Repeated local simulations have failed to find any evidence for a
purely hydrodynamic local nonlinear instability. The absence of
magnetic fields has always resulted in the decay of any imposed
turbulence, leading to the conclusion that there is no turbulent
in unmagnetized disks. While there are known global
instabilities in hydrodynamic disks such as the Papaloizou-Pringle
instability, or local violations of the Høiland criteria, the
circumstances under which these manifest themselves are known, and
these instabilities are unlikely to be generally significant.
As a control a hydrodynamic cylindrical disk is computed. This disk begins with a Keplerian disk that is MHD turbulent. The simulation consists of turning off the magnetic forces and following the subsequent hydrodynamic evolution. The outcome is straightforward: the disk turbulence dies out promptly. All vertical structure decays away and the disk becomes two dimensional. Global spiral waves continue to propagate, but their amplitudes are reduced from those seen with active MHD, particularly for larger azimuthal wavenumbers.
Title Page
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5.3 Stress at the marginally stable orbit
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5.5 Coherent Structures