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-   -   momentum loss (http://www.cfd-online.com/Forums/fluent/99612-momentum-loss.html)

congsu April 7, 2012 11:06

momentum loss
 
hi
Can someone tell me:
how to calculate energy or momentum of fluid loss behind the cylinders. ( flow past the cylinder)
thank so much!

banty April 7, 2012 16:19

Hi,
U can write a UDF for energy calculation at particular time
E = (rho* Velocity*Velocity)/2

congsu April 8, 2012 10:18

Thank a lot, and I think i must spent more time to learn UDF. i'm a beginner

kingjewel1 April 8, 2012 13:16

Quote:

Originally Posted by congsu (Post 353693)
Thank a lot, and I think i must spent more time to learn UDF. i'm a beginner

  • You don't need a UDF for such a simple thing. Use a Custom Defined Function. It's like a calculator with all the different variables.
  • Alternatively you can open your solved case into CFDpost and use a CEL function like
  • ave(u-velocity^2)@point behind cylinder/2*rho

That's not the exact command because you have to define that point in either Fluent or in CFD post.

If you want to calculate the average momentum or energy in a volume, you need to cut that volume out of the domain. If that's what you want to do, I'll help further.

congsu April 8, 2012 22:49

Quote:

Originally Posted by kingjewel1 (Post 353699)
If you want to calculate the average momentum or energy in a volume, you need to cut that volume out of the domain. If that's what you want to do, I'll help further.

Thank a lot kingjewel1. Yes, i want to calculate the average momentum or energy . Can use help me how to cut that volume out of the domain and how to calculate it, more detail, please..

kingjewel1 April 9, 2012 05:25

When you say you want to calculate the average momentum loss behind a cylinder what exactly do you mean? can you draw it for us?

Is it the momentum you want to calculate in a given volume behind the cylinder?

If that's the case what you need to do is decide what size this volume needs to be and the bounding coordinates (x0 y0 z0) and (x1 y1 z1).
Go to Adapt -Region - Put in those coordinate and click MARK.
Then go to Mesh -Separate- Zones choose the two fluid zones you want to split and that's your volume created. It's still perfectly transparent to the fluid, just has a separate name.

Then go to Report/Volume/Volume-Average choose which volume you want to calculate in (ie your new one) and then choose your variable. Since you have a particular equation, you need to define this first in Custom Field Functions for it to appear in the list available in report.

You can also set this as a volume monitor during calculation eg for a convergence criterium.

Hope that helps.

congsu April 11, 2012 07:55

thank you for replying
it is very useful.
I did it.


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