Role of Pipe Length in Poiseuille's Law

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Discussion Overview

The discussion revolves around the role of pipe length in Poiseuille's Law, specifically how increasing the length of a pipe affects the pressure drop across it when fluid flows through. The scope includes conceptual understanding and technical reasoning related to fluid dynamics.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about why the length of the pipe (L) increases the change in driving pressure (Delta P) according to Poiseuille's Law, suggesting that incompressibility of the fluid should mean Delta P remains unchanged.
  • Another participant clarifies that if the volumetric throughput rate (Q) is held constant while increasing the length of the pipe, the pressure drop must increase, questioning the initial assumption about incompressibility's relevance to pressure drop.
  • A participant notes that a longer pipe results in more drag on the fluid due to the pipe wall.
  • Another participant acknowledges that drag from the pipe wall is a key factor in understanding the pressure drop, reflecting on the relationship between force, fluid flow, and distance.
  • One participant states that at a steady flow rate, the pressure difference across the pipe must balance the drag forces from the walls.

Areas of Agreement / Disagreement

Participants generally agree that the drag from the pipe walls contributes to the pressure drop, but there is some initial confusion regarding the implications of fluid incompressibility on this relationship. The discussion does not reach a consensus on the conceptual understanding of these dynamics.

Contextual Notes

Some assumptions about fluid behavior, such as the effects of drag and incompressibility, remain unresolved. The discussion does not clarify how these factors quantitatively interact within the framework of Poiseuille's Law.

Willis92
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I can't figure out why the length the a pipe (L) increases the change in driving pressure with respect to this law:

Delta P = (8*mu*L*Q)/(pi*(r^4))

I would think that delta P wouldn't change because the fluid is incompressible.

Does anyone have a conceptual explanation for the simple fact I can't seem to find?

Thanks in advance.
 
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Willis92 said:
I can't figure out why the length the a pipe (L) increases the change in driving pressure with respect to this law:

Delta P = (8*mu*L*Q)/(pi*(r^4))

I would think that delta P wouldn't change because the fluid is incompressible.

Does anyone have a conceptual explanation for the simple fact I can't seem to find?

Thanks in advance.
If you want to hold the volumetric throughput rate Q constant while you increase the length of the pipe, you have to increase the pressure drop. Why do you think that the pressure drop from one end of the pipe to the other end of the pipe wouldn't change if the fluid is incompressible? The incompressibility of the fluid means that its density doesn't change when you change the pressure. That doesn't have any bearing on the pressure drop/flow rate behavior of the fluid.

Chet
 
The longer the pipe the more drag on the fluid from the pipe wall.
 
Ok, so it's just the drag from the pipe wall then? That makes sense.

I got hung up thinking about if you have a certain force pushing an amount of fluid through at a time, the same comes out at the other end, regardless of how far away that end is.

But it makes sense if drag is the only thing preventing that.
 
Yes; when the fluid is flowing at a steady rate, the difference in pressures at either end of the pipe times the cross-sectional area must balance the drag forces from the pipe walls.
 

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