# momentum loss

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 April 7, 2012, 11:06 momentum loss #1 New Member   suong Join Date: Apr 2012 Posts: 7 Rep Power: 7 hi Can someone tell me: how to calculate energy or momentum of fluid loss behind the cylinders. ( flow past the cylinder) thank so much!

 April 7, 2012, 16:19 #2 Member   banty Join Date: Mar 2012 Posts: 52 Rep Power: 7 Hi, U can write a UDF for energy calculation at particular time E = (rho* Velocity*Velocity)/2 congsu likes this.

 April 8, 2012, 10:18 #3 New Member   suong Join Date: Apr 2012 Posts: 7 Rep Power: 7 Thank a lot, and I think i must spent more time to learn UDF. i'm a beginner

April 8, 2012, 13:16
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Quote:
 Originally Posted by congsu 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.

April 8, 2012, 22:49
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suong
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Quote:
 Originally Posted by kingjewel1 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..

 April 9, 2012, 05:25 #6 Senior Member   Join Date: Jul 2009 Posts: 246 Rep Power: 11 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.

 April 11, 2012, 07:55 #7 New Member   suong Join Date: Apr 2012 Posts: 7 Rep Power: 7 thank you for replying it is very useful. I did it.

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