Magnetoconvection at Very Strong Magnetic Fields

Abstract

Magnetoconvection is the type of flows in which the effects of thermal convection and magnetohydrodynamics (MHD) are combined. Magnetoconvection dramatically affects the nature of flows of electrically conducting fluids such as liquid metals and plasmas. This physical phenomenon is investigated in the dissertation for two configurations. The first is the Rayleigh-Bénard convection (RBC), which plays an important role in numerous systems found in technology and nature. The results are consistent with those of earlier experimental and numerical data. As anticipated, the heat transfer rate and kinetic energy are suppressed by a strong magnetic field. At the same time, their growth with the Rayleigh number is found to be faster in flows at higher values of Hartmann numbers. Exceptionally high values of the Nusselt number are observed in such flows. This behavior is attributed to the newly discovered flow regime that is explored in high-resolution direct numerical simulations (DNS). The regime is characterized by prominent quasi-twodimensional (Q2D) structures reminiscent of vortex sheets observed earlier in simulations of magnetohydrodynamic turbulence. Another studied case is the mixed convection in a horizontal duct (Poiseuille-Rayleigh-Bénard flow (PRBF)), in which unique regimes characterized by extreme temperature gradients and high-amplitude fluctuations (the so-called magneto-convective fluctuations (MCFs)) have been recently discovered. DNS and linear stability analysis are carried out to further explore this phenomenon which is relevant to the design of liquid-metal components of future nuclear fusion reactors. Also, the two-dimensional (2D) approximation is validated and applied in the asymptotic limit of a very strong magnetic field effect in order to investigate the instability in the previously unexplored range of control parameters corresponding to the typical conditions of a liquid metal blanket of a nuclear fusion reactor. Additionally, the modal analysis of magnetoconvection is performed to seek for an effective low-dimensional representation and retrieve physically dominant features of the fluid flows. The modal decomposition technique provides an efficient approach to exploring the common flow features that emerge over a range of flow parameters that can be utilized to facilitate reduced-order flow modeling.