Blundell Thermal Physics · Problem 32.3

Shock waves

Physics — Statistical Mechanics · rankine-hugoniot-monatomic-strong-shock

Problem

For a shock wave in a monatomic gas show that ρ2ρ1→4,p2p1→54M12,T2T1→532M12,(32.31)\frac{\rho_2}{\rho_1} \to 4, \qquad \frac{p_2}{p_1} \to \frac{5}{4}M_1^2, \qquad \frac{T_2}{T_1} \to \frac{5}{32}M_1^2, \tag{32.31} in the limit M1≫1M_1 \gg 1.
Intuitive explanation

Shock waves involve a discontinuous jump in fluid properties, but the key is that conservation laws—mass, momentum, and energy—still hold across the discontinuity. In the strong-shock limit (M1≫1M_1 \gg 1), the upstream flow is supersonic and carries enormous kinetic energy that gets converted into pressure, density, and heat on the downstream side. By applying the Rankine-Hugoniot relations and using the equation of state for a monatomic gas, you can show how each ratio scales with M1M_1 and find the asymptotic limits where the density ratio saturates at 4 (a geometric limit from the equations) while pressure and temperature grow quadratically with Mach number.

Intuitive explanation

Shock waves involve a discontinuous jump in fluid properties, but the key is that conservation laws—mass, momentum, and energy—still hold across the discontinuity. In the strong-shock limit (M1≫1M_1 \gg 1), the upstream flow is supersonic and carries enormous kinetic energy that gets converted into pressure, density, and heat on the downstream side. By applying the Rankine-Hugoniot relations and using the equation of state for a monatomic gas, you can show how each ratio scales with M1M_1 and find the asymptotic limits where the density ratio saturates at 4 (a geometric limit from the equations) while pressure and temperature grow quadratically with Mach number.

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