天然地下水系统在Darcy尺度下的混沌混合
Natural Groundwater Systems Can Display Chaotic Mixing at the Darcy Scale
摘要：
虽然稳定的各向同性达西流本质上是层流和非混合的，但众所周知，通过工程泵送方案的瞬态强迫可以引起地下水中的快速混沌混合。在这项研究中，我们探讨了这种混合的倾向，出现在自然地下水系统的周期性强迫，如潮汐或季节性影响。使用传统的线性地下水流模型的潮汐强迫，我们表明，在一定条件下，这些流量产生拉格朗日运输和混合现象（混沌平流）的潮汐边界附近。我们发现，含水层的异质性，storativity，和强迫的大小造成逆转的流动方向的强迫循环，反过来，产生连贯的拉格朗日结构和混乱。这些功能显着增强流体的混合和运输，导致异常的停留时间分布，流动分离，并深刻改变反应动力学的潜力。我们定义的无量纲参数组，管理这种现象，并探讨这些群体与一组特征良好的沿海系统。在现场研究中，必须认识到拉格朗日混沌在放电边界附近存在的可能性并进行评估。
英文摘要：
Although steady, isotropic Darcy flows are inherently laminar and non-mixing, it is well understood that transient forcing via engineered pumping schemes can induce rapid, chaotic mixing in groundwater. In this study we explore the propensity for such mixing to arise in natural groundwater systems subject to cyclical forcings, e.g. tidal or seasonal influences. Using a conventional linear groundwater flow model subject to tidal forcing, we show that under certain conditions these flows generate Lagrangian transport and mixing phenomena (chaotic advection) near the tidal boundary. We show that aquifer heterogeneity, storativity, and forcing magnitude cause reversals in flow direction over the forcing cycle which, in turn, generate coherent Lagrangian structures and chaos. These features significantly augment fluid mixing and transport, leading to anomalous residence time distributions, flow segregation, and the potential for profoundly altered reaction kinetics. We define the dimensionless parameter groups which govern this phenomenon and explore these groups in connection with a set of well-characterised coastal systems. The potential for Lagrangian chaos to be present near discharge boundaries must be recognized and assessed in field studies.
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