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-   -   what's the definition of (Velocity u.Gradient X) PLS? (http://www.cfd-online.com/Forums/cfx/73033-whats-definition-velocity-u-gradient-x-pls.html)

rystokes February 25, 2010 00:23

what's the definition of (Velocity u.Gradient X) PLS?
 
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 03:04

anyone help me please?

longbow February 25, 2010 14:21

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 23:21

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 05:40

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 07:04

Hi Glenn,

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

ghorrocks February 26, 2010 07:19

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 09:10

Hi Glenn,

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


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