Fluid model in SU2
 

In case of fluid model multple options are present. By default it takes FLUID_MODEL= STANDARD_AIR with GAMMA and GAS_CONSTANT parameter values provided.
If I want to use IDEAL_GAS instead of STANDARD_AIR, what all parameters do I need to define (like gamma etc) and what is the main difference between the two?

Assuming you use same fluid, e.g., Nitrogen (N2), then its gamma and gas constant should be same, regardless you choose standard air or ideal air.

For ideal gas, we compute density based on P=RT/v calculation.

For standard air, we obtain density based on experiment data.

Hope this helps.