Differences in out. file and function results

Luk_Fiz · September 22, 2009, 11:56

Hi all,
I was digging through the forum and found similar problems with no answer. I am trying to simulate viscous flow around the cylinder. There are differences in drag forces from .out files and in postprocessor using force_x@cylinder functions? Can anyone tell why? Even adding viscous forces (given in .out file also) there are still discrepancies in order of up to 7%.

Luk

cfxmar · September 28, 2009, 06:55

Hi!

I´m also calculating forces over a cylinder and with a monitor point over the surface of the cylinder I obtain a force on 10^7 N while Morrison equation give me 10^5.
But I haven´t seen force values in the .out file ?¿
Wich values do you obtain?

ghorrocks · September 28, 2009, 07:05

I guess because nobody on the forum knows the answer to your question. Have you asked CFX support? The two values should be the same so it sounds like a bug, so CFX support is the only way of doing something about it.

cfxmar's issue seems to simply be of simulation accuracy. Looks like you need to do the normal mesh, timestep and convergence sensitivity.

cfxmar · September 28, 2009, 07:17

Thank you ghorrocks,
I have seen that the .out doesn´t give me pressure force values for the pile (a smooth non slip wall), just for the floor (free slip wall).
Could it be the problem?

cfxmar · September 28, 2009, 07:23

Sorry, It was a mistake. The .out file gives me values for the Pressure Force on walls and Viscous Force on walls in x, y, z axis.
Would Morrison equation Force (the addition of the Inertial and drag forces) be the same as the addition of total Pressure and viscous forces that CFX gives me?

Luk_Fiz · September 28, 2009, 08:15

Thanks, for all answer. To Glenn - for me the answer is not very urgent and not that meaningful so I didynt asked him. From my point of view it is rather strange, since there are at least few notices about that at CFX and no one knows

I have noticed that (in POST) "Integral (Pressure x Direction)" gives very close results to (in .out) "Pressure Force" - close but not exact again. Similar (in POST) "Force" gives similar results to (in .out) "Pressure Force"+"Shear Force" - it is very close but not the same.

Luk

joey2007 · September 28, 2009, 17:08

Hi Luk,
is it 11 or 12?

Luk_Fiz · September 28, 2009, 17:24

From forum I see that it occures in various versions.

September 22, 2009, 11:56	Differences in out. file and function results	#1
Luk_Fiz Member Lukasz Join Date: Mar 2009 Posts: 67 Rep Power: 17	Hi all, I was digging through the forum and found similar problems with no answer. I am trying to simulate viscous flow around the cylinder. There are differences in drag forces from .out files and in postprocessor using force_x@cylinder functions? Can anyone tell why? Even adding viscous forces (given in .out file also) there are still discrepancies in order of up to 7%. Luk

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September 28, 2009, 06:55		#2
cfxmar Member Join Date: Aug 2009 Posts: 42 Rep Power: 17	Hi! I´m also calculating forces over a cylinder and with a monitor point over the surface of the cylinder I obtain a force on 10^7 N while Morrison equation give me 10^5. But I haven´t seen force values in the .out file ?¿ Wich values do you obtain?

September 28, 2009, 07:05		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	I guess because nobody on the forum knows the answer to your question. Have you asked CFX support? The two values should be the same so it sounds like a bug, so CFX support is the only way of doing something about it. cfxmar's issue seems to simply be of simulation accuracy. Looks like you need to do the normal mesh, timestep and convergence sensitivity.

September 28, 2009, 07:17		#4
cfxmar Member Join Date: Aug 2009 Posts: 42 Rep Power: 17	Thank you ghorrocks, I have seen that the .out doesn´t give me pressure force values for the pile (a smooth non slip wall), just for the floor (free slip wall). Could it be the problem?

September 28, 2009, 07:23		#5
cfxmar Member Join Date: Aug 2009 Posts: 42 Rep Power: 17	Sorry, It was a mistake. The .out file gives me values for the Pressure Force on walls and Viscous Force on walls in x, y, z axis. Would Morrison equation Force (the addition of the Inertial and drag forces) be the same as the addition of total Pressure and viscous forces that CFX gives me?

September 28, 2009, 08:15		#6
Luk_Fiz Member Lukasz Join Date: Mar 2009 Posts: 67 Rep Power: 17	Thanks, for all answer. To Glenn - for me the answer is not very urgent and not that meaningful so I didynt asked him. From my point of view it is rather strange, since there are at least few notices about that at CFX and no one knows I have noticed that (in POST) "Integral (Pressure x Direction)" gives very close results to (in .out) "Pressure Force" - close but not exact again. Similar (in POST) "Force" gives similar results to (in .out) "Pressure Force"+"Shear Force" - it is very close but not the same. Luk

September 28, 2009, 17:08		#7
joey2007 Senior Member Join Date: Mar 2009 Location: Europe Posts: 169 Rep Power: 17	Hi Luk, is it 11 or 12?

September 28, 2009, 17:24		#8
Luk_Fiz Member Lukasz Join Date: Mar 2009 Posts: 67 Rep Power: 17	From forum I see that it occures in various versions.