Pressure in fluids and effect of gravity, BR cbt 3 question 10 PS

[SaCkO]

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Ok guys look at the attachment i drew a pipe with different heights.
My question is since fluid always goes from High to lower pressure and according to Bernoulli's equation if we increase the height we will decrease the pressure, why then if we have fluid in such a pipe the flow of the fluid will go down and not up the pipe.
I encountered this question in BR cbt 3 question 10 , and in their explanation they said that gravity aids in emptying a press chamber which is situated above a reservoir
Then why would the fluid flow from the top to bottom there.

 

[circulus vitios]

Lower energy states are more stable. Compare the potential energy at the top of the pipe and at the bottom of the pipe.

 

[milski]

The high to low pressure is applicable only if there are no other forces acting on the fluid. In this case you have gravity, so it will move down.

At the low end of the pipe, the fluid interfaces with the open atmosphere which has lower pressure - that will start moving the end parcel of fluid out of the pipe and will result in a lower pressure right behind it, moving the next parcel of fluid and so on, creating the flow in the pipe.

 

[SaCkO]

The high to low pressure is applicable only if there are no other forces acting on the fluid. In this case you have gravity, so it will move down.

At the low end of the pipe, the fluid interfaces with the open atmosphere which has lower pressure - that will start moving the end parcel of fluid out of the pipe and will result in a lower pressure right behind it, moving the next parcel of fluid and so on, creating the flow in the pipe.

Yes but gravity is always acting on the flow of liquids in which ever pipe you have..
I understand that it will go from high potential to low potential, but according to bernolli's equation it when height increases then pressure decreases . So why is bernoulli's eqaution not working on this kind of situation?

 

[Tatiana3325]

Yes but gravity is always acting on the flow of liquids in which ever pipe you have..
I understand that it will go from high potential to low potential, but according to bernolli's equation it when height increases then pressure decreases . So why is bernoulli's eqaution not working on this kind of situation?

Can you put the question you are struggling with in this thread? I was not able to see your drawing for some reason.

I saw another post a few days ago by someone who also was struggling with bernoulli. Hopefully this will help: As fluid speed increases, an unbalanced force is required to produce this acceleration. The pressure must decrease. You're probably used to thinking of motions as the consequence of given forces. In fluid dynamics, the velocity of flow along a pipe of variable cross section is determined by the equation of continuity. The resulting pressure distribution is such to produce forces to maintain the flow.

Bernoullis principle assumes:
Points 1 and 2 are on the same streamline
Fluid has constant density
The flow is steady
There is no friction

To understand this you must:
Know and understand what a streamline is.
Know what constitutes steady flow
Know what continuity is

Bernoullis principle also applies to gasses (fluid= liquid and gas). A way of understanding Bernoulli is by considering how a plane is launched from an aircraft carrier. To avoid sending a plane (an F18 or something) off the edge of a carrier, it must be traveling at approximately 165 mph when it leaves the deck. Newtonian mechanics describe the process of doing this. Configuring the steam pressure in the catapult, getting the engines up to maximum thrust, etc. What actually occurs that keeps the plane in the air when the deck drops off is Bernoullis principle. Watch the first ten seconds or so of the following video and then finish reading this.

http://www.youtube.com/watch?v=_mWtTFPdtkk&feature=youtube_gdata_player

You might have noticed that when the plane reached the end of the deck it dropped a little before lifting. What ultimately caught the plane and lifted it was the decrease in pressure on the top of the wing. Some don't understand this concept and firmly believe that the lift is Newtonian. But it's actually Bernoullis principle at work. When fluid forces flow over a solid body the streamlines get closer together. The flow velocity then increases. It's the tight airstreams contributing to Bernoullis principle. So, regarding the video of the jet, if there were sudden cross wind or something else that disrupted flow at the end of the flight deck it would result in a small loss of lift (like you saw) because of the disrupted streamlines. Even if speed was not decreased. It is NOT the speed of air flowing over the wing that causes lift. Tight streamlines, possible under the Bernoulli principle, cause the decrease in pressure. Now, bernoullis principle becomes impossible approaching Mach 1. Beyond Mach 1 a totally different set of things are going on.

Remember to look at the criteria that must be satisfied for Bernoulli to apply. Let me know if I have confused you further or if you have any questions 🙂