Decay of plane shock waves in equilibrium flows

A new model is presented for the decay of plane shock waves in equilibrium flows with an arbitrary equation of state. A fundamental challenge for the accurate prediction of shock propagation using analytical modelling is to account for the coupling between a shock’s motion and the post-shock flow. Our model accomplishes this by neglecting only higher-order perturbations to the second velocity gradient, $u_{xx}$ , in the incident simple wave. The second velocity gradient is generally small and exactly zero for centred expansion waves in a perfect gas, so neglecting its effect on the shock motion provides an accurate closure criterion for a shock-change equation. This second-order shock-change equation is derived for a general equation of state. The model is tested by comparison with numerical simulations for three problems: decay by centred waves in a perfect gas, decay by centred waves in equilibrium air and decay by the simple wave generated from the constant deceleration of piston in a perfect gas. The model is shown to be exceptionally accurate for a wide range of conditions, including small $\gamma$ and large shock Mach numbers. For a Mach 15 shock in equilibrium air, model errors are less than 2 % in the first 60 % of the shock’s decay. The analytical results possess a simple formulation but are applicable to fluids with a general equation of state, enabling new insight into this fundamental problem in shock wave physics.