Non-local amplification of intense vorticity in turbulent flows

TL;DR

This study reveals that in turbulent flows, intense vorticity is mainly amplified by non-local strain interactions beyond a certain scale, challenging existing theories of turbulence intermittency.

Contribution

It demonstrates the dominant role of non-local strain in vorticity amplification using high-resolution simulations and introduces a scale-dependent understanding of this process.

Findings

Vorticity amplification is primarily driven by non-local strain beyond a characteristic scale.

Vorticity aligns with the most extensive eigenvector of non-local strain.

Local strain tends to attenuate vorticity and disrupt scale-invariance.

Abstract

The nonlinear and nonlocal coupling of vorticity and strain-rate constitutes a major hindrance in understanding the self-amplification of velocity gradients in turbulent fluid flows. Utilizing highly-resolved direct numerical simulations of isotropic turbulence in periodic domains of up to $12288^3$ grid points, and Taylor-scale Reynolds number $R_\lambda$ in the range $140-1300$, we investigate this non-locality by decomposing the strain-rate tensor into local and non-local contributions obtained through Biot-Savart integration of vorticity in a sphere of radius $R$. We find that vorticity is predominantly amplified by the non-local strain coming beyond a characteristic scale size, which varies as a simple power-law of vorticity magnitude. The underlying dynamics preferentially align vorticity with the most extensive eigenvector of non-local strain. The remaining local strain aligns vorticity with the intermediate eigenvector and does not contribute significantly to amplification; instead it surprisingly attenuates intense vorticity, leading to breakdown of the observed power-law and ultimately also the scale-invariance of vorticity amplification, with important implications for prevailing intermittency theories.