Ideal flow

[chiddler]

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This is false:

If the fluid is ideal, flow is always from high pressure to low pressure.

ie it's possible for fluid to flow from low to high pressure. When is this ever possible?

 

[MedPR]

This is false:

If the fluid is ideal, flow is always from high pressure to low pressure.

ie it's possible for fluid to flow from low to high pressure. When is this ever possible?

I think it's false because fluid can flow between regions of equal pressure. For instance down a vertical pipe, not because "fluids can flow from low pressure to high pressure".

 

[SaintJude]

Could it be that this is false b/c the statement doesn't consider situations where ΔP=0?

Edit: MedPr, faster by a split second haha

 

[milski]

Down a vertical pipe cannot be at the same pressure - you will have pressure difference between the top and the bottom.

Ideal fluid can flow between two equal pressures - for example a horizontal pipe. It's very similar to uniform motion with no friction - you'll need some pressure/force to make it stop (or to make it start moving that way).


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[chiddler]

Do you mean random motion between two equal pressures? Or actual translational motion.

I can't imagine the latter happening.

 

[MedPR]

Down a vertical pipe cannot be at the same pressure - you will have pressure difference between the top and the bottom.

Ideal fluid can flow between two equal pressures - for example a horizontal pipe. It's very similar to uniform motion with no friction - you'll need some pressure/force to make it stop (or to make it start moving that way).


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Do you mean random motion between two equal pressures? Or actual translational motion.

I can't imagine the latter happening.

If you inclined a pipe the ends would have different pressures? Different because of the additional height of the higher end? I guess that's true, I didn't think about that.

 

[milski]

Do you mean random motion between two equal pressures? Or actual translational motion.

I can't imagine the latter happening.

You still need to have some initial pressure at the beginning to start the movement. If it's an ideal fluid with no viscosity (friction) it will not stop moving after you remove the pressure - same way that an object keeps moving without friction.

 

[milski]

If you inclined a pipe the ends would have different pressures? Different because of the additional height of the higher end? I guess that's true, I didn't think about that.

Yes, that's what would create the difference in pressures in that case.

 

[chiddler]

You still need to have some initial pressure at the beginning to start the movement. If it's an ideal fluid with no viscosity (friction) it will not stop moving after you remove the pressure - same way that an object keeps moving without friction.

aha. that's a good point.

So there must be some initial push, but when it stops movement can continue even in 0 pressure difference. Can you give any example with such a scenario? I can't think of anything.

 

[MedPR]

aha. that's a good point.

So there must be some initial push, but when it stops movement can continue even in 0 pressure difference. Can you give any example with such a scenario? I can't think of anything.

Hmm.. What about a pipe with one open end and one closed end laying horizontally on the ground. The pipe is frictionless and the fluid is ideal.

You shoot in some water with a water hose, then close the open end of the pipe. The water will eventually hit the closed end, then reflect back towards the previously open end. This should repeat and create a wave, right?

 

[chiddler]

yes, that sounds like it'll work.

thanks for all the responses.

 

[milski]

aha. that's a good point.

So there must be some initial push, but when it stops movement can continue even in 0 pressure difference. Can you give any example with such a scenario? I can't think of anything.

You put some amount of ideal fluid in an ideal horizontal tube, blow at the end of the tube. The liquid accelerates in the tube. Once you stop blowing the liquid will continue at the same speed inside the tube.

You won't get a real world example since this time the fluid being ideal is important to the experiment and you cannot estimate it with a real world experiment.

 

[milski]

Hmm.. What about a pipe with one open end and one closed end laying horizontally on the ground. The pipe is frictionless and the fluid is ideal.

You shoot in some water with a water hose, then close the open end of the pipe. The water will eventually hit the closed end, then reflect back towards the previously open end. This should repeat and create a wave, right?

Won't work, since the water moving against the closed end of the pipe will increase the pressure of the trapped air. If you leave the closed end open, it will work - you shoot the water in and it happily comes out of the other end with the same velocity.

 

[chiddler]

unless you're in a vacuum ;D

 

[milski]

unless you're in a vacuum ;D

👍 😎

 

[MedPR]

Won't work, since the water moving against the closed end of the pipe will increase the pressure of the trapped air. If you leave the closed end open, it will work - you shoot the water in and it happily comes out of the other end with the same velocity.

It's blatantly obvious how much more you and MT know and understand than most of us. If you guys aren't in the 40+ range, then I'm screwed.

 

[chiddler]

It's blatantly obvious how much more you and MT know and understand than most of us. If you guys aren't in the 40+ range, then I'm screwed.

lol

amen

 

[milski]

It's blatantly obvious how much more you and MT know and understand than most of us. If you guys aren't in the 40+ range, then I'm screwed.

We are just very good at masking what we don't know. Sometimes I read some of your bio discussions and have a complete "what on Earth is going on here" feeling. 😳