On the irregular jet formation of shock-accelerated spherical heavy gas bubbles

Abstract

The behaviors of shock-accelerated heavy gas bubbles are numerically investigated, focusing on clarifying the forming mechanisms of the bubble jets in different types. The present study categorizes heavy bubble jets into two types, regular jets, and irregular jets. The present shock-accelerated multi-component flows are simulated by solving inviscid compressible Euler equations. An upwind characteristic space-time conservation element solution element scheme is adopted, and a five-equation model is used to treat the gas interface. Bubbles of R22, SF6, and Kr in ambient N2 and air are examined, and the incident shock Mach numbers are 1.1 and 1.23. The numerical results demonstrate that the bubble jet formation and its shape are very sensitive to the test gas species and incident shock strength. It is found that the tiny upstream jet formed in the single-shocked SF6/air scenario results from a very small Mach stem impingement onto the bubble upstream interface, the type II shock-shock interaction features the flow mechanism. While the large upstream jet formed in the re-shock SF6/air scenario is a combined result of the re-shock convergence and later vortex stretching. For the complex Kr/air scenario, the upstream jet results from the vorticity-induced inward jet stretching, and the downstream hollow jet results from the slip line guided tip extension. The measurements of bubble volumes, gas mixings, and material line lengths suggest that, although the jet formation greatly changes the bubble morphology, it makes a minor contribution to the bubble overall integral properties.