水下超声速喷嘴起动/关机过程射流的动态不稳定性研究

Instability of a Transient Submerged Jet in Supersonic Nozzle During Its Start-Up and Shut-Down Process

关键词:  喷嘴  水下射流  超声速  起动  关机  不稳定性 

Key words:  Nozzle  Submerged jet  Supersonic  Start-up  Shut-down  Instability

摘要:

为了研究水下超声速喷嘴出口燃气泡的发展及射流的动态不稳定机制,应用高速摄像的流场可视化方法,开展了水下欠膨胀超声速喷嘴起动和关机过程的冷流试验研究。试验结果表明:喷嘴起动初期高压气体喷出后很快建立起射流形态,没有出现间歇性断裂泡流形态；喷嘴起动阶段的射流动态不稳定特征仅出现在较低压比下的过膨胀状态,而关机阶段的超声速射流在较高压比下的欠膨胀状态即开始出现高度不稳定振荡,射流在欠膨胀到过膨胀的过渡阶段回击频率高达10Hz；起动与关机阶段动态不稳定性的差异与气/水之间相互作用过程有关,入口压力的持续升高有助于抑制射流边界的颈缩,而入口压力的降低过程则进一步加剧了水环境对射流边界的作用,射流表现出更高的不稳定性。 

Abstract:

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 10Hz 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. 