Dynamic pressure and pitot tube

[david316]

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?

 

[rcgldr]

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.

 

[david316]

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?

 

[rcgldr]

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.

 

[boneh3ad]

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.

 

[david316]

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?

 

[Nidum]

Do the experiment and find out . Water manometer made from clear plastic tube and a bent straw pitot is all you need .

 

[boneh3ad]

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.

 

[david316]

Thanks