Numerical Study On The Irregular Jet Formation Of Heavy Gas Bubbles Under The Impact Of Shock Wave

The shock-bubble interaction（SBI）happens when a gaseous bubble,which contains a test gas different from the surrounding one,is impulsively accelerated by a shock wave.The bubble deformation,shock reflection and transmission emerge during the shock and bubble interacting.Complex physical processes are involved in the SBI,such as shock-shock interaction,shock-interface interaction,vortex development,and the final turbulent flow.The underlying mechanisms therein exist in many natural phenomena and man-made industries,including supernova explosion,inertial confinement fusion,supersonic combustion,shock wave lithotripsy,and high-energy physics.A very interesting structure that formed in many of the heavy bubble scenarios is the bubble jet.Great progress has been made on the heavy bubble deformation and jet formation mechanism in the most typical outward jet occurred on the downstream interface.However,various bubble jets arise under the different gas pair scenarios.Investigation on the multiple jet types contributes to further understanding of shock-bubble interaction.In the present study,the behaviors of shock-accelerated heavy gas bubbles are numerically investigated with an upwind characteristic space-time conservation element and solution element scheme adopted.The present study highlights the internal mechanism of jet formation,jet contribution to the overall bubble integral characteristic and physical insight of jet types.The main contents are as follows:1.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.2.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.The measurement of the jet local circulation suggests that the regular jet contributes to the bubble circulation,and the contribution increase as the evolution of bubble deformation.3.The heavy bubbles in different gas pairs are examined to discuss the physical insight of bubble deformation,and pattern of bubble material line lengths and bubble volume evolution.It is found that the jet types are ascribed to the specific heat ratio and density of the gas on both sides of the interface when a weak shock wave（incident shock Mach number at 1.0～2.0）impinges the heavy bubbles.And a rough partition of the jet types in the specific heat ratio-density space is proposed.The bubble material line lengths follow the exponent stretching law.Under the same shock strength,the exponent law is related to the bubble jet types.The bubble volume compression ratio is estimated by incident shock Mach number and the specific heat ratio of the test gas.