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When the cross section of the flow tube decreases, the flow speed increases, and therefore the pressure decreases.

can someone explain to me why this is true, i would think that as the cross section decreases the pressure would also increase .

Катя
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    I'm voting to close this question as off-topic because it is a physics question, not a computer programming question. – Raymond Chen Apr 07 '18 at 06:26
  • I'm voting to close this question as off-topic because it is about fluid dynamics / physics instead of programming / coding / programming tools / software algorithms. – Pang Dec 12 '18 at 07:36
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    I'm voting to close this question as off-topic because it's about physics, not programming. – Lasse V. Karlsen Jan 04 '20 at 21:21
  • I’m voting to close this question (again) because it's about physics, not programming. – Jeremy Caney Dec 01 '21 at 18:44

2 Answers2

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This is related to "Continuity Equation" of fluid mechanisam.(Assuming fluid as incompressible)

if we have two cross-sections of areas A1 and A2 having velocities V1 and V2 respectively .Then according to continuity equation

A1*V1=A2*V2 or we can write

V2=(A1*V1)÷A2

V2 Is inversly proportional to the A2.

so velocity increases as the area decreases.

further we have a theorem in fluid mechanics called "Bernouli's theorem".

which states that the sum of all energies at any cross-section is constant.

So if the velocity(i.e kinetic energy) increases at any section there will be decrease in pressure(i.e pressure energy)

Sudoss
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Think of it this way, what is pressure in the first place.! Well pressure is the force acting perpendicular to a unit area write ? So the fluid whatsoever particles are exerting force on that unit area, that's fine.. image five 10 people standing in an elevator standing next to each other, these guys are too much to fit inside the width of the elevator, thus they would push themselves towards the wall of the elevator making huge amount of force on the adjacent walls write ? what was that again ? aha!! huge force per area which are in this examples the walls of the elevator. hence they are too much pressure, okay.. now imagine that these people instead of standing 10 next to each others they formed themselves as groups of twos, so 5 rows of twos instead one of ten, i bet that they will feel more comfortable right? they won't push themselves that much to the wall and hence the wall will have small force on it and then small pressure, that was an example for proving that physics isn't just some numbers that define what is going to happen, Bernoulli's equation predicted that the pressure will decrease based on logic. science works :D

Amr Hossam
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