Measuring Scale-dependent Shape Anisotropy by Coarse-Graining: Application to Inhomogeneous Rayleigh-Taylor Turbulence

TL;DR

This paper introduces a multi-dimensional filtering spectrum method to analyze scale-dependent shape anisotropy in inhomogeneous Rayleigh-Taylor turbulence, revealing distinct anisotropic behaviors in 2D and 3D flows.

Contribution

It generalizes the filtering spectrum to measure shape anisotropy across scales in inhomogeneous flows, providing new insights into RT turbulence.

Findings

3D RT exhibits large-scale vertical to horizontal aspect ratio of about 4:3.

3D RT tends toward isotropy at small scales.

2D RT is isotropic at large scales and becomes more anisotropic at smaller scales.

Abstract

We generalize the `filtering spectrum' [1] to probe scales along different directions by spatial coarse-graining. This multi-dimensional filtering spectrum quantifies the spectral content of flows that are not necessarily homogeneous. From multi-dimensional spectral information, we propose a simple metric for shape anisotropy at various scales. The method is applied to simulations of 2D and 3D Rayleigh-Taylor (RT) turbulence, which is inhomogeneous and anisotropic. We show that 3D RT has clear shape anisotropy at large scales with approximately $4:3$ vertical to horizontal aspect ratio, but tends toward isotropy at small scales as expected [2,3,4]. In sharp contrast, we find that RT in 2D simulations, which are still the main modeling framework for many applications, is isotropic at large scales and its shape anisotropy increases at smaller scales where structures tend to be horizontally elongated. While this may be surprising, it is consistent with recent results in [5]; large-scale isotropy in 2D RT is due to the generation of a large-scale overturning circulation via an upscale cascade, while small scale anisotropy is due to the stable stratification resultant from such overturning and the inefficient mixing in 2D.