# Piston stroke cutoff

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 March 3, 2010, 10:00 Piston stroke cutoff #1 New Member   Chester Join Date: Feb 2010 Posts: 24 Rep Power: 8 Dear all, I can read the following in the user's guide about the piston stroke cutoff : "... you need to specify the dynamic layering zone adjacent to the piston surface to move with the piston until some specified distance from the TDC position. Beyond this cutoff distance, the motion of the layering zone is stopped and the piston wall is allowed to continue to the BDC position." But it is not clear how to specify the layering motion (first rigid and then deforming). Do I have to write an UDF or create an event? My geometry was mesh in ICEM CFD with piston at TDC. My case is 3D and I have specified the motion of the piston head to rigid body with the built-in function "piston-full". Thanks in advance, Chester

 March 7, 2010, 12:20 #2 New Member   Chester Join Date: Feb 2010 Posts: 24 Rep Power: 8 Ok the solution is to use the built-in UDF **piston-limit** for the layering and **piston-full** for the piston head.

March 28, 2010, 02:00
#3
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Quote:
 Originally Posted by ChesterP Ok the solution is to use the built-in UDF **piston-limit** for the layering and **piston-full** for the piston head.
so what does fluent do when runing with **piston-limit** or **piston-full**.
thanks

March 29, 2010, 17:22
#4
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Chester
Join Date: Feb 2010
Posts: 24
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Quote:
 Originally Posted by yubaibai88 so what does fluent do when runing with **piston-limit** or **piston-full**. thanks
Hi,

**piston-limit** makes the body moves from TDC until the cut-off distance that you defined.
**piston-full** makes the body moves from TDC to BDC.

Hope it helps

 March 31, 2010, 23:33 #5 Member   李逾 Join Date: Mar 2010 Posts: 65 Rep Power: 8 hi 'cut off' is still the problem which I don't know until now,what does it mean in a piston moving process and what effect does it have during the moving

 April 1, 2010, 03:57 #6 New Member   Chester Join Date: Feb 2010 Posts: 24 Rep Power: 8 Ok the cut-off distance is the distance until which the layering part of your mesh will move as a rigid body.It will then stop moving when reaching that distance but the piston head will continue to move until it will reach BDC and as a consequence the layering process will begin from that cutoff distance. You need that in order to create a space in the upper part of your cylinder that is meshed with tetrahedra and that is big enough (greater than you dead volume) to ensure that your valves can fully move in that zone and that the smoothing and remeshing process can happen. In other words your valves cannot touch your HEX layer. Is that clear enough? Have a nice day

 April 1, 2010, 10:29 #7 Member   李逾 Join Date: Mar 2010 Posts: 65 Rep Power: 8 thank you very much! finally can I understand it until you describe!

 April 1, 2010, 22:58 nice explanation #8 New Member   sunqiang Join Date: Nov 2009 Posts: 23 Rep Power: 8 I've been looking for the answer for days, it's really great to get it here,thanks

 Tags dynamic, in-cylinder, layering, remeshing

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