Abstract: To obtain more in-depth understanding on the physics，especially the instantaneous near-field characteristics of highly underexpanded jets，three-dimensional large eddy simulation(LES)of highly underexpanded nitrogen jets with nozzle pressure ratio(NPR)from 5.60，7.47，9.34 to 11.21 and Reynolds numbers about 105 are carried out utilizing the AstroFoam solver developed based on OpenFOAM function libraries. The time-averaged near-field flow characteristics are examined to verify the reliability of the LES results，which successfully capture the typical near-field wave structures in comparison with experimental measurements and literature data. The formation and time evolution of near-field shock structures of highly underexpanded jets which are difficult to be measured experimentally are revealed，which is found to be more influenced by the initial field conditions rather than the NPR. A kind of diamond-like cellular structures of small scales in the Mach barrel is reported，and confirmed by high-speed pulsed schlieren images. The oscillation characteristics of the Mach disk for highly underexpanded jets are found to be highly transient and nonlinear，and are investigated quantitatively. The oscillation cycles of Mach disk under different NPRs are almost identical，and mainly dominated by the axisymmetric mode with f =5.298 kHz.

Key words: Highly underexpanded jets；Large eddy simulation；Near-field structures；Instantaneous unsteady characteristics

摘要： 为增加对高度欠膨胀射流物理流动规律，特别是对其近场瞬态结构特征的认识，利用基于OpenFOAM函数库开发的可压缩流动求解器astroFoam，对喷压比(NPR)分别为5.60，7.47，9.34，11.21，雷诺数Re约105的高度欠膨胀射流进行了三维大涡模拟。首先讨论了射流近场的时均流动特征，发现大涡模拟计算成功捕捉到了高度欠膨胀射流近场的典型结构，并与实验测量和文献结果吻合较好。重点研究了高度欠膨胀射流近场的瞬态非定常特征，给出了实验难以测量的射流近场波系结构的建立过程，发现其主要受初始条件的主导，喷压比的影响较为有限。揭示了马赫桶内精细的小尺度胞格结构，并通过高速脉冲纹影图片予以初步证实。定量地研究了射流马赫盘的振荡特征，发现其具有强瞬态和高非线性的特点，不同喷压比下马赫盘的振荡周期相同，主要受f =5.298kHz的轴对称模态的主导。

关键词: 高度欠膨胀射流；大涡模拟；近场结构；瞬态非定常特征