Visual analysis of two-dimensional magnetohydrodynamics

TLDR

Magnetohydrodynamics (MHD) enables study of two‑dimensional turbulent flows, where a strong external magnetic field perpendicular to the flow suppresses velocity gradients and yields a quasi‑two‑dimensional approximation. The authors use an experimental setup with a spatially extended probe to visualize two‑dimensional flow kinematics in an opaque liquid metal, studying the wake behind a circular cylinder up to Re = 10 000 and determining stability limits for transitions to non‑stationary patterns. Unexpected vortex patterns were observed in the cylinder wake, confirming theoretical predictions and validating the quasi‑two‑dimensional MHD approach.

Abstract

Magnetohydrodynamics (MHD) offers a unique opportunity to study the behavior of two-dimensional turbulent flows. A strong external magnetic field B perpendicular to the flow direction of an electrically conducting fluid will suppress velocity gradients in the direction of B. The resulting approximation is known as quasi-two-dimensional MHD. An experimental configuration is presented which meets this requirement, along with a spatially extended probe used to visualize the two-dimensional flow kinematics inside the opaque liquid metal flow. As a prototypical example, the wake behind a circular cylinder is investigated for Reynolds numbers up to R=10 000. New and unexpected vortex patterns are observed that deviate significantly from usual hydrodynamic flows. Also, stability limits for the transition from stationary to nonstationary flow patterns are experimentally determined for the cylinder wake and another type of shear flow profile. These results confirm existing theoretical predictions and thus validate the quasi-two-dimensional approach.