I was doing the air flow in a 90 degree bend experiment.

On the first run, I connected the manometer to the tappings on the outer wall, then I turned off the bench and changed the connections to the inner wall. Then, I turned on the air flow again.

Each time I done this I got a different reading on the manometer for the airbox pressure and the atmospheric pressure.

The air flow settings weren't changed and no change in room temperature or nothing. It was like 10 minutes difference between each run.

Why is this?

  • $\begingroup$ what was the manometer difference? one difference is 10 minutes to each run as you said. If we go by the observer effect, it is impossible to observe anything without altering it at least a little. $\endgroup$
    – Abel
    Apr 21, 2022 at 11:33

2 Answers 2


Not an answer per se, so don't downvote - just too much for a comment without formatting...

Different readings for atmospheric pressure seems odd.

Do you have a barometer in the lab - usually mercury, 1 metre high with a vernier scale and a temperature correction chart.

If you are measuring the inside and outside values relative to atmospheric without an absolute reference of atmospheric then not sure what you are actually getting.

Perhaps a diagram would help us all.


"Euler’s equation expresses the relationship between the velocity and the pressure fields in inviscid flow. Written in terms of streamline coordinates, this equation gives information about not only about the pressure-velocity relationship along a streamline (Bernoulli’s equation), but also about how these quantities are related as one moves in the direction transverse to the streamlines. The transverse relationship is often overlooked in textbooks, but is every bit as important for understanding many important flow phenomena, a good example being how lift is generated on wing"

Bold mine.

Try here - Equation of Motion in Streamline Coordinates, Ain A. Sonin, MIT 2.25 Advanced Fluid Mechanics


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