Variational closures for composite homogenised fluid flows

Theo Diamantakis, Ruiao Hu
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Abstract

The Stochastic Advection by Lie Transport is a variational formulation of stochastic fluid dynamics introduced to model the effects of unresolved scales, whilst preserving the geometric structure of ideal fluid flows. In this work, we show that the SALT equations can arise from the decomposition of the fluid flow map into its mean and fluctuating components. The fluctuating component is realised as a prescribed stochastic diffeomorphism that introduces stochastic transport into the system and we construct it using homogenisation theory. The dynamics of the mean component are derived from a variational principle utilising particular forms of variations that preserve the composite structure of the flow. Using a new variational principle, we show that SALT equations can arise from random Lagrangians and are equivalent to random coefficient PDEs. We also demonstrate how to modify the composite flow and the associated variational principle to derive models inspired by the Lagrangian Averaged Euler-Poincare (LAEP) theory.
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复合均质流体流动的变量闭包
李氏输运随机吸附是随机流体动力学的一种变分公式,用于模拟未解决的尺度效应,同时保留理想流体流的几何结构。在这项工作中,我们证明了 SALT 方程可以通过将流体流图分解为平均分量和波动分量而产生。波动分量被视为一种规定的随机差分,它将随机传输引入系统,我们利用均质化理论构建了波动分量。均值分量的动力学原理来自变分原理,利用特定的变分形式保留了流动的复合结构。利用新的变分原理,我们证明了 SALT 方程可以从随机拉格朗日衍生出来,并等价于随机系数 PDE。我们还演示了如何修改复合流和相关的变分原理,以推导出受拉格朗日平均欧拉-平卡理论(LAEP)启发的模型。
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