Fluid Velocity and Pressure in a *Closed* System

In summary, the conversation discusses the confusion around the relationship between fluid velocity, pressure drop, and flow rate in a closed fluidic system with a pump. While fluid velocity is constant throughout the system, there is a decrease in pressure due to viscous effects in a horizontal pipe. This raises questions about how a constant velocity can result in a constant flow rate. The conversation also mentions Bernoulli's principle and the need to account for viscous losses in real-world situations. It is suggested that additional constraints, such as a constant pipe diameter, may be necessary to make the statement about constant velocity true.
  • #1
joh_eng
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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.
 
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  • #2
joh_eng said:
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. :smile:
joh_eng said:
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? :smile:
 
  • #3
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?
 
  • #4
joh_eng said:
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
 
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  • #5
berkeman said:
Welcome to the PF. :smile:

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

Then, fluid velocity increases which increases dynamic pressure to decrease static pressure? Thanks
berkeman said:
Welcome to the PF. :smile:

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

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

Fluid velocity, also known as flow velocity, is the rate of change of fluid position with respect to time. It is a measure of how fast the fluid is moving in a given direction within a closed system.

What factors affect fluid velocity?

The main factors that affect fluid velocity in a closed system are the fluid's density, viscosity, and the pressure gradient within the system. Other factors that can influence velocity include the size and shape of the system, as well as any obstacles or obstructions in the fluid's path.

How is fluid velocity related to pressure?

In a closed system, fluid velocity and pressure are directly related. As fluid velocity increases, the pressure within the system decreases, and vice versa. This relationship is described by Bernoulli's principle, which states that as the velocity of a fluid increases, its pressure decreases.

What is the equation for calculating fluid velocity in a closed system?

The equation for calculating fluid velocity in a closed system is v = Q/A, where v is the fluid velocity, Q is the volume flow rate, and A is the cross-sectional area of the system. This equation is based on the principle of conservation of mass, which states that the mass of a fluid entering a system must equal the mass exiting the system.

Why is understanding fluid velocity and pressure important in a closed system?

Understanding fluid velocity and pressure in a closed system is essential for many scientific and engineering applications. It allows us to predict the behavior of fluids in various systems, such as pipes, pumps, and turbines. This knowledge is also crucial for designing efficient and safe systems, as well as troubleshooting and diagnosing potential issues within existing systems.

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