how to know if a deflection in a supersonic isentropic flow causes compression (oblique shock) or expansion (Prandtl-Meyer expansion) Thanks in advance.
Really easy: Calculate the deflection angle beta. If the angle is positive, then it's an oblique shockwave, but if it's negative, then you have an expansion. If the angle beta is zero, then there's neither compression nor compression and if beta is 90º (π/2) you have a normalmente shockwave.
An expression that may help you (I did a report on this and demonstrated the formulæ):