Negative absolute pressure for air
 

I am new to FLUENT; While trying to simulate scavenging in IC engine using the available FLUENT tutorial and with the given mesh file, I accomplish the simulation and the results sounds good.

Now I am trying it with my own model which is modeled in Pro-E and meshed in ANSYS - Meshing (using cutcell method and inflation for the piston surface).
On calculating, In the first saved case file, the cylinder has negative absolute pressure . In successive cases, the pressure gradually increases to the exhaust pressure.
I am facing this trouble when the initial position of the piston is either two degree before the Exhaust Port Open(EPO) or exactly at EPO.
When both the inlet and the exhaust port is closed, the pressure value varies uncontrollably.

If you are using a constant density material then there's no bounds on the pressure. Using an Ideal Gas if you expect large pressure changes.

Can you please explain your answer elaborately?

Using a constant density for a material is an assumption/simplification. You are saying that given the expected pressure range in your simulation, the density changes are so small that you can assume constant density. If you are getting negative absolute pressure then this assumption is likely not valid. You should use an ideal gas or other equation of state so that density can very with pressure.

Thanks a lot....
Now we are out of the trouble "Negative absolute pressure" :) :)

But now we are facing another Error "Floating point Exception" :(
Please help us If you are familiar with this error.
Thanks in advance