Zero Prandtl-number rotating magnetoconvection (1712.07340v1)

Abstract: We investigate instabilities and chaos near the onset of Rayleigh-B\'{e}nard convection (RBC) of electrically conducting fluids with free-slip, perfectly electrically and thermally conducting boundary conditions in presence of uniform rotation about vertical axis and horizontal external magnetic field by considering zero Prandtl-number limit ($\mathrm{Pr} \rightarrow 0$). Direct numerical simulations (DNS) and low dimensional modeling of the system are done for the investigation. Values of the Chandrasekhar number ($\mathrm{Q}$) and the Taylor number ($\mathrm{Ta}$) are varied in the range $0 < \mathrm{Q}, \mathrm{Ta} \leq 50$. Depending on the values of the parameters in the chosen range and choice of initial conditions, onset of convection is found be either periodic or chaotic. Interestingly, it is found that chaos at the onset can occur through four different routes namely homoclinic, intermittency, period doubling and quasi-periodic routes. Homoclinic and intermittent routes to chaos at the onset occur in presence of weak magnetic field ($\mathrm{Q} < 2$), while period doubling route is observed for relatively stronger magnetic field ($\mathrm{Q} \geq 2$) for one set of initial conditions. On the other hand, quasiperiodic route to chaos at the onset is observed for another set of initial conditions. However, the rotation rate (value of $\mathrm{Ta}$) also plays an important role in determining the nature of convection at the onset. Analysis of the system simultaneously with DNS and low dimensional modeling helps to clearly identify different flow regimes concentrated near the onset of convection and understand their origins. The periodic or chaotic convection at the onset is found to be connected with rich bifurcation structures involving subcritical pitchfork, imperfect pitchfork, supercritical Hopf, imperfect homoclinic gluing and Neimark-Sacker bifurcations.