Conditions for turbulent Ekman layers in precessionally driven flow
Conditions for turbulent Ekman layers in precessionally driven flow
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DOI：
10.1093/gji/ggab088
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发表时间：
2021-03-25
影响因子：
2.8
通讯作者：
Buffett, B. A.
中科院分区：
文献类型：
--
作者：
Buffett, B. A.
埃克曼层在地球流体核心的边界处发展，以响应岁差。当基于Ekman层厚度的局部雷诺数Re超过临界值时，这些层中的不稳定性导致湍流。湍流的转捩通常是用稳定的埃克曼层实验来评估的，其中内部的地转流与时间无关。旋进驱动流在昼夜时间尺度上变化，因此在不同的Re值下可能发生向湍流的过渡。我们使用3-D数值计算在一个本地的笛卡尔几何形状，以评估在旋进流的湍流过渡。计算保留了旋转矢量的水平分量，并考虑了流体分层的影响。在中性分层流体中，湍流的转变发生在Re = 500附近，该值高于稳定埃克曼层通常引用的Re = 150。然而，它与地球上岁差流的标称值相当。由于流体分层或磁场的复杂性可以抑制向湍流的过渡，从而减少地球核心中湍流埃克曼层的可能性。
Ekman layers develop at the boundaries of the Earth's fluid core in response to precession. Instabilities in these layers lead to turbulence when a local Reynolds number, Re, based on the thickness of the Ekman layer, exceeds a critical value. The transition to turbulence is often assessed using experiments for steady Ekman layers, where the interior geostrophic flow is independent of time. Precessionally driven flow varies on diurnal timescales, so the transition to turbulence may occur at a different value of Re. We use 3-D numerical calculations in a local Cartesian geometry to assess the transition to turbulence in precessional flow. Calculations retain the horizontal component of the rotation vector and account for the influence of fluid stratification. The transition to turbulence in a neutrally stratified fluid occurs near Re = 500, which is higher than the value Re = 150 usually cited for steady Ekman layers. However, it is comparable to the nominal value for precessional flow in the Earth. Complications due to fluid stratification or a magnetic field can suppress the transition to turbulence, reducing the likelihood of turbulent Ekman layers in the Earth's core.