube, the latter can be used as a pointer to align the tip properly. When the Pitot tube is correctly aligned, the pressure indication will be maximum."
    https://www.dwyer-inst.com/Products/AirVelocityIntroduction.cfm

    Hope this helps!
     
  12. Mar 27, 2014 #11
    When you measure stagnation and static pressure you say you are removing the other tube. When you measure static pressure are you leaving the static tube on the negative input and just removing the stagnation tube? If you are this would mean when measuring static pressure you are measuring (atmospheric - static) and when you measure stagnation pressure you are measuring (stagnation - atmospheric)
     
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