Ideal Gas EOS

One of the simplest forms of equation of state is that for an ideal polytropic gas which may be used in many applications involving the motion of gases. This may be derived from the laws of Boyle and Gay-Lussac and expressed in the form

This form of equation is known as the “Ideal Gas” equation of state and only the value of the adiabatic exponent γ needs to be supplied.

In order to avoid complications with problems with multiple materials where initial small pressures in the gas would generate small unwanted velocities the equation is modified for use in these cases

where pshift is a small initial pressure defined to give a zero starting pressure.

The definition of a non-zero adiabatic constant, c, will turn the energy dependent ideal gas equation of state into the following energy independent adiabatic equation of state


Note:  This equation of state can only be applied to solid bodies. A specific heat capacity should be defined with this property to allow the calculation of temperature.


Table 124:  Input Data

NameSymbolUnitsNotes
Adiabatic exponentγNone 
Adiabatic constantcNone 
Pressure shiftPshiftPressure 

This equation of state can only be used with solid elements. Custom results variables available for this model:

NameDescriptionSolidsShellsBeams
PRESSUREPressureYesNoNo
DENSITYDensityYesNoNo
COMPRESSIONCompressionYesNoNo
INT_ENERGYInternal EnergyYesNoNo
TEMPERATURETemperatureYesNoNo

Release 16.2 - © SAS IP, Inc. All rights reserved.