Investigation on flow instability and shock wave structures of submerged supersonic gas jet based on dynamic adaptive mesh refinement

Submerged supersonic gas jets are investigated using dynamic adaptive mesh refinement, to capture the transient Kelvin-Helmholtz instability and high-resolution shock wave structures. The gas evolution is identified with two stages: Stage I of gas cavity enclosing gas jet divided from surrounding gas recirculation; Stage II of submerged gas jet appearing after cavity necking, with gas-liquid interface acting as jet boundary. The gas jet morphology is significantly affected by nozzle expansion ratio and back pressure, i.e., the under-expanded jet is consistent with self-excited oscillation, whereas the highly over-expanded jet is compressed into slender shape and full of intermittent break-off. Two kinds of K-H instabilities are resolved: the first kind of positive vortex train with relatively low frequency on gas cavity interface, and the second kind occurring on the gas-liquid interface near nozzle exit with much higher frequency. Three types of shock wave behaviors are discussed, including quasi-steady barrel and diamond shock structures, complex cellular shock system, and shock retraction. The vortices induced by K-H instability leads to periodic disturbance on the shock system, with secondary shock waves continuously separated from the leading shock layer by layer. Complex cellular shock structure is formed and evolved through shock stacking and intensification into multiple segments. Three-dimensional effect reveals that the jet is filled with intertwined structures of circumferential and streamwise vortexes.