The development of a separated bottom boundary layer in the footprint of a large-amplitude internal solitary wave of depression, propagating against an oncoming barotropic current, is examined in detail using high-resolution implicit Large-Eddy-Simulation. The wave is supported by a continuous two-layer stratification. The Reynolds number based on the water column height is $1.6\times {10}^5$. This numerical simulation is the first to reproduce the self-sustained three-dimensional vortex shedding, resultant transition and turbulence under an ISW, which have long been hypothesized to occur in field experiments. No artificial noise is inserted to the flow domain. Part I focuses on a structural description of the sequence of flow regimes developing from a wave-induced, long, high-aspect ratio, laminar separation bubble. Three illuminating topological features are identified: (a) The spatial development of the self-sustained turbulence is comprised of three transitional stages: (i) Spontaneous excitation of a global instability in the separation bubble that emanates trailing vortices; (ii) Vortex breakup and degeneration into turbulent clouds; and (iii) Relaxation to a spatially developing turbulent boundary layer. (b) In the separation bubble, there exists a three-dimensional linear global oscillator, which is primarily excited by the two-dimensional absolute instability of the separated shear layer. This global mode possesses a transverse coherent structure. The transverse perturbation subsequently excites an elliptic instability mode inside the shed vortex, resulting in an axial distortion of the vortex core. (c) A shortwave secondary instability is excited in the form of a series of coherent streamwise vortex streaks that wrap around each shed vortex, leading to a rapid breakup/burst of the vortex.

中文翻译：

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内部孤立波足迹中的长分离气泡引起的自持不稳定性，过渡和湍流第一部分：流动拓扑

使用高分辨率的隐含大涡模拟，详细研究了大振幅内部孤波的凹陷区中分离的底部边界层的发展，该凹陷区是在迎面而来的正压电流下传播的。该波由连续的两层分层支撑。基于水柱高度的雷诺数为$1.6\times {10}^5$。这种数值模拟是第一个重现自保持的三维涡旋脱落，在ISW下产生的过渡和湍流的方法，长期以来人们一直假设这种方法会在野外实验中发生。没有人为的噪音插入到流域。第一部分着重描述了由波浪引起的长高纵横比层流分离气泡形成的流态序列的结构。确定了三个启发性的拓扑特征：（a）自持湍流的空间发展包括三个过渡阶段：（i）自发激发分离气泡中整体的不稳定性，该不稳定性产生了尾随的涡旋；（ii）涡旋破裂并退化成湍流云；（iii）放宽到在空间上发展的湍流边界层。（b）在分离气泡中，存在一个三维线性全局振荡器，该振荡器主要由分离的剪切层的二维绝对不稳定性所激发。这种整体模式具有横向相干结构。随后，横向扰动激发了脱落涡旋内部的椭圆形不稳定性模式，从而导致涡旋核的轴向变形。（c）短波次级不稳定性以一系列连贯的流向涡流条纹的形式被激发，这些条纹环绕着每个流下的涡流，导致涡流迅速破裂/爆发。随后，横向扰动激发了脱落涡旋内部的椭圆形不稳定性模式，从而导致涡旋核的轴向变形。（c）短波次级不稳定性以一系列连贯的流向涡流条纹的形式被激发，这些条纹环绕着每个流下的涡流，导致涡流迅速破裂/爆发。随后，横向扰动激发了脱落涡旋内部的椭圆形不稳定性模式，从而导致涡旋核的轴向变形。（c）短波次级不稳定性以一系列连贯的流向涡流条纹的形式被激发，这些条纹环绕着每个流下的涡流，导致涡流迅速破裂/爆发。