The effects of boundary proximity on Kelvin–Helmholtz instability and turbulence
The effects of boundary proximity on Kelvin–Helmholtz instability and turbulence
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DOI：
10.1017/jfm.2023.412
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发表时间：
2023-06
影响因子：
3.7
通讯作者：
Chih-Lun Liu;A. Kaminski;W. Smyth
中科院分区：
文献类型：
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作者：
Chih-Lun Liu;A. Kaminski;W. Smyth
开尔文-亥姆霍兹（KH）不稳定性的研究通常将初始流动建模为一个孤立的剪切层。然而，在地球物理情况下，不稳定性往往发生在边界附近，因此可能受到边界接近效应的影响。为了了解边界接近对不稳定性演变和湍流产生的影响，进行了直接数值模拟。集合平均用于降低对初始条件小变化的敏感性。当切变层靠近边界时，向湍流的过渡和由此产生的湍流混合都发生了改变：不稳定和湍流开始的时间尺度变长，KH巨浪的高度降低。边界抑制了次谐波不稳定性，因为由于KH和次谐波模式的相速度发散，阻止了锁相。此外，三维二次不稳定性对配对的破坏性影响随着两个事件的重合越近而更加深刻。当切变层远离边界层时，以切变对向对流失稳为主；然而，当剪切层靠近边界时，次级中心-核心不稳定接管，为过渡到湍流提供了另一种途径。边界接近效应大大降低了混合效率和湍流扩散系数。
Abstract Studies of Kelvin–Helmholtz (KH) instability have typically modelled the initial flow as an isolated shear layer. In geophysical cases, however, the instability often occurs near boundaries and may therefore be influenced by boundary proximity effects. Ensembles of direct numerical simulations are conducted to understand the effect of boundary proximity on the evolution of the instability and the resulting turbulence. Ensemble averages are used to reduce sensitivity to small variations in initial conditions. Both the transition to turbulence and the resulting turbulent mixing are modified when the shear layer is near a boundary: the time scales for the onset of instability and turbulence are longer, and the height of the KH billow is reduced. Subharmonic instability is suppressed by the boundary because phase lock is prevented due to the diverging phase speeds of the KH and subharmonic modes. In addition, the disruptive influence of three-dimensional secondary instabilities on pairing is more profound as the two events coincide more closely. When the shear layer is far from the boundary, the shear-aligned convective instability is dominant; however, secondary central-core instability takes over when the shear layer is close to the boundary, providing an alternate route for the transition to turbulence. Both the efficiency of the resulting mixing and the turbulent diffusivity are dramatically reduced by boundary proximity effects.