Passive scalars: mixing, diffusion and intermittency in helical and non-helical rotating turbulence

TL;DR

This study uses numerical simulations to analyze how helicity and rotation influence passive scalar mixing, diffusion, and intermittency in turbulent flows, revealing spectral scaling laws and anisotropic diffusion behaviors.

Contribution

It demonstrates the impact of helicity on passive scalar scaling and intermittency in rotating turbulence, providing new insights into anisotropic diffusion and scalar intermittency.

Findings

Helicity alters the inertial range scaling of passive scalars in rotating turbulence.

Passive scalar intermittency is stronger in helical flows, with non-Gaussian statistics.

Horizontal diffusion decreases with helicity, while vertical diffusion increases.

Abstract

We use direct numerical simulations to compute structure functions, scaling exponents, probability density functions and turbulent transport coefficients of passive scalars in turbulent rotating helical and non-helical flows. We show that helicity affects the inertial range scaling of the velocity and of the passive scalar when rotation is present, with a spectral law consistent with $\sim k_{\perp}^{-1.4}$ for the passive scalar variance spectrum. This scaling law is consistent with the phenomenological argument presented in \cite{imazio2011} for rotating non-helical flows, wich states that if energy follows a $E(k)\sim k^{-n}$ law, then the passive scalar variance follows a law $V(k) \sim k^{-n_{\theta}}$ with $n_{\theta}=(5-n)/2$. With the second order scaling exponent obtained from this law, and using the Kraichnan model, we obtain anomalous scaling exponents for the passive scalar that are in good agreement with the numerical results. Intermittency of the passive scalar is found to be stronger than in the non-helical rotating case, a result that is also confirmed by stronger non-Gaussian tails in the probability density functions of field increments. Finally, Fick's law is used to compute the effective diffusion coefficients in the directions parallel and perpendicular to the rotation axis. Calculations indicate that horizontal diffusion decreases in the presence of helicity in rotating flows, while vertical diffusion increases. We use a mean field argument to explain this behavior in terms of the amplitude of velocity field fluctuations.