Pressure higher than calculated value

arun1994 · July 10, 2016, 11:00

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?

ghorrocks · July 10, 2016, 20:28

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

arun1994 · July 13, 2016, 09:22

h0w d0 I res0lve this err0r?

ghorrocks · July 14, 2016, 07:05

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?

July 10, 2016, 11:00	Pressure higher than calculated value	#1
arun1994 Member Arun subramanian Join Date: Jun 2016 Location: Florence,Italy Posts: 48 Rep Power: 10	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=pirrL = 3.142202050 =62840 mm^3 Now I compress it till L=13.5 New V= 3.142202013.5 = 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?

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July 10, 2016, 20:28		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	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 14, 2016, 07:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	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?