Pitot tube confusion

[bluequestions]

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Below is an attached image of pitot tube from KA lecture, however I'm confused about the way it works. For the top right chamber, we call that pressure where the purple dot is the static pressure and the green dot stagnation pressure. Can someone explains exactly what the distinction between the two is. Also for the velocity we used at the green spot, it seems that right chamber is sealed off also just like the left chamber, so why is it that fluid could flow into only the right chamber but not the left? Thank you!

 

[sazerac]

You seem to have either your dot colors, your directions, or your terms confused.

The inside tube, the purple tube, the stagnant tube, the lower left tube, the tube with the light blue lines across it, this tube has an opening that faces into the wind. Air is getting smushed into this tube. You can measure the pressure at either purple dot. It will be higher the the pressure in the wind. This purple pressure is called the stagnant (i.e. zero velocity) pressure.

The outside tube, the green tube, the static tube, the upper right tube, the tube with the dotted green line in it, this tube also has an opening but this opening is 90 degrees to the wind. Air does not flow into this tube, rather air flows past this tube's opening. The pressure inside this tube will be the same as pressure in the free flowing wind itself. This is called the static pressure. This seems like a lame name to me. I would have called it the "flowing pressure" to indicate it is different from the stagnant pressure. Oh well.

The whole point of this device is that it is pretty easy to insert a membrane between these two tubes and measure the pressure difference between the two tubes. This number (deltaPresure) is easy to measure but not very interesting by itself. However, with a little math, you can convert this deltaPressure into V, the velocity of the fluid. Knowing the fluid velocity is very valuable if you are driving a boat or an airplane through the fluid.

I suspect you may have more questions. Feel free to ask some more if you need it.

 

[bluequestions]

Hi! Sorry for the late response, thanks for you help first of all (and hope you'll be enjoying your Thanksgiving haha)! Sorry I've got couple more questions to make sure I understand what's going on:
1) For the stagnant tube, is the velocity of air particle zero throughout the entire tube? If so, could you explain why, because it seems to me that the air would still be traveling at some velocity when they just entered the tube? (in fact, it seems to be they would be traveling at a higher speed since they are entering into a small cross section? so according to continuity equation, speed should go up? or am I just confusing the concepts.....)

Anyway, aside from that, so saying that the pitot tube inside a boat measuring the velocity of water, does this mean the static pressure being measured is the hydrostatic pressure at that depth of water, equals to rho(g)(h)?

2) For the upper static tube, you mentioned that "The pressure inside this tube will be the same as pressure in the free flowing wind itself", is this because since the entry point is perpendicular to the wind flow, the pressure inside the static tube will just equilibrate with the external pressure by the wind flow? So that the air particles inside the static tube are traveling at the same speed as air particles in the wind flow?

Thanks for the help!

 

[sazerac]

Hi! Sorry for the late response, thanks for you help first of all (and hope you'll be enjoying your Thanksgiving haha)! Sorry I've got couple more questions to make sure I understand what's going on:
1) For the stagnant tube, is the velocity of air particle zero throughout the entire tube? If so, could you explain why, because it seems to me that the air would still be traveling at some velocity when they just entered the tube? (in fact, it seems to be they would be traveling at a higher speed since they are entering into a small cross section? so according to continuity equation, speed should go up? or am I just confusing the concepts.....)

Anyway, aside from that, so saying that the pitot tube inside a boat measuring the velocity of water, does this mean the static pressure being measured is the hydrostatic pressure at that depth of water, equals to rho(g)(h)?

2) For the upper static tube, you mentioned that "The pressure inside this tube will be the same as pressure in the free flowing wind itself", is this because since the entry point is perpendicular to the wind flow, the pressure inside the static tube will just equilibrate with the external pressure by the wind flow? So that the air particles inside the static tube are traveling at the same speed as air particles in the wind flow?

Thanks for the help!

The stagnant tube has an entrance but no exit. Because it's opening is towards the oncoming fluid, new fluid will try to enter the tube, raising the pressure in there. There might be a little turbulence at the opening of the tube, but the tube is essentially "full" and the sensor at the far end and it will see fluid at rest.

As for the static tube, the particles in there will also be standing still, just like in the stagnant tube. The difference is, because the opening is on the side, nothing is trying to mash more fluid into the tube. The pressure equilibrates with the total pressure outside the tube, which will be less than that in the stagnant tube.

 

[bluequestions]

Thank you!