Abstract: Based on the compressible Navier-Stokes equation，the initial flow structures of an iso-expanded subsonic square jet at Ma=0.6 was investigated numerically with the use of large eddy simulation (LES) and combination of the high-order hybrid WENO/TCD schemes. Numerical results illustrate clearly the formation and three-dimensional evolution of the primary vortex loop (PVL). It has the general flow characteristics of plane and circular jets，however，the azimuthal curvature discrepancy of the square nozzle leads to the non-uniform translational velocity，which results in the self-induced Biot-Savart deformation of PVL and generates 4 sets of counter-rotating streamwise vortices on the square cross-section of PVL. Therefore，the self-induced Biot-Savart deformation of PVL and the induced velocities of streamwise vortex pairs combine together to make the axis switching of the square vortex loop. Furthermore，the induced velocities of streamwise vortices dominant the axis-switching in iso-expanded subsonic jets.

Key words: Subsonic jet；Square jet；Vortex loop；Axis-switching；Large-eddy simulation

摘要： 基于可压缩Navier-Stokes方程，采用大涡模拟方法与高精度WENO/TCD混合格式，对Ma=0.6的亚声速等膨胀方管射流的初始流场进行了数值研究。数值结果清晰地描述了初始主涡环的形成与三维演变过程。方管射流具有平面管与圆管射流的一般流场特征，但方管周向曲率的不一致导致轴向流动速度不均匀，使方形主涡环出现复杂的Biot-Savart自诱导变形，并在方形涡环截面上诱导形成4对反向流向涡对。在主涡环的Biot-Savart自诱导变形与流向涡对的诱导速度共同作用下致使主涡环截面形状相对于初始时旋转45°，完成轴置换。另外，在亚声速等膨胀射流中，流向涡对的诱导速度在轴置换中占主要作用。

关键词: 亚声速射流；方管射流；涡环；轴置换；大涡模拟