Postshock

Postshock refers to the region downstream of a shock wave, where the flow has passed through the discontinuity and properties such as pressure, density, and temperature are altered. In supersonic flows, a shock compresses, heats, and slows the gas, producing a postshock state with higher pressure, density, and temperature, and reduced velocity. The description of these changes is given by the Rankine–Hugoniot jump conditions.

For an ideal gas with ratio of specific heats gamma and a normal shock with upstream Mach number M1, the standard relations are: the density ratio rho2/rho1 = ((gamma+1) M1^2) / ((gamma-1) M1^2 + 2), the pressure ratio P2/P1 = (2 gamma M1^2 - (gamma-1)) / (gamma+1), and the temperature ratio T2/T1 = (P2/P1) / (rho2/rho1). The downstream velocity satisfies V2/V1 = rho1/rho2. In oblique shocks, postshock properties depend on the shock angle and flow deflection, while in magnetized plasmas the magnetic field also changes across the shock, modifying the postshock state through magnetohydrodynamic (MHD) effects.

Applications and contexts include high-speed aerodynamics, supersonic jets, meteoroid entry, and astrophysical phenomena such as supernova remnants, stellar winds, and planetary bow shocks. In many astrophysical cases, radiative cooling can cause the postshock gas to condense into dense shells, adding complexity to the downstream evolution.