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The importance of nonlocality of mean scalar transport in 2D Rayleigh-TaylorInstability (RTI) is investigated. The Macroscopic Forcing Method (MFM) isutilized to measure spatio-temporal moments of the eddy diffusivity kernelrepresenting passive scalar transport in the ensemble averaged fields.Presented in this work are several studies assessing the importance of thehigher-order moments of the eddy diffusivity, which contain information aboutnonlocality, in models for RTI. First, it is demonstrated through a comparisonof leading-order models that a purely local eddy diffusivity is insufficient incapturing the mean field evolution of the mass fraction in RTI. Therefore,higher-order moments of the eddy diffusivity operator are not negligible.Models are then constructed by utilizing the measured higher-order moments. Itis demonstrated that an explicit model based on the Kramers-Moyal expansion ofthe eddy diffusivity kernel is insufficient. An implicit model constructionthat matches the measured moments is shown to offer improvements relative tothe local model in a converging fashion.

Nonlocality of Mean Scalar Transport in Two-Dimensional Rayleigh-Taylor Instability Using the Macroscopic Forcing Method