Abstract: Based on Large-eddy simulation，combined with the high order hybrid schemes，the initial flow fields of moderately and highly underexpanded circular jets under two different nozzle pressure ratios (n) of 1.4 and 4.0 are investigated numerically，respectively. Our numerical results illustrated clearly the initial flow structures of two underexpanded jet regimes，such as the generation and evolution of the vortices and shock structures，and the reason for the generation of the secondary vortex rings in highly underexpanded jets is found to be the rolling up of the slip lines originating from the triple point of the Mach disk，which is different from that of the moderately condition. In addition，the evolution of the secondary vortex rings after their formation and its effects on the main vortex core during its rolling over the periphery of the main vortex rings have been discussed. Furthermore，as n=4.0，the length of the barrel shock and diameter of the Mach disk are equal to 24mm and 0.84mm，respectively，which agree well with the value predicted by the empirical formula.

Key words: Underexpanded jet；Circular jet；Vortex ring；Large-eddy simulation

摘要： 基于大涡模拟方法与高精度混合格式，对出口压力比(n)为1.4与4.0的弱与强欠膨胀圆管射流初始流场进行了数值模拟。结果清晰地描述了两种压力比条件下欠膨胀射流初始流场的结构特征，包括涡与激波结构及它们演变过程，发现强欠膨胀射流中二次涡环是由三波点向下游形成的滑移层失稳而直接卷起，此与弱欠膨胀情况不同。另外，还讨论了二次涡环在绕主涡环外缘向后翻转过程中的演变过程及其对主涡核的影响。当n=4.0时，得到射流域内桶形激波长度与马赫盘直径分别为24mm与0.84mm，与经验公式的计算结果相符。

关键词: 欠膨胀射流；圆管射流；涡环；大涡模拟