Much of my work has concerned studies of the local hydrodynamic and MHD processes that can lead to outward angular momentum transport in accretion disks. As computers become more powerful, previous studies of local patches of an accretion flow are being expanded into global studies that encompass the entire disk. Angular Momentum Transport Via Vertical Convection Vertical convection can occur in an accretion disk if a sufficiently steep entropy gradient is established via cooling of the surface layers by optically thin radiation. To study whether hydrodynamical turbulence generated by strong convection can transport angular momentum in disks, 3D direct numerical simulations of nonlinear convective in a local patch of a shearing disk were performed. The image shows the magnitude of vertical velocities on three slice planes through the computational volume.
Red regions are updrafts, blue regions are downdrafts.
The convective eddies are clearly stretched into long, thin sheets in the azimuthal direction via shear. Surprisingly, the time-averaged Reynolds stress for driven convective turbulence in a Keplerian disk is both small and negative, indicating angular momentum is transported inwards, if at all. To study the nonlinear regime of the MRI in vertically stratified disks, 3D MHD simulations were performed. As in unstratified flows, the MRI saturates as anisotropic MHD turbulence. The image below again shows the density, as well as magnetic field vectors along a horizontal slice just below the disk midplane.