Alignment of the scalar gradient in evolving magnetic fields

TL;DR

This study uses simulations to show that in magnetized turbulent flows, scalar gradients tend to align perpendicular to magnetic fields, affecting mixing efficiency over time.

Contribution

It reveals the persistent perpendicular alignment of scalar gradients and magnetic fields during turbulent mixing, highlighting the magnetic field's role in resisting compression and slowing mixing.

Findings

Scalar gradients are biased perpendicular to magnetic fields.

Magnetic fields resist compression, affecting scalar gradient growth.

Mixing is likely slowed by magnetic back reaction at late times.

Abstract

We conduct simulations of turbulent mixing in the presence of a magnetic field, grown by the small-scale dynamo. We show that the scalar gradient field, $\nabla C$, which must be large for diffusion to operate, is strongly biased perpendicular to the magnetic field, ${\mathbf B}$. This is true both early-on, when the magnetic field is negligible, and at late times, when the field is strong enough to back react on the flow. This occurs because $\nabla C$ increases within the plane of a compressive motion, but ${\mathbf B}$ increases perpendicular to it. At late times the magnetic field resists compression, making it harder for scalar gradients to grow and likely slowing mixing.