欠膨胀启动射流的流动结构和噪声特性

Flow Structure and Noise Characteristics in Underexpanded Starting Jet

高速射流在材料切割、表面清洗和火箭发射等方面有着广泛的应用。稳态自由射流已经得到了广泛的研究,但瞬态射流并没有受到同样的关注,因此欠膨胀启动射流是主要关注的问题。对喷嘴压比为2.7的超声速欠膨胀自由射流进行了大涡模拟,瞬态流动特性与试验结果吻合较好。射流刚喷出时,整个流场是轴对称的,稳定之后整个流场是非对称的。通过分析涡结构的演化,给出了射流下游发生非对称的原因。采用动力学模态分解方法,首次研究了射流启动过程中啸叫频率和啸叫模态的变化。

High-speed jets have been widely used in material cutting,surface cleaning and rocket launching.Steady free jet has been widely studied,but transient jet has not received the same attention,so the underexpanded starting jet is the main concern.The large eddy simulation of supersonic underexpanded free jet with nozzle pressure ratio of 2.7 is carried out.The transient flow characteristics are in good agreement with the experimental results.When the jet just ejects from the nozzle,the whole flow field is axisymmetric,and after jet stabilizes,the whole flow field is asymmetric.By analyzing the evolution of vortex structure,the reason why the downstream of jet becomes asymmetric is given.Using the dynamic mode decomposition method,the changes of screech frequency and screech mode during the starting process are studied for the first time.