Learning Forum

[Gronhwi Lee]

Hello,

I'm currently in the process of implementing an ideal gas equation of state (EOS) using an Autodyn subroutine. During this process, I encountered a question regarding the initialization of internal energy.

I've noticed that for existing EOS models in Autodyn (excluding the ideal gas EOS), when I input the parameters of the equation of state, the initial specific internal energy of the material is automatically calculated based on certain criteria.

I wanted to achieve similar functionality when implementing the ideal gas EOS using a subroutine. Specifically, I aimed to input a 'Reference Pressure' and have the initial specific internal energy automatically calculated according to the formula below, so that it would be incorporated into the initial conditions.

Unfortunately, I'm currently unsure about the method to accomplish this. 

Below is the code snippet I've written.

I would greatly appreciate any advice or guidance on how to achieve automatic calculation and input of initial specific internal energy based on a reference pressure when implementing the ideal gas EOS using a subroutine.

Thank you for your assistance.

Best regards

Geonhwi Lee

[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.