Reply To: Query Regarding Initial Internal Energy Calculation in Autodyn Subroutine

[Chris Quan]

Although Internal Energy is not part of the parameters in the Ideal Gas EOS, it is a required field when filling an ideal gas material to an Eulerian domain. Internal energy is part of the initial conditions and cannot be zero for ideal gas materials. 

The Specific Internal Energy is Energy per mass so it has the unit of J/Kg. The Internal Energy in other EOS such as JWL EOS is Energy per volume so it has the unit of J/m^3. To convert the internal energy from J/m^3 to J/Kg, you need to divide the Internal Enegy J/m^3 by material Reference Density Kg/m^3. 

Now to back-calculate the Specific Internal Energy from the specified initial pressure when filling an ideal gas material, you can follow the procedures below:

The ideal gas EOS can be written as: 

Pressure = (gama – 1) * Density * Internal Energy.

 

Thus, Internal Energy = Pressure / (gama -1) / Density. You can calculate the new internal energy based upon the actual material density and actual pressure.

 

For example, one atmospheric pressure is 101.3 KPa. Gama = 1.4, Density = 1.225E-3 g/cm^3. After calculation, internal energy is 2.068E5 J/Kg. This is the Specific Internal Energy that needs to be filled with the ideal gas material to create the initial condition of one atomospherical pressure in air.