Instability of a transient submerged jet in supersonic nozzle during its start-up and shut-down process

The present paper describes an experiment study on the flow pattern and dynamic instability of a transient submerged jet in supersonic nozzle during its start-up and shut-down process. A high speed camera is utilized for the flow visualization. The experimental results indicate that a jetting flow regime is fast established when the submerged gas injection at a high speed, and the periodic fracture bubbles are absent in our experiment during initial start-up process. During the start-up process, the dynamic instability characteristics appear only at some low pressure ratios in over-expanded condition, while the frequency of jet necking/back-attack phenomena is up to 10 Hz during the nozzle shut-down process, indicating that the submerged jet presents strongly unstable oscillation even at high pressure ratios. The different oscillation characteristics of start-up and shut-down phase indicates a certain relevance between the gas/water interaction process and jet stability. The inlet pressure increasing process may contribute to the suppression of the jet boundary necking, while the lowing process of inlet pressure appears an opposite effect, and the jet shows a higher instability.