Solution
ID: past-exam-of-the-astrophysical-fluid-dynamics-course-of-the-university-of-cambridge/2017/3/b/solution
Past exam of the astrophysical fluid dynamics course of the University of Cambridge 2017 3 b Solution by
Codex 0 2026-10-18
Interpret as the normal upstream speed in the shock rest frame and as its upstream mass density. If the quoted jet speed is in another frame, the shock speed must first be supplied to obtain this relative velocity. Use a calorically perfect gas of mean particle mass .
In the strong-shock limit, . The Strong-shock Rankine-Hugoniot conditions giveThe pressure expression also follows from the supplied ratio by inserting . The ideal gas law then gives the immediate postshock temperatureThe upstream thermal contribution is negligible at this order. This is a temperature in kelvin, rather than a temperature change per unit time.
To turn the postshock thermal energy into a rate, the processed mass per unit time is . Its downstream specific internal energy isThus the thermal internal energy carried into the shocked gas per unit time isPer unit area, divide this expression by . Its dimensions are watts; the mean molecular weight cancels because the heat capacity per unit mass is inversely proportional to it, while is proportional to it.
There is a second common convention for shock heating power: the loss of directed bulk kinetic energy flux. The energy fluxes of a strong adiabatic shock giveThis equals the downstream enthalpy flux, and includes pressure work as well as the advected internal energy. For , the compression ratio is , , , and . Specifying the energy convention avoids conflating these rates. A rate for a fixed slab cannot be inferred without its participating mass and thermal evolution; the supplied data fix the postshock temperature and the processed energy fluxes.
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