How water flows even after adverse pressure gradient?

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Discussion Overview

The discussion revolves around the phenomenon of fluid flow in the presence of an adverse pressure gradient, particularly in the context of a Venturimeter. Participants explore the conditions under which fluid can flow despite opposing pressure forces, examining concepts such as energy gradients and momentum.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that a gradient is necessary for flow, citing examples like temperature gradients for heat transfer and pressure differences for fluid flow.
  • One participant suggests that flow can occur due to an energy gradient rather than solely due to pressure differences.
  • Another participant argues that fluid momentum allows flow against an adverse pressure gradient, especially when kinetic energy is significant.
  • Participants discuss specific cases of fluid flow, such as gravity-driven flow in open channels and flow induced by centrifugal pumps, highlighting different mechanisms at play.
  • There is mention of localized adverse pressure gradients not being sufficient to reverse fluid momentum, particularly in the context of Venturimeters.
  • A metaphor comparing fluid flow against a pressure gradient to a ball rolling uphill is introduced to illustrate the concept of momentum in overcoming resistance.

Areas of Agreement / Disagreement

Participants express various viewpoints on the prerequisites for fluid flow and the role of pressure gradients, indicating that multiple competing views remain. The discussion does not reach a consensus on the underlying principles governing fluid behavior in adverse pressure conditions.

Contextual Notes

Participants note that the specific physical situation influences the prerequisites for fluid flow, suggesting that different scenarios may yield different outcomes regarding flow dynamics.

Ravi Singh choudhary
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In nature, gradient is always required for flow; whether it is temperature gradient for heat transfer or pressure difference for fluid flow. There is a case of Venturimeter in which we have throat section. After throat there is a divergent section. How could flow even happen in that adverse pressure gradient?

Someone says it is due to energy gradient. That means we should not speak due to pressure gradient fluid flows.
 
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Ravi Singh choudhary said:
In nature, gradient is always required for flow; whether it is temperature gradient for heat transfer or pressure difference for fluid flow. There is a case of Venturimeter in which we have throat section. After throat there is a divergent section. How could flow even happen in that adverse pressure gradient?

Someone says it is due to energy gradient. That means we should not speak due to pressure gradient fluid flows.
The fluid has momentum, and higher pressure downstream compared to upstream enables the flow to slow down. So, when fluid momentum (kinetic energy) is significant, you can have flow against an adverse pressure gradient.
 
So What is the prerequisite for fluid flow?
 
Ravi Singh choudhary said:
So What is the prerequisite for fluid flow?
Why do you need to have a prerequisite for fluid flow? That will depend on the specific physical situation. If you wish to identify some specific physical situations and as whether the fluid will be flowing through them, we can analyze that.
 
1st case: Fluid is flowing due to gravity only as in case of open channel. There is no pressure difference I think.

2nd case:Fluid is being pumped to the top using centrifugal pump; so creating pressure difference using pump so that fluid flow is happening.

In the second case consider a venturi section in the vertical pipe. Fluid will flow because of net pressure difference between source and the sink. What I am thinking is that localized adverse pressure gradient is not able to reverse the momentum of the fluid. Similarly venturimeter is the instrument placed in a pipeline is not able to do the same.
 
Ravi Singh choudhary said:
1st case: Fluid is flowing due to gravity only as in case of open channel. There is no pressure difference I think.
You have a gravitational force acting on the fluid, so if, if viscous resistance is negligible, the fluid velocity can accelerate downward just as with a body in free fall. If substantial viscous resistance is present, this balances the gravitational force by viscous shear, but only if the fluid is flowing. So the fluid has to flow to balance the gravitational force.
2nd case:Fluid is being pumped to the top using centrifugal pump; so creating pressure difference using pump so that fluid flow is happening.

In the second case consider a venturi section in the vertical pipe. Fluid will flow because of net pressure difference between source and the sink. What I am thinking is that localized adverse pressure gradient is not able to reverse the momentum of the fluid. Similarly venturimeter is the instrument placed in a pipeline is not able to do the same.
Yes and yes.
 
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Not a direct answer to your question but you should enjoy this video of water flowing uphill :



Severn Bore
 
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Pressure gradients result in a net force on fluid elements. If the fluid is already moving (e.g. due to some force previously exerted on it) then encountering an adverse pressure gradient will slow that fluid down. You might think of it as a ball rolling up a hill. If it has enough momentum then it will make it up the hill against gravity but will be much slower for the effort. This is essentially the same as a fluid moving against a pressure gradient.
 
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Perfect explanation for me. Thanks a lot.
 

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