2009ApJ...702.1536B

Using a 2.5D time-dependent numerical code we recently developed, we solve the full compressible Navier-Stokes equations to determine the structure of the boundary layer (BL) between the white dwarf (WD) and the accretion disk in nonmagnetic cataclysmic variable systems. In this preliminary work, our numerical approach does not include radiation. In the energy equation, we either take the dissipation function (Φ) into account or we assume that the energy dissipated by viscous processes is instantly radiated away (Φ = 0). For a slowly rotating nonmagnetized accreting WD, the accretion disk extends all the way to the stellar surface. There, the matter impacts and spreads toward the poles as new matter continuously piles up behind it. We carry out numerical simulations for different values of the alpha-viscosity parameter (α), corresponding to different mass accretion rates. In the high viscosity cases (α = 0.1), the spreading BL sets off a gravity wave in the surface matter. The accretion flow moves supersonically over the cusp making it susceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz shearing instabilities. This BL is optically thick and extends more than 30° to either side of the disk plane after only 3/4 of a Keplerian rotation period (t_K= 19 s). In the low viscosity cases (α = 0.001), the spreading BL does not set off gravity waves and it is optically thin.