Refraction of cylindrical converging shock wave at an air/helium gaseous interface

Abstract

Refraction of a cylindrical converging shock wave at an inclined air/helium interface is investigated. Experimentally, based on the shock dynamics theory, a special wall profile is designed to generate a perfectly cylindrical converging shock wave. A soap film technique is developed to form an inclined discontinuous air/helium interface, and high-speed schlieren photography is adopted to capture the flow. Numerical simulations are also performed to compare with the experimental counterparts and to show details of refraction. In this work, two initial incident angles (45° and 60°) are considered. As the incident shock converges inward, the shock intensity increases while the incident angle decreases, causing possible transitions among the wave patterns. For the case of 45°, an irregular refraction of free precursor refraction (FPR) first occurs and gradually transits into regular refraction, while for the case of 60°, various irregular refractions of twin von Neumann refraction (TNR), twin regular refraction (TRR), free precursor von Neumann refraction (FNR), and FPR occur in sequence. The transition sequences do not belong to any groups described in the planar counterpart, indicating that the classification of the refraction phenomenon in the planar case is not exhaustive or cannot be applied to the converging case. It is also the first time to observe the transition from FNR to FPR, providing an experimental evidence for the previous numerical results. It is deemed that the difference between the velocities of the incident and transmitted shocks propagating along the interface is the primary factor that induces the transitions among wave patterns.