Numerical simulations of MHD flows around a 180-degree sharp bend under a strong transverse magnetic field

The quasi-two-dimensional flow of a liquid metal subjected to a strong transverse magnetic field around a 180-degree sharp bend is investigated by means of parametric numerical simulations where the Reynolds number Re, Hartmann number Ha and the gap ratio beta (defined as the ratio of the gap thickness to the inlet width) vary in the respective ranges [100-50 000], [100-2000] and [0.04-1]. Both steady-state flow solutions and the evolution of unsteady flow regimes can be captured within this parameter space. The critical Reynolds number for transition from steady to unsteady flow increases as Ha increases for all beta. It is shown, for 0.04 <= beta <= 0.25, the critical Reynolds number remains almost linear relationship with the parameter Re/Ha(0.9), whereas for beta = 1, the key parameter is dominated by Re/Ha(0.6). The present simulations aim to investigate the physical mechanism of this phenomenon and characterizing the position where the vortices are shed from the free shear layer. We discover that the vortices shedding is originated in the outlet region for 0.04 <= beta <= 0.25 other than the turning part in bend region for beta = 1. Additionally, the free shear layer separates the recirculation bubble from mainstream and its instability is proposed to interpret the transition, commonly known as Kelvin-Helmholtz instability. The effect of a strong transverse magnetic field on flow characteristics is considered such as the length of recirculation bubbles and the pressure drop between inlet and outlet. A further frequency analysis reveals that at the end of vortices shedding, the oblique waves resonance exists, or a new vortex street consisting of the vortices detached from the boundary layer and upstream fluctuations appears. Finally, according to the influence of beta on the transition, we present a modified map of fluid regimes for prediction, which provides useful information for improved mixing and heat transport.