I Shouldn't the pressure difference along a pipe be a negative number?

AI Thread Summary
In fluid dynamics, pressure drops occur due to friction between the fluid and pipe walls, resulting in a lower pressure at the exit compared to the entrance. The pressure difference (ΔP) is defined as P2 - P1, which is negative when P1 is greater than P2. However, the term "pressure drop" is often used positively to indicate the magnitude of the decrease. The confusion arises from terminology; while the pressure gradient can be negative, the term "pressure drop" is conventionally treated as a positive value. Understanding this distinction is crucial for accurate communication in fluid mechanics.
lost captain
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We know that across a horizontal pipe, the pressure will become smaller due to friction. So shouldn't the pressure difference be a negative number?
Across a horizontal pipe with fluid flowing inside, the pressure will drop due to the friction between the fluid and the pipe walls
So if for example the fluid flows from left to right and we take 2 points: P1 (pressure on the left) and P2 (pressure on the right) then P1 should be bigger than P2.
And the pressure difference should be
ΔP = P2 - P1 <0
So pressure difference or pressure drop should be negative. Why is it positive?
 
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Why is it positive you think? Where did you read that?

It is entirely a matter of definition. If the pipe is along the x-axis and the flow in positive x direction than the pressure gradient (dp/dx) is indeed negative. If on the other hand the flow is in negative x-direction then the gradient is positive.

The way you describe it is however the common way to do this.
 
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PipeRoughness-768x355.png



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echanics-hagen-poiseuille-pipe-flow-pressure-drive.jpg
 
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lost captain said:
So pressure difference or pressure drop should be negative. Why is it positive?

By the way: if you call it a pressure drop, it should be positive. A negative pressure drop is a rise in pressure.
 
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Arjan82 said:
By the way: if you call it a pressure drop, it should be positive. A negative pressure drop is a rise in pressure.
Thank you, very much🙇‍♂️😊
Yes understand that the word "drop" in "pressure drop" basically means subtracting.
 
In pipe flow, the entrance pressure is higher than the exit pressure. So the pressure at the exit minus the pressure at the entrance is negative.
 
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Chestermiller said:
In pipe flow, the entrance pressure is higher than the exit pressure. So the pressure at the exit minus the pressure at the entrance is negative.
Yeah exactly, but when we calculate ΔP we take the positive value
 
lost captain said:
Yeah exactly, but when we calculate ΔP we take the positive value
It’s just terminology.
 
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Chestermiller said:
It’s just terminology.
Thank you very much for taking the time to reply 😊🙇‍♂️
 
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