What is the advantage of using ideal gas in CFD simultion..?
 

which problem physics i should/should not consider the ideal gas as my medium...?

Use ideal gas when your fluid can be approximated as an ideal gas, don't use the ideal gas approximation when it isn't one.

Basically you want to use the model when your fluid is compressible in the ideal gas region of the material, usually low to medium pressures (encompassing atospheric for air). This is good if you want to capture bouyancy forces and density changes.