The response of a turbulent accretion disc to an imposed epicyclic shearing motion

We excite an epicyclic motion, the amplitude of which depends on the vertical position, z, in a simulation of a turbulent accretion disc. An epicyclic motion of this kind may be caused by a warping of the disc. By studying how the epicyclic motion decays, we can obtain information about the interaction between the warp and the disc turbulence. A high-amplitude epicyclic motion decays first by exciting inertial waves through a parametric instability, but its subsequent exponential damping may be reproduced by a turbulent viscosity. We estimate the effective viscosity parameter, α v, pertaining to such a vertical shear. We also gain new information on the properties of the disc turbulence in general, and measure the usual viscosity parameter, α h, pertaining to a horizontal (Keplerian) shear. We find that, as is often assumed in theoretical studies, α v is approximately equal to α h and both are much less than unity, for the field strengths achieved in our local box calculations of turbulence. In view of the smallness (∼0.01) of α v and α h we conclude that for β p gas p mag∼10 the time-scale for diffusion or damping of a warp is much shorter than the usual viscous time-scale. Finally, we review the astrophysical implications.