[Xatkor]

Hi!


A did a simulation in Fluent about a straight pipe with a velocity profile as inlet = 0.05 m/s as maximum velocity, and pressure outlet = 0 at Outlet. The fluid is water included in Fluent database.


Using the Darcy equation:


DeltaP = rho * f * L * V^2 / D


where


Re = 498
f = 64 / Re = 0.128

I get DeltaP = 3.2 Pa (same as calculator online). The problem is that I get 1.6 Pa in Fluent. I have used Laminar flow, k-w SST and k-e realizable. All of them obtain the same value and mass flow net is equal to e-11. I had change the mesh and I did refinement but value does not change. Residuals go below 1e-6.



What am I missing?


The pipe has dimensions:


Long = 200mm
Diameter = 10mm

[CFDKareem]

Originally Posted by Xatkor

Hi!


A did a simulation in Fluent about a straight pipe with a velocity profile as inlet = 0.05 m/s as maximum velocity, and pressure outlet = 0 at Outlet. The fluid is water included in Fluent database.


Using the Darcy equation:


DeltaP = rho * f * L * V^2 / D


where


Re = 498
f = 64 / Re = 0.128

I get DeltaP = 3.2 Pa (same as calculator online). The problem is that I get 1.6 Pa in Fluent. I have used Laminar flow, k-w SST and k-e realizable. All of them obtain the same value and mass flow net is equal to e-11. I had change the mesh and I did refinement but value does not change. Residuals go below 1e-6.



What am I missing?


The pipe has dimensions:


Long = 200mm
Diameter = 10mm

Are you using an inlet velocity profile? The Darcy equation assumes the flow is fully developed. In your simulation your velocity profile, by default, will be L_laminar=0.0575*Re*D. If your entrance length is longer then your flow channel then the flow will never fully develop before the exit.

[Xatkor]

In the inlet I have applied a name expression:

f(r) = Umax * ( 1 - (r/R)**2)

Where Umax is the velocity 0.05 m/s, r is defined as sqrt(z**2 + x**2) and R = D/2. Pipe is along Y axis.

This function I understand is for a developed laminar flow so I can use Darcy equation, isn’t it?




Edit: if I use the mean velocity = 0.025 m/s I get DeltaP = 1.6 Pa which is the value obtained in Fluent. So mean velocity is needed in equation right?