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首页> 外文期刊>Nonlinear processes in geophysics >Competition between chaotic advection and diffusion: stirring and mixing in a 3-D eddy model
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Competition between chaotic advection and diffusion: stirring and mixing in a 3-D eddy model

机译：混沌对流与扩散之间的竞争：3-D涡流模型中的搅拌和混合

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The importance of chaotic advection relative to turbulent diffusion is investigated in an idealized model of a 3-D swirling and overturning ocean eddy. Various measures of stirring and mixing are examined in order to determine when and where chaotic advection is relevant. Turbulent diffusion is alternatively represented by (1)?an explicit, observation-based, scale-dependent diffusivity, (2)?stochastic noise, added to a deterministic velocity field, or (3)?explicit and implicit diffusion in a spectral numerical model of the Navier–Stokes equations. Lagrangian chaos in our model occurs only within distinct regions of the eddy, including a large chaotic “sea” that fills much of the volume near the perimeter and central axis of the eddy and much smaller “resonant” bands. The size and distribution of these regions depend on factors such as the degree of axial asymmetry of the eddy and the Ekman number. The relative importance of chaotic advection and turbulent diffusion within the chaotic regions is quantified using three measures: the Lagrangian Batchelor scale, the rate of dispersal of closely spaced fluid parcels, and the Nakamura effective diffusivity. The role of chaotic advection in the stirring of a passive tracer is generally found to be most important within the larger chaotic seas, at intermediate times, with small diffusivities, and for eddies with strong asymmetry. In contrast, in thin chaotic regions, turbulent diffusion at oceanographically relevant rates is at least as important as chaotic advection. Future work should address anisotropic and spatially varying representations of turbulent diffusion for more realistic models.
机译：在3D涡旋和倾覆海洋涡旋的理想化模型中，研究了混沌对流相对于湍流扩散的重要性。为了确定何时和何处与对流有关，研究了搅拌和混合的各种措施。湍流扩散也可以用（1）表示，即基于观测的显式，与比例有关的扩散率表示;（2）随机噪声，添加到确定性速度场中;或（3）频谱数值模型中的显式和隐式扩散表示Navier–Stokes方程组。在我们的模型中，拉格朗日混沌仅发生在涡流的不同区域内，包括一个大的混沌“海”，它充满了涡流的周长和中心轴附近的大部分体积，以及较小的“共振”带。这些区域的大小和分布取决于诸如涡流的轴向不对称程度和Ekman数之类的因素。混沌对流和湍流扩散在混沌区域内的相对重要性可通过以下三种方法进行量化：拉格朗日Batchelor规模，紧密分布的流体包裹的扩散速度以及中村有效扩散率。通常发现，在对流式无源示踪剂的搅拌中，混沌对流的作用在较大的混沌海中，中间时间，扩散率小以及对于具有强不对称性的涡旋中最为重要。相比之下，在稀薄的混沌区域中，以海洋学相关速率进行的湍流扩散至少与混沌平流同等重要。未来的工作应该针对更现实的模型来解决湍流扩散的各向异性和空间变化表示。

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