Spectral analysis for magnetohydrodynamic channel turbulence at low magnetic Reynolds number

Abstract

In this study, direct numerical simulations were conducted to investigate turbulent channel flow under the influence of a spanwise magnetic field, at a friction Reynolds number italic Re Subscript tau Baseline equals 180 Re τ = 180 $ \textit{Re}_{\tau } = 180$ and Hartmann numbers italic Ha Ha $\textit{Ha}$ ranging from 0 to 15. The study elucidates how the spanwise magnetic field modulates energy spectra, near-wall streaks and energy transfer. With increasing italic Ha Ha $\textit{Ha}$ , near-wall streamwise streaks elongate markedly and energetic spectral peaks move away from the wall, whereas the spanwise length of streak enlarges only marginally. At italic Ha equals 15 Ha = 15 $\textit{Ha}=15$ , dual spectral peaks appear near the wall, reflecting coexisting high- and low-momentum regions that alternate to form a quasi-steady organisation. Concurrently, an additional very large-scale peak arises in the outer layer ( y greater than 0.3 h y > 0.3 h $y\gt 0.3h$ ), indicating a very large-scale structure that abruptly amplifies the streamwise normal stress there beyond the zero-magnetic-field level. Temporal statistics show that stronger fields reduce streak density but substantially prolong streak lifetimes, attributable to suppression of the nonlinear energy cascade and the attendant decrease in turbulent decay rates. Budget analyses further reveal that, as the field intensifies, the influence of outer-layer large scales on the viscous sublayer weakens; the dominant contributors to wall-normal energy transport shift towards larger scales; and principal energy-transfer pathways migrate away from the near-wall region. Finally, the streamwise elongation of streaks is primarily driven by the wall-normal gradient of the electric potential, while their energetic amplification is effectively damped by Joule dissipation.