Shock velocity increase due to a heterogeneity produced by a two-gas layer.

Shock tube experiments are performed in order to study shock propagation along a two-gas layer in a confined geometry and to compare it to the case of a homogeneous density equivalent mixture. The analysis of the homogeneous case gives values for the adiabatic coefficient and density of the mixture of both gases, while the comparison between heterogeneous and homogeneous media with the same averaged density shows modifications of the shock front shape and velocity. In the two-gas layer, the shock propagates faster than in the homogeneous medium. The shock front is curved with a triple point which appears close to the shock-tube wall, in the slow medium, while it stays planar during its whole propagation in the homogeneous mixture. A correlation is found between the angle of curvature and the shock velocity increase. It is confirmed by two-dimensional Eulerian numerical calculations. Experiments and calculations exhibit very good agreement on all the measurements when molecular diffusion is taken into account in the numerical calculations. A sustained irregular refraction pattern of the shock front at the diffuse interface of both gases is obtained experimentally and confirmed by the calculations.

Investigation of the Richtmyer-Meshkov instability with stereolithographed interfaces.

A novel method to set highly accurate initial conditions has been designed in the context of shock tube experiments for the Richtmyer-Meshkov instability study. Stereolithography has been used to design the membrane supports which initially materialize the gaseous interface. The visualizations of both heavy-light and light-heavy sinusoidal interfaces were carried out with laser sheet diagnostics. Experiments are in very good agreement with theory and simulations for the heavy-light case, but probably due to the membrane effects, quickly deviate from them in the light-heavy configuration.

High-amplitude single-mode perturbation evolution at the Richtmyer-Meshkov instability.

The single-mode Richtmyer-Meshkov hydrodynamic instability at light/heavy (air/SF6 and air/CO2), close density (air/N2), and heavy/light (air/He) interfaces has been experimentally studied for a low incident shock wave Mach number. Two identical 2D half sinusoidal initial perturbations, with a relatively high initial amplitude, were considered in order to rapidly reach the nonlinear regime and check the reduction of the initial growth rate compared to that predicted by the small-amplitude theory. The growth rate measurements for the air/SF6 and air/CO2 cases are in excellent agreement with the nonlinear model of Sadot et al. coupled with a reduction factor suggested by Rikanati et al. In the air/N2 case, the reversal phase can be precisely described by the linear theory. Finally, the heavy/light experiment is well described by the Vandenboomgaerde model also coupled with a smaller reduction factor.

Distortion of a spherical gaseous interface accelerated by a plane shock wave.

The evolution of a spherical gaseous interface accelerated by a plane weak shock wave has been investigated in a square cross section shock tube via a multiple exposure shadowgraph diagnostic. Different gaseous bubbles, i.e., helium, nitrogen, and krypton, were introduced in air at atmospheric pressure in order to study the Richtmyer-Meshkov instability in the spherical geometry for negative, close to zero, and positive initial density jumps across the interface. We show that the bubble distortion is strongly different for the three cases and we present the experimental velocity and volume of the developed vortical structures. We prove that at late times the bubble velocities reach constant values which are in good agreement with previous calculations. Finally, we point out that, in our flow conditions, the gaseous bubble motion and shape are mainly influenced by vorticity and aerodynamic forces.