Numerical simulations have been performed to study the near and far fields of an axisymmetric zero-net-mass flux (ZNMF) jet in the laminar-transitional regime, before the three-dimensional vortex breakdown that promotes transition to turbulence. The main frequencies driving the flow and their associated spatial modes have been calculated using the iterative, multidimensional higher order dynamic mode decomposition algorithm. The results show that the flow is periodic in the near field, but it is quasi-periodic in the far field, where two additional high frequency traveling modes appear. One of them is localized downstream a saddle point (a topology pattern characteristic of the suction phase defining the jet flow oscillations) and seems to be responsible for a shear layer instability. The second high frequency traveling mode, instead, extends further downstream and is associated with the overall wake of the ZNMF jet, which behaves in this sense as continuous jet. Evidence of non-linear coupling between the different spatio-temporal modes is presented in this work, which suggests the existence of a mechanism of energy transition between the periodic modes in the near field and the quasi-periodic modes in the far field previous to transition.

已经进行了数值模拟，以研究层流过渡状态下的轴对称零净质量通量（ZNMF）射流的近场和远场，然后进行了三维涡旋破坏，从而促进了向湍流的过渡。使用迭代的多维高阶动态模式分解算法已计算出驱动流动的主要频率及其相关的空间模式。结果表明，流动在近场中是周期性的，但在远场中是准周期性的，在那里出现了两个附加的高频行进模式。其中之一位于鞍形点的下游（定义喷射流振荡的吸力相的拓扑特征），似乎是剪切层不稳定的原因。而是第二种高频行驶模式 进一步向下游延伸，并与ZNMF射流的整体尾流相关联，ZNMF射流在这种意义上表现为连续射流。这项工作提供了不同时空模式之间非线性耦合的证据，这表明在转换之前，近场的周期性模式与远场的准周期性模式之间存在能量转换的机制。