ABSTRACT
Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments, and is broadly applicable to natural convection systems.
ABSTRACT
The Cartesian dynamo model of Childress and Soward [Phys. Rev. Lett. 29, 837 (1972)] is studied numerically in the regime of low viscosity. Dynamos with Ekman numbers E in the range 10(-4) > or =E> or =5 x 10(-7) are discussed and compared with the corresponding nonmagnetic states and with results obtained for imposed magnetic fields. We find that in the range of investigated Ekman numbers, a transition occurs from a flow regime where the planform of convection is only weakly affected by the dynamo generated field to a regime where the typical length scales of the flow are largely controlled by the Lorentz forces. The magnetic field acts to facilitate convection and leads to an increase in both the heat transport and in the amplitude of the flow. We demonstrate that this convection promoting effect allows for dynamo action even for Rayleigh numbers below the critical Rayleigh number for the onset of nonmagnetic convection.
