# Pressure higher than calculated value

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 July 10, 2016, 10:00 Pressure higher than calculated value #1 New Member   Arun subramanian Join Date: Jun 2016 Posts: 25 Rep Power: 3 Dear friends, I am trying to compress a simple cylinder with " total energy " option . I believe this is similar to adiabatic compression. PV^gamma = constant. . . . . . (1) Pinitial= 0.1 MPa, V=pi*r*r*L = 3.142*20*20*50 =62840 mm^3 Now I compress it till L=13.5 New V= 3.142*20*20*13.5 = 16967 mm^3 from (1) , P2=P1*(V1/v2)^gamma P2 = 0.6259 MPa In my CFX simulation , I get higher values like 0.78 MPa. At lower stroke lengths, I get almost perfect results. Also, I had specified a sinusoidal motion but I get something like in the figure below for higher stroke length. It works normally for small stroke lengths. Can someone please tell me what is the reason that this happens?

 July 10, 2016, 19:28 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,535 Rep Power: 104 Your error is almost certainly that you have not conducted a sensitivity analysis of the important variables. You must check that your convergence criteria, time step size and mesh are adequate to get accurate results. If you just guessed these parameters then you almost certainly have guessed wrong. This is discussed in the FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

July 13, 2016, 08:22
#3
New Member

Arun subramanian
Join Date: Jun 2016
Posts: 25
Rep Power: 3
h0w d0 I res0lve this err0r?
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 July 14, 2016, 06:05 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,535 Rep Power: 104 That error message is for pitch change across GGI interfaces for rotating frames of reference. Why are you using rotating frames of reference on a piston compression model?

 Tags cfx, cylinder, pressure, total energy, transient

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