The Liutex core line method, first combined with the snapshot proper orthogonal decomposition (POD), is utilized in a supersonic micro-vortex generator (MVG) wake flow at Ma = 2.5 and Re_θ = 5 760 to reveal the physical significance of each POD mode of the flow field. Compared with other scalar-based vortex identification methods, the Liutex core line identification is verified to be the most appropriate approach that is threshold-free and provides full information of a fluid rotation motion. Meanwhile, the Liutex integration is employed to quantitatively track the evolution of the vortices in MVG wake and is applied to the determination of the effective control section of the MVG wake for the optimization study of MVG design. The physical mechanism of each POD mode for multi-scale and multi-frequency vortical structures is investigated by using Liutex core line identification to give some revelations. For the mean mode (mode 0) indicating the time-averaged velocity flowfield of the MVG wake flow, a pair of primary counter-rotating streamwise vortices and another pair of secondary vortices is uniquely identified by two pairs of Liutex core lines with Liutex magnitude. In contrast, mode 1 is featured by a fluctuated roll-up motion of streamwise vortex, and the streamwise component of the MVG wake is demonstrated to be dominant in terms of the total kinetic energy contribution. Meanwhile, a dominant shedding frequency of St = 0.072 is detected from the temporal behavior of mode 2, which has the organized arc-shaped vortex structures shedding from MVG induced by the K-H instability. Additionally, mode 4 subjects to low-frequency oscillations of the wall vortices and thus takes a relatively lower frequency of St = 0.044.

中文翻译：

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Liutex核心线识别对MVG尾气中涡旋结构的POD分析

所述Liutex芯线的方法，先用适当的快照正交分解（POD）相结合，在一个超声波微涡流发生器被用来在（MVG）尾流马= 2.5和再_θ= 5760，以揭示流场的每个POD模式的物理意义。与其他基于标量的涡旋识别方法相比，Liutex核心线识别方法被证明是最合适的方法，该方法无阈值并提供了流体旋转运动的完整信息。同时，Liutex积分用于定量跟踪MVG尾流中涡流的演变，并用于确定MVG尾流的有效控制段，以优化MVG设计。利用Liutex核心线识别技术，研究了多尺度，多频涡旋结构各POD模式的物理机制，并给出了一些启示。对于表示MVG尾流的时间平均速度流场的平均模式（模式0），一对初级反向旋转的涡流和另一对次级涡流由两对具有Liutex大小的Liutex核心线唯一地标识。相比之下，模式1的特征是沿流涡旋的起伏向上运动，而MVG尾流的沿流分量在总动能贡献方面占主导地位。同时，主要的脱落频率为从模式2的时间行为中检测到St = 0.072，该模式2具有由KH不稳定性引起的从MVG脱落的有组织的弧形涡旋结构。另外，模式4经受壁涡旋的低频振荡，因此采用相对较低的St＝ 0.044的频率。