Direct numerical simulations of rotating Rayleigh-Taylor instability under the influence of magnetic fields.

The combined effects of the imposed vertical mean magnetic field (B₀) and rotation on the heat transfer phenomenon driven by the Rayleigh-Taylor (RT) instability are investigated using DNS. In the hydrodynamic (HD) case (B₀ = 0), as the rotation rate f  increases from 4 to 8, the Coriolis force suppresses the growth of the mixing layer height (h) and the vertical velocity fluctuations (u′₃), leading to a reduction in the heat transport, characterized by the Nusselt number (Nu). In non-rotating magnetohydrodynamic (MHD) cases, we find a significant delay in the onset of RT instability with increasing B₀ (= 0.1, 0.15, 0.3), consistent with the linear theory in the literature. The imposed B₀ forms vertically elongated thermal plumes that exhibit larger u′₃ and efficiently transport heat between the bottom hot fluid and the upper cold fluid with limited horizontal mixing. Therefore, due to higher u′₃, we observe an enhancement in heat transfer in the initial regime of unbroken elongated plumes in non-rotating MHD cases compared to the corresponding HD case. In the mixing regime of broken small-scale structures, the flow is collimated along the vertical magnetic field lines due to the imposed B₀, resulting in a decrease in u′₃ and an increase in the growth of h compared to the non-rotating HD case. This increase in h enhances heat transfer in the mixing regime of non-rotating MHD over the corresponding HD case. When rotation is added along with the imposed B₀, the growth and breakdown of vertically elongated plumes are inhibited by the Coriolis force, reducing h and u′₃. Consequently, heat transfer is also reduced in the rotating MHD cases compared to the corresponding non-rotating MHD cases. Interestingly, the heat transfer in the rotating MHD cases remains higher than in corresponding rotating HD cases due to the vertical stretching and collimation of flow structures along the vertical magnetic field lines. This also suggests that the mean magnetic field mitigates the instability- suppressing effect of the Coriolis force. The turbulent kinetic energy budget reveals the conversion of the turbulent kinetic energy, generated by the buoyancy flux, into turbulent magnetic energy.