Nonlinear evolution of magnetorotational instability in a magnetized Taylor-Couette flow: scaling properties and relation to upcoming DRESDYN-MRI experiment

Abstract

Magnetorotational instability (MRI) is the most likely mechanism driving angular momentum transport in astrophysical disks. However, despite many efforts, a conclusive experimental evidence of MRI is still missing. Recently, performing 1D linear analysis of the standard MRI (SMRI) in a cylindrical Taylor-Couette (TC) flow with an axial magnetic field, we showed that SMRI can be detected in the upcoming DRESDYN-MRI experiment based on a magnetized TC flow of liquid sodium. In this study, also related to DRESDYN-MRI experiments, we focused on the nonlinear evolution and saturation properties of SMRI and analyzed its scaling behavior with respect to the main parameters of the TC flow. We did a detailed analysis over the extensive ranges of magnetic Reynolds number $Rm\in [8.5, 37.1]$, Lundquist number $Lu\in[1.5, 15.5]$ and Reynolds number, $Re\in[10^3, 10^5]$. We considered small magnetic Prandtl numbers, $Pm \ll 1$, down to $Pm\sim 10^{-4}$, aiming at values typical of liquid sodium in the experiments. In the saturated state, the magnetic energy of SMRI and torque due to perturbations on the cylinders, which characterizes angular momentum transport, both increase with $Rm$ for fixed $(Lu, Re)$, while for fixed $(Lu, Rm)$, the magnetic energy decreases and torque increases with increasing $Re$. We studied the scaling of the magnetic energy and torque in the saturated state as a function of $Re$ and find a power law dependence $Re^{-0.6...-0.5}$ for the magnetic energy and $Re^{0.4...0.5}$ for the torque at all $(Lu, Rm)$ and high $Re\geq 4000$. We also explored the dependence on Lundquist number and angular velocity of the cylinders. These scaling laws will be instrumental in the subsequent analysis of more realistic finite-length TC flows and comparison of numerical results with those obtained from the DRESDYN-MRI experiments to unambiguously identify SMRI in laboratory.