Main Question or Discussion Point

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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Chestermiller
Mentor
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?

Chestermiller
Mentor
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.

Chestermiller
Mentor
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.

Nidum
Gold Member
Not a direct answer to your question but you should enjoy this video of water flowing uphill :

Severn Bore