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Pressure drop in laminar pipe flow obtained by Fluent is not correct!

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Old   September 21, 2021, 10:33
Default Pressure drop in laminar pipe flow obtained by Fluent is not correct!
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Hamideh Hayati
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Hello,

I faced an strange issue with pressure drop in a laminar water flow in a pipe.

the case is as follows:

Pipe diameter: 9.525 mm

Pipe length: 686 mm

Re=1284

Inlet velocity: 0.117 m/s

Water density 998.2 kg/m3; viscosity 0.001 Pa.s

Model: Laminar

Flow direction: from bottom to top

Based on the analytical solution, for the vertical pipe, the pressure drop should be about 7600 pa. but what Fluent gives me is 1.18 pa!!!! It seems not accounting elevation pressure! and It reports the same pressure drop for vertical and horizontal pipes!! which is not correct, right?

This is not the only case, I ran multiple laminar flow simulations (different inlet velocities), and Fluent gave me the same pressure drop for vertical and horizontal orientations!!!

Am I missing something?

I would appreciate it if you could help me.



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Old   September 21, 2021, 12:10
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Lorenzo Galieti
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Did you include gravity in the model?
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Old   September 21, 2021, 12:14
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Yes, I did.
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Old   September 21, 2021, 15:23
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You sure you did? Can I see a screenshot of the gravity settings? Also, 7600 is quite large, is it static or total pressure drop? I think total?

Last edited by LoGaL; September 22, 2021 at 01:37.
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Old   September 21, 2021, 16:15
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Yes, I did.
Actually, I dug deeper a little bit and I realized that when Fluent is reporting the static pressure, it is not showing the hydrostatic head, and the static pressure drop is only from friction loss. with that in mind, the static pressure drop that I get from Fluent is correct, and for adding the hydrostatic head, I should add it manually.
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Old   September 22, 2021, 01:32
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You should alwais check pressure drops with total pressure, not static.
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Old   September 22, 2021, 10:05
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Even total pressure does not show the hydrostatic head. The total pressure is the summation of static pressure and dynamic pressure, correct?
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Old   September 23, 2021, 12:54
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It should include also the gravity term. For incompressible flow it is the sum of the three terms in the bernoulli equation.

So perhaps this gravity term is all converted into velocity at basically constant pressure? Could you extract the mean velocity at inlet and outlet section and then calculate (rhoV^2)_outlet - (rhoV^2)_inlet? Is it equal to the pressure drop you are looking for?

I am really not expert in gravity affected flows, sorry xD
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Old   September 23, 2021, 14:54
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Total pressure also does not include the hydrostatic head.

This isn't a bug btw, it's a feature. Otherwise, to specify pressure (whether static or total) along any boundary you would need to account for the hydrostatic part as well, which would then require a profile, custom field function, or worse a UDF for even the most simple of problems.


You can complain that Fluent should have better options to display pressure including the hydrostatic part, which is fair. But Fluent giving you access to the primitive gauge "pressure" field that it utilizes at the solver level isn't an error.
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