The SST based Improved Delayed Detached Eddy Simulation (IDDES) is implemented and utilized with a 3D compressible Navier-Stokes solver to investigate the physics of flow separation. The implementation is first validated with the standard zero pressure gradient subsonic flat plate case and the results agree well with the SST model prediction. After the validation, the wall-mounted NASA hump and NACA 0012 airfoil post-stall flow cases are simulated with the SST and SST-IDDES. The high-fidelity IDDES predicts the separation size as well as the aerodynamic performance in close agreement with the experiment. Apart from the quantitative results, its efficacy is also observed in more realistic 3-D turbulent flow structures.