Causal Viscosity in Accretion Disc Boundary layers

Details Causal Viscosity in Accretion Disc Boundary layers Causal Viscosity in Accretion Disc Boundary layers

Feb 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997mnras.285..239k&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 285, Issue 2, pp. 239-252.

Astronomy and Astrophysics

Astronomy

12

Accretion, Accretion Discs: Hydrodynamics

Scientific paper

We study the structure of the boundary layer region between the disc and a comparatively slowly rotating star using a causal prescription for viscosity. We allow the vertically integrated viscous stress to relax towards its equilibrium value on a relaxation time-scale tau. This naturally yields a finite speed of propagation for viscous information. For a standard alpha prescription with alpha in the range 0.1-0.01, and ratio of viscous speed to sound speed in the range 0.02-0.5, details in the boundary layer are strongly affected by the causality constraint. We study both steady-state polytropic models and time-dependent models taking into account energy dissipation and transport. Steady-state solutions are always subviscous with a variety of Omega profiles which may exhibit near discontinuities. For alpha=0.01 and small viscous speeds the boundary layer adjusted to a near steady state. Sometimes a long-wavelength oscillation generated by viscous overstability could be seen near the outer boundary. Being confined there, the boundary layer remained almost stationary. However, for alpha=0.1, and large viscous speeds, short-wavelength disturbances were seen throughout which could significantly affect the power output in the boundary layer. Behaviour of this kind is potentially important in producing time-dependent behaviour in accreting systems such as cataclysmic variables (CVs) and protostars.

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