Atwood effects on nonlocality of mean scalar transport in three-dimensional Rayleigh-Taylor Instability*

Previous work used the Macroscopic Forcing Method (MFM), a numerical method for determining closure operators, to show the importance of nonlocality in modeling mean scalar transport for the 2D low-Atwood (A=0.05) Rayleigh-Taylor (RT) instability (Lavacot et al., JFM, 2023). In this work, nonlocality of mean scalar transport in 3D variable density Rayleigh-Taylor instability is investigated. Three cases of different Atwood numbers (A=0.05, A=0.5, A=0.8) are studied, and MFM is extended to the variable density problem. In higher-Atwood cases (A>0.05), asymmetry of the eddy diffusivity moments is observed, and nonlocality is found to increase in importance as Atwood number increases. Implications of these results on modeling variable density RT mixing are discussed.