Fluid Velocity and Pressure in a *Closed* System

[joh_eng]

In a closed fluidic system (with a pump), fluid velocity is constant throughout the system but what puzzles me is the pressure drop due to viscous effect. In a horizontal pipe, pressure decreases gradually (assuming low friction) down the pipe due to viscous effect. In this concepts, it's hard to comprehend constant velocity (=constant flowrate). In Bernoulli's principles, where total energy is constant along the streamline, if there is pressure drop (static) in a horizontal pipe (where potential energy is zero), dynamic pressure is increased meaning velocity has increased. Can someone pinpoint what I am missing here? Thanks.

 

[berkeman]

In a closed fluidic system (with a pump), fluid velocity is constant throughout the system but what puzzles me is the pressure drop due to viscous effect. In a horizontal pipe, pressure decreases gradually (assuming low friction) down the pipe due to viscous effect. In this concepts, it's hard to comprehend constant velocity (=constant flowrate). In Bernoulli's principles, where total energy is constant along the streamline, if there is pressure drop (static) in a horizontal pipe (where potential energy is zero), dynamic pressure is increased meaning velocity has increased. Can someone pinpoint what I am missing here? Thanks.

Welcome to the PF.

fluid velocity is constant throughout the system

That is incorrect. Would you like to put additional constraints on the diameter of the pipes carrying this flow to make that statement true?

 

[Chestermiller]

What is it that is hard to comprehend about the volumetric throughput rate remaining constant while the pressure is decreasing. You are aware that the Bernoulli equation is for an inviscid fluid, correct? Are you familiar with the version of the Bernoulli equation that includes viscous heat loss?

 

[russ_watters]

In a closed fluidic system (with a pump), fluid velocity is constant throughout the system...

Clarification: this is true with a constant pipe size, but more generally (with a variable pipe size), it is volumetric flow rate that is constant...

... but what puzzles me is the pressure drop due to viscous effect. In a horizontal pipe, pressure decreases gradually (assuming low friction) down the pipe due to viscous effect. In this concepts, it's hard to comprehend constant velocity (=constant flowrate). In Bernoulli's principles, where total energy is constant along the streamline, if there is pressure drop (static) in a horizontal pipe (where potential energy is zero), dynamic pressure is increased meaning velocity has increased. Can someone pinpoint what I am missing here? Thanks.

Simply put, the basic versions of Bernoulli's equation don't apply. Bernoulli's equation is a conservation of flow energy statement, so it requires lossless and therefore inviscid flow.

But you can add a term to Bernoulli's equation to represent the loss and preserve the conservation of energy in a real-world situation. Welcome to my world!

http://my.me.queensu.ca/People/Sellens/LossesinPipes.html

 

[joh_eng]

Welcome to the PF.

That is incorrect. Would you like to put additional constraints on the diameter of the pipes carrying this flow to make that statement true?

Then, fluid velocity increases which increases dynamic pressure to decrease static pressure? Thanks

Welcome to the PF.

That is incorrect. Would you like to put additional constraints on the diameter of the pipes carrying this flow to make that statement true?

I believe this is correct if diameter is constant. Constant velocity and diameter --> constant flowrate throughout the system