Problems Verifying Laminar Flow in Pipe

dreamchaser · September 16, 2014, 20:45

Hello,

I am trying to do a tutorial titled "Laminar Flow in a Pipe". The details of the problem can be found here: https://confluence.cornell.edu/displ...inar+Pipe+Flow

The tutorial does the simulation in Fluent. I am trying to perform this tutorial in CFX. The axial velocity I am getting is 1.48m/s while the correct result from the tutorial is 2m/s. I am trying to figure out what I am doing wrong and I would really appreciate your help. I have explained the steps I performed below in case I have made a mistake.

1) I made my 2-D geometry (8mx.1m) in ANSYS workbench. After, I exported the mesh as a Fluent.msh. I did this because when you open the mesh in CFX stand alone mode, it automatically extrudes the mesh in the z-direction.

2) I closed workbench and opened CFX in stand alone mode. I imported my Fluent.msh. After opening in CFX it was extruded in the z-direction.

3) I did set my boundary conditions as the tutorial instructs. Something that is confusing in the tutorial is that they set the outlet pressure to be 1 atm. However, if you look under "Numerical Results" on the tutorial, the pressure in the axial direction varies from 12Pa to 0 Pa.

I tried setting my outlet pressure to 1atm and the solution diverged. I assume it did because I have set the pressure everywhere else to be 0 Pa.

I am really confused regarding the pressure. I have attached my variation of the pressure in the axial direction for you to see. In their solution the pressure at the inlet is around 12 Pa while mines is around 5Pa. I am not sure what I am doing wrong for my pressure to be lower. I believe this is where my problem is.

I would appreciate any insight for this problem.

Thanks!

ghorrocks · September 17, 2014, 06:48

When you imported the mesh did it extrude it in the z direction, or did it rotate it about the central axis?

As this is a 2D axisymmetric pipe you are modelling you need to do a rotation about the axis. I think by default CFX does an extrusion in the z direction. This would explain your dodgy results if this is what it is doing.

dreamchaser · September 17, 2014, 10:07

Quote:

Originally Posted by ghorrocks

When you imported the mesh did it extrude it in the z direction, or did it rotate it about the central axis?

As this is a 2D axisymmetric pipe you are modelling you need to do a rotation about the axis. I think by default CFX does an extrusion in the z direction. This would explain your dodgy results if this is what it is doing.

Hi Ghorrocks,

Thanks for the reply. Yes, when I imported the mesh, it extruded in the z-direction. How do I do a rotation about the central axis since this is axis symmetric? This is probably why my results are not matching.

Thanks!

ghorrocks · September 17, 2014, 18:32

So if that is what it has done then this is exactly why your results are not matching.

In CFX-Pre when you import the mesh, go to advanced options, "Override Default 2D mesh settings". You will have planar set, change that to axisymmetric. 5 degrees is a good angle for most applications but you can reduce this is if you want higher accuracy (and less stable simulations). Also make sure Remove Duplicate nodes at axis is selected.

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September 17, 2014, 06:48		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143	When you imported the mesh did it extrude it in the z direction, or did it rotate it about the central axis? As this is a 2D axisymmetric pipe you are modelling you need to do a rotation about the axis. I think by default CFX does an extrusion in the z direction. This would explain your dodgy results if this is what it is doing.

September 17, 2014, 18:32		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143	So if that is what it has done then this is exactly why your results are not matching. In CFX-Pre when you import the mesh, go to advanced options, "Override Default 2D mesh settings". You will have planar set, change that to axisymmetric. 5 degrees is a good angle for most applications but you can reduce this is if you want higher accuracy (and less stable simulations). Also make sure Remove Duplicate nodes at axis is selected.