## LS Dyna

Topics relate to LS-DYNA, Autodyn, Explicit STR and more

#### How to write ideal gas equation of state in ansys autodyn using fortran?

• LI Hailong
Subscriber
How to write ideal gas equation of state (EOS) in autodyn using fortran?

I modified the Autodyn user subroutine mdeos_user_1.f90, writing a custom ideal state equation. Ea0 (E0) is treated as a constant, 2.068 × 105Kj/kg, and the simulation is still more consistent with the no user subroutines results. My code is as follows，
IF_SIMULTANEOUS = 0
ARHO = (1.4_real8-ONE) * E0 * DEN
BRHO = ZERO
DADETA = ((1.4_real8 - ONE)* E0)/ RHOREF
DBDETA = ZERO
******
(1) Properties of Air
The air and TNT gas product media were assumed to behave as an ideal gas, thus the default Equations of state (1) were specified. The air was specified with an internal energy of 2.068 × 105Kj/kg in AUTODYN 2D.
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where, ‘ρ’ is the density of gas, ‘cv’ and ‘cp’ are specific heats at constant volume and pressure, respectively; ‘T’ is the gas temperature, ‘P’ is the pressure, ‘γ’ is the ratio of specific heats and ′Ea0′ is the specific energy. The air model was taken from AUTODYN 2D material library as shown in Table 2.
Reference：Numerical Simulations of Exposed Circular Surface of Rolled
Homogeneous Armor Steel Plates Subjected to Blast Loadings by Using AUTODYN 2D
• Chris Quan
Ansys Employee

The EOS (Equation Of State) in Autodyn defines the relation between the hydrostatic pressure, the local density, and local specific energy.

Thus, every EOS, including user-defined EOS, should calculate the current pressure from the current internal energy, update the internal energy for pressure calculation in the next time step, and finally update the material sound speed based upon the current condition (pressure, density, energy, etc).