# Pressure drop does not match theory

Make sure the turbulence model is switched off. In the 'Viscous Model' tab, choose 'Laminar'.

Also note that the Darcy-Weisbach equation is valid for developed flow, so at the start of the bend you should have Poiseuille flow. You should have a sufficiently long piece of straight pipe before and after the bend in your simulation. Do you have this?
Viscos-Laminar model has been chosen all the time.
And I think it is long enough in start and end. L_e=0.06*Re*D=0.06*190*0.01905=0.218 m
My pipe geometry is exactly like this: 1 meter straight pipe, a 90 degree bend, 11 cm straight pipe, one more 90 degree bend, and 1 meter straight pipe again (almost U-shaped). So there's one meter in start which is 5 times greater than L_e and enough for being fully developed.

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Let's keep our eye on the ball. The value of the viscosity is irrelevant to the comparison we are trying to make.
Have you checked that you didn't accidentally mix up radius and diameter in your Fluent simulation?
Make sure the turbulence model is switched off. In the 'Viscous Model' tab, choose 'Laminar'.

Also note that the Darcy-Weisbach equation is valid for developed flow, so at the start of the bend you should have Poiseuille flow. You should have a sufficiently long piece of straight pipe before and after the bend in your simulation. Do you have this?
I finally found the problem !!! So weird. for straight pipes the answer is wrong also UNLESS you model them as AXISYMMETRIC ! for my geometry (U-shaped pipe) is it possible to model it axisymmetric? if not what can I do now???

256bits
Gold Member
I finally found the problem !!! So weird. for straight pipes the answer is wrong also UNLESS you model them as AXISYMMETRIC ! for my geometry (U-shaped pipe) is it possible to model it axisymmetric? if not what can I do now???
I was just going to ask if you poked on 2D or axisymmetric.

I would think the following should apply:
2d would be a calculation for a depth of 1 m on the z-axis. ie z-axis into the page, x-axis left to right, y -axis top to bottom.

Axisymmetric would be around an x-axis of radius D/2, for say a round tube.
Ansys should allow you to set up your grid for complicated shapes.

Omish
I was just going to ask if you poked on 2D or axisymmetric.

I would think the following should apply:
2d would be a calculation for a depth of 1 m on the z-axis. ie z-axis into the page, x-axis left to right, y -axis top to bottom.

Axisymmetric would be around an x-axis of radius D/2, for say a round tube.
Ansys should allow you to set up your grid for complicated shapes.
yes. I tried to use the centerline as my axis BC but Fluent gives errors and doesn't even run for 1 Iteration. The error is about diverging, but I'm pretty sure it's irrelevant; The cause I guess is that the axis is not straight but twisted because of U-shape.
any other suggestions?

can help me how to show pressure drop value in ansys fluent

Chestermiller
Mentor
can help me how to show pressure drop value in ansys fluent
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