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Pressure higher than calculated value

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Old   July 10, 2016, 10:00
Default Pressure higher than calculated value
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Arun subramanian
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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?
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Old   July 10, 2016, 19:28
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Glenn Horrocks
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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
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Old   July 13, 2016, 08:22
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Arun subramanian
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h0w d0 I res0lve this err0r?
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Old   July 14, 2016, 06:05
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Glenn Horrocks
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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?
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cfx, cylinder, pressure, total energy, transient

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