Quantifying the temporal-spatial transition of rayleigh-taylor mixing

Abstract

In fusion engineering, the mixing transition induced by Rayleigh-Taylor (RT) instability significantly influences mixing evolution, necessitating accurate prediction of both temporal and spatial characteristics. However, the transition is a strong nonlinear process, and it is hard to accurately characterize and model the spatial-temporal transition. To address the challenges, we perform implicit large-eddy simulations of RT mixing across various density ratios. To comprehensively capture both temporal and spatial dynamics, this study proposes a novel approach by leveraging the evolution of flow structures as the foundation for characterizing a temporal-spatial transition process. The transition process is inherently linked to the evolution of flow structures, leading to the introduction of the intermittency factor $\gamma$, which quantifies the probability of turbulent events generated by these structures of transition. A specialized methodology for extracting $\gamma$ in RT mixing is developed and the evolution of $\gamma$ is analyzed. The temporal evolution of $\gamma$ reveals the basic mechanism of the onset of transition and turbulence and its spatial distribution can reflect the relationship between turbulence, mixing and entrainment. Ultimately, we have uncovered a scaling law for the temporal evolution and a universal quantitative formula for the spatial characteristics of the transition process, both of which are independent of the density ratio.