Dynamic pressure and pitot tube

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Discussion Overview

The discussion revolves around the behavior of a pitot tube when it is oriented against the direction of airflow. Participants explore the implications for measuring static and dynamic pressure, considering various factors that may influence the readings, such as vortex flow and the geometry of the probe.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that if a pitot tube is reversed, it will primarily measure static pressure and not dynamic pressure.
  • Others argue that reversing the pitot tube would result in sensing pressure lower than ambient static pressure, influenced by vortex flow at the tube's end.
  • A participant questions whether, ignoring vortex effects, the pressure would be lower by the amount of dynamic pressure.
  • Another participant emphasizes that vortex effects cannot be ignored and that a rear-facing pitot tube would not measure static pressure in general.
  • One participant suggests that turning the pitot tube 180 degrees leads to uncertainty in measurements, indicating that it would likely capture static pressure combined with other effects.
  • It is noted that inserting the probe affects the flow field, and the measurements depend on the nature of the wake created by the probe.

Areas of Agreement / Disagreement

Participants express differing views on the effects of reversing the pitot tube, with no consensus reached regarding the exact nature of the measurements obtained in this configuration.

Contextual Notes

Participants acknowledge the complexity of the flow field around the pitot tube and the influence of vortex effects, but specific mathematical or empirical details remain unresolved.

david316
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Hello,

Normally a pitot tube points in the direction of airflow so that it measures static and dynamic airflow. If it points in the opposite direction my assumption is that it will not measure the dynamic pressure but will only measure the static pressure.

Is this correct?
 
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If reversed, the end of the pitot tube would sense pressure lower than ambient static pressure. Any vortice flow at the end of the pitot tube would also be a factor.
 
rcgldr said:
If reversed, the end of the pitot tube would sense pressure lower than ambient static pressure. Any vortice flow at the end of the pitot tube would also be a factor.

Ignoring the effects of vortices flow would it be lower by dynamic pressure amount?
 
david316 said:
Ignoring the effects of vortices flow would it be lower by dynamic pressure amount?
It would be different, but I'm not sure of all the effects involved.
 
david316 said:
Ignoring the effects of vortices flow would it be lower by dynamic pressure amount?

You can't ignore those effects. There will be a wake produced but a rear-facing Pitot tube and it would not measure static pressure in general. It wouldn't differ by the dynamic pressure, either. If you want all three, your best bet is still the old standard of the Pitot-static tube.
 
Consider that I have a tube with air moving through it. If I have a right angle pitot tube in the airflow so that the opening of the pitot tube is inline with the airflow the stagnation pressure should be the static pressure plus the dynamic pressure.
http://www.engineeringtoolbox.com/pitot-tubes-d_612.html
https://en.wikipedia.org/wiki/Stagnation_pressure

So if I turn the pitot tube around 180 degrees so its not facing the flow there is no sure answer of what I will actually measure? It sounds like static pressure combined with some other effects?
 
Last edited:
Do the experiment and find out . Water manometer made from clear plastic tube and a bent straw pitot is all you need .
 
david316 said:
Consider that I have a tube with air moving through it. If I have a right angle pitot tube in the airflow so that the opening of the pitot tube is inline with the airflow the stagnation pressure should be the static pressure plus the dynamic pressure.
http://www.engineeringtoolbox.com/pitot-tubes-d_612.html
https://en.wikipedia.org/wiki/Stagnation_pressure

So if I turn the pitot tube around 180 degrees so its not facing the flow there is no sure answer of what I will actually measure? It sounds like static pressure combined with some other effects?

No, there is no easy answer for what you will measure. Inserting the probe affects the flow field. When you face it backward, you are placing the opening in the location experiencing the greatest effects of the presence of the probe in the flow. Like I said, the opening would then be in the probe's wake. What it measures will depend on the nature of the wake and therefore the geometry of the probe and flow conditions.
 
Thanks
 

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