what's the definition of (Velocity u.Gradient X) PLS?

rystokes · February 24, 2010, 23:23

Hi,

I'm wondering of the exact definition of some variable calculators provided by CFX POST, such as (Velocity u.Gradient X), (Velocity v.Gradient Y), (Velocity w.Gradient Z) please?

Do they simply mean the velocity component gradient on each direction please?　In that case, the value should have a unit of "s", however, I noticed there is not such unit. Besides, I sum the above three terms in order to verify the conservation of mass (density is constant), while the result is far from zero (in solve process, the calculation reached convergency). Similar thing also happens when I　tried to check momentum equation balance.

Could anyone give me advice or help please？

Thanks!

rystokes · February 25, 2010, 02:04

anyone help me please?

longbow · February 25, 2010, 13:21

The unit of those variables should be [s^-1] instead of [s]. Where did you sum those three terms, over a surface or volume?

rystokes · February 25, 2010, 22:21

Hi,

Yes, you are right. I just simply created several points in the fluid domain, and for each point, I can obtain the three terms by calculators in post, and summed them, because I think the continuity equation should work for each point. Do you mean I should sum over a surface or volume please?

ghorrocks · February 26, 2010, 04:40

When you set a point in CFD-post it just interpolates the value from the surrounding nodes. This interpolation is not conservative - meaning that conservation is not enforced. Conservation is enforced at the control volume level, or at least you converge towards conservation.

You have not explained what you want to do. Do you want to check conservation at a point (ie in an individual control volume)? Or do you want to check global conservation (ie inlet flow equals the outlet flow)? Both of these are reported in the output file - the first as the residual, and the second as imbalances.

rystokes · February 26, 2010, 06:04

Hi Glenn,

Thanks a lot for your input. Yes, I just want to check the conservation at a point.

ghorrocks · February 26, 2010, 06:19

OK, well first of all you can't do conservation at a point. The smallest unit is the control volume and conservation is achieved by the fluxes through the control volume faces. You do not have access to the fluxes in the control volume faces in CFD-Post, you will have to use fortran to access them.

But why do you want to do this? If you want to see how good the convergence is then just look at the residuals. Have a look in the documentation for what the residual means.

rystokes · February 26, 2010, 08:10

Hi Glenn,

I got it. Thank you very much for your kind help.

February 24, 2010, 23:23	what's the definition of (Velocity u.Gradient X) PLS?	#1
rystokes New Member harry Join Date: May 2009 Posts: 29 Rep Power: 16	Hi, I'm wondering of the exact definition of some variable calculators provided by CFX POST, such as (Velocity u.Gradient X), (Velocity v.Gradient Y), (Velocity w.Gradient Z) please? Do they simply mean the velocity component gradient on each direction please?　In that case, the value should have a unit of "s", however, I noticed there is not such unit. Besides, I sum the above three terms in order to verify the conservation of mass (density is constant), while the result is far from zero (in solve process, the calculation reached convergency). Similar thing also happens when I　tried to check momentum equation balance. Could anyone give me advice or help please？ Thanks!

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
UDF velocity and temperature	Raj	FLUENT	3	February 1, 2009 18:29
How to judge the velocity boundary layer thick...	xumustc	Main CFD Forum	4	September 29, 2008 02:15
calculate the average velocity of particles	robert	FLUENT	0	August 1, 2008 09:44
How to calculate Velocity?	Kali Sanjay	Phoenics	1	May 1, 2006 13:57
particle velocity and velocity profile	HGG	FLUENT	2	June 10, 2001 16:32

February 25, 2010, 02:04		#2
rystokes New Member harry Join Date: May 2009 Posts: 29 Rep Power: 16	anyone help me please?

February 25, 2010, 13:21		#3
longbow New Member Join Date: Apr 2009 Posts: 14 Rep Power: 17	The unit of those variables should be [s^-1] instead of [s]. Where did you sum those three terms, over a surface or volume?

February 25, 2010, 22:21		#4
rystokes New Member harry Join Date: May 2009 Posts: 29 Rep Power: 16	Hi, Yes, you are right. I just simply created several points in the fluid domain, and for each point, I can obtain the three terms by calculators in post, and summed them, because I think the continuity equation should work for each point. Do you mean I should sum over a surface or volume please?

February 26, 2010, 04:40		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	When you set a point in CFD-post it just interpolates the value from the surrounding nodes. This interpolation is not conservative - meaning that conservation is not enforced. Conservation is enforced at the control volume level, or at least you converge towards conservation. You have not explained what you want to do. Do you want to check conservation at a point (ie in an individual control volume)? Or do you want to check global conservation (ie inlet flow equals the outlet flow)? Both of these are reported in the output file - the first as the residual, and the second as imbalances.

February 26, 2010, 06:04		#6
rystokes New Member harry Join Date: May 2009 Posts: 29 Rep Power: 16	Hi Glenn, Thanks a lot for your input. Yes, I just want to check the conservation at a point.

February 26, 2010, 06:19		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	OK, well first of all you can't do conservation at a point. The smallest unit is the control volume and conservation is achieved by the fluxes through the control volume faces. You do not have access to the fluxes in the control volume faces in CFD-Post, you will have to use fortran to access them. But why do you want to do this? If you want to see how good the convergence is then just look at the residuals. Have a look in the documentation for what the residual means.

February 26, 2010, 08:10		#8
rystokes New Member harry Join Date: May 2009 Posts: 29 Rep Power: 16	Hi Glenn, I got it. Thank you very much for your kind help.