Normal Shocks
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 that bound this volume parallel to the shock wave (with one surface on the pre-shock side of the fluid medium and one on the post-shock side). The two surfaces are seperated by a very small depth such that the shock itself is entirely contained between them. At such control surfaces, 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.
Read more about this topic: Shockwave
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