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I have a difficulty to understand Bernoulli's Principle which states increased fluid speed decreases the pressure. But in a wind tunnel when we measure the pressure with a pitot tube, both the pressure and the velocity of the wind is higher at the narrow section.

Does the pitot tube measure another kind of pressure than the one in Bernoulli's principle?

What is P1 and P2 in the figure at this link: http://eidein.edublogs.org/2008/07/29/36-bernoullis-principle/

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  • $\begingroup$ 16307-Hi, You'll need to draw a diagram of your pitot tube, show where the "narrow section" is, and what other point you are comparing it to. $\endgroup$ – akrasia Sep 13 '14 at 17:21
  • $\begingroup$ look at my link in my question. the narrow section is where P2 and V2 is where the tunnel is contracted. is that clear now? pls answer my question $\endgroup$ – user16307 Sep 13 '14 at 17:58
  • $\begingroup$ I am afraid that's not a pitot tube, as you can see from the fact that that the text does not include the words "pitot" or "tube". Now that I think of it, that probably answers your question! $\endgroup$ – akrasia Sep 13 '14 at 18:33
  • $\begingroup$ Keep in mind the causal relationship that is implied by Bernoulli's principle. The pressure drop causes the velocity increase, the two variables aren't simply "related" $\endgroup$ – Bryson S. Sep 13 '14 at 22:07
  • $\begingroup$ check user3814483's answer that was i was looking for. glad that some people really understood what i asked. $\endgroup$ – user16307 Sep 13 '14 at 22:19
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There's static pressure (P) and dynamic pressure ($\frac{1}{2} \rho v^2$). A pitot tube placed in the center section of your drawing will measure the stagnation pressure, which is the sum of the two terms.

Physically, the dynamic pressure will be larger at the narrow section, and the static pressure will be smaller there. The sum will be constant throughout the wide and narrow regions.

I interpret P1 and P2 in your image to be the static pressure.

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  • $\begingroup$ thank you very much. i think i have to learn 2 different types of pressure. if pitot measures dynamic pressure. which instrument measure static pressure? a barometer? is static pressure atmospheric air pressure if i understood you right? $\endgroup$ – user16307 Sep 13 '14 at 22:01
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    $\begingroup$ @user16307 I think a pitot measures static + dynamic pressure (you'd have to decouple the two using another instrument). A manometer can be used. Look up "Prandtl tube" - this is a combination device that is essentially a Pitot tube with a separate static pressure measurement. $\endgroup$ – user3814483 Sep 13 '14 at 22:38
  • $\begingroup$ Static pressure is the pressure you measure when the flow is not decelerated at all, Stagnation pressure is the pressure you measure when it is slowed down completely. Moreover, static pressure is the pressure you would measure if you were riding along with the flow. $\endgroup$ – Bryson S. Sep 13 '14 at 23:01
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I once had the same problem understanding. But one day I realized pressure is just a measure of energy and Bernoulli's law is just another way of expressing the conservation of energy. The analogy: Total Pressure = Dynamic Pressure + Static Pressure >> Total Energy = Kinetic Energy + Potential Energy.

In either case we assume no losses, or otherwise adjust the equations to account for loss.

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