2D parabolic velocity expression

violethill · December 21, 2016, 18:49

Thank you to everyone that has taken the time to read this.

My geometry set up is simple, I have a 2D axisymmetric pipe (31 cm long) and 0.5 mm thick (radius). The velocity magnitude at the inlet vary from 0.08-0.31 m/s. I have noticed that when I run at a constant velocity, the flow profile is disturbed and the pressure drop is not matching the experimental data.

I am thinking that one of the reasons is that I don't have a fully developed flow right at the inlet and at different speed it takes different lengths for the flow to be fully developed.

I am using Fluent and I understand that you need to insert a UDF file with the correct expression for parabolic velocity profile.

Can anyone here please tell me what kind of expression I can use for my case? I have seen a lot of examples but those examples pertain to different geometries and I am not sure what to do in my specific case.

I would really appreciate some insight and guidance instead of just directing me to different tutorials that haven't been much of a help.

Thank you!

KevinZ09 · December 22, 2016, 05:06

What fluid is flowing through your pipe? Air? Water? Or something else? Either way, it seems your Reynolds number is only like O(100) so pretty laminar still. In that case your velocity profile looks something like:

v(r) = Vm (1 - r^2/R^2), where Vm is your maximum velocity and R your radius. You can simply code this using an UDF or profile file.

However, I'm a bit surprised you say your data is off. By how much? You can estimate the entrance length, which should be for laminar flow about 0.06ReD. In your case that's only like 3mm. On a total length of 31cm (100 times the entrance length) that shouldn't affect the results that much.

What settings are you using? And the solvers?

December 21, 2016, 18:49	2D parabolic velocity expression	#1
violethill New Member Violet Join Date: Jun 2016 Posts: 8 Rep Power: 10	Thank you to everyone that has taken the time to read this. My geometry set up is simple, I have a 2D axisymmetric pipe (31 cm long) and 0.5 mm thick (radius). The velocity magnitude at the inlet vary from 0.08-0.31 m/s. I have noticed that when I run at a constant velocity, the flow profile is disturbed and the pressure drop is not matching the experimental data. I am thinking that one of the reasons is that I don't have a fully developed flow right at the inlet and at different speed it takes different lengths for the flow to be fully developed. I am using Fluent and I understand that you need to insert a UDF file with the correct expression for parabolic velocity profile. Can anyone here please tell me what kind of expression I can use for my case? I have seen a lot of examples but those examples pertain to different geometries and I am not sure what to do in my specific case. I would really appreciate some insight and guidance instead of just directing me to different tutorials that haven't been much of a help. Thank you!

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December 22, 2016, 05:06		#2
KevinZ09 Senior Member Kevin Join Date: Dec 2016 Posts: 138 Rep Power: 9	What fluid is flowing through your pipe? Air? Water? Or something else? Either way, it seems your Reynolds number is only like O(100) so pretty laminar still. In that case your velocity profile looks something like: v(r) = Vm (1 - r^2/R^2), where Vm is your maximum velocity and R your radius. You can simply code this using an UDF or profile file. However, I'm a bit surprised you say your data is off. By how much? You can estimate the entrance length, which should be for laminar flow about 0.06ReD. In your case that's only like 3mm. On a total length of 31cm (100 times the entrance length) that shouldn't affect the results that much. What settings are you using? And the solvers?