In this paper, the separation-induced transition in an LPT (low-pressure turbine) cascade is investigated at low Reynolds number with DNS (direct numerical simulation). The transition process is accurately predicted giving good agreements between the DNS and experimental results. To illustrate the secondary instability of separation-induced transition in a low-disturbance environment, the results are comprehensively analyzed in both Fourier space and physical space. It is illustrated that the effect of hyperbolic instability dominates around the saddle point of hyperbolic streamlines. This instability mechanism is responsible for the emergence of the streamwise vortices in the braid region. Elongated and intensified because of the “stretching” effect of the background flow, these vortices become the most noticeable characteristic of the flow field. Fundamental modes of small spanwise wavelength are excited in the braid region, so as some low-frequency modes. The elliptical instability plays a minor role than hyperbolic instability. It is also observed that the fundamental mode with a larger spanwise wavelength is unstable in the vortex core which is associated with the deformation of the vortex core via elliptical instability. There is no convincing evidence for the existence of subharmonic instability.

中文翻译：

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低扰动环境下低压涡轮叶栅中分离诱导过渡的数值研究

在本文中，利用DNS（直接数值模拟）研究了低雷诺数下LPT（低压涡轮机）叶栅中分离引起的过渡。可以准确预测过渡过程，从而在DNS和实验结果之间达成良好的协议。为了说明在低扰动环境中由分离引起的跃迁的次要不稳定性，对结果进行了傅立叶空间和物理空间的综合分析。可以看出，双曲线不稳定性的影响在双曲线流线的鞍点附近占主导地位。这种不稳定性机制是造成辫状区域中流向涡旋的原因。由于背景流的“拉伸”效应而伸长并增强，这些涡流成为流场最明显的特征。在编织区域中会激发小跨度波长的基本模式，因此会激发一些低频模式。椭圆不稳定性比双曲线不稳定性起较小的作用。还观察到，具有较大跨度波长的基本模式在涡旋核中是不稳定的，这与通过椭圆形不稳定性引起的涡旋核的变形有关。没有令人信服的证据表明存在亚谐波不稳定。