This work considers the numerical study of unsteady MHD convection of cobalt-kerosene ferro fluid in a linearly heated two-sided cavity and in the presence of constant and alternating magnetic elds. An accurate nite volume method is adapted to solve the governing equations for this problem. The fluid flow and heat transfer
characteristics are studied for a wide range of Richardson numbers (0:01  Ri  10) and Hartmann numbers (0  Ha  100). The volumetric fraction of nanoscale ferromagnetic particles in ferro fluid also varies between 0:0    0:2, while the size of those nanoparticles is fixed at 45 nm. In addition, different alternating magnetic fields with various periods and phase deviations are utilized to study the effect of time-periodic magnetic field on the flow characteristics and heat transfer. The entropy generation and Bejan number are also evaluated to study the eect of key parameters and temporal variation of magnetic field on various fluid irreversibilities. The results show that applying time-periodic magnetic field has remarkably influenced the fluid characteristics, the
fluid irreversibilities, and heat transfer within the cavity. Meanwhile, the influence of phase deviation on heat transfer seems to be insignicant.