Velocity at wall in an internal flow

rsmartins · July 13, 2011, 15:50

Hello, CFX-users.

I've been using ANSYS CFX for a while and I've just found something inconsistent (in my opinion). When I run a turbulent pipe flow and create a line to check out the velocity profile the velocity at the wall is not zero. Of course I've double checked the no-slip condition and the exactly points to create the line.

The point is, when I run a laminar case using the same geometry and mesh the flow profile is OK (null velocity at the wall). On the other hand, none of the turbulent cases presents the same behavior.

Actually, nondimensionalization of velocity (nondimensionalized by the maximum velocity) presents wall velocities around 0.4-0.5 (which means 40-50% of the maximum velocity).

I've been using v. 13.0 and k- $\epsilon$ turbulence model. I've already tested k- $\omega$ and $\omega$ -RSM turbulence models.

I've also tested several y $^{+}$ (from 1 to 1000, approximately).

I would like to know if any of you has any similar trouble. I thank you for any contribution already.

Regards,

Ramon Silva Martins

ghorrocks · July 13, 2011, 19:32

This question has been asked many times on the forum. We should do an FAQ on it some day.

CFX uses control volumes centred on the nodes, so the nodes on the walls generate control volumes whose centroids is off the walls. That is why the wall control volumes have non-zero velocities.

To make it display as expected in post processing, CFX has added the "hybrid" variables, where it forces the velocity to zero (or more generally the wall velocity) at the wall. The "conservative" variables show the actual control volume values.

rsmartins · July 15, 2011, 14:54

Dear Mr. Horrocks,

Thank you very much for your help and explanation. I do thing that this issue deserve a FAQ, due to its relevance.

I appreciate your reply.

Best regards,

Ramon Silva Martins

ghorrocks · July 16, 2011, 06:58

FAQ done. Thanks for the nudge.

http://www.cfd-online.com/Wiki/Ansys...t_the_walls.3F

July 13, 2011, 15:50	Velocity at wall in an internal flow	#1
rsmartins New Member Ramon Silva Martins Join Date: May 2011 Posts: 5 Rep Power: 14	Hello, CFX-users. I've been using ANSYS CFX for a while and I've just found something inconsistent (in my opinion). When I run a turbulent pipe flow and create a line to check out the velocity profile the velocity at the wall is not zero. Of course I've double checked the no-slip condition and the exactly points to create the line. The point is, when I run a laminar case using the same geometry and mesh the flow profile is OK (null velocity at the wall). On the other hand, none of the turbulent cases presents the same behavior. Actually, nondimensionalization of velocity (nondimensionalized by the maximum velocity) presents wall velocities around 0.4-0.5 (which means 40-50% of the maximum velocity). I've been using v. 13.0 and k- $\epsilon$ turbulence model. I've already tested k- $\omega$ and $\omega$ -RSM turbulence models. I've also tested several y $^{+}$ (from 1 to 1000, approximately). I would like to know if any of you has any similar trouble. I thank you for any contribution already. Regards, Ramon Silva Martins

July 13, 2011, 19:32		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	This question has been asked many times on the forum. We should do an FAQ on it some day. CFX uses control volumes centred on the nodes, so the nodes on the walls generate control volumes whose centroids is off the walls. That is why the wall control volumes have non-zero velocities. To make it display as expected in post processing, CFX has added the "hybrid" variables, where it forces the velocity to zero (or more generally the wall velocity) at the wall. The "conservative" variables show the actual control volume values. rsmartins likes this.

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July 15, 2011, 14:54		#3
rsmartins New Member Ramon Silva Martins Join Date: May 2011 Posts: 5 Rep Power: 14	Dear Mr. Horrocks, Thank you very much for your help and explanation. I do thing that this issue deserve a FAQ, due to its relevance. I appreciate your reply. Best regards, Ramon Silva Martins

July 16, 2011, 06:58		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	FAQ done. Thanks for the nudge. http://www.cfd-online.com/Wiki/Ansys...t_the_walls.3F