What causes the liquid to rise in this picture?

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1. Sep 6, 2015

Ranul P

This is a diagram of a pitot-static tube. My question is however not related to its applications but rather, what causes the liquid to rise up the static tube? The static tube is at right angles to the fluid flow. I understand that this is a very basic question but I can't seem to get my head around it.

Many thanks.

2. Sep 6, 2015

3. Sep 6, 2015

Ranul P

Is that the only possible explanation? If it is due to capillary action, then is it implied in calculations involving pitot-static tubes that the diameters of the pitot and static tubes are the same, and also that these diameters are rather small?

4. Sep 6, 2015

ShayanJ

That's the accepted explanation and its old enough to be considered a well-established explanation. So there is no point in looking for anything else.

The diameters don't need to be the same Its just that if the diameters are not the same, you can't have a meaningful comparison between two cases.
But a small diameter is needed for capillarity to work because as you increase the diameter, gravity defeats capillarity at a smaller height because a less height is needed to provide the enough mass of water and for a large enough diameter, the height can become undetectable.

5. Sep 6, 2015

Thank you :)

6. Sep 6, 2015

gsal

Nope, not capillarity.

Total pressure at some given point consists of two components: static pressure and dynamic pressure. When you get to a tank or chamber and things stop flowing, dynamic pressure is zero and total pressure has the same magnituce as static pressure...when the fluid is flowing and it has speed, then dynamic pressure has a value greater than zero.

What makes the fluid rise in the static tube is precisely the static pressure at the entrance to the tube.

What makes the fluid rise in the Pitot tube is precisely the pressure exerted by the fluid (dynamic pressure) crashing into the entrance to the Pitot tube/

7. Sep 6, 2015

Merlin3189

Totally agree with gsal.
The same thing happens if the fluid is a gas and the pressure is measured with a U-tube manometer. There one side goes up and the other down - not capillarity.

8. Sep 6, 2015

Staff: Mentor

gsal is correct: it is static pressure, not capillary action.

9. Sep 6, 2015

billy_joule

10. Sep 6, 2015

Staff: Mentor

11. Sep 6, 2015

rcgldr

The bottom end of the static tube is incorrectly located. It's bottom end needs to be flush mounted with the upper wall of the tube so that it senses the static pressure of the boundary layer between the bottom open end of the tube and the fluid flowing across below, which will be the same (within reason) as the static pressure of the moving fluid. With the tube located as shown in the diagram, vortices are formed by the fluid flowing across the open end of the tube, and typically the static pressure of the vortices would be lower than the static pressure of the flowing fluid. This is how some atomizers work (like the old flit gun sprayer).

12. Sep 7, 2015

Ranul P

Oh okay...this makes sense now. I always only thought of it as having a taller column of liquid causes a greater pressure at the bottom of the column. This requires me to think of the opposite as well; a greater pressure causing a taller column of liquid. Thank you everyone! This really helped my perspective.

13. Sep 8, 2015

DocZaius

Isn't it rather precisely the dynamic pressure plus the static pressure?

14. Sep 8, 2015

gsal

Sorry, yes, it would have to be. Should the fluid not be moving, both columns of water would have the same height as static pressure is expressed in all directions; it is when the fluid moves that its dynamic pressure adds to the static pressure, when the tube faces the incoming flow....it the tube was pointing in the opposite direction, then the dynamic pressure would be taking away from the static and the column of water in this tube would actually be lower than the static tube.