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In elementary fluid mechanics utilizing ideal gasses, a shock wave is treated as a discontinuity where entropy increases over a nearly infinitesimal region. Since no fluid flow is discontinuous, a control volume is established around the shock wave. With the control surfaces the exit lines into and out of the region. At the control surfaces assuming these are parallel to the shock wave, momentum, mass flux, and energy are constant.  It is assumed the system is adiabatic  (no heat exits or enters the system) and no work is being done.

Taking into account the established assumptions, in a system where the downstream properties are becoming subsonic: the upstream and downstream flow properties of the fluid are considered isentropic. Since the total amount of energy within the system is constant, the stagnation enthalpy remains constant over both regions. Though, entropy is increasing this must be accounted for by a drop in stagnation pressure of the downstream fluid.